Shrabana Sarkar

Nidia Torres Ponce

Aparna Banerjee
email: abanerjee@ucm.cl

Rajib Bandopadhyay

Saravanan Rajendran
email: saravanan3.raj@gmail.com

Eric Lichtfouse
email: eric.lichtfouse@gmail.com

Green polymeric nanomaterials for the photocatalytic degradation of dyes: a review

Keywords: Biopolymer, Nanomaterial, Photocatalyst, Dye degradation

Pure and drinkable water will be rarer and more expensive as the result of pollution induced by industrialisation, urbanisation and population growth. Among the numerous sources of water pollution, the textile industry has become a major issue because effluents containing dyes are often released in natural water bodies. For instance, about two years are needed to biodegrade dye-derived, carcinogenic aromatic amines, in sediments. Classical remediation methods based upon physicochemical reactions are costly and still generate sludges that contain amine residues. Nonetheless, recent research shows that nanomaterials containing biopolymers are promising to degrade organic pollutants by photocatalysis. Here, we review the synthesis and applications of biopolymeric nanomaterials for photocatalytic degradation of azo dyes. We focus on conducting biopolymers incorporating metal, metal oxide, metal/metal oxide and metal sulphide for improved biodegradation. Biopolymers can be obtained from microorganisms, plants and animals. Unlike fossil-fuel-derived polymers, biopolymers are carbon neutral and thus sustainable in the context of global warming. Biopolymers are often biodegradable and biocompatible.

Introduction

According to the latest report of World Health Organization (WHO), approximately 844 million people worldwide lack the access to basic drinkable water [START_REF] Wutich | Measuring human water needs[END_REF]. Waterborne pathogens in the form of disease-causing bacteria, virus or protozoa spread many diseases including cholera, typhoid, hepatitis, giardia and COVID-19 [START_REF] Sharma | Environmental chemistry is most relevant to study coronavirus pandemics[END_REF]. Unsafe water causes epidemics in developing countries due to improper management of water pollution [START_REF] Alhamlan | Recommended advanced techniques for waterborne pathogen detection in developing countries[END_REF]. Organic pollutants in wastewater are potentially harmful for all living organisms. Regular consumption of untreated or poorly treated waters induces carcinogenesis or prolonged illness in humans and other animals [START_REF] Sarkar | Degradation of synthetic azo dyes of textile industry: a sustainable approach using microbial enzymes[END_REF]). As a consequence, wastewater remediation and water recirculation are now the major research focus [START_REF] Wen | Polymeric nanocomposite membranes for water treatment: a review[END_REF]Karimi-Maleh et al. 2020a). Particularly, the regulation of water contamination and recycling the wastewater should be improved in drought-affected countries [START_REF] Gholami | The electrochemical removal of bacteria from drinking water[END_REF].

The negative health effects of water pollution are a major source of mortality worldwide [START_REF] Wang | Industrial water pollution, water environment treatment, and health risks in China[END_REF][START_REF] Sarkar | Degradation of synthetic azo dyes of textile industry: a sustainable approach using microbial enzymes[END_REF]. In particular, water pollution has historically impacted food safety [START_REF] Lu | Impacts of soil and water pollution on food safety and health risks in China[END_REF]. The textile industry represents a threat when dye effluents are released into water bodies. Textile wastewater contains various contaminants such as synthetic azo dyes. Therefore, environmental legislation commonly obligates textile factories to treat effluents before discharge [START_REF] Yaseen | Textile dye wastewater characteristics and constituents of synthetic effluents: a critical review[END_REF]. Dye effluents are high in colour, pH, suspended solids (SS), chemical oxygen demand (COD), biochemical oxygen demand (BOD) [START_REF] Yaseen | Shallow pond systems planted with Lemna minor treating azo dyes[END_REF], metals [START_REF] Sharma | A comparative study on characterization of textile wastewaters (untreated and treated) toxicity by chemical and biological tests[END_REF][START_REF] Sekomo | Heavy metal removal in duckweed and algae ponds as a polishing step for textile wastewater treatment[END_REF], temperature [START_REF] Santos | Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology[END_REF][START_REF] Shah | Enhanced bioactivity of Ag/ZnO nanorods-a comparative antibacterial study[END_REF]) and salts [START_REF] Yaseen | Textile dye wastewater characteristics and constituents of synthetic effluents: a critical review[END_REF]. Synthetic textile dyes are often recalcitrant and carcinogenic by nature due to the presence of -N=N-bond [START_REF] Singh | Enzymatic decolorization and degradation of azo dyes-a review[END_REF]. Those dyes mainly consist of complex aromatic structures that are hardly biodegradable.

Wastewater treatment involves a step of physicochemical fractionation, which separates hydrophilic and hydrophobic matter [START_REF] Kim | Characterization of natural organic matter in conventional water treatment processes for selection of treatment processes focused on DBPs control[END_REF]. Techniques and adsorbents used for wastewater treatments have been recently compared (Crini and Lichtfouse 2019;Crini et al. 2019a). Methods for the treatment of dye-contaminated waters include reverse osmosis, coagulation, flocculation, ion exchange, activated carbon adsorption, advanced oxidation, ozonation, photocatalysis, Fenton process, photo-Fenton, electrochemical oxidation [START_REF] Lade | Biodegradation and detoxification of textile azo dyes by bacterial consortium under sequential microaerophilic/aerobic processes[END_REF] and filtration [START_REF] Singh | Enzymatic decolorization and degradation of azo dyes-a review[END_REF]. These processes are often expensive and generate amine residues found in sludges after treatment. Alternatively, semiconductors such as titanium dioxide and zinc oxide have shown excellent photocatalytic activity due to a positive band position that develops more electrons and holes under UV light [START_REF] Fujishima | Electrochemical photolysis of water at a semiconductor electrode[END_REF][START_REF] Mclaren | Shape and size effects of ZnO nanocrystals on photocatalytic activity[END_REF][START_REF] Xu | Nanostructured materials for photocatalysis[END_REF]. Recently, the photocatalytic capacity has been improved by modifying material surfaces using metal doped , non-metal doped and coupled systems [START_REF] Gnanasekaran | Synthesis and characterization of metal oxides (CeO 2 , CuO, NiO, Mn 3 O 4 , SnO 2 and ZnO) nanoparticles as photo catalysts for degradation of textile dyes[END_REF][START_REF] Gnanasekaran | Efficient photocatalytic degradation of toxic dyes using nanostructured TiO 2 / polyaniline nanocomposite[END_REF][START_REF] Sulaiman | Effects of photocatalytic activity of metal and non-metal doped TiO 2 for hydrogen production enhancement-a review[END_REF][START_REF] Kuo | Effect of non-metal doping on the photocatalytic activity of titanium dioxide on the photodegradation of aqueous bisphenol A[END_REF]. Composite or coupled systems are now used for solar cells, opto-electronics, bio-electrochemical sensors, electro-oxidation and disinfection [START_REF] Li | Synergistic bactericidal activity of Ag-TiO 2 nanoparticles in both light and dark conditions[END_REF][START_REF] Devi | A review on non metal ion doped titania for the photocatalytic degradation of organic pollutants under UV/solar light: role of photogenerated charge carrier dynamics in enhancing the activity[END_REF][START_REF] Rokhmat | Performance improvement of TiO 2 /CuO solar cell by growing copper particle using fix current electroplating method[END_REF][START_REF] Karimi-Maleh | Electrochemical sensors, a bright future in the fabrication of portable kits in analytical systems[END_REF][START_REF] Karimi-Maleh | Simultaneous determination of cholesterol, ascorbic acid and uric acid as three essential biological compounds at a carbon paste electrode modified with copper oxide decorated reduced graphene oxide nanocomposite and ionic liquid[END_REF]Karimi-Maleh et al. 2020b, c). Here, we review green polymeric nanomaterials for photocatalytic dye degradation with special emphasis on recent developments, biopolymers and applications in wastewater remediation.

Synthesis of biopolymeric nanomaterials

Conventional methods to synthesise polymeric nanomaterial employ chemical compounds that may cause environmental toxicity later due to their long-term stability. By contrast, biopolymers are usually composed of safe monomers and are carbon neutral for the climate. Biopolymers facilitate the synthesis of nanomaterials because biomass morphology is often structured at the nanolevel. Biopolymers are found in various organisms such as plants, algae, fungi, bacteria and animals. Macromolecules include starch, alginate, chitosan, dextran and chitin (Fig. 1). Chitosan, starch, dextran and cellulose are polysaccharides derived from plants and microbial biofilms, and these biopolymers are common for nanomaterial synthesis [START_REF] Banerjee | Use of dextran nanoparticle: a paradigm shift in bacterial exopolysaccharide based biomedical applications[END_REF][START_REF] Farshchi | Photocatalytic/biodegradable film based on carboxymethyl cellulose, modified by gelatin and TiO 2 -Ag nanoparticles[END_REF][START_REF] Kolangare | Antibiofouling hollow-fiber membranes for dye rejection by embedding chitosan and silver-loaded chitosan nanoparticles[END_REF]. In particular, chitosan has been used for dye removal and wastewater treatment (Crini et al. 2019b;[START_REF] Lichtfouse | Chitosan for direct bioflocculation of wastewater[END_REF].

Biopolymers are unique in composition and have various physiological properties. Biopolymeric nanomaterials can be formed by attachment of metals to biopolymers. In particular, biopolymers form molecular capsules by intramolecular hydrogen bonding. For example, starch may incorporate metals or metal oxide, thus forming polymeric nanocomposites. Chitosan is also used for nanotechnologyrelated applications due to its wide compatibility [START_REF] Vanaamudan | Chitosan-Guar gum blend silver nanoparticle bionanocomposite with potential for catalytic degradation of dyes and catalytic reduction of nitrophenol[END_REF][START_REF] Morin-Crini | Applications of chitosan in food, pharmaceuticals, medicine, cosmetics, agriculture, textiles, pulp and paper, biotechnology, and environmental chemistry[END_REF]. Silver (Ag) can be incorporated within starch in a supramolecular way to form nanomaterials [START_REF] Raveendran | Completely "green" synthesis and stabilization of metal nanoparticles[END_REF]. Nanomaterials can be incorporated in biopolymers by both sorption and impregnation [START_REF] Shankar | Bionanocomposite films for food packaging applications[END_REF]. Polymeric nanomaterials are solid colloidal particles within the size range of 10 nm-1 µm.

Physical properties of nanomaterials can be drastically different from the corresponding macro-sized, bulk material because nanomaterials have much higher surface area and reactivity [START_REF] Sreedharan | Biodegradation of Textile Azo Dyes[END_REF]. Either nanospheres or nanocapsules can be prepared, depending on the preparation method [START_REF] Sharma | Transdermal and intravenous nano drug delivery systems: present and future[END_REF]. Biopolymeric nanomaterials are characterised by microscopy, spectroscopy and other techniques (Fig. 2). A list of green polymeric nanomaterials used for textile dye degradation is presented in Table 1.

Photocatalytic degradation of dyes by biopolymeric nanomaterials

Biopolymers such chitosan act as support material of metallic photocatalysts. Owing to strong adsorption and high surface area, chitosan reduces the amount of intermediates during photocatalytic reactions. In addition, chitosan allows quick and trouble-free recovery of the photocatalyst, which can be recycled with or without any regeneration [START_REF] Adnan | Mitigation of pollutants by chitosan/metallic oxide photocatalyst: a review[END_REF]. Photocatalysis is different versus general catalysis in a way that during photocatalysis photons induce catalysis at the time of reaction [START_REF] Bahal | Investigations on amphoteric chitosan/TiO 2 bionanocomposites for application in visible light induced photocatalytic degradation[END_REF]). In the presence of photon (λ), oxygen acts as an electron acceptor and electrons are generated photocatalytically by the breakage of complex dyes [START_REF] Yang | Mechanism of TiO 2 -assisted photocatalytic degradation of dyes under visible irradiation: photoelectrocatalytic study by TiO 2 -film electrodes[END_REF]. In response to visible light, polymeric nanomaterials have been shown to degrade dye-containing wastewater photocatalytically within very short period [START_REF] Bahal | Investigations on amphoteric chitosan/TiO 2 bionanocomposites for application in visible light induced photocatalytic degradation[END_REF], which is both eco-friendly and inexpensive.

The concept of using TiO 2 nanoparticles for photocatalytic dye degradation was developed several decades ago [START_REF] Fujishima | Electrochemical photolysis of water at a semiconductor electrode[END_REF]. Green-synthesised silver nanomaterials have also been used as photocatalysts to treat dyes and other organic chemicals [START_REF] Sharma | Silver nanoparticles: green synthesis and their antimicrobial activities[END_REF]. At that time, nanotechnology was not popular for wastewater remediation, but now it is due to evidence of high performances [START_REF] Durgalakshmi | Current role of nanomaterials in environmental remediation[END_REF].

Electron affinity is a major parameter for photocatalytic degradation of reactive textile dyes [START_REF] Saravanan | Enhanced photocatalytic activity of ZnO/ CuO nanocomposite for the degradation of textile dye on visible light illumination[END_REF]) as the ionic nature or the presence of lone pair electrons in

Metal-incorporated biopolymeric nanomaterials

The synthesis of metal-incorporated biopolymeric nanomaterials is outlined in Fig. 3. Metal-incorporated nanomaterials display high efficacy for the photocatalytic degradation of azo dyes. This can be attributed to their pore size, chemistry of surface plane and ideal mechanical rigidity [START_REF] Opoku | Metal oxide polymer nanocomposites in water treatments. Descriptive inorganic chemistry researches of metal compounds[END_REF]. The basics of photocatalytic degradation involve an electron transfer process coupled with a redox reaction. If the semi-conductivity is modified with metal-incorporated nanoparticles, then the system endorses the charge transport at interface and, in turn, decreases the oxidation of the metal [START_REF] Subramanian | Semiconductor-metal composite nanostructures To what extent do metal nanoparticles improve the photocatalytic activity of TiO 2 films?[END_REF]. This process increases the lifetime of electron followed by the augmentation of the reactivity. Surface plasmon resonance (SPR) increases the co-existence of electrons due to the small particle size [START_REF] Sankar | Ultrarapid photocatalytic activity of Azadirachta indica engineered colloidal titanium dioxide nanoparticles[END_REF]. Polymeric nanomaterials with metal incorporation act as a stabilizer for itself. Metal nanoparticles incorporated with polymeric materials such as resin have found industrial applications as reaction catalysts [START_REF] Kralik | Catalysis by metal nanoparticles supported on functional organic polymers[END_REF].

Metal nanoparticles can also be grafted on different polymeric materials, which increases compactness and stability [START_REF] Tamayo | Polymer composites with metal nanoparticles: synthesis, properties, and applications[END_REF]) and provides a different functionality than that of the metal monomeric nanomaterials (Van Berkel and Hawker 2010). Usually, metal nanoparticles vary in size, whereas incorporation of polymer makes nanoparticle sizes more homogeneous and renders the material more stable. This has been shown during the integration of bacterial cellulose fibres with gold nanoparticles (AuNP).

Gold nanoparticles, of 74.32 nm, incorporated in biomaterials from fresh fruiting bodies of the Enoki mushroom degrade nearly 75% of the methylene blue dye in 4 h [START_REF] Rabeea | Mycosynthesis of gold nanoparticles using the extract of Flammulina velutipes, Physalacriaceae, and their efficacy for decolorization of methylene blue[END_REF]). Due to their wide substrate specificity, Au nanoparticles are able to treat diverse types of organic dyes [START_REF] Tamayo | Polymer composites with metal nanoparticles: synthesis, properties, and applications[END_REF]. For lanthanum (La), the f-orbital electron of metal ions interacts with different functional groups of different biopolymers and forms complexes with greater surface areas. Lanthanum incorporation in biopolymers results in photocatalytic activity with better adsorption capacity, specifically for organic compounds such as azo dyes [START_REF] Sirajudheen | Facile synthesis of chitosan-La 3+graphite composite and its influence in photocatalytic degradation of methylene blue[END_REF]. A composite of lanthanum (La) metal and chitosan has degraded 90% of methylene blue in 40 min [START_REF] Sirajudheen | Facile synthesis of chitosan-La 3+graphite composite and its influence in photocatalytic degradation of methylene blue[END_REF]. Here, the chemical oxygen demand (COD) decreased nearly 8 times, indicating mineralisation of methylene blue.

Metal oxide-incorporated biopolymeric nanomaterials

Synthesis of metal oxide-incorporated biopolymeric nanomaterials for photodegradation of dye is illustrated in Fig. 4.

Nanomaterials made up of metal oxide and biopolymers have also extensive photocatalytic activity that can degrade complex chemical structure of azo dyes. Incorporation of biopolymer and metal oxide nanomaterials improves the physicochemical properties of the nanomaterials. Conversely, the presence of metal oxide within the biopolymeric structure enhances the properties of polymer too [START_REF] Prasanna | Metal oxide based nanomaterials and their polymer nanocomposites[END_REF].

ZnO is one of the most efficient nanomaterials for photocatalytic dye degradation because of ZnO semi-conductivity, Fig. 3 Synthesis of metal-incorporated biopolymeric nanoparticles for photocatalytic dye degradation stability and activity [START_REF] Ravishankar | Comparison of the photocatalytic degradation of trypan blue by undoped and silver-doped zinc oxide nanoparticles[END_REF]. Starch-based ZnO nanomaterial has been reported for its improved conductive and dielectric properties, compared to pure metal oxide nanoparticles [START_REF] Ravishankar | Comparison of the photocatalytic degradation of trypan blue by undoped and silver-doped zinc oxide nanoparticles[END_REF]. Cellulosebased ZnO nanomaterials display higher thermal stability than that of the pure metal oxide, and can be used for remediation of dye-containing wastewater at large scale (Ravishankar et al.2014;[START_REF] Azizi | Synthesis, antibacterial and thermal studies of cellulose nanocrystal stabilized ZnO-Ag heterostructure nanoparticles[END_REF]. A chitosan-acrylic acid biopolymer grafted with nano-TiO 2 was reported to degrade more than 90% of malachite green present in wastewater under neutral pH, through a visible light-mediated photocatalytic way [START_REF] Bahal | Investigations on amphoteric chitosan/TiO 2 bionanocomposites for application in visible light induced photocatalytic degradation[END_REF].

Chitosan incorporating nano-iron oxide (Fe 3 O 4 ) has been used for purification of dye-containing wastewater [START_REF] Prasanna | Metal oxide based nanomaterials and their polymer nanocomposites[END_REF][START_REF] Ngah | Adsorption of dyes and heavy metal ions by chitosan composites: a review[END_REF]. Chitosan/nano-Fe 3 O 4 nanomaterial is also increasing frictions due to magnetic dipole-dipole interactions during the degradation of the hazardous X-3B dye [START_REF] Adnan | Mitigation of pollutants by chitosan/metallic oxide photocatalyst: a review[END_REF]). TiO 2 is widely used for preparing biopolymeric nanomaterials due to TiO 2 advantageous surface properties and photocatalysis under visible light [START_REF] Bahal | Investigations on amphoteric chitosan/TiO 2 bionanocomposites for application in visible light induced photocatalytic degradation[END_REF]. Cyclodextrin, an oligosaccharide produced from enzymatic conversion of starch, has been used for wastewater treatment after modification with nano-TiO 2 [START_REF] Khaoulani | Wastewater treatment by cyclodextrin polymers and noble metal/mesoporous TiO 2 photocatalysts[END_REF]. ZnO/carbon black grafted in cellulose acetate has been used to treat azo dyes such as congo red, methyl orange and methylene blue [START_REF] Khan | A facile synthesis of CuAg nanoparticles on highly porous ZnO/carbon black-cellulose acetate sheets for nitroarene and azo dyes reduction/degradation[END_REF].

MnO 2 /cellulose nanoparticles of size lower than 100 nm degrade 90% indigo carmine within 25 min under ambient light and acidic pH [START_REF] Oliveira | Free-standing cellulose film containing manganese dioxide nanoparticles and its use in discoloration of indigo carmine dye[END_REF]. Here, the biopolymeric nanomaterials can be recovered from solution and recycled for at least 10 times without compromising the decolourisation efficiency. This provides evidence that in the presence of photons, metal oxide-incorporated biopolymeric nanomaterials degrade complex azo dyes within a very short time through an eco-friendly, recyclable process. Few bio-sourced enzymes degrade reactive azo dyes. Laccase shows good potential for bioremediation of dye-containing wastewater [START_REF] Wang | Rapid decolorization of phenolic azo dyes by immobilized laccase with Fe 3 O 4 /SiO 2 nanoparticles as support[END_REF][START_REF] Sarkar | Structural-functional analyses of textile dye degrading azoreductase, laccase and peroxidase: a comparative in silico study[END_REF]. Nano-Fe 3 O 4 /SiO 2 supported with immobilized laccase has achieved nearly complete degradation of the azo dye procion Red MX-5B within 20 min [START_REF] Wang | Rapid decolorization of phenolic azo dyes by immobilized laccase with Fe 3 O 4 /SiO 2 nanoparticles as support[END_REF].

Metal sulphide-incorporated biopolymeric nanomaterials

Synthesis of metal sulphide-incorporated biopolymeric nanoparticles is depicted in Fig. 5. The biopolymer helps to crystallize ZnS nanoparticles [START_REF] Tiwari | Stabilization of ZnS nanoparticles by polymeric matrices: syntheses, optical properties and recent applications[END_REF]. ZnS-incorporated chitosan nanomaterials of 4 nm size display a photocatalytic activity and are used in photodegradation of organic dyes in wastewater [START_REF] Das | Biological synthesis of metallic nanoparticles: plants, animals and microbial aspects[END_REF]. Cadmium (Cd) and lead (Pb) are also used for the formation of metal sulphide nanomaterials. For instance, Klebsiella pneumonia has the ability to synthesise electron-dense nano-CdS materials on its cell membrane, which can induce photoreduction of methyl orange by subsequent electron transfer [START_REF] Das | Biological synthesis of metallic nanoparticles: plants, animals and microbial aspects[END_REF].

Nano-chitosan has been used to remove hazardous dyes [START_REF] Mansur | Nano-photocatalysts based on ZnS quantum dots/chitosan for the photodegradation of dye pollutants[END_REF]. Chitosan-based quantum dots, a nano-photocatalyst and ZnS are able to remove methylene blue [START_REF] Mansur | Nano-photocatalysts based on ZnS quantum dots/chitosan for the photodegradation of dye pollutants[END_REF]. Here, ZnS acts as a semiconductor and thus enhances the removal. This ZnS/ chitosan-based nanomaterial of nearly 3.5 nm size induces 90% photodegradation of methylene blue by oxidation under UV irradiation within 90 min.

Other biopolymeric nanomaterials

Metal/metal oxide nanoparticles are of special interest because the metal centre of the metal-metal oxide nanomaterial increases the semi-conductivity and rate of separation of electron holes, which in turn increases the photon irradiation, followed by escalation of the photocatalytic activity [START_REF] Malagutti | Deposition of TiO 2 and Ag: TiO 2 thin films by the polymeric precursor method and their application in the photodegradation of textile dyes[END_REF]. For example, 0.25% Ag-TiO 2 thin film incorporation in a resin biopolymer drastically increases Fig. 4 Synthesis of metal oxideincorporated biopolymeric nanoparticles for the photodegradation of dyes the photocatalytic activity of rhodamine B, compared to sole TiO 2 , as a result of reduced electron hole recombination [START_REF] Malagutti | Deposition of TiO 2 and Ag: TiO 2 thin films by the polymeric precursor method and their application in the photodegradation of textile dyes[END_REF]). TiO 2 /Ag hybrid, modified by the incorporation of carboxymethyl cellulase and gelatine of 50-100 nm size, has shown improved photocatalytic activity towards benzene and NH 3 present in the chemical structure of organic pollutants [START_REF] Farshchi | Photocatalytic/biodegradable film based on carboxymethyl cellulose, modified by gelatin and TiO 2 -Ag nanoparticles[END_REF].

Various other chemicals are integrated with diverse polymers for the synthesis of photocatalytic nanomaterials. Several coupled systems expand the photocatalytic performances, such as metal-incorporated metal oxide and dual metal oxide systems [START_REF] Gnanasekaran | Synthesis and characterization of metal oxides (CeO 2 , CuO, NiO, Mn 3 O 4 , SnO 2 and ZnO) nanoparticles as photo catalysts for degradation of textile dyes[END_REF][START_REF] Gnanasekaran | Efficient photocatalytic degradation of toxic dyes using nanostructured TiO 2 / polyaniline nanocomposite[END_REF][START_REF] Sulaiman | Effects of photocatalytic activity of metal and non-metal doped TiO 2 for hydrogen production enhancement-a review[END_REF][START_REF] Kuo | Effect of non-metal doping on the photocatalytic activity of titanium dioxide on the photodegradation of aqueous bisphenol A[END_REF]). These systems have two different band positions under sunlight or UV light exposure, which avoids electrons-holes recombination, and thus improves the photocatalytic activity [START_REF] Gnanasekaran | Synthesis and characterization of metal oxides (CeO 2 , CuO, NiO, Mn 3 O 4 , SnO 2 and ZnO) nanoparticles as photo catalysts for degradation of textile dyes[END_REF][START_REF] Gnanasekaran | Efficient photocatalytic degradation of toxic dyes using nanostructured TiO 2 / polyaniline nanocomposite[END_REF][START_REF] Sulaiman | Effects of photocatalytic activity of metal and non-metal doped TiO 2 for hydrogen production enhancement-a review[END_REF][START_REF] Kuo | Effect of non-metal doping on the photocatalytic activity of titanium dioxide on the photodegradation of aqueous bisphenol A[END_REF].

Unique properties of nanocomposites have created a revolution in the field of bioremediation [START_REF] Mohanraj | Facile synthesis of paper based graphene electrodes for point of care devices: a double stranded DNA (dsDNA) biosensor[END_REF]. Recently, several research groups have tried diverse biopolymeric nanocomposites such as metal/metal oxide/biopolymer or metal oxide/conducting polymer for improved degradation of synthetic azo dyes. One example of biopolymer/metal oxide nanocomposite is chitosan/ ZnO/AgCl nanocomposite based on hydrogel beads, which permits complete photocatalytic degradation of methylene blue [START_REF] Taghizadeh | ZnO, AgCl and AgCl/ZnO nanocomposites incorporated chitosan in the form of hydrogel beads for photocatalytic degradation of MB, E. coli and S. aureus[END_REF]. The presence of chitosan in nanocomposites or other biopolymeric nanomaterials significantly increases the degradation activity, as a consequence of hydrophilic adsorption of organic pollutants [START_REF] Adnan | Mitigation of pollutants by chitosan/metallic oxide photocatalyst: a review[END_REF]. Furthermore, chitosan-coated CuSO 4 nanocomposite, synthesised from Psidium guajava aqueous leaf extract, induced more than 90% oxidative photodegradation of congo red and methylene blue within 150 min [START_REF] Sathiyavimal | Eco-biocompatibility of chitosan coated biosynthesized copper oxide nanocomposite for enhanced industrial (Azo) dye removal from aqueous solution and antibacterial properties[END_REF]. Here, electrons are generated from the valance bond due to the presence of sunlight.

Cellulose nanocrystals have been prepared from bleached bagasse pulp and reacted with ZnO/CuO to synthesise biopolymeric dual metal oxide nanocomposites, which can degrade rose bengal more than 99% in 40 min [START_REF] Elfeky | Multifunctional cellulose nanocrystal/metal oxide hybrid, photo-degradation, antibacterial and larvicidal activities[END_REF]. Immobilised dye-degrading enzyme laccase conjugated with thiolated chitosan-Fe 3 O 4 hybrid has been reported to have magnetic properties, and it can remove more than 80% of reactive blue 171 and acid blue 74 within short periods of time [START_REF] Ulu | Laccase-conjugated thiolated chitosan-Fe 3 O 4 hybrid composite for biocatalytic degradation of organic dyes[END_REF].

Conclusion

The long-term fate of textile dyes in river and sea sediments is not clear. A recent report suggests that aromatic amines could be naturally degraded in more than 2 years by the sediment bacterial community [START_REF] Ito | Long-term natural remediation process in textile dye-polluted river sediment driven by bacterial community changes[END_REF]). Yet this process is slow and most probably never complete in natural anaerobic environments. Therefore, wastewater should be treated to remove all pollutants before discharge of residual waters in rivers. Here, promising techniques should be based on biosourced tools such as microorganisms and dye-degrading enzymes, e.g. laccase, azoreductase or peroxidase [START_REF] Sarkar | Structural-functional analyses of textile dye degrading azoreductase, laccase and peroxidase: a comparative in silico study[END_REF]. The current success of photocatalytic dye degradation using biopolymers is attributed to the green process, uniform deposition of the nanoparticles, less cytotoxicity and recyclable nature.

Fig. 1

 1 Fig. 1 Different sources of biopolymers and their usage in the synthesis of green polymeric nanomaterials. M + : metal ions

Fig. 2

 2 Fig. 2 Properties and analysis of biopolymeric nanomaterials

Fig. 5

 5 Fig. 5 Rationale behind the synthesis of metal sulphide-incorporated biopolymeric nanoparticles as photocatalysts for dye degradation. BP: biopolymer, MS: metal sulphide

Table 1

 1 Biopolymeric nanomaterials used for the degradation of textile dyes degradation

	Type of nanomaterial Catalyst	Biopolymer	Degradation of dyes	References
	Metal	Silver (Ag)	κ-Carrageenan gum	Mineralisation and catalytic	Pandey et al. (2020)
				degradation of industrially
				significant organic dyes such
				as methylene blue and rhoda-
				mine B
		Palladium (Pd)	Chitosan c-Nanotube supported Congo red, methylene blue,	Sargin et al. (2020)
				methyl orange, methyl red
		Chitosan/Fe	Chitosan	Basic dye	Kasiri (2019)
		Au	Alginate beads	Discoloration of azo dye acidic	Ahmed (2019)
				orange 7 and reactive orange 5
		Lanthanum (La) Chitosan	Photocatalytic degradation of	Sirajudheen and Meenakshi
				azo dye (methylene blue)	(2019)
		Gold (Au)	Bacterial cellulosic fibre	Azo dye degradation	Tamayo et al. (2019), Vilela et al.
					(2018)
		Cupper (Cu)	Chitosan	Congo red	Ali et al. (2018)
		Silver (Ag)	Chitosan	Ponceau BS dye	Sultana et al. (2017)
		Palladium (Pd)	Carboxymethyl cellulose	Degradation of azo dye	Li et al. (2017a)
		Zirconium (Zr)	Gelatine	Methylene blue and fast green	Thakur et al. (2017)
		Palladium (Pd)	Glucuronoarabinogalactan	Coomassie brilliant blue G-250,	Kora and Rastogi (2016)
			polymer and gum olibanum	rhodamine B, methylene blue
			(Boswellia serrata)	and 4-nitrophenol
		Cupper (Cu)	Chitosan-coated cellulosic	Methyl orange and congo red	Kamal et al. (2016)
			microfibres
	Metal oxide	ZnO	Chitosan in the form of hydro-	Methylene blue	Taghizadeh et al. (2020)
			gel beads
		ZnO	Quince seed mucilage	Photocatalytic degradation of	Moghaddas et al. (2020)
				methylene blue
		Fe 3 O 4	Chitosan	Hazardous dye X-3B	Adnan et al. (2020)
		MnO 2	Cellulose	Indigo carmine dye solution	Oliveira et al. (2020)
		Alumina (Al 2 O 3 ) Chitosan	orange sulfonated azo dye methyl	Kasiri (2019)
		ZnO	Chitosan	Chromium complex dye, Direct	Kasiri (2019)
				Blue 78, Acid Black 26
		ZnO	Cellulose	Dye-containing wastewater	Bahal et al. (2019)
				remediation
		TiO 2	mer Chitosan-acrylic acid biopoly-	Malachite green	Bahal et al. (2019)
		ZnO	Cellulose acetate polymeric	Congo red, methyl orange,	Khan et al. (2019)
			sheet	methylene blue
		TiO 2	Bacterial cellulose	Photocatalytic dye degradation Vilela et al. (2018)
		TiO 2	of Komagataei bacterxylinus-Cellulose by the fermentation	Reactive red X-3B	Li et al. (2017b)
			immobilized laccase
		TiO 2	Oak gall tannin	Direct yellow 86	Binaeian et al. (2016)
		TiO 2	Chitosan	acid blue 113, reactive yellow Acid orange 7, acid red 18, C.I.	Škorić et al. (2016)
				17, reactive black 5, direct
				blue 78
		ZnO	Conducting polyalanine poly-	Methylene blue and malachite	Riaz et al. (2015)
			mer	green
	Metal sulphide	ZnS	Chitosan	Photodegradation of organic	Das et al. (2017)
				dyes (methyl orange)
		ZnS	Chitosan	Around 90% photodegradation	Mansur and Mansur (2015)
				of methylene blue under UV
				irradiation

Table 1 (

 1

	continued)
	Type of nanomaterial Catalyst	Biopolymer	Degradation of dyes	References
	Others	AgCl	Chitosan in the form of hydro-	Methylene blue	Taghizadeh et al. (2020)
			gel beads
		Fe 3 O 4	lus sp. MSK-01 conjugated Immobilised laccase from Bacil-	organic dyes (Reactive Blue Biocatalytic degradation of	Ulu et al. (2020)
			with thiolated chitosan	171 and Acid Blue 74)
		TiO 2	Chitosan-epichlorohydrin	Reactive Red 120	Jawad et al. (2020)
		CuSO 4	from Psidium guajava aque-Chitosan-coated nanocomposite	Congo red and methylene blue Sathiyavimal et al. (2020)
			ous leaf extract
		ZnO	Arabic gum-grafted polyacryla-	Complete degradation of mala-	Mittal et al. (2020)
			mide hydrogel	chite green
		ZnO/CuO	Cellulose nanocrystal from	Rose Bengal (RB)	Elfeky et al. (2020)
			bleached bagasse pulp
		Ag/TiO 2	gelatine Carboxymethyl cellulase and	Organic dye pollutant	Farshchi et al. (2019)
		SiO2	Chitosan/carbon nanotubes	Direct Blue 71, Reactive	Kasiri (2019)
				Blue 19
		AgNO 3	Chitosan and guar gum	Binary dye	Vanaamudan et al. (2018)
		ZnS	Chitosan	Photocatalytic degradation of	Das et al. (2017)
				organic dye
		AgNO 3	carbohydrate polymers Tangerine peel containing	orange Catalytic reduction of methyl	Alzahrani (2015)
		Pt-TiO 2	Conjugated polymer	azo dye Photocatalytic degradation of	Dong et al. (2015)

Acknowledgements SS and RB are thankful to UGC-CAS, Department of Botany, The University of Burdwan for pursuing research activities. SS is thankful to SVMCM-Non-Net fellowship for the financial assistance. AB acknowledges the support of FONDECYT Iniciación No. 11190325 by Govt. Of Chile and Centro de Biotecnología de los Recursos Naturales (CenBio), Universidad Católica del Maule for the laboratory facilities. The author (S.R.) acknowledges the support of ANID through the project ANID/FONDAP/15110019 and FONDECYT Government of Chile (Project No.: 11170414). All the authors are thankful to biorender.com for preparation of the images.


Liqiang Wang

Mengying Li

Shaocai Yu
email: shaocaiyu@zju.edu.cn

Xue Chen

Zhen Li

Yibo Zhang

Linhui Jiang

Yan Xia

Jiali Li

Weiping Liu

Pengfei Li

Eric Lichtfouse

Daniel Rosenfeld

John H Seinfeld

Unexpected rise of

Keywords: COVID-19, Hangzhou, Lockdown, Air quality, Emission reductions, Coronavirus, PM 2.5, PM 10, SO 2, CO, NO 2, O

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

In early December 2019, the novel coronavirus pneumonia named COVID-19 by the World Health Organization (WHO) was initially identified in Wuhan, the capital city of Hubei province [START_REF] Huang | Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China[END_REF][START_REF] Guan | Clinical characteristics of coronavirus disease 2019 in China[END_REF]WHO, 2019). As of May 28, 2020, COVID-19 had spread rapidly on a global scale with 5,491,678 confirmed cases, 349,190 confirmed deaths in 217 countries, areas or territories, as reported by the WHO (https ://www.who.int/emerg encie s/disea ses/novel -coron aviru s-2019). To curb the spread of COVID-19, Wuhan and other cities of Hubei province announced lockdowns on January 23 and 24, 2020, respectively, followed by other provinces in China, by declaring the highest level of a public health emergency on January 26, 2020 [START_REF] Tian | An investigation of transmission control measures during the first 50 days of the COVID-19 epidemic in China[END_REF]. As part of the national emergency response, the Chinese government established nationally strict curfews and isolation policies, named 'lockdown', for instance:

• extending the Spring Festival holiday,

• suspending all public transportation including city buses, subways, ferries and long-distance coaches, • closing outbound channels at airports and railway stations, • closing schools and entertainment venues, • banning public gatherings, • enhancing social distancing, and • issuing stay-at-home orders and closing non-essential businesses.

Future analyses will evaluate the extent to which these approaches have been effective, and whether potential benefits have outweighed the costs, e.g., economic losses. Non-essential businesses in most areas resumed work after February 10, although non-essential businesses in Wuhan remained closed until March 20.

The all-out massive efforts of the Chinese government by sealing off all cities in a bid to curb the spread of the COVID-19 provide an unprecedented opportunity to study the possible effects of anthropogenic emission on air quality. NASA and European Space Agency (ESA) pollution monitoring satellites, i.e., the Tropospheric Monitoring Instrument (TROPOMI) on ESA's Sentinel-5 satellite, detected significant decreases in NO 2 over China based on the data from January 1-20, 2020, before the quarantine, to February 10-25, during the quarantine (NASA 2020). There is evidence that the changes were at least partly related to the economic slowdown following the coronavirus outbreak. By comparison of NO 2 values in 2020 with the average values detected at this time of year from 2005 to 2019, NASA determined that NO 2 concentrations in eastern and central China in 2020 were 10-30% lower than those normally observed for this time period (NASA 2020).

On the other hand, despite reduction in activities at the outbreak of COVID-19, the Beijing-Tianjin-Hebei cluster remained shrouded in smog during the Spring Festival holiday in 2020 despite the expectation that air pollution would remain low (MEE 2020;China Daily 2020). On Wednesday, February 5, 2020, hourly concentrations of PM 2.5fine particles with an aerodynamic diameter lower than 2.5 μm-in most districts in Beijing exceeded 200 μg m -3 with worsening haze over the Lantern Festival on Saturday, February 9 (MEE 2020). It was estimated that from January 24 to February 2, 2020, pollutant emissions, with 77% fewer freight vehicles and 39% fewer cars on the roads, decreased by at least 40% in the 28 major cities in the Beijing-Tianjin-Hebei (BTH) cluster (MEE 2020). However, emissions from industries such as steel, nonferrous metals, glass, cooking and heating in the BTH cluster remained generally unchanged, despite a suspension of activities by many restaurants and labor-intensive factories (MEE 2020). There was a decrease of only 10% in emissions by steel plants and coal-burning power generation sectors with no decrease in other smokestack industries. About 10 million rural households in the BTH region relied on coal for heating, with some shift to cleaner energy like natural gas. Coal consumption for heating increased as reflected by more than 10% enhancement of CO concentrations, relative to last year in the BTH rural areas. Another major factor for poor air quality in the BTH region during the cold periods was the unfavorable meteorological conditions for pollutant dispersal, such as mixing layer heights of only 300 to 500 m, half to one-third of their normal heights in the same season in MEE (2020).

Elevated PM 2.5 is a major factor controlling regional climate and human health [START_REF] Van Donkelaar | Global estimates of ambient fine particulate matter concentrations from satellite-based aerosol optical depth: development and application[END_REF][START_REF] Yu | Origin of air pollution during a weekly heavy haze episode in Hangzhou, China[END_REF]Wang et al. 2014a;[START_REF] Rosenfeld | Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds[END_REF]Pope 2000;[START_REF] Seinfeld | Atmospheric chemistry and physics: from air pollution to climate change[END_REF][START_REF] Chen | Evidence on the impact of sustained exposure to air pollution on life expectancy from China's Huai River policy[END_REF]. The main driver of decreases in PM 2.5 in China between 2013 and 2017 was strict emission control policies [START_REF] Zhang | Drivers of improved PM 2.5 air quality in China from 2013 to 2017[END_REF][START_REF] Ding | Significant reduction of PM 2.5 in eastern China due to regional-scale emission control: evidence from SORPES in 2011-2018[END_REF]. Efforts to improve air quality during large international events, such as the 2008 Beijing Olympics, the 2014 Beijing Asia-Pacific Economic Cooperation (APEC) summit, the 2015 Grand Military Parade, and the 2016 Hangzhou G20 summit, have included short-term closure of power plants and factories, restriction of traffic, and reduction in construction activities [START_REF] Xing | Modeling study on the air quality impacts from emission reductions and atypical meteorological conditions during the 2008 Beijing Olympics[END_REF][START_REF] Liu | A paradox for air pollution controlling in China revealed by "aPEC Blue" and "parade Blue[END_REF][START_REF] Li | High reduction of ozone and particulate matter during the 2016 G-20 summit in Hangzhou by forced emission controls of industry and traffic[END_REF][START_REF] Wang | Characteristics and emission-reduction measures evaluation of PM 2.5 during the two major eventsAPEC and Parade[END_REF].

Hangzhou, the capital of Zhengjiang Province situated in the Yangtze River Delta and one of the most famous and prosperous cities in China (Fig. 1 62 days for PM 2.5 for the secondary Chinese Ambient Air Quality Standards (CAAQS): PM 2.5 : 75 μg m -3 for 24 h average and O 3 : 75 ppbv for daily maximum 8 h [START_REF] Zhang | Characterization of atmospheric trace gases and particulate matter in Hangzhou, China[END_REF]. During a severe haze episode from December 3-9, 2013, PM 2.5 mean concentrations reached as high as 293.4 ± 103.2 μg m -3 [START_REF] Yu | Origin of air pollution during a weekly heavy haze episode in Hangzhou, China[END_REF]. Local emissions, complex atmospheric conditions, and regional transport exert combined effects on air pollution occurrence in Hangzhou [START_REF] Yu | Origin of air pollution during a weekly heavy haze episode in Hangzhou, China[END_REF][START_REF] Li | Ozone source apportionment at urban area during a typical photochemical pollution episode in the summer of 2013 in the Yangtze River Delta[END_REF][START_REF] Wu | Residential emissions predicted as a major source of fine particulate matter in winter over the Yangtze River Delta, China[END_REF][START_REF] Chen | Common source areas of air pollution vary with haze intensity in the Yangtze River Delta, China[END_REF]. On the other hand, Zhejiang, known for its thriving private sector, was the first province both to declare a highest level response to coronavirus on January 23, 2020 and to relax control measures and support work resumption (https ://www.scmp.com/econo my/china econo my/artic le/30498 21/coron aviru s-chine se-provi nce-order s-relax ation -exces sive). As the capital of Zhejiang province, Hangzhou city declared the first-level public health emergency response on January 23, 2020 (http://www.zj.gov.cn/ art/2020/1/26/art_12289 96604 _41859 419.html).

A recent editorial underlines the importance and relevance of research in environmental chemistry to understand pandemics [START_REF] Sharma | Environmental chemistry is most relevant to study coronavirus pandemics[END_REF]). Here we study the impacts of the city lockdowns due to the COVID-19 outbreak on air quality in Hangzhou from January 1 to February 28, 2020. Our hypothesis was that air pollutants should have decreased during lockdown due to the absence of activities and traffic. This report reveals that, on the contrary, the concentrations of some air pollutants increased in both urban and rural areas.

Experimental

Hourly observational data of the six pollutants, PM 2.5 , PM 10 , SO 2 , CO, NO 2 , O 3 , at the monitoring sites from January 1 to February 28, 2020, were collected from the website of China National Environmental Monitoring Center (CNEMC) (http://www.mee.gov.cn/hjzl/dqhj/). In addition data were acquired at 11 air quality monitoring stations are set up in Hangzhou, as shown in Fig. 1a, including 10 urban sites (Binjiang, Xiasha, Wolongqiao, Zhejiangnongda, Zhaohuiwuqu, Hemuxiaoxue, Linpingzhen, Xixi, Yunqi and Chengxiangzhen), and one rural background site (Qiandaohu), a lake park located about 170 km away from Hangzhou center. Average concentrations at the 10 urban sites were estimated to represent urban conditions, while the concentrations at the rural site were used to represent rural conditions. According to the China National Ambient Air Quality Standard (CNAAQS, GB3095-2012), the 24-hour average levels of each pollutant are regarded as valid only under if at least 20 h of hourly data are acquired. The 8 h-O 3 concentrations could be used only with more than 6 h available data for each 8 h period. In this study, the data in 2016, 2017, 2018 and 2019 were also used for comparison purpose.

Data of population (7.03 millions) and area (16,596 km 2 ) of Hangzhou city were acquired from the Civil Affairs Ministry of the People's Republic of China (http://xzqh.mca. gov.cn/). The change of the Baidu Migration Scale Index to characterize the daily travel intensity index inside Hangzhou was downloaded from the public Baidu Maps website (http://qianx i.baidu .com/). Daily traffic density was calculated using the travel intensity index multiplied by population, and then divided by area.

Results and discussion

Trends of air pollutants in early 2020

Overall trends

Figure 1b shows the time series of hourly concentrations of six pollutants at the urban and rural sites from January 1 to February 28, 2020, with the corresponding statistical values summarized in Table S1. To analyze the temporal variations of concentration and impacts of the lockdown, the entire study period was divided into three periods: before lockdown (BL, January 1-23), during lockdown (DL, January 24-February 15), and resumption (February 16-28). Results show that when comparing pollutant levels before and during lockdown, concentrations of PM 2.5 , PM 10 , CO, NO 2 decreased significantly, as expected. However, results also show an overall increase in ozone (O 3 ) levels both in both urban and rural areas. After February 15, these trends began to reverse due to the resumption of work and production activities.

Particulate matter

In the urban area, hourly average concentrations of PM 2.5 decreased from 45.9 ± 31.3 μg m -3 before lockdown to 29.6 ± 19.5 μg m -3 during lockdown (Table S1). Similarly, at the rural site hourly PM 2.5 levels decreased from 25.0 ± 24.0 to 20.1 ± 16.9 μg m -3 . In the same way, mean PM 10 decreased significantly from 70.5 to 48.4 μg m -3 in the urban area and from 31.0 to 24.1 μg m -3 at the rural site. Then, both PM 2.5 and PM 10 levels rose to higher values after resumption. It is known that coal combustion, biomass burning, vehicle emissions, and industrial emissions are the main anthropogenic sources of PM 2.5 [START_REF] Chow | Review of PM 2.5 and PM 10 apportionment for fossil fuel combustion and other sources by the Chemical Mass Balance receptor model[END_REF][START_REF] Cheng | Impact of biomass burning on haze pollution in the Yangtze River delta, China: a case study in summer 2011[END_REF][START_REF] Wu | Potential Sources and Formations of the PM 2.5 Pollution in Urban Hangzhou[END_REF]Wang et al. 2016a;[START_REF] Mehmood | Spatial and temporal distributions of air pollutant emissions from open crop straw and biomass burnings in China from 2002 to 2016[END_REF][START_REF] Mehmood | Relative effects of open biomass burning and open crop straw burning on haze formation over central and eastern China: modeling study driven by constrained emissions[END_REF]. Therefore, our observed reduction in PM 2.5 concentrations in Hangzhou city during lockdown is mainly due to limited industrial activity and traffic [START_REF] Wang | Potential source analysis for PM 10 and PM 2.5 in autumn in a Northern City in China[END_REF][START_REF] Lyu | Chemical characteristics and causes of airborne particulate pollution in warm seasons in Wuhan, central China[END_REF][START_REF] Zhang | Dust deposition and ambient PM 10 concentration in northwest China: spatial and temporal variability[END_REF]). In the rural area, reduction in PM 2.5 and PM 10 can be ascribed to restricted vehicle travel and lower impacts of regional transports. The consistent patterns of PM 2.5 and PM 10 in both urban and rural areas suggested the important contributions of anthropogenic emission sources and impacts of regional transport during the lockdown.

Carbon monoxide

CO decreased in both urban areas from 1.0 to 0.59 mg m -3 , and in rural areas from 0.78 to 0.60 mg m -3 . Since CO mainly originates from fossil fuel combustion and biomass burning [START_REF] Zhang | Asian emissions in 2006 for the NASA INTEX-B mission[END_REF][START_REF] Chai | Spatial and temporal variation of particulate matter and gaseous pollutants in 26 cities in China[END_REF], decreasing anthropogenic emissions after reduction in industrial activities explain at least partly the CO patterns in both urban and rural areas.

Sulfur dioxide

SO 2 mean concentrations in the urban area decreased from 6.3 to 5.3 μg m -3 after lockdown, then increased to 5.6 μg m -3 in the resumption period. However, surprisingly, SO 2 mean concentrations at the rural site increased from 4.7 to 6.1 μg m -3 continuously during the entire period (Table S1). The unexpected increase in SO 2 at the rural site is explained by the increase in coal consumption for heating coupled with inputs from the traditional fireworks during Spring Festival Eve and the Lantern Festival. Overall, SO 2 concentrations remained low, which resulted from the implementation of control of SO 2 emission [START_REF] Fang | Managing air quality in a rapidly developing nation: china[END_REF], as observed in the Yangtze River Delta [START_REF] Huang | Typical types and formation mechanisms of haze in an Eastern Asia megacity, Shanghai[END_REF].

Nitrogen dioxide

Concentrations of NO 2 at the urban sites dropped from 44.2 to 13.0 μg m -3 after lockdown, then slightly increased to 21.8 μg m -3 during resumption (Fig. 1, Table S1). At the rural site, NO 2 levels decreased sharply from 21.0 μg m -3 to 6.8 μg m -3 during lockdown, then remained at very low levels of 6.4 μg m -3 during the resumption period. These findings are explained by lower NO x emissions from local vehicle emissions, power plants and industrial activities (Wang et al. 2014b(Wang et al. , 2016b) ) during the lockdown, whereas rural decreases in NO 2 concentrations might result from less vehicle emissions and less pollution transport from urban areas.

Ozone

Contrary to the dramatic decreases in NO 2 during the lockdown, relatively low O 3 levels in the urban area occurred before lockdown with almost all the values below 100 μg m -3 , then began to reach higher concentrations during lockdown with the maximum over 139 μg m -3 as shown in Fig. 1. Mean concentrations of O 3 increased from 24.6 μg m -3 before lockdown to 60.6 μg m -3 during lockdown in the urban area, whereas a lower increase in O 3 levels from 42.0 to 62.9 μg m -3 occurred in the rural area (Table S1). Then, during the resumption, the mean O 3 concentrations increased slightly in both urban and rural areas. As a secondary pollutant formed by photochemical reactions of nitrogen oxides (NO x ) and volatile organic compounds (VOC) [START_REF] Seinfeld | Atmospheric chemistry and physics: from air pollution to climate change[END_REF], O 3 will decrease with decrease in NO x in the NO x sensitive regime. The unexpected increase in O 3 levels during lockdown with the decreases in NO 2 is explained by the weakening of chemical titration of O 3 by NO during the nighttime, resulting in increases in background O 3 concentrations, as shown in Fig. 2.

In 2020, from 20:00 p.m. to 9:00 a.m. local time, the mean O 3 concentrations for the non-photochemical reaction time period increased from 18.8 μg m -3 before lockdown to 53.7 μg m -3 during lockdown in urban areas, and from 35.6 to 58.0 μg m -3 in rural areas (Fig. 2). By comparison, in 2019, the mean O 3 concentrations for the same time periods increased from 10.0 to 35.9 μg m -3 in urban areas, and from 30.6 to 47.8 μg m -3 in rural areas. The higher levels during the lockdown time period are partly explained by higher atmosphere temperatures for late January/early February versus January 1-23. Apart from higher levels in 2020 for the whole time period, Fig. 2 shows meaningful higher O 3 concentrations at night in 2020 relative to those in 2019. This confirms indirectly the fact that the O 3 increase in 2020 during lockdown is due to the weakening of chemical titration of O 3 by NO during the nighttime, resulting in increases in background O 3 concentrations.

Comparisons of air quality between 2019 and 2020

Daily average concentrations of ambient PM 2.5 , PM 10 , CO, NO 2 , SO 2 and daily maximum 8 h-O 3 concentrations in 2020 relative to those in the same period of 2019 in the urban and rural areas are displayed in Fig. 3 and Table S1. PM 2.5 and PM 10 concentrations in the urban areas decrease sharply on January 24, and then remained below 75% during a few days January 24-27 (Fig. 3a,b). Similar drops of 96% for PM 2.5 and 95.6% for PM 10 were observed at the rural site on January 26. CO and SO 2 exhibited no significant change in the urban areas, whereas rising trends were observed at the rural site. This is explained by increasing biomass burning for residential heating or cooking in the rural area. At both urban and rural sites, NO 2 decreased and the maximum drop appeared during January 24-30, as expected. Daily maximum 8 h-O 3 concentrations were enhanced during almost all days in January and February in 2020 relative to 2019, although the larger enhancements existed before lockdown in both urban and rural areas (Fig. 3).

For the urban area, before the lockdown period, January 1-23, 2020/2019 decreases amounted to 24.8% for PM 2.5 , 19.8% for PM 10 , 29.2% for SO 2 , 14.1% for CO and 13.7% for NO 2 . By comparison, the decreases were much higher during the lockdown period, January 24-February 15: 42.7% for PM 2.5 , 47.9% for PM 10 , 22.3% CO and 58.4% for NO 2 ; except for SO 2 (28.6%) (Table S1). Higher decreases after January 24 reveal the reductions in industrial activities and traffic in Hangzhou. The similar SO 2 changes are in agreement with the contribution of coal combustion for heating and firework emissions during lockdown.

For the rural area, only PM 2.5 , PM 10 , and NO 2 showed significantly higher decreases of -18.5%, -39.6%, -48.0% during the lockdown period, respectively, compared with the corresponding 4.4%, -27.0%, -4.8% before lockdown (Table S1). Whereas SO 2 and CO exhibited weak decreases during lockdown, O 3 concentrations exhibited an increase during the whole period at all sites relative to those in 2019. With similar decrease percentages of NO 2 at the urban sites (-58.4%) and rural site (-48.0%) during January 24-February 15, larger enhancement percentages of O 3 were observed at the urban (22.2%) relative to rural (15.7%) sites in 2020 in comparison to those in 2019. Moreover, the weekly mean concentrations of NO 2 and PM 2.5 in 2020 were consistently lower than those in the same periods of 2016, 2017, 2018 and 2019, due to the emission reductions because of the city lockdown (see Fig. 3b) in addition to the possible effects from the differences in meteorological conditions for each year.

Correlations between NO 2 and city traffic index in Hangzhou

NO x , including NO and NO 2 , are major precursors of PM 2.5 and O 3 , and are primarily emitted as NO from anthropogenic combustion sources including transportation, power plants, industries and residential combustion, and from natural sources such as soil, lightning, and wildfire. NO x is an excellent tracer of human activity [START_REF] Chen | Evidence on the impact of sustained exposure to air pollution on life expectancy from China's Huai River policy[END_REF][START_REF] Yu | Origin of air pollution during a weekly heavy haze episode in Hangzhou, China[END_REF]. Our results show that NO 2 concentrations in both urban and rural areas in Hangzhou dropped sharply during the lockdown (Figs. 1,2). Higher temperatures, stronger sunlight and increasing precipitation in February can decrease NO 2 relative to those in January for fixed emissions in Hangzhou.

To understand the impact of traffic flows on the NO 2 in Hangzhou, Fig. 4a displays changes in daily traffic density against the changes in daily NO 2 mean concentrations in 2020 relative to those in 2019. We found a significant positive correlation (correlation coefficient (r) = 0.51) between the change in traffic density and NO 2 in the urban area. This finding reinforces the hypothesis by which traffic reduction is at least partly responsible for NO 2 decreases. By contrast, traffic density and NO 2 is very weakly correlated at the rural site (r = 0.18). This suggests that NO 2 does not mainly originate from local traffic in rural areas, but may come from long-distance transport. It has been shown that regional transport of NO x and VOCs from urbanized upwind areas can lead to high O 3 levels [START_REF] Sikder | Surface ozone and carbon monoxide levels observed at Oki, Japan: regional air pollution trends in East Asia[END_REF][START_REF] Monteiro | Investigating a high ozone episode in a rural mountain site[END_REF][START_REF] Yu | Performance and diagnostic evaluation of ozone predictions by the eta-community multiscale air quality forecast system during the 2002 new England air quality study[END_REF][START_REF] Li | Ozone source apportionment at urban area during a typical photochemical pollution episode in the summer of 2013 in the Yangtze River Delta[END_REF]. Moreover, NO 2 exhibits a strong correlation with PM 2.5 in the urban area of Hangzhou (r = 0.51, P < 0.05) (Fig. 3d), indicating local vehicle emissions are an important source of PM 2.5 .

Inter-species correlations in the observations

Negative correlations are found between daily maximum 8-h O 3 and mean NO 2 levels in both urban and rural areas (Fig. 4b). This suggests a VOC-limited regime of O 3 formation prevailing during winter time periods in Hangzhou, in agreement with conclusions reported in Shanghai and Nanjing [START_REF] Ding | Ozone and fine particle in the western Yangtze River Delta: an overview of 1 yr data at the SORPES station[END_REF][START_REF] Gu | Observed dependence of surface ozone on increasing temperature in Shanghai[END_REF]. Notably, a significantly weaker positive response is observed under low NO 2 levels, below 20 μg m -3 , showing the shift of O 3 production to the transition regime or even NO x -limited regime, especially in the rural area. At low VOC/NO x ratios, chain termination reaction of OH and NO 2 produces a VOC-limited regime. Conversely, at high VOC/NO x ratios, the primary chain termination and propagation reactions involve OH and VOCs, and oxidation of NO by HO 2 and RO 2 , respectively, producing an NO x -limited regime [START_REF] Seinfeld | Atmospheric chemistry and physics: from air pollution to climate change[END_REF].

Little correlation was observed between daily mean PM 2.5 and maximum 8-h O 3 concentrations (Fig. 4c). Here, we displayed boxplots of maximum 8-h O 3 concentrations versus daily mean PM 2.5 concentrations. The initial PM 2.5 bin was 5 μg m -3 and rose to 10 or 15 μg m -3 under high PM 2.5 concentrations. Overall, median maximum 8-h O 3 concentrations increased along with daily mean PM 2.5 at low to moderate PM 2.5 levels, then declined for daily PM 2.5 concentrations > 40 μg m -3 as shown in Fig. 4c. The positive correlation between PM 2.5 and O 3 is associated with the conversion of gaseous organics to PM 2.5 under OH and O 3 oxidation, forming secondary organic aerosols [START_REF] Seinfeld | Atmospheric chemistry and physics: from air pollution to climate change[END_REF]. The observed decline in O 3 driven by high PM 2.5 concentrations could be attributed to reduced solar radiation due to extinction by particulate matter, depositional 2016, 2017, 2018, 2019and 2020. Before Lockdown (BL), During Lockdown (DL), and Resumption denote the periods of January 1-January 23, January 24-February 15, and February 16-February 28, respectively. The diurnal variations of mean O 3 concentrations for the periods of BL and DL in (a, b) clearly indicate that the mean O 3 concentrations for the non-photochemical reaction times from 20:00 PM to 9:00 AM (local time) during lockdown in 2020 were 53.7 and 58.0 μg m -3 in the urban and rural areas, respectively, much higher than the corresponding values (i.e., 18.8 and 35.6 μg m -3 in the urban and rural areas, respectively) before lockdown ◂ sink of HO 2 and NO x radicals, and chemical titration of NO [START_REF] Buysse | Relationships between particulate matter, ozone, and nitrogen oxides during urban smoke events in the western US[END_REF][START_REF] Li | A two-pollutant strategy for improving ozone and particulate air quality in China[END_REF][START_REF] Yu | Origin of air pollution during a weekly heavy haze episode in Hangzhou, China[END_REF].

Conclusion

We investigated the effects of city lockdowns on air quality based on ground-based observations in both urban and rural areas in Hangzhou. Results indicate that during lockdown relative to the period before lockdown, concentrations of PM 2.5 , PM 10 , CO, and NO 2 displayed significantly decreasing trends, as expected, while an overall rising trend was observed for O 3 in both urban and rural areas, which is explained by the 'absence' of NO x . The sharp decreases in the NO 2 concentrations in the urban area during the lockdown were mainly a result of direct traffic emission reductions, whereas in the rural area decreases were mainly from less regional transport. SO 2 mean concentrations in the urban area decreased from 6.3 to 5.3 μg m -3 , as expected, but increased in rural areas from 4.7 to 5.8 μg m -3 , a consequence of increases in coal combustion for heating and firework emissions. Mean O 3 concentrations increased significantly from 24.6 before lockdown to 60.6 μg m -3 during lockdown in the urban area, whereas the corresponding rural O 3 levels modestly increased from 42.0 to 62.9 μg m -3 . Here the O 3 increase in the urban areas is explained by the reduction in NO x emissions. Overall, the city lockdown improved air quality 3a and Fig. 3b, PM 2.5 and PM 10 concentrations in the urban areas experienced a sharp decrease on January 24, and then remained relatively larger negative percentages (below -75%) during a few days from January 24-27. The simultaneous quick drops after January 24 also occurred for PM 2.5 and PM 10 levels at the rural site with the minimum values as low as -96% and -95.6% on January 26, respectively by reducing PM 2.5 , PM 10 , CO, and NO 2 , but also decreased air quality by augmenting O 3 in both urban and rural areas and SO 2 in the rural area in Hangzhou. 4a indicates that NO 2 in the rural area did not arise mainly from direct traffic emissions rather from regional transport

Fig. 2 a

 2 Fig. 2 a Diurnal variations of the mean O 3 concentrations for the periods of before (BL) and during (DL) lockdowns in the urban and rural areas in Hangzhou in 2020 and 2019; b Comparisons of the weekly mean values of NO 2 and PM 2.5 concentrations in the years of2016, 2017, 2018, 2019 and 2020. Before Lockdown (BL), During Lockdown (DL), and Resumption denote the periods of January 1-January 23, January 24-February 15, and February 16-February 28, respectively. The diurnal variations of mean O 3 concentrations for the periods ofBL and DL in (a, b) clearly indicate that the mean O 3 concentrations for the non-photochemical reaction times from 20:00 PM to 9:00 AM (local time) during lockdown in 2020 were 53.7 and 58.0 μg m -3 in the urban and rural areas, respectively, much higher than the corresponding values (i.e., 18.8 and 35.6 μg m -3 in the urban and rural areas, respectively) before lockdown

Fig. 3

 3 Fig.3Change percentages of the daily average concentrations of ambient PM 2.5 , PM 10 , CO, NO 2 , SO 2 and daily maximum 8 h-O 3 concentration during January and February in 2020 relative to those in 2019 in Hangzhou in (a) the urban area and (b) the rural area. As indicated in Fig.3aand Fig.3b, PM 2.5 and PM 10 concentrations in the urban areas experienced a sharp decrease on January 24, and then remained relatively larger negative percentages (below -75%) during a few days from January 24-27. The simultaneous quick drops after January 24 also occurred for PM 2.5 and PM 10 levels at the rural site with the minimum values as low as -96% and -95.6% on January 26, respectively

Fig. 4 a

 4 Fig. 4 a Scatter plots and linear regressions of the changes in daily traffic density against changes in daily concentrations of NO 2 during January and February in 2020 relative to 2019 at urban and rural sites. b Scatter plots and linear regression of daily (24-h) NO 2 against maximum 8-h O 3 (MDA8 O 3 ) during January 1-February 28, 2020, at urban and rural sites. c Scatter plots of 24-h PM 2.5 against MDA8 O 3 and boxplots of MDA8 O 3 binned by daily (24-h) PM 2.5 (at least three data points required) during January 1-February 28, 2020 at urban and rural sites. The boxes represent the 75th, 50th, and 25th percentiles from top to bottom, the whiskers and solid squares denote

State Key Laboratory of Multiphase Flow in PowerEngineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, People's Republic of China

Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel

Acknowledgements

This work was partially supported by the Department of Science and Technology of China (Nos. 2016YFC0202702, 2018YFC0213506, and 2018YFC0213503), National Research Program for Key Issues in Air Pollution Control in China (No. DQGG0107), and National Natural Science Foundation of China (Nos. 21577126 and 41561144004). Part of this work was also supported by the "Zhejiang 1000 Talent Plan" and Research Center for Air Pollution and Health in Zhejiang University. Pengfei Li is supported by Initiation Fund for Introducing Talents of Hebei Agricultural University (412201904).

SUPPLEMENTARY MATERIAL

Unexpected rises of ozone in urban and rural areas and sulfur dioxide in rural areas during the coronavirus city lockdown in Hangzhou, China: Implications for air quality Liqiang Wang, Mengying Li, Shaocai Yu, Xue Chen, Zhen Li, Yibo Zhang, Linhui Jiang, Yan Xia, Jiali Li, Weiping Liu, Pengfei Li, Eric Lichtfouse, Daniel Rosenfeld and John H. Seinfeld Environmental Chemistry Letters (2020) 18:1713-1723 https://doi.org/10.1007/s10311-020-01028-3 Table S1. Statistical summary of mean concentrations (mean ± standard deviation) in 2020 and concentration changes (percentages) of PM2.5, PM10, CO, NO2, SO2, and O3 in 2020 relative to those in the same periods of 2019 at urban and rural sites in Hangzhou. Before Lockdown (BL), During Lockdown (DL), and Resumption denote the periods of January 1-January 23, January 24-February 15, and February 16-February 28, respectively. Hourly average concentrations of PM2.5 in the urban area were 45.9±31.3 and 29.6±19.5 µg m -3 for the BL and periods, respectively. The simultaneous drop of hourly PM2.5 levels from 25.0±24.0 to 20.1±16.9 µg m -3 was observed at the rural site. Similar patterns were detected for PM10 with mean values decreasing significantly from 70.5 to 48.4 µg m -3 in the urban area and slightly from 31.0 to 24.1 µg m -3 at the rural site.

Pollutants

O3 (µg m -3 ) NO2 (µg m -3 ) CO (mg m -3 ) SO2 (µg m -3 ) PM10 (µg m -3 ) PM2.5 (µg m -3 )


Pulse. Dwelling in Sound

Jean-Paul Thibaud What happens when we question contemporary cities, everyday spaces and situated experiences with sound, according to and from a sonic perspective? Our bearings and our models of intelligibility are no longer exactly the same. Porosity tends to replace limits, soft envelopes take precedence over strict lines, rhythms and tones regain importance compared to face-to-face and distancing. Assurely, resonance is a key feature of sonic experience. Hence, Sam Auinger and Dietmar Offenhuber perfectly demonstrate how "the plaza is a place of resonance", giving voice to a great diversity of social activities as well as various soundful infrastructures of transportation. From this perspective, the plaza -for instance Alexanderplatz in Berlin -is a perfect place to listen to the complexity, the richness and the dynamic of a social form of life. In a way resonance blurs the separation between the subject and its immediate surroundings since "resonance is both the resonance of a sonic body for its own sake and the resonance of the sound in a listening body, which resonates itself as it listens." 1 With sound, we are not only immersed in a space. We are permeated by a medium, both inside and out, imbued with the current situation and penetrated with its atmospheric content. Therefore our ways of thinking habitation are bound to move and to transform. Our very idea of dwelling has to be reconsidered. We must then give voice to diffuse, vibratory, molecular and transitory phenomena. And still, it is our sensitivity itself that is put to the test. Are we not bound to become more attentive of ordinary and unnoticed ambiances? Are we not led to give special attention to the enigma of attunement? Do we not have to take seriously our capacity to be affected by our surroundings and to act upon it in return? Such questions emerge as one seeks to understand our sonic ways of dwelling in the world. In other words, sound has the ability to set the tone of lived spaces and to install common moods. One of the challenges is thus to describe how a space acquires an atmospheric charge and manages to resonate with a specific Stimmung. Perhaps this is one of the lessons that can be drawn from Andi Schoon's discussion of workspaces. As it is righly noted, in a post-fordist area "work itself is diffused into the everyday". Thus, a new field of research opens, questioning the way in which ambiances and affective atmospheres are embedded in the ongoing process of emergence of a control society. One can wonder then how sound embodies the diffuse quality and the affective tonality of a situation. Whether related to domestic spaces, urban squares, street life or workplaces, sound is closely intertwined with ordinary gestures, bodily affects, social interactions and background practices. It is always embedded in "the whole hurly-burly of human actions" 2 . It can help to test our sense of home or to highlight the features of a public space, to reveal the affordances of a place or to explore the resources of a company. But then, new ways of listening have to be explored. For instance, one can rely on sensobiographic walks inspired by new materialism (Juhana Venäläinen, Sonja Pöllänen & Rajko Muršič) or on resonance chambers to think with our ears (Sam Auinger & Dietmar Offenhuber). One can mobilize a tapestry of sonic ritual to highlight a realistic home (Jacqueline Waldock) or the spirit of a whole company inspired by the fabbrica diffusa (Andi Schoon). These various approaches constitute original propositions that enable to explicit the role of sound in our habitual modes of dwelling. They help to better understand how sound adjusts to a place and configures it at the same time, accommodates to it while activating it, is both perceived and produced. In other words, sound is not only spatialized but also spatializing. But also, as proposed by Juhana Venäläinen, Sonja Pöllänen and Rajko Muršičnous, we must bear in mind that sound is not only a social feature of the lived world. It involves also fundamentally a material dimension. Analysing the streets of Turku and Ljubljana, they enable us "to make sense of the materialities of sound". Beyond the extreme variety of contexts, conditions, and social forms of life, sound has the power to hold together the heterogeneous components of a situation. As Erwin Straus has perfectly shown, "space filled with tone and homogenized by a single pervading movement -and in this the homogeneity of the acoustic mode of the space differs from that of empty metrical space -has itself a presentic character" 3 . We are dealing here with the capacity of sound to give consistency to an overall situation, to unify in a single diffuse quality the numerous features of a situation. It is of course possible to discriminate and distinguish the sounds of an environment, to identify them one by one, to find out what each one of them relates to. No doubt sound involves a cognitive activity that makes it possible to face problems of a practical and functional nature. To live in a place, one must be able to identify the sounds that compose it and to act in accordance, to be able to orient oneself in the everyday sonic environment. But sound also has a power of "atmosphericization" and homogenization -a power of coalescence -which endows each situation with a singular pervasive quality 4 . When we enter a new space or when we discover a new city, we are immediately caught in its ambiance. We are not only led to perceive and interpret a sonic environment, but also to immediately feel and sense an atmosphere. A first overall impression emerges, which appeals to solicits the whole body and suggests qualities of movement. Before being oriented, our sense of hearing is floating, peripheral and defocused. Let us not forget that if an acoustic signal may transform into an information in referring to its source, it is first and foremost a medium, constitutive of an ambient milieu. The experience of sound consists as much in feeling a global tone of the place as hearing a set of clearly distinct sounds. It is not just about identifying the various sound signals, but also letting oneself traversed and transported by a whole acoustic environment. To dwell in sound requires as much to immerse oneself in the situation and to resonate with the surrounding environment, as to seek to understand and interpret what is going on. In addition to the power of atmospherization of sound previously mentioned, we also have to consider its power of "tonalization" 5 . This means that sound is not reducible to its cognitive dimension. It opens up to the pathic and affective feature of sensory experience. But if this is the case, it is because sound imposes on us and takes possession of us, overwhelms us, constantly insists and persists. No doubt that is why sound makes it possible to wonder what it means to feel at home. By asking this question, Jacqueline Waldock manages to give a very thorough account of what sound can teach us in this matter: "both nurturing and damaging experiences are a reality of home". And finally, regardless of the spaces considered and the activities at stake, we cannot help but to vibrate in unison with the world, to reverberate and to attune to the surroundings in which we are immersed. We are inevitably "ensounded"6 .

Jean-Luc Nancy Listening. translated by C. Mandell, New York, Fordham University Press,

2007, p. 77. 2 Ludwig Wittgenstein Zettel. second edition, edited by G.E.M. Anscombe and G.H.V. Wright.Oxford, Blackwell, 1981, n°567.

Erwin Straus [1930] « The Forms of Spatiality » In Phenomenological Psychology of ErwinStraus. New York, Basic Books Inc., 1966, p. 35.

On the notion of diffuse quality, refer in particular to JohnDewey [1934] Art as Experience. New York, Perigee Books, 1985. See also John Dewey « Qualitative Thought ». In Philosophy andCivilization, New York, Minton, Balch & Co, 1931, p. 93-116.

About the idea of "tonalization" and the image of the Eolian Harp, see Gaston Bachelard[1957] The poetics of space.Boston, Beacon Press, 1994.

Tim Ingold Being alive. Essays on movement, knowledge and description, Abingdon, Routledge, 2011.