Jean Secondi

Aurélie Davranche

Marc Théry

Nathalie Mondy

Thierry Lengagne

Assessing the effects of artificial light at night on biodiversity across latitude -Current knowledge gaps

Keywords: ALAN, light pollution, intertropical, latitude, global change, biogeography, exposure

1

ALAN as an emergent global risk for biodiversity

Habitat loss, climate change, pollution, invasive species, and overexploitation negatively affect a large part of the Earth's ecosystems, if not all of them (Millenium Ecosystem Assessment, 2005). Artificial light at night (ALAN) is emerging as another potential threat to biodiversity worldwide (Bennie et al., 2015b;[START_REF] Davies | Why artificial light at night should be a focus for global change research in the 21st century[END_REF]. ALAN is mainly generated by urbanized areas and transport infrastructures, and exposure happens either via direct lights or via skyglow. The latter is produced by the scattering of light by atmospheric particles and aerosols from distant sources. It can be detected dozens of kilometres away from a major source like a city by animals [START_REF] Kocifaj | Quantitative analysis of night skyglow amplification under cloudy conditions[END_REF][START_REF] Aubé | Physical behaviour of anthropogenic light propagation into the nocturnal environment[END_REF], whose sensitivity can be very high for nocturnal species [START_REF] Kelber | Nocturnal colour vision-not as rare as we might think[END_REF][START_REF] Kelber | Thresholds and noise limitations of colour vision in dim light[END_REF]. ALAN now alters the natural photic regime across a large fraction of Earth [START_REF] Cinzano | The first World Atlas of the artificial night sky brightness[END_REF][START_REF] Longcore | Ecological light pollution[END_REF]. Recently, it has been estimated that 23 % of land surfaces between 60°S and 75°N are exposed to light polluted skies [START_REF] Falchi | The new world atlas of artificial night sky brightness[END_REF].

ALAN has been gaining attention from the scientific communities, authorities, and more generally societies. Concerns have initially sprung from astronomers that warned about the loss of quality of the nocturnal sky, and spread to other fields. New issues have been raised and expressed in reviews [START_REF] Longcore | Ecological light pollution[END_REF][START_REF] Navara | The dark side of light at night: Physiological, epidemiological, and ecological consequences[END_REF][START_REF] Hölker | Light pollution as a biodiversity threat[END_REF][START_REF] Gaston | The ecological impacts of nighttime light pollution: A mechanistic appraisal[END_REF][START_REF] Gaston | The biological impacts of artificial light at night: the research challenge[END_REF] and opinions [START_REF] Nadis | Biologists join drive to turn down the lights[END_REF][START_REF] Smith | Time to turn off the lights[END_REF] for the last 15 years. ALAN generates a broad array of effects on animals, and as far as we know to a lesser extent on plants or micro-organisms [START_REF] Hölker | Microbial diversity and community respiration in freshwater sediments influenced by artificial light at night[END_REF][START_REF] Ffrench-Constant | Light pollution is associated with earlier tree budburst across the United Kingdom[END_REF][START_REF] Bennie | Artificial light at night alters grassland vegetation species composition and phenology[END_REF]. These effects range from the molecular [START_REF] Honnen | Sex-specific gene expression in the mosquito Culex pipiens f. molestus in response to artificial light at night[END_REF] to the community (Rich & Longcore, 2006a;[START_REF] Hölker | Microbial diversity and community respiration in freshwater sediments influenced by artificial light at night[END_REF][START_REF] Bennie | Artificial light at night alters grassland vegetation species composition and phenology[END_REF], and ecosystem level [START_REF] Lewanzik | Artificial light puts ecosystem services of frugivorous bats at risk[END_REF][START_REF] Sanders | Artificial nighttime light changes aphid-parasitoid population dynamics[END_REF][START_REF] Knop | Artificial light at night as a new threat to pollination[END_REF]. As for the other major risk factors for biodiversity, ecosystem exposure to ALAN has been assessed at the global scale (Bennie et al., 2015b;[START_REF] Davies | Stemming the tide of light pollution encroaching into marine protected areas[END_REF][START_REF] Falchi | The new world atlas of artificial night sky brightness[END_REF]Kyba et al., 2017a;[START_REF] Guetté | Worldwide increase in artificial light at night around protected areas and within biodiversity hotspots[END_REF]. The field is expanding fast as revealed by the recent bloom of publications [START_REF] Davies | Why artificial light at night should be a focus for global change research in the 21st century[END_REF]. Nevertheless,no framework has been developed to predict whether and how the intensity of the effects caused by ALAN varies across the globe yet.

Photoperiod is crucial for many biological processes and ecological interactions. It is a major environmental cue that resets circadian rhythms and prime seasonal change in activity [START_REF] Coppack | Photoperiodic response and the adaptability of avian life cycles to environmental change[END_REF][START_REF] Hut | Latitudinal clines: an evolutionary view on biological rhythms[END_REF]. Photoperiod varies widely with latitude and this affects the importance of light as a cue depending on the location on Earth. Similarly, a general biogeographic rule is the occurrence of latitudinal biodiversity clines, the intertropical zone hosting more species than temperate or arctic environments [START_REF] Willig | Latitudinal gradients of biodiversity: pattern, process, scale, and synthesis[END_REF][START_REF] Hillebrand | On the generality of the latitudinal diversity gradient[END_REF][START_REF] Mittelbach | Evolution and the latitudinal diversity gradient: Speciation, extinction and biogeography[END_REF]. Exceptions have been observed [START_REF] Willig | Latitudinal gradients of biodiversity: pattern, process, scale, and synthesis[END_REF] but the statement holds true for vertebrates such as freshwater fishes [START_REF] Abell | Freshwater Ecoregions of the World: A New Map of Biogeographic Units for Freshwater Biodiversity Conservation[END_REF], birds, mammals, amphibians [START_REF] Jenkins | Conservation implications of divergent global patterns of ant and vertebrate diversity[END_REF], insects [START_REF] Guenard | Global models of ant diversity suggest regions where new discoveries are most likely are under disproportionate deforestation threat[END_REF], and vascular plants [START_REF] Kreft | Global patterns and determinants of vascular plant diversity[END_REF]. Surprisingly, while the major biodiversity hotspots are mainly located within the intertropical zone, we do not know whether the effects of ALAN in biodiversity are stronger at low than at mid or high latitudes.

From a literature survey on Scopus (accessed on 22/09/2019) using the combination of keywords (tropic* OR equat*) and "artificial light", "artificial" AND "light", or "light pollution", we identified eleven references [START_REF] Erkhert | Light-induced activity optimum in night monkeys (Aotus trivirgatus)[END_REF][START_REF] Thakurdas | Light at night alters the parameters of the eclosion rhythm in a tropical fruit fly, Drosophila jambulina[END_REF][START_REF] Lewanzik | Artificial light puts ecosystem services of frugivorous bats at risk[END_REF]Bennie et al., 2015b;[START_REF] Dorado-Correa | Anthropogenic noise, but not artificial light levels predicts song behaviour in an equatorial bird[END_REF][START_REF] González-Bernal | Toads in the backyard: why do invasive cane toads (Rhinella marina) prefer buildings to bushland?[END_REF][START_REF] Gupta | Increased Incidence of Breast Cancer Due to Long Exposure of Light[END_REF][START_REF] De Freitas | Exposure of tropical ecosystems to artificial light at night: Brazil as a case study[END_REF][START_REF] Gineste | Tropical Shearwater population stability at Reunion Island, despite light pollution[END_REF][START_REF] Rivas | Effects of light and temperature on daily activity and clock gene expression in two mosquito disease vectors[END_REF][START_REF] Frank | Impact of artificial lights on foraging of insectivorous bats in a Costa Rican cloud forest[END_REF]. Another search using the combination ("artificial light" OR "light pollution") AND "latitud*" accessed on Scopus on the same date yielded 59 hits. Four additional and partly relevant references were found. These were three reviews in which artificial lighting was not the main topic [START_REF] Higuchi | adaptation to natural and artificial light: Variation in circadian photosensitivity. Human Variation: From the Laboratory to the Field[END_REF][START_REF] Borniger | Photoperiodic regulation of behavior: Peromyscus as a model system[END_REF][START_REF] Brelsford | The influence of spectral composition on spring and autumn phenology in trees[END_REF], and one research paper on humans [START_REF] Martín-Olalla | Seasonal synchronization of sleep timing in industrial and pre-industrial societies[END_REF]. In comparison, a search using "light pollution" OR "artificial light", and restricted to the Agricultural and Biological Sciences section yielded 233 relevant references. More information is certainly present but not necessarily in papers addressing directly ALAN as previously noted (Rich & Longcore, 2006b). Nevertheless, these figures highlight the publication bias between biomes and the fact that authors do not explicitly consider the biogeographical context.

Assessing the biological and ecological effects of ALAN at a global scale is now urgent. Yet, neither the systematic investigation of environmental factors varying at a biogeographic scale and affecting the sensitivity of organisms to ALAN, nor the identification of the areas/ecosystems/biomes the most threatened now or in the near future, has been undertaken. We need a biogeographic framework to assess how the exposure risk for biodiversity varies across ecosystems or regions, and we present here the first attempt in that direction. We will focus on the latitudinal variation of environmental factors that affect the photic regime, summarize what biological processes vary with latitude, and provide a prospective view of the evolution of ALAN with a particular emphasis on the intertropical zone.

2

The geographical extent of ALAN and its effect on biodiversity ALAN exposure at a given site depends on the distance to the light sources, their power and spectral composition [START_REF] Luginbuhl | The impact of light source spectral power distribution on sky glow[END_REF], the natural light regime (moon phase, visual barriers), and the atmosphere composition, including the presence and altitude of clouds. Cloud cover reflects artificial light downward and over large distances. For this reason, it has been identified as a major factor enhancing nocturnal light levels [START_REF] Kyba | Cloud coverage acts as an amplifier for ecological light pollution in urban ecosystems[END_REF][START_REF] Kocifaj | Quantitative analysis of night skyglow amplification under cloudy conditions[END_REF]. ALAN intensity, as measured by satellites, has kept increasing worldwide for the last decades, with local or regional variations [START_REF] Falchi | The new world atlas of artificial night sky brightness[END_REF], and changes were still recently detected over a short 5-year period for most countries (Kyba et al., 2017a). All major ecosystems are now exposed (Bennie et al., 2015b), including the intertropical zone (Figure 1). To highlight this point, we used the data from the last published World Atlas [START_REF] Falchi | The new world atlas of artificial night sky brightness[END_REF] and plotted the latitudinal distribution of ALAN in 5° bins.

The highest mean values were observed between 30° and 40° in both hemispheres. The standard deviation was maximal for northern temperate latitudes. However, the largest variation occurred around 10°N which is explained by the large expanses of darker land in Africa and brightly lit areas in northern Venezuela, Southern India, and Malaysia. ALAN keeps on increasing in this area. Between 2012 and 2016, an elevation of upward radiance in the intertropical zone was detected at the country level (Kyba et al., 2017a), while up to 35 % of the area covered by tropical ecosystems in Brazil experienced higher brightness at night between [START_REF] Mills | Latitudinal gradients of biologically useful semi-darkness[END_REF]and 2012[START_REF] De Freitas | Exposure of tropical ecosystems to artificial light at night: Brazil as a case study[END_REF].

Insert Figure 1

Light intensities consistent with field measurements of ALAN alter the physiology of organisms, the behaviour of animals, and ecological processes. We will only outline the main traits and processes relevant to this article. For more extensive treatment, several review articles and book chapters have been published that can be referred to [START_REF] Longcore | Ecological light pollution[END_REF]Rich & Longcore, 2006a;[START_REF] Perry | Effects of artificial night lighting on amphibians and reptiles in urban environments[END_REF][START_REF] Gaston | The ecological impacts of nighttime light pollution: A mechanistic appraisal[END_REF][START_REF] Gaston | The biological impacts of artificial light at night: the research challenge[END_REF][START_REF] Gaston | Impacts of artificial light at night on biological timings[END_REF][START_REF] Stevens | Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem? Philosophical transactions of the Royal Society of London[END_REF][START_REF] Grubisic | Insect declines and agroecosystems: does light pollution matter?[END_REF][START_REF] Owens | The impact of artificial light at night on nocturnal insects: A review and synthesis[END_REF][START_REF] Desouhant | Mechanistic, ecological, and evolutionary consequences of artificial light at night for insects: review and prospective[END_REF]. ALAN affects activity and time partitioning (Rich & Longcore, 2006a;[START_REF] Tallec | Light pollution modifies the expression of daily rhythms and behavior patterns in a nocturnal primate[END_REF][START_REF] Dominoni | Individual-based measurements of light intensity provide new insights into the effects of artificial light at night on daily rhythms of urban-dwelling songbirds[END_REF][START_REF] Da Silva | Light pollution alters the phenology of dawn and dusk singing in common European songbirds[END_REF], orientation [START_REF] Salmon | Artificial lighting and sea turtles[END_REF][START_REF] Pawson | LED lighting increases the ecological impact of light pollution irrespective of color temperature[END_REF][START_REF] Thums | Artificial light on water attracts turtle hatchlings during their near shore transit[END_REF], space use [START_REF] Moore | Urban light pollution alters the diel vertical migration of Daphnia[END_REF][START_REF] Stone | Street lighting disturbs commuting bats[END_REF], foraging [START_REF] Dwyer | Shedding light on light: Benefits of anthropogenic illumination to a nocturnally foraging shorebird[END_REF][START_REF] Cravens | Illuminating prey selection in an insectivorous bat community exposed to artificial light at night[END_REF][START_REF] Van Langevelde | Artificial night lighting inhibits feeding in moths[END_REF], social interactions including sexual behaviour [START_REF] Kurvers | Bright nights and social interactions: A neglected issue[END_REF][START_REF] Van Geffen | Artificial light at night inhibits mating in a Geometrid moth[END_REF][START_REF] Firebaugh | Experimental tests of light-pollution impacts on nocturnal insect courtship and dispersal[END_REF], and lifehistory traits [START_REF] Van Geffen | Artificial light at night causes diapause inhibition and sex-specific life history changes in a moth[END_REF]. Foraging behaviour can be modified in opposite ways depending on the focal species [START_REF] Dwyer | Shedding light on light: Benefits of anthropogenic illumination to a nocturnally foraging shorebird[END_REF][START_REF] Cravens | Illuminating prey selection in an insectivorous bat community exposed to artificial light at night[END_REF]. At the sensory level, the effects of ALAN primarily depend on the spectrum and intensity of ambient light, and the photoreceptor characteristics [START_REF] Endler | On the measurement and classification of color in studies of animal color patterns[END_REF]. Individuals, in insects or amphibians for instance, can be attracted or repelled by unusual brightly lit areas [START_REF] Hailman | Phototactic responses to spectrally dominant stimuli and use of colour vision by adult anuran amphibians: A comparative survey[END_REF][START_REF] Owens | The impact of artificial light at night on nocturnal insects: A review and synthesis[END_REF], but more subtle process can be altered, like colour vision. Dim-light colour vision based on cones in insects and reptiles [START_REF] Kelber | Nocturnal colour vision-not as rare as we might think[END_REF], and rods in amphibians [START_REF] Gomez | The role of nocturnal vision in mate choice: female prefer conspicuous males in the European treefrog (Hyla arborea)[END_REF][START_REF] Kelber | Thresholds and noise limitations of colour vision in dim light[END_REF] has now been demonstrated. Therefore, the capacity of individuals to detect and select food items, or catch prey can be modified under ALAN because the contrast between the target and the background is altered.

Regarding physiology, ALAN is a major source of circadian disruption that generates metabolism, sleep and cognitive disorders, depression syndrome, and may increase risk of some cancers [START_REF] Stevens | Adverse health effects of nighttime lighting: Comments on american medical association policy statement[END_REF][START_REF] Borniger | Acute dim light at night increases body mass, alters metabolism, and shifts core body temperature circadian rhythms[END_REF][START_REF] Bedrosian | Endocrine effects of circadian disruption[END_REF][START_REF] Chellappa | Daily circadian misalignment impairs human cognitive performance task-dependently[END_REF]. It inhibits the production of melatonin, a hormone that modifies stress response, mitochondria functioning, oxidative stress, metabolism, and immune defence [START_REF] Carrillo-Vico | A review of the multiple actions of melatonin on the immune system[END_REF][START_REF] Tan | The changing biological roles of melatonin during evolution: From an antioxidant to signals of darkness, sexual selection and fitness[END_REF][START_REF] Tallec | Melatonin concentrations and timing of seasonal reproduction in male mouse lemurs ( Microcebus murinus ) exposed to light pollution[END_REF]. Melatonin holds a central role in these processes because of its multifarious effects on circadian timekeeping and the regulation of biological rhythms [START_REF] Hardeland | Melatonin-A pleiotropic, orchestrating regulator molecule[END_REF]. The circadian variation of melatonin has been observed from bacteria to vertebrates [START_REF] Tan | The changing biological roles of melatonin during evolution: From an antioxidant to signals of darkness, sexual selection and fitness[END_REF], and the effects of this molecule on clock genes have been demonstrated in vertebrates and arthropods. These molecular mechanisms likely date back at least to the cnidarian-bilaterian divergence [START_REF] Peres | Developmental and light-entrained expression of melatonin and its relationship to the circadian clock in the sea anemone Nematostella vectensis[END_REF], which illustrates the taxonomic breadth of groups potentially affected by ALAN. The strength of the effects may depend on the diel activity patterns, with cathemeral species being probably less sensitive to changes in photoperiod than diurnal, nocturnal or crepuscular species. In plants, effects on physiological processes, including photosynthesis, and photoperiodism are expected [START_REF] Aubé | Evaluating potential spectral impacts of various artificial lights on melatonin suppression, photosynthesis, and star visibility[END_REF][START_REF] Singhal | Eco-physiological Responses of Artificial Night Light Pollution in Plants[END_REF]. Individuals need to be relatively close to the light sources but physiological effects have been observed [START_REF] Bennie | Ecological effects of artificial light at night on wild plants[END_REF][START_REF] Bennie | Artificial light at night alters grassland vegetation species composition and phenology[END_REF][START_REF] Ffrench-Constant | Light pollution is associated with earlier tree budburst across the United Kingdom[END_REF]. The uncoupling between the thermal and photic cues caused by ALAN may affect the timing of seasonal processes like dormancy [START_REF] Aubé | Evaluating potential spectral impacts of various artificial lights on melatonin suppression, photosynthesis, and star visibility[END_REF][START_REF] Hut | Latitudinal clines: an evolutionary view on biological rhythms[END_REF][START_REF] Ffrench-Constant | Light pollution is associated with earlier tree budburst across the United Kingdom[END_REF][START_REF] Solano-Lamphar | Numerical research on the effects the skyglow could have in phytochromes and RQE photoreceptors of plants[END_REF]. However, the actual consequences for plant population largely remain to be investigated [START_REF] Knop | Artificial light at night as a new threat to pollination[END_REF][START_REF] Macgregor | Effects of street lighting technologies on the success and quality of pollination in a nocturnally pollinated plant[END_REF].

Because of the biological effects cited above, ALAN is expected to affect ecological processes like energy or nutrient fluxes or relationships in trophic networks [START_REF] Longcore | Ecological light pollution[END_REF][START_REF] Perkin | The influence of artificial light on stream and riparian ecosystems: questions, challenges, and perspectives[END_REF][START_REF] Gaston | The ecological impacts of nighttime light pollution: A mechanistic appraisal[END_REF][START_REF] Meyer | Bright lights, big city: Influences of ecological light pollution on reciprocal stream-riparian invertebrate fluxes[END_REF]Bennie et al., 2015a;[START_REF] Mathews | Barriers and benefits: implications of artificial night-lighting for the distribution of common bats in Britain and Ireland[END_REF][START_REF] Sanders | Artificial nighttime light changes aphid-parasitoid population dynamics[END_REF][START_REF] Knop | Artificial light at night as a new threat to pollination[END_REF][START_REF] Macgregor | The dark side of street lighting: impacts on moths and evidence for the disruption of nocturnal pollen transport[END_REF][START_REF] Miller | Combined effects of night warming and light pollution on predator -Prey interactions[END_REF]. More generally, the synchronisation between the components of any interspecific interactions can be altered (Bennie et al., 2015a;[START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF][START_REF] Kronfeld-Schor | Chronobiology of interspecific interactions in a changing world[END_REF]. Therefore, behavioural and physiological effects on individuals may scale up to the population, community and ecosystem level [START_REF] Sanders | How ecological communities respond to artificial light at night[END_REF]. Evidence of disruption on micro-organism, arthropod, and plant community has been provided [START_REF] Meyer | Bright lights, big city: Influences of ecological light pollution on reciprocal stream-riparian invertebrate fluxes[END_REF][START_REF] Pawson | LED lighting increases the ecological impact of light pollution irrespective of color temperature[END_REF][START_REF] Hölker | Microbial diversity and community respiration in freshwater sediments influenced by artificial light at night[END_REF][START_REF] Bennie | Artificial light at night alters grassland vegetation species composition and phenology[END_REF][START_REF] Knop | Artificial light at night as a new threat to pollination[END_REF], but so far no evidence of large scale effects on ecosystems have been reported.

Nevertheless, studies showing reduced efficiency of plant pollination by insects [START_REF] Knop | Artificial light at night as a new threat to pollination[END_REF][START_REF] Macgregor | The dark side of street lighting: impacts on moths and evidence for the disruption of nocturnal pollen transport[END_REF] and bats [START_REF] Lewanzik | Artificial light puts ecosystem services of frugivorous bats at risk[END_REF], or increased predation on agricultural pest by insect [START_REF] Miller | Combined effects of night warming and light pollution on predator -Prey interactions[END_REF] suggest that large effects may occur in natural environments and agrosystems.

3

Natural factors modulating exposure to ALAN at the global scale

To understand how ALAN affects organisms and populations depending on their position on the globe, the geographical variation of factors that naturally determine the light-dark regime should be considered: photoperiod, lunar cycle, twilight, and climate. Photoperiod varies in a predictable way, with annual variation in daylength increasing with latitude, from about 0 % at the Equator to 100 % at the Arctic Circle.. In other words, photoperiod is more constant towards the Equator and more variable towards the poles.

Moonlight is the brightest nocturnal light source in pristine environments. The lunar cycle lasts 29.52 days and is synchronous at all latitudes. While Moon phase and Earth-Moon distance change, the former parameter contributes much more to the variation of illuminance. Natural maximal illuminance on Earth and under optimal conditions is 0.32 lux and in the intertropical zone only. Because of its orbital plane, the Moon can be seen at the sky zenith, where illuminance is maximal, only at locations lower than 28° latitude (Kyba et al., 2017b). Thus, the Moon is less bright at higher latitudes.

Illuminance lies typically within 0.05-0.2 lux at mid-latitude (Kyba et al., 2017b). Brightness varies with season because of the change in the distance to the Sun and elevation. Moon elevation is higher in winter than in summer.

The variation in duration and intensity of moonlight cannot be used to determine the exact time or date. Yet, moonlight is more and more recognised as a major feature on the nocturnal environment for animals. Effects on activity, orientation, and communication have been observed in invertebrates and vertebrates [START_REF] Dacke | Insection orientation to polarized moonlight[END_REF][START_REF] Grant | The lunar cycle: a cue for amphibian reproductive phenology?[END_REF][START_REF] Kronfeld-Schor | Chronobiology by moonlight[END_REF]. In particular, moonlight is expected to suppress activity to reduce predation risk, especially in open habitat species.

This prediction was confirmed in amphibians [START_REF] Vignoli | Better in the dark: Two Mediterranean amphibians synchronize reproduction with moonlit nights[END_REF]) and all groups of nocturnal mammals but primates for which higher activity during bright nights may reflect a better ability to detect predators and forage [START_REF] Prugh | Does moonlight increase predation risk? Meta-analysis reveals divergent responses of nocturnal mammals to lunar cycles[END_REF]. One direct consequence of ALAN is that elevated brightness tends to mask the natural photic variation as moonlight is attenuated against the skyglow background ( Davies et al., 2013a).

Twilight is the transition between day and night when Sun drops under the horizon. It is split into three periods that are characterized by the Sun declination and the colour of ambient light. During the civil twilight the sun declination lies between 0 and 6°. Light level drops sharply during the nautical twilight (declination 6-12°) and less quickly during the astronomical twilight (declination 12-18°).

Irradiance turns from a red dominant spectrum to a blue dominant spectrum [START_REF] Johnsen | Measuring Light. The Optics of Life: A Biologist's Guide to Light in[END_REF]. After twilight, the irradiance spectrum of the sky depends on the Moon and other light sources. There is a latitudinal cline in twilight duration that is shorter at lower latitudes (Figure 2). Some authors have considered as "semi-darkness" the merged periods of twilight and lunar phases where at least 26 % of the Moon is visible [START_REF] Mills | Latitudinal gradients of biologically useful semi-darkness[END_REF]. Darkness is a relatively vague concept given the variable visual capacity of species under dim light [START_REF] Kelber | Nocturnal colour vision-not as rare as we might think[END_REF]. Nevertheless, the duration of natural semidarkness remains constant around 4.5h at the Equator and varies widely from 0h to 9h at 75° [START_REF] Mills | Latitudinal gradients of biologically useful semi-darkness[END_REF] and forms a gradient parallel to the photoperiod gradient. ALAN generates light levels in the same range of those observed during natural twilight and can, therefore, extend the duration of this period.

Circadian rhythms which are reset with twilight. The rapid changes in light intensity and colour are reliable cues to determine the onsets and the ends of activities like foraging or mating [START_REF] Daan | Circadian rhythms of locomotor activity in captive birds and mammals: Their variations with season and latitude[END_REF][START_REF] Andersson | Light, predation and the lekking behaviour of the ghost swift Hepialus humuli (L.) (Lepidoptera, Hepialidae)[END_REF][START_REF] Boulos | Season-and latitude-dependent effects of simulated twilights on circadian entrainment[END_REF].

The importance of cloud cover in the global assessment of ALAN has probably been overlooked.

Clouds amplify light emitted from the ground, extend the photophase in light-polluted areas whereas they mask celestial objects, dim their lights, and shorten the photophase in non-polluted areas. It is important to keep in mind that global models of ALAN underestimate exposure at ground level because they use upward radiance recorded from satellite images. The representation of a cloud-free world elicited by these images may thus bias our perception of the phenomenon. The spatial extent of ALAN during overcast nights is much broader [START_REF] Kyba | Cloud coverage acts as an amplifier for ecological light pollution in urban ecosystems[END_REF][START_REF] Kocifaj | Quantitative analysis of night skyglow amplification under cloudy conditions[END_REF]. A logical conclusion is that, all other factors being equal, the intensity, spread, and the annual number of days with high levels of ALAN should be related to the annual number of days with overcast skies, although rain or mist could strongly attenuate light transmission. Because cloud cover is crucial to determine the spatial extent of ALAN, it is important to consider how uneven its annual distribution on Earth is. A recent multi-year study of cloud cover mapped the average cloud fraction (number of pixels covered) across the World during the day and at night. Figure 1 shows the latitudinal distribution of cloud fraction between July 2002 and April 2018 excluding seas and large lakes. Higher values were observed in the Arctic and subarctic regions and within the intertropical zone where cloud cover was particularly high in Central Africa, the Amazonian basin and Central America, India, and south-east Asia [START_REF] King | Spatial and temporal distribution of clouds observed by MODIS onboard the terra and aqua satellites[END_REF]. The annual variation in cloud cover is also the lowest for both areas. Owing to latitude and cloud cover, organisms in the intertropical zone often experience a stable photoperiod and on average a darker visual environment than in any other region on the globe. In the tropics, the variation in cloud cover is maximal and cloud cover low due to the presence of areas with arid or humid climates. Changes in the fraction of high clouds, and precipitation have been observed in the intertropical convergence zone but the expected trends for the next decades are still debated [START_REF] Legates | Arctic albedo changes are small compared with changes in cloud cover in the tropics[END_REF][START_REF] Mauritsen | Missing iris effect as a possible cause of muted hydrological change and high climate sensitivity in models[END_REF][START_REF] Norris | Evidence for climate change in the satellite cloud record[END_REF][START_REF] Wodzicki | Long-term characterization of the Pacific ITCZ using TRMM, GPCP, and ERA-Interim[END_REF]. Rainfall is also expected to decrease or increase depending on regional trends [START_REF] Kitoh | Monsoons in a changing world: A regional perspective in a global context[END_REF]IPCC, 2014), and uncertainty is higher for rainfall than temperature models especially in the tropics [START_REF] Corlett | Climate change in the tropics: The end of the world as we know it?[END_REF]. Furthermore, a consensual prediction is a higher frequency of extreme rainfall events, largely in the form of storms shorter than a day [START_REF] Westra | Future changes to the intensity and frequency of short-duration extreme rainfall[END_REF], which may not affect the annual number of overcast days. More robustness in climate models related to cloud cover and precipitation is needed to more precisely assess the exposure of ecosystems to ALAN across the Earth.

Insert Figure 2 4 Biological traits vary with latitude

Because of its influence on annual daylength variation, climate, and its current correlation with economic development, latitude is a key factor that is expected to modulate the sensitivity of populations to ALAN. We illustrate how the latitudinal variation in photoperiod influences variation in circadian rhythms, photoperiodism for major life-history traits like reproduction or migration [START_REF] Hut | Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod[END_REF][START_REF] Van Geffen | Artificial light at night causes diapause inhibition and sex-specific life history changes in a moth[END_REF][START_REF] Gaston | Impacts of artificial light at night on biological timings[END_REF][START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF], or the characteristics of sensory systems [START_REF] Yammouni | A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish[END_REF]. Annual life-history events are often synchronised with photoperiod so that organisms anticipate changes in forthcoming environmental conditions. Latitudinal variation in photoperiod determines changes in the timing of major events like migration, reproduction in birds, or diapause in insects. In birds, photoperiod is a major cue for the onset of gonadal maturation, moult, and migratory activity [START_REF] Rowan | Relation of light to bird migration and developmental changes[END_REF][START_REF] Gwinner | Circannual clocks in avian reproduction and migration[END_REF][START_REF] Dawson | Photoperiodic control of seasonality in birds[END_REF][START_REF] Coppack | Photoperiodic response and the adaptability of avian life cycles to environmental change[END_REF][START_REF] Bauer | The relevance of environmental conditions for departure decision changes en route in migrating geese[END_REF]. Great tits Parus major need longer days to initiate sexual maturation at mid-latitude than at higher latitudes, causing a cline in the onset of male gonadal maturation [START_REF] Silverin | Photoperiodic adaptation to breeding at different latitudes in great tits[END_REF]. The required daylength variation can be as small as 1h or less to initiate sexual maturation in temperate [START_REF] Dawson | Seasonality in a temperate zone bird can be entrained by near equatorial photoperiods[END_REF] and subtropical species [START_REF] Lewis | Photoperiodic Responses of a Subtropical Population of the Finch (Zonotrichia capensis hypoleuca )[END_REF][START_REF] Hau | A neotropical forest bird can measure the slight changes in tropical photoperiod[END_REF][START_REF] Coppack | Photoperiodic response and the adaptability of avian life cycles to environmental change[END_REF].

Photoperiod also largely contributes to induce diapause and to regulate voltinism in insects. Critical photoperiod for diapauses increases with latitude [START_REF] Danilevskii | Photoperiodism and seasonal development of insects[END_REF][START_REF] Bradshaw | Evolution of Dormancy and its Photoperiodic Control in Pitcher-Plant Mosquitoes[END_REF][START_REF] Hut | Latitudinal clines: an evolutionary view on biological rhythms[END_REF] and the number of diapausing individuals with shorter daylength [START_REF] Schmidt | Demographic processes underlying population growth and decline in Salamandra salamandra[END_REF][START_REF] Hut | Latitudinal clines: an evolutionary view on biological rhythms[END_REF][START_REF] Paolucci | Adaptive latitudinal cline of photoperiodic diapause induction in the parasitoid Nasonia vitripennis in Europe[END_REF]. Photoperiod and local environmental conditions determine the number of generations per year, more generations being produced at lower latitudes [START_REF] Altermatt | Climatic warming increases voltinism in European butterflies and moths[END_REF].

For instance, in the water strider Aquarius remigi, univoltinism is less frequent in southern populations, and change in growth occurred across a narrow range of 5° [START_REF] Blanckenhorn | Life history adaptation along a latitudinal cline in the water strider Aquarius remigis (Heteroptera: Gerridae)[END_REF]. In mammals, latitudinal variation in the timing of the breeding period was observed in the broadly distributed Peromyscus deermice and Odocoileus deers, and photoresponsiveness decreased with latitude in a mouse [START_REF] Bronson | Mammalian reproductive strategies: genes, photoperiod and latitude[END_REF].

Circadian clocks are found from cyanobacteria to plants and animals [START_REF] Hut | Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod[END_REF]. They generate endogenous circadian rhythms that synchronize physiological processes and activities of organisms to the diel variation of environmental conditions. Light is the main cue used to reset circadian clocks, which explains why activity patterns and other behaviours change with latitude. In mammals, the occurrence of nocturnal and diurnal species is higher in the intertropical zone whereas crepuscular and cathemeral species are distributed at higher latitudes, thus reflecting the adaptation to larger variation in annual daylength and longer twilight periods [START_REF] Bennie | Biogeography of time partitioning in mammals[END_REF]. The photic regime of Arctic regions favours behavioural plasticity. For instance, populations of Reindeer Rangifer tarandus above 70°N lose their circadian rhythm of activity in summer and recover it in autumn and spring [START_REF] Van Oort | Circadian organization in reindeer[END_REF], while the Arctic sandpiper Calidris melanotos undergo a drastic reduction of sleep during the mating period as a likely result of prolonged sexual activity under long days conditions [START_REF] Lesku | Adaptive sleep loss in polygynous pectoral sandpipers[END_REF]. A large comparative study on shorebirds detected a latitudinal cline in incubation behaviour [START_REF] Bulla | Unexpected diversity in socially synchronized rhythms of shorebirds[END_REF]. The influence of photoperiod was lower at higher latitudes. Although the dataset is impressive, the study included temperate, boreal and arctic populations between 35° and 75°, and none from the intertropical zone. Latitudinal variation in diel activity patterns is observed at the intraspecific level too. In Drosophila melanogaster, ovoposition peaks at dusk at the beginning of the dark period. In a rare study ranging from 0° to 60°N, authors showed that the percentage of eggs laid during the dark period decreases from 80 % in Equatorial populations to less than 50 % in Northern Scandinavia [START_REF] Allemand | The circadian rhythm of oviposition in Drosophila melanogaster: A genetic latitudinal cline in wild populations[END_REF]. Similarly, an earlier study showed that the onset and end of locomotor activity of five bird and three mammal species held in captivity at 47°N and 66°N varied more between days at higher latitude [START_REF] Daan | Circadian rhythms of locomotor activity in captive birds and mammals: Their variations with season and latitude[END_REF]. Variation was reduced when species activity occurred during the civil twilight when light spectrum changes rapidly. Another study ranging from 37°N to 65°N across Europe found that the onset of dawn singing depended on latitude in some passerines. Under ALAN conditions, it was advanced for latitude-insensitive species only (Da Silva & Kempenaers, 2017).

Sensory systems experience circadian and seasonal regulation. In vertebrates, the expression of photoreceptors, and the detection of colour stimuli follow rhythms that are synchronized by photoperiod [START_REF] Terman | A circadian pacemaker for visual sensitivity ?[END_REF][START_REF] Cahill | Circadian rhythmicity in vertebrate retinas-Cahill and Besharse 1995.pdf[END_REF]. In fishes [START_REF] Shimmura | Seasonal changes in color perception[END_REF] and humans [START_REF] Welbourne | Human colour perception changes between seasons[END_REF] seasonal changes in colour perception occur. The visual system also varies with latitude. In fishes retinomotor movements are observed during which rods and cones move in opposite ways across the retina depending on the time of the day [START_REF] Yammouni | A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish[END_REF]. Retinomotor movements during twilight track the changes in illumination and keep vision functional. They follow endogenous rhythms as demonstrated in individuals kept in the dark or exposed to light during the dark period. Fishes may rely more on changes in ambient illumination and less on endogenous oscillators at higher than at lower latitudes [START_REF] Yammouni | A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish[END_REF].

A predictive biogeographical framework for ALAN is lacking

Beyond the characteristics of the light sources, exposure to ALAN depends on the time of the year, latitude, and climate. Logically, individuals should be less exposed during long-day periods than during short-day periods. Figure 2 illustrates for three towns at low (6°N), medium (43°N), and high (64°N) latitude the monthly variation in sunrise and sunset, as well as the time of the astronomical sunrises and sunsets. Before astronomical sunrise and after astronomical sunset, the sun does not contribute to the ambient light level, and it is a conservative approach to determine when sky brightness will be for sure elevated beyond its natural value by ALAN, at least during moonless nights or under overcast skies. We represented the number of extra-lit hours due to ALAN before and after the astronomical twilight for two lighting scenarios. Lights are turned off at midnight in the first scenario, as an energy-saving measure, and remain turned on all night in the second. The pattern is the same for the two scenarios, but the number of extra-lit hours due to ALAN is larger for the second. At high latitude, there are no extra-lit hours for about 5 months, which period is used for breeding in many arctic species. One can predict the disruption to be mild or non-existent then. In contrast, ALAN duration is maximal during wintertime. Effects could be positive if artificial light allows longer foraging periods or negative if it raises activity and metabolic cost beyond sustainable levels for organisms. At low latitude, the extent of artificial lighting is constant throughout the year. Therefore, disruption may affect any lifehistory events regardless of annual timing. At mid-latitudes, the number of extra-lit hours is minimal around summer equinox, which often corresponds to the end of breeding for birds or amphibians, but is the main active period for insects or bats. The extension of ALAN is maximal in winter and the concern is the same as for arctic organisms with the addition that reproduction occurs in winter for some mammals too. In temperate species, the seasonal effect of ALAN may be more diverse than at lower or higher latitudes and depend on the taxonomic group.

We have knowledge about the latitudinal variation of some traits and expectations about how exposure to ALAN may change across latitudes and seasons. Yet, we still do not know whether and how latitude determines the short or long-term responses of individuals and populations to ALAN. We make some propositions to go forward in that direction by pointing to candidate traits that would deserve more attention. We also make some predictions about the evolutionary responses that could be expected at different latitudes. Most physiological or behavioural traits are potentially affected but we can nevertheless identify some traits to address key issues, namely whether and how the strength of ALANinduced effects varies with latitudes, if some periods of the year are more critical for biodiversity exposed to ALAN, and whether these periods change latitudinally. A major pending question is whether latitude can predict at least qualitatively the time taken by populations to develop responses to ALAN exposure.

As mentioned above, a longer photoperiod could have beneficial or detrimental effects on individuals in terms of time and energy budget. Thus, it is important to determine whether diurnal and nocturnal species respectively prolong or delay their activity period, and how they partition key activities like foraging or reproduction during a day. In this regard, light pollution may contribute to the stronger homogenizing effect of urbanization on nocturnal moths than on diurnal butterflies [START_REF] Merckx | Urbanization-driven homogenization is more pronounced and happens at wider spatial scales in nocturnal and mobile flying insects[END_REF]. The latitudinal effect on incubating bouts in shorebirds [START_REF] Bulla | Unexpected diversity in socially synchronized rhythms of shorebirds[END_REF], where ALAN exposure was not assessed, or on the advanced onset of dawn singing in passerines (Da Silva & Kempenaers, 2017), illustrate which type of behaviour could be targeted. Focusing on activity alone may not unravel the complexity of the disruption process for organisms. The diel distribution of feeding bouts was altered in mice, resulting in obese individuals [START_REF] Fonken | Light at night increases body mass by shifting the time of food intake[END_REF], while activity and metabolism were uncoupled in amphibians exposed to light at night [START_REF] Touzot | Artificial light at night disturbs activity and energy allocation of common toad during the breeding period[END_REF]. These lab studies highlight the need for analysing together activity and metabolic budgets or relevant proxies when other physiological measurements cannot be made. Because physiological constraints may be stronger at higher than at lower latitudes, short-term effects on survival may be quicker. More generally, we do not know well how circadian clocks are affected by ALAN in most species. Even less do we know about how the clocks of populations located at different latitudes respond to novel photic regimes. Birds provide a good illustration of our limited capacity to predict the effects of ALAN in animals. Life-history traits like the onset of breeding, migration, dispersal, moult are important for individual fitness and population growth rate. They are all potentially affected by ALAN even in populations at low latitudes.

The increase in the apparent daylength by ALAN may lead to maladaptive decision by desynchronizing the onset of biological events and the time window of optimal climate/trophic conditions.

Desynchronization may affect the reproductive output and survival of individuals in resident intertropical species and migratory species. For instance, the extension of the photoperiod by ALAN could affect photorefractoriness, i.e. the loss of sensitivity to long days that prevents the initiation of spring migration and maturation. Slight changes in migration timing in a context of climate change may have negative consequences for bird populations. We simply do not know whether the sensitivity to the disturbance generated by ALAN depends on latitude and hardly what to predict. At the sensory level, ALAN is expected to affect the efficiency of visual tasks carried out at night or under crepuscular conditions like orientation [START_REF] Somanathan | Nocturnal bees learn landmark colours in starlight[END_REF], or the selection of food sources [START_REF] Kelber | Scotopic colour vision in nocturnal hawkmoths[END_REF], or mates [START_REF] Gomez | The role of nocturnal vision in mate choice: female prefer conspicuous males in the European treefrog (Hyla arborea)[END_REF]. Adaptation to the light/dark transitions may be partly inefficient in populations that do not use photic cues to reset the circadian rhythm of their visual system [START_REF] Yammouni | A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish[END_REF].

The novel pattern of annual daylength variation generated by ALAN may not be necessarily harder to overcome, at the evolutionary scale, than the colonization of higher latitudes for a species though. Latitudinal clines in activity illustrate the evolutionary potential of some populations to adjust to spatially heterogeneous conditions even over a few degrees of latitude [START_REF] Blanckenhorn | Life history adaptation along a latitudinal cline in the water strider Aquarius remigis (Heteroptera: Gerridae)[END_REF]. In the long-term, populations are expected to develop higher resistance to this novel disturbance [START_REF] Hopkins | Artificial light at night as a driver of evolution across urban-rural landscapes[END_REF]. For instance, urban populations of a moth developed a lower attraction to light than populations from less non-polluted areas (Altermatt 2018). A major issue is which response type should be favoured at different latitudes and what time scale is relevant for such changes to occur. The stability of the natural daylength and the ALAN period should favour adaptation to a longer photophase at lower latitudes unless the physiological and ecological costs of plasticity are small [START_REF] Dewitt | Cost and limits of phenotypic plasticity[END_REF]. In contrast, plastic phenotypes able to respond to varying daylength should be selected at higher latitudes. In this regard, it has been argued that individuals rely more on transitions between the light and dark period than on internal oscillators at higher latitudes, whereas the opposite pattern should be observed at lower latitudes [START_REF] Gaston | Impacts of artificial light at night on biological timings[END_REF][START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF]. Whether plasticity is adaptive in winter time, when the higher numbers of extra-lit hours may require a higher energetic demand, remains to be tested. We observed a limit to the studies on the latitudinal variation of biological traits. The behavioural and physiological clines observed at the interspecific or intraspecific level spanned a fraction of the latitudinal range between temperate and higher latitudes. Information about the transition between the intertropical and the temperate zones is lacking. However, such knowledge is necessary if we want to build a biogeographical framework robust enough to predict the latitudinal effect of ALAN.

This publication bias has been noticed for the study of the evolution of breeding in mammals [START_REF] Bronson | Mammalian reproductive strategies: genes, photoperiod and latitude[END_REF]. It is likely that the historical distribution of laboratories and the difficulty to identify species or groups of species spanning the entire latitudinal range, as well as the logistic constraint of these studies, account for the current situation. Up to now, only a few species distributed across the whole range like D. melanogaster have been tested (but see [START_REF] Bronson | Mammalian reproductive strategies: genes, photoperiod and latitude[END_REF] for mammals).

6 Prospective view about the geographical spread of ALAN: a concern for intertropical biomes? 2017). Larger use of lighting can also be predicted according to prospective studies that forecast the expansion of urban areas and road networks. Roads generate light by their traffic, but they also facilitate new settlements and the expansion of existing settlements. A recent study predicted an increase of 185 % of the global urban extent between 2000 and 2030 for the land category with the highest probability of being urbanized [START_REF] Seto | Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools[END_REF]. In Africa a 590 % increase of urbanized areas is expected with a concentration of this process in five areas, four of which are located in the intertropical zone.

Furthermore, the percentage of urbanization in 34 World major biodiversity hotspots is predicted to triple during that period. The largest changes are expected in regions undisturbed by urban development so far, and all concern inter-tropical areas [START_REF] Seto | Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools[END_REF]. Regarding road networks only 43 % of the emerged land (except Antarctica and Greenland), were considered roadless, i.e. distant more than 5 km from a road, in 2016 [START_REF] Ibisch | A global map of roadless areas and their conservation status[END_REF], a distance at which a nocturnal light source is detected. In tropical and subtropical countries, one-fifth of national parks is crossed by at least one main road [START_REF] Caro | Compromise solutions between conservation and road building in the tropics[END_REF]. Expansion of logging creates roads and gaps that are likely to increase exposure to light in the vicinity of human settlements [START_REF] Laporte | Expansion of Industrial Logging in Central Africa[END_REF]. Therefore, population growth, urbanized land, and road networks are expected to keep growing in the next decades especially in areas of high conservation value and in the intertropical zone (Figure 3).

International initiatives seeking to increase the access to electricity in developing countries are welcome (see for instance www.AREI.org), but they may contribute to the increase of night brightness in preserved environments. There is little doubt that a larger fraction of land in the intertropical zone will experience higher levels of ALAN in the next decades. The increasing use of LEDs for outdoor lighting may further deteriorate the nocturnal environment and generate more deleterious effects for biodiversity than older technical solutions [START_REF] Cajochen | Evening exposure to a light-emitting diodes (LED)-backlit computer screen affects circadian physiology and cognitive performance[END_REF]Davies et al., 2013b;[START_REF] Pawson | LED lighting increases the ecological impact of light pollution irrespective of color temperature[END_REF]. For a given power, LEDs provide better lighting conditions than older sodium lamps because they stimulate a broader range of the visual system of many organisms (Davies et al., 2013b). In addition, this cheaper technology tends to favour greater use of nocturnal light (Kyba et al., 2017a). Countries in the intertropical zone are developing their territory and will likely use the technology that is the most disrupting for biodiversity. By doing so, they will generate a more sudden change than the one experienced by wildlife earlier in other parts of the World. Such a rapid transition in lighting has been documented in Peru (Kyba et al., 2017a). At a broader scale, an increase in nocturnal light was detected by satellites in the tropical/subtropical biomes. The proportion of the area that experienced higher brightness raised by 2 % between 1992-1996 and 2008-2012. The largest increase was observed for needle-leaf and mixed forests that occupy a restricted fraction of the total area but host rich endemic biodiversity (Bennie et al., 2015b).

Insert Figure 3 7

Conclusion

We are clearly lacking a biogeographical framework to predict how species adapt or acclimate to ALAN and by which way. We do not know in which biomes ecological networks are the most sensitive to the elevation of nocturnal brightness, which ecosystems will be the most strongly exposed in the near future, and to which extent climate modulates exposure to ALAN at a given latitude. Only fragmentary information is available to generate testable hypotheses. Climate has probably a still unappreciated influence on the intensity of the biological and ecological effects of ALAN. In particular, more attention should be paid to the geographical variation in cloud cover. A higher sensitivity to change in nocturnal ambient light is expected in organisms from ecosystems where cloud cover is frequent because nocturnal light intensity is often low. Furthermore, within the same latitudinal range, areas where rain, fog, or mist is frequent may be less exposed than arid areas where moonlight is more present most of the nights, and negative effects on organisms and ecosystems may be milder. In forest, light exposure depends on the position of the organism relative to canopy. A large fraction of tropical forest biodiversity lies at the canopy level where individuals are more easily exposed to distant sources of ALAN. Under the canopy where the light level is very low, species are probably less exposed, but they could be highly sensitive to small changes in light intensity. Species naturally living in forest edges, clearings, riparian or open habitats may be more exposed and suffer more from new lighting conditions. In this regard, roads or clearings in tropical forest generate a barrier effect, many organisms avoiding gaps as narrow as 30 meters [START_REF] Laurance | Impacts of roads and linear clearings on tropical forests[END_REF]. The occurrence of ALAN in an otherwise dark environment could strongly enhance the barrier effect for these species [START_REF] Lewanzik | Artificial light puts ecosystem services of frugivorous bats at risk[END_REF].

Building a theoretical framework that describes how biogeographical factors modulate ALAN is a necessary step but it may not be sufficient. The spread of light at night co-occurs with the other drivers of biodiversity decline such as habitat fragmentation, chemical pollution, invasive species, that all generate stress for individuals. Negative synergistic effects of multiple stressors on organisms and populations are well known [START_REF] Sih | Two stressors are far deadlier than one[END_REF]. Climate change is likely the main global factor to focus on. A recent review highlighted the negative effects on animal fitness of changing climate under stable photoperiod [START_REF] Walker | Global climate change and invariable photoperiods: A mismatch that jeopardizes animal fitness[END_REF]. The evolution towards a less frequent or less reflecting cloud cover would reduce exposure of organisms in terms of intensity and duration but also the size of the areas experiencing light pollution, compensating in a way the expansion of urbanized area. In contrast, more clouds are likely to enhance the biological and ecological effects of ALAN. Temperature, the main focus of attention of climate change, is also relevant. It modulates biological rhythms entrained by light [START_REF] Underwood | The pineal and melatonin: Regulators of circadian function in lower vertebrates[END_REF][START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF], resets circadian clocks [START_REF] Chen | Drosophila Ionotropic Receptor 25a mediates circadian clock resetting by temperature[END_REF], affects gene expression in insects and mammals [START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF], alters the daily expression peak of melatonin in lower vertebrates [START_REF] Mayer | Melatonin in non-mammalian vertebrates: Physiological role in reproduction?[END_REF], and induces seasonal shift from nocturnality to diurnality in insects and mammals [START_REF] Van Der Vinne | Cold and hunger induce diurnality in a nocturnal mammal[END_REF][START_REF] Helm | Two sides of a coin: ecological and chronobiological perspectives of timing in the wild[END_REF]. The sensitivity of insects to warming is believed to be the highest in the intertropical zone because individuals are already close to their thermal tolerance threshold [START_REF] Deutsch | Impacts of climate warming on terrestrial ectotherms across latitude[END_REF]. The annual covariation of temperature and photoperiod at a location determines an environmental envelope that enlarges in both dimensions with latitude [START_REF] Hut | Latitudinal clines: an evolutionary view on biological rhythms[END_REF], as a result of increasing temperature and daylength range. Global warming changes one dimension of the envelope while ALAN changes the other dimension, giving rise to entirely new conditions with unknown consequences about the desynchronization of physiological and behavioural rhythms at the individual level, and the timing and intensity of interspecific relationships at the ecological level (but see [START_REF] Miller | Combined effects of night warming and light pollution on predator -Prey interactions[END_REF]). Effects are expected at any location on the globe but predicting the outcome of the interaction remains difficult since temperature is rising faster at higher latitudes [START_REF] Hansen | Global temperature change James[END_REF] and light at night strongly changes the photic regime year-round at lower latitudes.

The demographic and macro-economic projections for the next decades support the expansion of ALAN across the World and particularly within the intertropical zone. Because synergies with other drivers of global decline are not known so far, even if some may be anticipated, prevention and caution remain the most appropriate approaches. We did not attempt a meta-analysis of the papers published on biological rhythms or ALAN for species in the intertropical zone but it is no doubt that there are many less than papers devoted to temperate or high-latitude species. The time is ripe to shift the balance and start investigating the effects of ALAN at different latitudes. In particular, attention should be given to the capacity of intertropical species to respond to the novel conditions generated by ALAN and climate change. A good starting point could be to focus on groups originating from the intertropical zone. Habitat loss, climate change, overexploitation have received so far the main attention in many areas for obvious reasons. It should be acknowledged that ALAN might be a more insidious form of threat to local biodiversity. Eco-tourism has been a major source of income for several countries and it is becoming a major contribution for new ones particularly in countries benefiting from hotspots. It may also contribute to accelerate the change of the photic environment at night. In this regard, a recent global study showed that the increase in ALAN was much higher around protected areas than farther away [START_REF] Guetté | Worldwide increase in artificial light at night around protected areas and within biodiversity hotspots[END_REF]. The demand for biodiversity will keep on growing with the global human population. A cautionary message would be to remind that increasing population, infrastructures and facilities, and more generally the use of electricity for nocturnal lighting may be harmful even in the most environment-friendly areas that value biodiversity as a resource for their economic development.

Figure 1

 1 Figure 1 Mean latitudinal distribution of ALAN and cloud cover fraction (A and C) as measured from

Figure 3

 3 Figure 3 Expected change in Urbanization/demography between 2018 and 2030 (Source UNICEF,

  Between 2017 and 2050, the intertropical region including Western, Middle and Eastern Africa should increase its population by 217 %. South-Eastern Asia, and the region including Central and Latin America (but excluding Argentina, Chile, and Uruguay), should grow respectively of 23 % and 21 %. In comparison population in Europe and Eastern Asia should decrease by 4 % and Northern America raise by 20 % (United Nations Department of Economic and social affair Population division,

	The increase of ALAN essentially results from demographic growth and economic development, i.e. the
	greater use of electricity. World population is forecast to grow from 7.5 billion in 2017 to 9.8 billion in
	2050 and over 11 billion in 2100 (United Nations Department of Economic and social affair Population
	division, 2017).

214, 501-508. Yovanovich, C.A.M., Koskela, S.M., Nevala, N., Kondrashev, S.L., Kelber, A. & Donner, K. (2017) The dual rod system of amphibians supports colour discrimination at the absolute visual threshold. Philosophical Transactions of the Royal Society B: Biological Sciences, 372,

20160066.

Data accessibility statement -Data be will made available in Repository upon acceptance of the manuscript Figure 2 (A-C) Annual variation in twilight for three towns at low, mid, and high latitudes. Twilight represents the periods of the day when sun declination is between 0° and 18° below horizon. When the declination is lower, the sun does contribute to ambient light. The right column shows for two lighting scenarios the annual variation in the time period when ALAN elevates the ambient light level outside the twilight periods. Artificial lights are turned off at midnight (D) or stay turned on all night (E). Sources : https://www.esrl.noaa.gov/gmd/grad/solcalc/ ; https://earthsky.org/?p=235797.


Saeed A Almheiri

Ahmad A L Ahmad

Benjamin Le Droumaguet

Rémy Pires

ah. Ahmed A Mohamed
email: mohamed@sharjah.ac.ae

Mohamed M Chehimi
email: chehimi@icmpe.cnrs.fr

Developed Latent Fingerprints via Aryldiazonium Tetrachloroaurate Salts on Copper Surfaces: An XPS Study

Keywords: Forensics, Diazonium Gold Salt, Copper Coins, XPS, Latent Fingerprints, Gold-Aryl Film

Surface studies of developed fingerprints have aided in the elimination of criminal cases before moving to the court. The combination of X-ray photoelectron spectroscopy (XPS) with the aryldiazonium gold(III), 4-O 2 NC 6 H 4 N 2 + AuCl 4 -, surface modifier has shown to be a novel approach in latent fingerprint detection and development for the quantification of film elements. The robust gold-aryl film was developed on the reducing chemicals excreted in the sebaceous fingerprints without the need for external stimuli, and at lesser extent after contacting the free metal surface. The concurrent reduction of the diazonium functional group and gold(III) from [AuCl 4 ] -developed a robust gold-aryl film which showed increasing gold(0) quantity in the time range 30 to 120 min over copper coins and model flat sheets. SEM and XRD support the presence of reduced gold on the top of the latent fingerprints and the presence of CuO resulting from the reaction of the diazonium salt with copper metal. This research combines the quantification of deposits using XPS, the surface sensitive technique for chemical analysis, in addition to the surface imaging.

Introduction

Alongside the early detection of the developed fingerprints on various types of surfaces, many efforts were put to quantitatively determine the chemical components existing in latent fingerprints. 1,2 One of such efforts includes the use of X-ray photoelectric spectroscopy (XPS). [START_REF] Schwoeble | Application of X-ray photoelectron spectroscopy (XPS) for the surface characterization of Gunshot Residue (GSR)[END_REF][START_REF] Fouetio Kengne | Self-assembled monolayers of thiols adsorbed on Au/ZnO-functionalized silica nanosprings: Photoelectron spectroscopy-analysis and detection of vaporized explosives[END_REF] Since the 1980s, XPS has gained momentum in forensic science in the analysis of explosives. [START_REF] Fouetio Kengne | Self-assembled monolayers of thiols adsorbed on Au/ZnO-functionalized silica nanosprings: Photoelectron spectroscopy-analysis and detection of vaporized explosives[END_REF][START_REF] Sharma | Fundamentals of X-ray Photoelectron Spectroscopy (XPS) and Its Applications to Explosives and Propellants[END_REF][START_REF] Sui | Determination of the radiation resistance order of high explosives by the two dimensional correlation X-ray photoelectron spectroscopy[END_REF][START_REF] Mahoney | Characterization of composition C4 explosives using time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy[END_REF][START_REF] Liyanage | Fabrication of a selfassembled and flexible SERS nanosensor for explosive detection at parts-perquadrillion levels from fingerprints[END_REF] According to Watts, 9 the use of XPS in fingerprint analysis was first recorded in 1970 by David Clark and David Briggs as an experiment performed to their local newspaper (Newcastle, England). The local newspaper at the time misunderstood that XPS may accurately track the ownership of the fingerprints in real time to catch escaping burglars. Despite this amusing misconception, Watts suggested that XPS may help identify potential suspect when characteristic materials persist on the suspect's fingerprints that may be readily transferred as well to the surface alongside with the fingerprints. An example would be the XPS peaks showing Si2p and Si2s from a sample of female fingerprints. Silicon is often used in cosmetics and skin products; thus, Si peaks may act as a potential route to identify the use of certain products by the suspect.

Such profiling information from XPS can considerably reduce the suspect pool.

There are a few techniques which aided the development of forensic science and moving this emerging field from being dependent on merely qualitative to quantitative. The recent progress utilized FTIR, 10,11 mass spectrometry, 12,13 fluorescence, 14,15 X-ray fluorescence 12, [START_REF] Zheng | Elemental analysis and imaging of sunscreen fingermarks by X-ray fluorescence[END_REF] in addition to imaging approaches. [START_REF] Boseley | Revealing the elemental distribution within latent fingermarks using synchrotron sourced X-ray fluorescence microscopy[END_REF] XPS has been introduced as surface technique for elemental analysis. [START_REF] Girard-Lauriault | Chemical and elemental depth profiling of very thin organic layers by constant kinetic energy XPS: a new synchrotron XPS analysis strategy[END_REF][START_REF] Viornery | Surface modification of titanium with phosphonic acid to improve bone bonding: characterization by XPS and ToF-SIMS[END_REF][START_REF] Banerjee | Effect of heat treatment on the surface chemical structure of glass: Oxygen speciation from in situ XPS analysis[END_REF] A recent report proposed an interesting method of demonstrating latent fingerprints on reactive metal surfaces using diazonium salts stabilized by tetrachloroaurate ion, NO 2 -aryldiazonium-AuCl 4 . [START_REF] Ahmad | Evaluation of diazonium gold(III) salts in forensic chemistry: Latent fingerprint development on metal surfaces[END_REF] Authors used imaging techniques such as SEM, XRF and optical microscopy to visualize the change on common metals of crime scenes. It was concluded from the study that copper surface is the most successful among the three reducing metals copper, aluminum and zinc. The gold film aided to visualize the three levels of primary, secondary and tertiary fingermarks at high level of details. Reduced gold on fingerprints can be beneficial in archiving and future analysis. ), on the one hand, and poor contrast images with scattered minor deposits slowly occurred after an elapsed time of 24h after deposition of the reactive aryldiazonium solution. Despite the successful use of aryldiazonium tetrachloroaurate for the visualization of the fingerprints, the recently published paper focused mainly on imaging techniques. As the demonstration of fingerprints using diazonium salts on easily reactive materials (e.g. copper) is a surface chemical reaction, we anticipated that the combination of diazonium chemistry and XPS would be an original and unique method to investigate. Indeed, it is well know that aryldiazonium salts reduce spontaneously on copper [START_REF] Belanger | Electrografting: a powerful method for surface modification[END_REF] and for the demonstration of proof of concept, this would be an ideal combination to track by XPS surface modification of copper coins (Euro cents) resulting from diazonium chemistry. The rationale for this motivation is that elsewhere, the surface analyses might take requires heat [START_REF] Bond | Visualization of latent fingerprint corrosion of metallic surfaces[END_REF] or application of an electrical potential to enhance visualization of the said fingerprints. [START_REF] Bond | Visualization of latent fingerprint corrosion on a discharged brass shell casing[END_REF] Herein, and as some of us have previously reported, the latent fingerprints are demonstrated spontaneously without any physico-chemical assitance, which facilitates the whole study and subsequent XPS analysis.

In this study, we have cleaned copper coins and model flat copper sheets and affixed fingerprints of a donor. The fingerprints were developed with aryldiazonium tetrachloroaurate aqueous solution for 1h. After washing, the copper coins and flat surfaces were analyzed by XPS. We will tackle two main aspects: (i) surface chemistry of aryldiazonium tetrafluoroborate salts, and (ii) XPS analysis of the positive and negative parts of the developed parts of the fingerprint on copper coin and model flat surface (Figure S1). We will then concentrate on our results of the selective binding phenomena between diazonium and fingerprints on top of copper metal. As our work is focused on fingerprints within the framework of forensic science, we have chosen to conduct surface chemical reactions on copper coins (Euro cents) in order to replicate a "real world" scenario. Some specific experiments have also been conducted on model, flat copper plate.

Experimental Section

Reagents. The reagents were used without any further purification: hydrochloric acid (Carlo Erba, 37%), nitric acid (Carlo Erba, 65%), 4-nitroaniline (Sigma-Aldrich, ≥ 99%) and sodium nitrite (Sigma-Aldrich, ≥97%). was dropped using a dropper on each of the prepared surfaces: Cu sheet+fingerprint (2 h). The surfaces were then washed gently with deionized water until the excess salt was completely washed away and dried. The samples formed a film just enough to cover the suspected area.

Synthesis and characterization

Characterization. Infrared spectra were recorded using a Nicolet Magna-IR 550 spectrometer in the attenuated total reflection (ATR-FTIR) mode in the 4000-450 cm -1 range.

XPS spectra were recorded using a K Alpha apparatus (Thermo) fitted with a monochromatic Al Kα X-ray source (spot size: 100 μm). The pass energy was set to 200 and 80 eV for the survey spectra and the narrow regions, respectively. The composition was determined using the manufacturer sensitivity factors. The copper coins and the model flat samples were mounted on the sample holder and clipped (Figure S2). The size of the microfocused beam was set to 100 µm, sufficiently low to analyze different parts of the coins, or the flat samples. Where reaction with diazonium occurred, black spots could be analyzed (Figure S3a). Also, the fingerprints showing alternating dark and light lines could individually be analyzed (Figure S3b).

NMR spectra were recorded using a Bruker Avance II spectrometer operating at a resonance frequency of 400 and 100 MHz, respectively. For 1 H NMR analysis, (CD 3 ) 2 CO was used as a solvent and internal standard. SEM investigations were performed using a Carl Zeiss MERLIN field emission-scanning electron microscope (FE-SEM). In order to reveal alternating section in composition of the surface, backscattered electrons (BSE) were collected and imaged. Energy dispersive X-ray analysis (EDS) were performed using a X-Max silicon drift detector from Oxford Instruments coupled with AZtec operating system.

Results and Discussion

Rationale and procedure for visualization of fingerprints using aryldiazonium tetrachloroaurate salt

In the recent years, aryldiazonium salts of tosylates, Aryldiazonium tetrachloroaurate salt was synthesized for the purpose of this study because it gives a specific NO 2 N1s peak in addition to Au4f from metallic gold obtained by reduction at the copper surface. These features are well distinguished in XPS from Cu2p peak due to the underlying copper coin or plate. [START_REF] Bécue | Emerging fields in fingermark (meta)detectiona critical review[END_REF] H NMR characterization in deuterated acetone showed two distinct doublet of doublet peaks at δ = 8.90 ppm and δ = 9.28 ppm, each with clear H integration of 2H. The first peak at δ = 8.90 ppm appears as a strong doublet which corresponds to the hydrogen atoms closer to N≡N + . The second strong doublet at δ = 9.28 ppm corresponds to the hydrogen atoms closer to NO 2 . It should also be noted that at the time of the analysis, two short peaks started to form at δ = 8.00 ppm and δ = 6.75 ppm, indicative of the reducible nature of diazonium salts. The analysis was redone after two days and the two peaks were stronger and sharper. This observation serves as a further evidence of the low shelf-life nature of the diazonium salt in solution at room temperature. FTIR spectra showed three significant peaks. First peak at 2280 cm -1 corresponds to the N≡N + (strong) stretching frequency.

The second peak at 1600 cm -1 corresponds to the aromatic C=C (weak) stretching frequency. The third peak came at 1400 cm -1 corresponds to the C-N (medium-weak) stretching frequency. diazonium and fingerprints was analyzed using XPS as a means of a preliminary assessment (Figure 2). Fingerprint samples from 12 different areas of the right thumb were collected after hand washing with soap, thorough tap water rinsing and hand drying. The composition of the deposited materials on the copper surface is summarized in Table S1. Cu2p 3/2 (avg. 932.5 ± 0.17 eV corresponds to the copper coin, whereas Au4f 7/2 (avg. 84.2 ± 0.02 eV) corresponds to gold deposits by spontaneous, in situ reduction of the aryldiazonium tetrachloroaurate salt. As shown in Figure 2b, the area under the curve of each position fluctuates as one moves from fingerprint to non-fingerprint region, these data initially showcase the selectivity nature of diazonium towards the fingerprint region. N1s region has two main peaks: one at (avg. 399.9 ± 0.13 eV), assigned to (-N=N-) from the aryl functional groups within the aryl layer and the fingerprints, and (avg. 406.0 ± 0.07 eV), assigned to the NO 2 group from the diazonium. The N1s region thus shows that the diazonium cation has been reduced and this region is left only with two main components from azo and NO 2 groups, and disappearance of diazonium N1s signals. Cu2p XPS spectra shows (Figure 2a) the relatively low atomic percentage because, compared to cleaned coins, the copper is attenuated by both the fingerprint and the aryl/gold deposits.

Surface study on model copper sheets.

Recently, Lafi. et al. [START_REF] Lafi | Forensic Nanotechnology: Engineering Polyaniline Nanocomposites for Latent Fingerprints Development[END_REF] discussed a phenomenon that occurs when applying aryldiazonium salt solution on a metal surface for a prolonged period.

The authors found that the salt solution may further react with the metal after initially binds with the fingerprint. As a result, the whole area of metal covered with the solution turned dark instead of the predicted appearance of observable dark print of the fingerprints. Thus, we were interested in investigating the driving force behind this phenomenon. Towards this end, four samples of copper sheets were treated with the aryldiazonium gold salt, each with different treatment time of 30, 60, 90 and 120 min. The samples were subsequently characterized using XPS (Figure 3 and Table 1). As reported in Table 1, the relative atomic percentage of Au (computed using Au 4f doublet with Au 4f 7/2 at ~84 eV due to Au(0)) increases with treatment time (Figure 3b). This is believed to be normal due to the natural tendency of Au deposition as the solution reacts with the metal for a longer period. What seems to be unusual, the relative atomic percentage of copper (computed using Cu 2p 3/2 at 932.5 eV) also increases as the treatment time increases (Figure 3c). This is surprising because it was expected that copper should have been covered by more deposits as time elapses, leading to less relative atomic percentage. Instead, the opposite case seems to persist, which indicates that there may be diffusion of copper species towards the surface. To further investigate the products of the interfacial reaction between the diazonium tetrachloroaurate and copper, we have closely inspected the narrow Cu2p copper region and have taken advantage of the method devised by Biesinger [START_REF] Biesinger | Advanced analysis of copper X-ray photoelectron spectra[END_REF] to deduce Cu(II) from the Cu2p doublet.

According to Biesinger, the Cu(0+I)/Cu(II) ratios for samples containing a mixture of Cu(0+I)

and Cu(II) rely on estimation of the main peak/shake-up peak areas (A 1s /B s ) for a 100% pure Cu(II) sample (Figure 4a). With a reliable value of A1s/Bs obtained for CuO (Table 2), the relative concentrations of Cu(0) and Cu(II) species present on the surface that contain both species (Figure 4b) can be obtained using Equations 1 and 2

% 𝑪𝒖(𝟎 + 𝑰) = 𝑨 -( 𝑨 𝟏𝒔 𝑩 𝒔 ) 𝑩 𝑨 + 𝑩 × 𝟏𝟎𝟎% ( 1) % 𝑪𝒖(𝑰𝑰) = 𝑩 (𝟏 + ( 𝑨 𝟏𝒔 𝑩 𝒔 )) 𝑨 + 𝑩 × 𝟏𝟎𝟎% ( 2)
where A is the total area of the main peak and B is the area of the shake-up peak of the sample. Next, a sample of a copper sheet treated with fingerprint and diazonium salt was analyzed (Figure 5 and Table S2). The reaction of the diazonium aqueous solution with copper yields positive dark lines after only 15-20 minutes, and bubbles are visible between the fingerprint lines (Figure 5e). As detailed in the Experimental section, 100 µm spot size was used in order to analyze the dark and the light lines without any interference. An XPS spectra was collected from four different areas of the now-visible fingerprint sample (P2D and P4D: fingerprint area, P1L

and P3L: non-fingerprint area), it was easy to differentiate the fingerprint (D for dark) side and the non-fingerprint (L for light) side of the copper sample by looking at the survey scan and observing the shape of the background in the range 700-900 eV (Figure 5). The composition of materials on the sample surfaces are summarized in Table S2.

Remarkably Au4d 5/2 (Figure 5d), centered at 335.1 eV, accounts for metallic state of gold and thus a reductive process affecting [AuCl 4 ] -anions. [START_REF]NIST XPS database[END_REF] Similarly, Figure 5c shows only two peaks for nitrogen, one centered at 400 eV and one at 406 eV assigned to -N=N-within the aryl layer and NO 2 , respectively. Therefore, we no longer observe the components due to N1s from the diazonium group N 2 + . This accounts for the spontaneous reduction of the diazonium cation on copper. These XPS results parallel the direct observation of bubbles (Figure 5e) when a drop of diazonium solution is gently deposited on copper with or without fingerprint.

Scanning electron microscopy of latent fingerprint after reaction with aryldiazonium tetrachloroaurate

Further SEM images display the morphology of the surfaces (Figures 6 and7). The SEM images of non-fingerprint area (Figure 6a) show minor gold nanoclustering (Figure 6b,c,f) on top of copper metal (Figure 6d,e). Fingerprint area investigated using EDS (Figure 7a) shows an interesting composition of what seems to be CuO (Figure 7b, O = 34.0% Cu = 32.8%) on top of copper (Figure 7c), in addition to a minor gold nanoclustering (Figure 7d).

Figure 1 . 4 -, 3 . 3 . XPS analysis of surfaces developed by fingerprints 3 . 3 . 1 .

 1433331 Figure 1. (a) XPS survey scan of 4-O 2 NC 6 H 4 N 2 + AuCl 4 -, (b) peak-fitted high-resolution Au4f spectra, (c) N1s.

Figure 2 .

 2 Figure 2. (a) High-resolution Cu2p, (b) Au4f spectra of fingerprint and 4-O 2 NC 6 H 4 N 2 + AuCl 4 -

Figure 3 .

 3 Figure 3. (a) XPS survey scan of 4-O 2 NC 6 H 4 N 2 + AuCl 4 -on copper sheet at multiple time increments, (black) clean Cu, (red) after 30 min, (blue) after 60 min, (green) after 90 min, and (violet) after 120 min, (b) high-resolution Au4f spectra, (c) Cu2p peak.

Figure 4 .

 4 Figure 4. (a) XPS survey scan of a pure CuO sample, (b) peak-fitted high-resolution Cu2p spectrum.

Figure 5 . 4 -

 54 Figure 5. (a) XPS survey scan of fingerprint and 4-O 2 NC 6 H 4 N 2 + AuCl 4 --treated copper sheet on four different areas, (b) high-resolution Cu2p, (c) N1s, (d) Au4d spectra, (e) Latent fingerprint demonstration using diazonium solution on a copper sheet. Bubbles correspond to the release of N 2 due to the spontaneous reduction of diazonium cation.

Figure 6 . 4 -

 64 Figure 6. SEM image (a) and EDS elemental spectra (b-f) of the nonfingerprint area of treated copper sheet with 4-O 2 NC 6 H 4 N 2 + AuCl 4 -.

Figure 7 .

 7 Figure 7. SEM image (a) and EDS elemental spectra (b, c, d) of the fingerprint area of treated copper sheet with 4-O 2 NC 6 H 4 N 2 + AuCl 4 -.

  It is to note that poor development of the latent fingerprint was

	noted with aryldiazonium tetrafluoroborate (4-O 2 NC 6 H 4 N 2	+ BF 4	-

of 4-O 2 NC 6 H 4 N 2 + AuCl 4 -

  Copper coins (€0.20) surfaces were prepared by cleaning with distilled white vinegar (~6-8%) and table salt to remove the oxides, sonification to remove any dirt and then washed with absolute ethanol. The following samples were prepared for XPS analysis: Cu coin (clean); 0.001 M 4-O 2 NC 6 H 4 N 2

	((CD 3 ) 2 CO, 298K): δ (ppm) 9.28 (d, 3 J = 8.8 Hz), 8.90 (d, 3 J = 8.8 Hz); 13 C NMR ((CD 3 ) 2 CO,
	298K): δ (ppm) 155.05, 135.65, 127.46, 122.12; and main ATR-FTIR bands at ν max (cm -1 ) 2280
	(NN stretch), 1537 (NO 2 asymmetrical stretch), 1349 (symmetrical stretch of NO 2 ), 1315 (C-H
	in plane deformation).
	Deposition of 4-O 2 NC 6 H 4 N 2	+ AuCl 4	-on copper coins.
			. 4-Nitroaniline (0.691 g, 5.0 mmol)
	was dissolved in 28.0 mL of 6.0 M HCl and the solution was cooled to 4.0 °C. Afterwards,
	NaNO 2 (0.345 g, 5.0 mmol) was dissolved in 5.0 mL of deionized water and added to the
	previous solution after it was cooled to 4.0 °C. The reaction mixture was stirred for 30 min at 4.0
	°C. Next, H[AuCl 4 ] was dissolved in 10.0 mL of water at 4.0 °C and added dropwise to the
	previous reaction mixture and was stirred for an additional 1 h at room temperature. The
	precipitate was recovered by suction filtration and dried to yield a bright yellow precipitate
	(1.076 g, 44.1%). Finally, the product was dissolved in water to form a 0.001M solution which
	was used in all fingerprint development studies. The salt was characterized by 1 H NMR

+ AuCl 4 -solution was dropped using a dropper on Cu coin+fingerprint (1 h). The surface was then washed gently with deionized water until the excess salt was completely washed away and dried. The sample formed a film just enough to cover the suspected area.

Deposition of 4-O

2 NC 6 H 4 N 2 + AuCl 4

	001M 4-O 2 NC 6 H 4 N 2	+ AuCl 4	-solution

-on copper sheets. Three copper sheets (1 cm 2 ), and one copper slit (5 × 1 cm 2 ) were prepared by cleaning with distilled white vinegar and table salt to remove the oxides, sonification to remove any dirt and then washed with absolute ethanol. The following samples were prepared for XPS analysis: Cu sheet (clean); Cu sheet+fingerprint; Cu slit was immersed partly (1 × 1 cm 2 ) for 30 min. The slit was then immersed further, and the process was repeated for a total of 2 h, resulting in a surface with multiple layers of diazonium salt (clean, immersion for 30, 60, 90, and 120 min). The 0.

. Synthesis and characterization of 4-O 2 NC 6 H 4 N 2 + AuCl 4 -salt.

  [START_REF] Filimonov | Unusually stable, versatile, and pure arenediazonium tosylates: their preparation, structures, and synthetic applicability[END_REF], triflates[START_REF] Bondarev | The first study of the thermal and storage stability of arenediazonium triflates comparing to 4nitrobenzenediazonium tosylate and tetrafluoroborate by calorimetric methods[END_REF] and tetrachloroaurate 27,[START_REF] Mohamed | Synthesis of gold organometallics at the nanoscale[END_REF] series emerged as very stable and safe, as they could be stored at room temperature; they are even stable up to 80-100 °C, triflates could be stable after decades as has just been claimed.[START_REF] Bondarev | The first study of the thermal and storage stability of arenediazonium triflates comparing to 4nitrobenzenediazonium tosylate and tetrafluoroborate by calorimetric methods[END_REF] This is a tremendous progress in the chemistry of diazonium salts. Whilst most of diazonium salts are soluble in aqueous media and react with surfaces to form surface-aryl interfacial bonds, what makes the tetrachloroaurate series unique is that the reduction of the salt affects both the diazonium cation and AuCl 4 This unique marker permitted to Pinson and co-workers in 1992 to demonstrate that passivation of electrodes was due to nitrophenyl groups.[START_REF] Delamar | Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts[END_REF] In this sense, XPS is a very surface sensitive technique and could be employed in this work with a microfocused beam, so that we could analyze both the positive and negative fingerprints after

	reaction of a aryldiazonium tetrachloroaurate aqueous solution with cleaned copper coins and
	model flat samples.
	3.2

-

. If the reduction is conducted in solution, one obtains gold nanoparticles grafted with aryl functional groups. However, if the reduction is conducted in the presence of a substrate, one is faced with a more complex situation in the sense that both gold and aryl groups react and deposit on the surface but not necessarily a film of gold-aryl nanoparticles is formed. This is what we will investigate in depth in the following sections. One way to contribute to the study and visualization of fingerprints is by combining diazonium salts and XPS. Diazonium salts react spontaneously on surfaces with reducing power; this is why we have selected copper coins (2 euro cents, unless otherwise stated). The choice of the nitrobenzenediazonium cation lies in the ability to provide a unique elemental marker for the aryl groups after reaction of the parent diazonium salt with the surface. As a matter of fact, NO 2 group is featured by N1s at ~406 eV.

Table 1 .

 1 Elemental composition of copper sheet at different time increments.

	Material		C 1s	NO 2	-N=N-Cu 2p 3/2 Au 4f
	Cu slit (Clean)	Peak BE	285.1	408.0	400.0	932.6	90.7
			Atomic %	74.1	0.4	4.9	20.6	0.0
	4-O 2 NC 6 H 4 N 2	+ AuCl 4 -on Cu	Peak BE	285.0	406.1	400.2	932.5	84.2
	slit (30 min)		Atomic %	84.6	6.9	3.3	1.4	3.9
	4-O 2 NC 6 H 4 N 2	+ AuCl 4 -on Cu	Peak BE	285.0	406.1	400.1	932.4	84.1
	slit (60 min)		Atomic %	83.6	7.2	3.5	1.4	4.3
	4-O 2 NC 6 H 4 N 2	+ AuCl 4 -on Cu	Peak BE	284.9	406.0	400.2	932.2	84.2
	slit (90 min)		Atomic %	82.2	6.7	3.1	1.6	6.3
	4-O 2 NC 6 H 4 N 2	+ AuCl 4 -on Cu	Peak BE	284.9	406.0	400.1	932.3	84.1
	slit (120 min)		Atomic %	80.6	6.1	3.8	2.0	7.6

Table 2 .

 2 Tabular representation of XPS data of a pure CuO sample (top) and ratio of the main peak/shake-up peak areas (A1s/Bs) for a 100% pure Cu(II) sample (bottom).

	Name	Peak BE	Atomic %
	C1s	284.6	14.7
	C1s B	285.8	2.13
	C1s C	287.53	1.33
	C1s D	288.8	1.51
	Cu2p 3/2	933.0	14.1
	Cu2p 3/2 B	934.7	7.82
	Cu2p 3/2 (Shake-up)	940.6	4.8
	Cu2p 3/2 (Shake-up) B	942.0	1.98
	Cu2p 3/2 (Shake-up) C	943.5	3.67
	Cu A	568.5	0.0
	O1s (CuO)	529.4	29.7
	O1s B	531.3	14.8
	O1s C	532.9	3.42
	A1	B1s	A1/B1s
	Cu2p 3/2	Cu2p 3/2 (SH)	Cu2p 3/2 / Cu2p 3/2 (SH)
	191643.74	90567.96	2.116

Acknowledgements

AAM acknowledges the University of Sharjah support of SEED grant VC-GRC-SR-83-2015, competitive grants 160-2142-029-P and 150-2142-017-P, Organometallic Research Group grant RISE-046-2016 and Functionalized Nanomaterials Synthesis Lab grant 151-0039. SAA thanks The General Department of Forensic Science and Criminology (Dubai Police HQ, Dubai) for financial support to visit ICMPE and perform surface modification and XPS analyses.

this what explains the occurrence of dark, black marks (the color of CuO). Initial sample preparation without fingerprints (Figure 8b, left) gives a slight change in the color of the copper coin, after 1 h wait and quenching, however without detection of Cu(II) diffusion and localized dark spots (see also Figure S2, top left coin). Obviously, it is the combination of the fingerprints and the aryldiazonium salt that permits the visualization of the latent fingerprints. Chloride ions in eccrine sweat are known to corrode copper and even noble metals after long contact (10 days) with fingerprints. [START_REF] Bond | The thermodynamics of latent fingerprint corrosion of metal elements and alloys[END_REF] However, for fast visualization, the fingerprints alone do not suffice since after 1h; the fingerprint on 1 euro cent clean copper coin does not lead to the developed black lines as shown in Figure S4.

Supporting Information

The Figures show mounting copper coins on sample holder, and selected areas analyzed by XPS. Tables reporting XPS-determined compositions of copper coins after reaction diazonium salts. This material is available free of charge via the Internet at http://pubs.acs.org.