Huizheng Che

Xiangao Xia

Hujia Zhao

Oleg Dubovik

Brent N Holben

Philippe Goloub

Emilio Cuevas-Agulló

Victor Estelles

Yaqiang Wang

Jun Zhu

Bing Qi

Wei Gong

Honglong Yang

Renjian Zhang

Leiku Yang

Jing Chen

Hong Wang

Yu Zheng

Ke Gui

Xiaochun Zhang

Xiaoye Zhang
email: xiaoye@cma.gov.cn

Spatial distribution of aerosol microphysical and optical properties and direct radiative effect from the China Aerosol Remote Sensing Network

Multi-year observations of aerosol microphysical and optical properties, obtained through ground-based remote sensing at 50 China Aerosol Remote Sensing Network (CARSNET) sites, were used to characterize the aerosol climatology for representative remote, rural, and urban areas over China to assess effects on climate. The annual mean effective radii for total particles (R effT ) decreased from north to south and from rural to urban sites, and high total particle volumes were found at the urban sites. The aerosol op-tical depth at 440 nm (AOD 440 nm ) increased from remote and rural sites (0.12) to urban sites (0.79), and the extinction Ångström exponent (EAE 440-870 nm ) increased from 0.71 at the arid and semi-arid sites to 1.15 at the urban sites, presumably due to anthropogenic emissions. Singlescattering albedo (SSA 440 nm ) ranged from 0.88 to 0.92, indicating slightly to strongly absorbing aerosols. Absorption AOD 440 nm values were 0.01 at the remote sites versus 0.07 at the urban sites. The average direct aerosol radiative ef-

Introduction

Atmospheric aerosols have important direct effects on climate because they can scatter and absorb radiant energy and, in so doing, affect the Earth's energy balance [START_REF] Charlson | Climate forcing by anthropogenic aerosols[END_REF][START_REF] Yang | Distinct impact of different types of aerosols on surface solar radiation in China[END_REF]. Meanwhile, the aerosols can serve as cloud condensation nuclei or ice nuclei to affect the climate indirectly through aerosol-cloud interactions [START_REF] Twomey | An assessment of the impact of pollution on the global cloud albedo[END_REF]Garrett and Zhao, 2006;[START_REF] Zhao | Temporal variability of the visibility, particulate matter mass concentration and aerosol optical properties over an urban site in Northeast China[END_REF][START_REF] Xie | Sensitivity of CAM5-Simulated Arctic Clouds and Radiation to Ice Nucleation Parameterization[END_REF]. The optical properties of the aerosol determine the particles' direct effects on the Earth's radiative balance and weather-climate change [START_REF] Ramanathan | Aerosol, climate, and hydrological cycle[END_REF]Eck et al., 2005;[START_REF] Myhre | Consistency between satellite-derived and modeled estimates of the direct aerosol effect[END_REF][START_REF] Zhao | Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols[END_REF]Che et al., 2019a;Li et al., 2016). Aerosol optical depth (AOD) is one of the key measures of the total aerosol extinction effects on climate [START_REF] Breon | Aerosols effect on the cloud droplet size monitored from satellite[END_REF], and the extinction Ångström exponent (EAE), with spectral dependence, can be used to obtain the information about aerosol size distributions (Gobbi et al., 2007;[START_REF] Eck | Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols[END_REF]Zheng et al., 2017). The aerosols' absorptivity depends on particle composition and is a key determinant to calculate the direct aerosol radiative effect (Haywood and Shine, 1995;Li et al., 2016), and the singlescattering albedo (SSA) is a parameter that has the spectral dependence to distinguish major aerosol particle types [START_REF] Jacobson | A physically based treatment of elemental carbon optics: implications for global direct forcing of aerosols[END_REF][START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF][START_REF] Gelencser | Carbonaceous Aerosol, Atmospheric and Oceanographic Sciences Libery[END_REF]Russell et al., 2010;[START_REF] Giles | An analysis of AERONET aerosol absorption properties and classifications representative of aerosol source regions[END_REF].

With the recognition of the importance for climate, the aerosol optical properties have been obtained from ground-based monitoring networks worldwide; some of the major networks include, the Aerosol Robotic Network (AERONET; [START_REF] Holben | AERONET -A Federated Instrument Network and Data Archive for Aerosol Characterization[END_REF] and its sub-networks, the PHOtométrie pour le Traitement Opérationnel de Normalisation Satellitaire (PHOTONS), the Canadian Sun Photometer Network (AEROCAN), and the Iberian Network for aerosol measurements (RIMA; [START_REF] Goloub | PHO-TONS/AERONET sunphotometer network overview: description, activities, results[END_REF][START_REF] Bokoye | Characterization of atmospheric aerosols across Canada from a Ground-based sun photometer network: Aerocan[END_REF]Prats et al., 2011); the SKYrad Network (SKYNET; [START_REF] Takamura | Overview of SKYNET and its activities[END_REF]Che et al., 2008); the Eu-ropean aerosol Lidar Network (EARLINET; Pappalardo et al., 2014); and the Global Atmosphere Watch Programmer-Precision Filter Radiometers network (GAW-PFR; [START_REF] Wehrli | Calibration of filter radiometers for the GAW Aerosol Optical Depth network at Jungfraujoch and Mauna Loa[END_REF]Estellés et al., 2012). The China Aerosol Remote Sensing NETwork (CARSNET), the Chinese Sun Hazemeter Network (CSHNET), and the Sun-Sky Radiometer Observation Network (SONET) have been established to measure aerosol optical properties in China (Che et al., 2009a[START_REF] Cheng | Seasonal variation and difference of aerosol optical properties in columnar and surface atmospheres over Shanghai[END_REF]Xin et al., 2007Xin et al., , 2015;;[START_REF] Li | Comprehensive Study of Optical, Physical, Chemical, and Radiative Properties of Total Columnar Atmospheric Aerosols over China: An Overview of Sun-Sky Radiometer Observation Network (SONET) Measurements[END_REF]. Furthermore, aerosol optical properties have also been used in comprehensive studies of aerosol physical characteristics and chemical composition in many regions of China (Che et al., 2009c[START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF][START_REF] Zhao | Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols[END_REF].

China has become one of the largest aerosol sources in the world associated with its rapid economic development, and this has caused significant effects on local environments and regional climate (Che et al., 2005;[START_REF] Lee | Aerosol optical depth measurements in eastern China and a new calibration method[END_REF]Li et al., 2016;[START_REF] Yang | Wintertime cooling and a potential connection with transported www[END_REF]Yang et al., , 2019b;;[START_REF] Zhao | Estimating the contribution of local primary emissions to particulate pollution using highdensity station observations[END_REF]Gui et al., 2019). There have been numerous studies that have focused on aerosol optical properties obtained though groundbased remote-sensing methods in China (Luo et al., 2002;Li et al., 2003;[START_REF] Duan | Study on the distribution and variation trends of atmospheric aerosol optical depth over the Yangtze River Delta[END_REF]. Some previous research has paid more attention to aerosol's optical properties and its radiative effects over the urban industrial areas, as well as at coastal sites in northeastern and eastern China [START_REF] Wang | Aerosol optical properties of regional background atmosphere in Northeast China[END_REF][START_REF] Xin | Trends in aerosol optical properties over the Bohai Rim in Northeast China from 2004 to 2010[END_REF][START_REF] Li | Overview of the East Asian Studies on tropospheric aerosols, an international regional experiment(EAST-AIRE)[END_REF][START_REF] Zhao | Investigation of the Optical Properties of Aerosols over the Coastal Region at Dalian, Northeast China[END_REF][START_REF] Wu | Aerosol optical properties observed at a semi-arid rural site in Northeastern China[END_REF][START_REF] Shen | Observed decrease of summer sea-land breeze in Shanghai from 1994 to 2014 and its association with urbanization[END_REF]. Many studies of aerosol optical properties were conducted in northern China with high aerosol loadings, such as the Beijing-Tianjin-Hebei region (Che et al., 2014;[START_REF] Xia | Climatological aspects of aerosol opticalproperties in North China Plain based on ground and satellite remote-sensing data[END_REF][START_REF] Fan | Analysis of column-integrated aerosol optical thickness in Beijing from AERONET observations[END_REF][START_REF] Xie | Characteristics of aerosol optical properties in pollution and Asian dust episodes over Beijing, China[END_REF]Zhang et al., 2019;Yang et al., 2019a;[START_REF] Zhao | Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols[END_REF][START_REF] Zheng | Five-year observation of aerosol optical properties and its radiative effects to planetary boundary layer during air pollution episodes in North China: Intercomparison of a plain site and a mountainous site in Beijing[END_REF]. Aerosol optical properties have also been investigated at Hefei, Shouxian, Nanjing, Taihu, Shanghai, and other sites in eastern China [START_REF] Lee | Aerosol optical depth measurements in eastern China and a new calibration method[END_REF]He et al., 2012;[START_REF] Zhuang | Optical properties and radiative forcing of urban aerosols in Nanjing over China[END_REF]Z. Wang et al., 2015;[START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF]. Some studies of aerosol optical properties have been made in southern and central China (L. C. [START_REF] Wang | Long-term observations of aerosol optical properties at Wuhan, an urban site in Central China[END_REF]Tao et al., 2014b), and those at remote and rural sites in China provide information on regional background conditions (Che et al., 2009b;[START_REF] Wang | Aerosol optical properties of regional background atmosphere in Northeast China[END_REF][START_REF] Zhu | Column-integrated aerosol optical and physical properties at a regional background atmosphere in North China Plain[END_REF][START_REF] Yuan | Using a new aerosol relative optical thickness concept to identify aerosol particle species[END_REF].

China's vast size, varied terrain, and heterogeneity of aerosol sources has led to strong temporal and spatial variability in aerosol optical and physical properties. The mixtures of aerosol types at most sites are complex, and aerosol populations' sizes and compositions are affected by their sources, transformations that occur during transportation, and removal processes [START_REF] Cao | Spatial and seasonal distributions of carbona-ceous aerosols over China[END_REF][START_REF] Wang | Variability of east Asia dust events and their long-term trend[END_REF]Zhang et al., 2013;[START_REF] Wan | Size distribution of carbonaceous aerosols at a high-altitude site on the central Tibetan Plateau (Nam Co Station, 4730m a.s.l.)[END_REF]. National-scale, groundbased measurements of aerosol microphysical and optical properties obtained from the sun photometer provide a better understanding of the aerosols' climate effects over the different regions of China. The measurements of greatest interest include aerosol size distributions (volume and aerosol effective radii) and optical properties (AOD, AE, SSA, absorption AOD) because those data can be used to evaluate aerosol direct radiative effect.

The aim of this study was focused on the investigation of the climatological spatial distribution of aerosol microphysical and optical properties over regional scales using spatial distribution data from the national CARSNET network. The data were collected at CARSNET sites, which includes sites in remote, rural, and urban areas, with the same calibration procedures and calculation algorithms used at all sites. As a result, the data are directly comparable among sites (Che et al., 2009a), and the results can be used to characterize the regional distribution and temporal variation in aerosol optical properties. This research focused on aerosol climate effects and regional environmental pollution, and the results should be useful for satellite validations and for the improvement of models in the future. The remainder of this paper is organized as follows: firstly, Sect. 2 describes the sites in detail and then introduces the methods for the data processing of the aerosol optical properties and the direct aerosol radiative effect calculation, through the retrieved aerosol optical parameters. Section 3 illustrates the aerosol microphysical and optical properties, as well as its direct aerosol radiative effect. An aerosol type classification method is proposed according to the aerosol optical parameters. Section 4 presents the conclusions of the study.

2 Site description, instruments, and data

Site description

Sun photometers (CE-318, Cimel Electronique, Paris, France; see Appendix A) were installed at 50 CARSNET sites (Fig. 1) from 2010 to 2017. The stations were classified as remote, rural, or urban sites based on administrative division (Appendix Table A1). Three of the remote stations were more than 3000 m above sea level on the Tibetan Plateau, far from the anthropogenic influences, and one of them was a northwestern regional background site in China. The 23 rural sites represent (i) 5 sites of desert regions affected by mostly dust aerosols rather than anthropogenic particles, (ii) 2 sites affected by both dust and anthropogenic activities on the Loess Plateau, and (iii) 16 sites located near or surrounding large cities with relatively strong impacts from anthropogenic activities in central and eastern China. The last category is 24 urban sites located in provincial capitals or heavily populated cities.

Instruments and calibration

The CE-318 sun photometers used in this study were calibrated annually, using the CARSNET calibration protocol, to verify the accuracy and reliability of the sky irradiance measurements [START_REF] Holben | AERONET -A Federated Instrument Network and Data Archive for Aerosol Characterization[END_REF]Che et al., 2009a;Tao et al., 2014a). The reference instruments for CARSNET were periodically calibrated at Izaña, Tenerife, Spain, lo-cated at 28.31 • N, 16.50 • W (2391.0 m a.s.l.), in conjunction with the AERONET program. There are several different types of Cimel instruments that have been used at the 50 sites in this network: (1) logical type CE-318 sun photometers (440, 675, 870, 940, 1020, and three polarization bands at 870 nm ), (2) numerical type 675,870,940, 1020 nm, and three polarization bands at 870 nm), (3) numerical type CE-318 sun photometers at eight wavelengths (340,380,440,500,675,870,940, and 1020 nm), (4) and numerical-type CE-318 sun photometers at nine wavelengths (340, 380, 440, 500, 675, 870, 940, 1020, and 1640 nm).

Measurements used to retrieve AODs were at 340, 380, 440, 500, 675, 870, 1020, and 1640 nm, while the total precipitable water content was obtained by using those measurements at 940 nm [START_REF] Holben | AERONET -A Federated Instrument Network and Data Archive for Aerosol Characterization[END_REF][START_REF] Dubovik | A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements[END_REF]. The cloud-screened AOD data were calculated by using the ASTPwin software, and extinction Ångström exponents (EAE) were calculated from the instantaneous AODs for wavelengths of 440 and 870 nm (Che et al., 2009a[START_REF] Cheng | Seasonal variation and difference of aerosol optical properties in columnar and surface atmospheres over Shanghai[END_REF]. Sites with more than three daily AOD observations and more than 10 monthly AOD observation days were used to calculate the daily and monthly mean AODs and extinction Ångström exponents. The fine-mode fraction (FMF) is described as the fraction of fine-mode particles of total AOD 440 nm (AOD fine440 nm /AOD 440 nm ).

Data processing

The aerosol microphysical and optical properties, including volume size distributions (dV (r)/dlnr); the total, fine, and coarse-mode aerosol effective radii (R effT , R effF , and R effC , respectively); single-scattering albedo (SSA); complex refractive indices; absorption AODs (AAODs); and absorption Ångström exponents (AAEs), were retrieved from the observational data from the sky scattering channel of the sun photometers at 440, 670, 870, and 1020 nm using the algorithms of [START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF][START_REF] Dubovik | Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust[END_REF]. In the process of retrieval, the data of surface albedo (SA) was interpolated or extrapolated to 440, 670, 870, and 1020 nm based on the daily MCD43C3 data, a product from the MODIS-Moderate Resolution Imaging Spectroradiometer surface reflectance (https://ladsweb. modaps.eosdis.nasa.gov/, last access: 31 March 2019). The algorithm used to calculate aerosol volume size distributions (dV (r)/dlnr) was under the assumption of a homogeneous distribution of non-spherical particles following the approach of [START_REF] Dubovik | Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust[END_REF]. The sphericity fraction retrieved from the inversions is defined as spherical particles/(spheroidal particles + spherical particles) (Giles et al., 2011). [START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF][START_REF] Dubovik | Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust[END_REF] defined that all the particles with effective radii < 0.992 µm were considered fine-mode particles, and those > 0.992 µm were considered coarse-mode particles. For the total (R effT ), fine-mode (R effF ), and coarsemode (R effC ) aerosols, the effective radii are calculated by

where r min denotes 0.05, 0.05, and 0.992 µm and r max denotes 15, 0.992, and 15 µm of the total, fine-mode, and coarse-mode particles, respectively. The coarse (PV C ) and fine aerosol particle volumes distributions (PV F ) are calculated according to a bimodal lognormal function described by [START_REF] Whitey | The physical characteristics of sulfur aerosols[END_REF], [START_REF] Shettle | Models for the aerosols of the lower atmosphere and the effects of humidity variations on their optical properties[END_REF], and [START_REF] Remer | Dynamic aerosol model: Urban/industrial aerosol[END_REF]:

dV (r) dlnr = 2 i=1 C v,i √ 2π σ i exp - ln r -ln r V ,i 2 2σ 2 i , (2)
where C v,i is the volume concentration, r V ,i is the median radius, and σ i is the standard deviation.

The volume median radius is computed by fine-mode and coarse-mode particles as follows: (3)

ln r V =
Then the standard deviation is calculated from the volume median radius:

σ V =
The volume concentration (µm 3 per µm 2 ) is speculated by the following equation:

C V = r max r min dV (r) d ln r d ln r. (5)
The SSA was retrieved only for AOD 440 nm > 0.40; this was done to avoid the larger uncertainty inherent in the lower AOD retrieval, according to [START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF][START_REF] Dubovik | Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust[END_REF]. The AAOD and AAE for wavelength λ were calculated as follows:

AAOD(λ) = [1 -SSA (λ)] × AOD(λ), ( 6
) AAE = -d ln[AAOD(λ)] d ln(λ) . ( 7
)
The total AODs' uncertainty was 0.01 to 0.02 according to [START_REF] Eck | Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols[END_REF]. The accuracy of SSA retrieved from AOD 440 nm > 0.50 with a solar zenith angle of > 50 was 0.03 [START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF]. The accuracy of the particle volume size distribution was 15 %-25 % between 0.1 µm ≤ r ≤ 7.0 µm and 25 %-100 % when r < 0.1 µm and r > 7 µm.

Direct aerosol radiative effect (DARE in W m -2 ) was calculated by the radiative transfer module under cloud-free conditions, which is similar to the inversion of AERONET [START_REF] García | Validation of AERONET estimates of atmospheric solar fluxes and aerosol radiative forcing by groundbased broadband measurements[END_REF][START_REF] García | Aerosol radiative forcing: AERONET based estimates, climate Models[END_REF]. The DARE at the bottom of the atmosphere (BOA) and the top of the atmosphere (TOA) was defined as the difference in the shortwave radiative fluxes with and without aerosol effects as follows:

DARE TOA = F ↑0 TOA -F ↑ TOA , (8)
DARE BOA = F ↓ BOA -F ↓0 BOA , (9)
where F and F 0 denote the broadband fluxes of including and excluding aerosols, respectively, at the BOA and TOA.

The "↑" and "↓"mean the upward fluxes and downward fluxes, respectively.

In the radiative transfer module, the absorption and multiple scattering effects are taken into account during flux calculations using the discrete ordinates (DISORT) approach [START_REF] Nakajima | Algorithms for radiative intensity calculations in moderately thick atmospheres using a truncation approximation[END_REF][START_REF] Stamnes | Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media[END_REF]. Gaseous distributions and single fixed aerosol vertical distributions (exponential to 1 km), taken from the multilayered US standard 1976 atmosphere, were used in the radiative flux calculations [START_REF] García | Validation of AERONET estimates of atmospheric solar fluxes and aerosol radiative forcing by groundbased broadband measurements[END_REF]. [START_REF] García | Validation of AERONET estimates of atmospheric solar fluxes and aerosol radiative forcing by groundbased broadband measurements[END_REF] pointed out that the error for the observed solar radiation at the surface in global was +2.1 ± 3.0 % for an overestimation of about +9 ± 12 Wm -2 . The data used in preparing the figures for the present paper have been made available as an Appendix.

Results and discussion

Spatial distribution of aerosol microphysical properties

A map showing the 50 CARSNET sampling sites and plots of the aerosol volume size distributions (dV (r)/dlnr) at each of the sites is shown in Fig. 1. Generally, the annual mean effective radius of total particles (R effT ) decreased from the inland northwestern areas to the southeastern coastal areas. Furthermore, the volume concentration of total particles was found to be substantially higher at the urban sites. The volume of the coarse-mode particles was considerably larger than that of the fine-mode particles at the remote, arid, and semi-arid sites and at those sites on the Chinese Loess Plateau (CLP) or nearby, indicating that those areas were most strongly affected by larger particles, most likely mineral dust, as discussed below.

The average (arithmetic mean) R effT at the remote sites was about 0.47 µm with the volume about 0.05 µm 3 per µm 2 (Table 1). A large R effT (0.64 µm) was found at Lhasa, and the total aerosol volume there was 0.05 µm 3 per µm 2 . These results are consistent with those reports by [START_REF] Li | Comprehensive Study of Optical, Physical, Chemical, and Radiative Properties of Total Columnar Atmospheric Aerosols over China: An Overview of Sun-Sky Radiometer Observation Network (SONET) Measurements[END_REF], who found high levels of coarse-mode particles at Lhasa due to the presence of mineral dust. The two other remote sites, Akedala and Shangri-La, had smaller average R effT values than Lhasa (0.36 and 0.39 µm, respectively), and corresponding volumes were 0.06 and 0.03 µm 3 per µm 2 . The average fine-mode effective radius (R effF ) was 0.14 µm at the remote sites, and fine-mode particle fractional volume (PV F ) was 0.01 µm 3 per µm 2 , while the average coarse-mode effective radii (R effC ) was 2.35 µm and the coarse-mode fractional volume (PV C ) was 0.03 µm 3 per µm 2 . These findings indicated that the contribution of coarse-mode particles to the total volume of aerosol was larger at the remote sites. A study by [START_REF] Cong | Aerosol optical properties at Nam Co, a remote site in central Tibetan Plateau[END_REF] at the remote Nam Co site on the Tibetan Plateau showed that dust particles mainly affected the site in spring, while anthropogenic aerosols were prevalent in the summer.

The average R effT at the arid and semi-arid sites (0.55 µm) was larger than at the remote sites, and the total volume of aerosols at the arid and semi-arid sites was also large (0.14 µm 3 per µm 2 ), nearly 3 times that at the remote sites. Large R effT values (0.71 µm) were found at Tazhong, which is near the northwestern deserts, and the aerosol volume there was also high, 0.30 µm 3 per µm 2 . Large PV C values were found at the arid and semi-arid sites (0.05-0.27 µm 3 per µm 2 ). The arithmetic mean R effT (0.49 µm) at the rural sites on or near the CLP had total aerosol volumes (0.15 µm 3 per µm 2 ) similar to those at the arid and semi-arid sites. These results also show a major contribution to the aerosol volumes by coarse-mode particles at the sites in or near the mineral dust source regions. [START_REF] Bi | Toward characterization of the aerosol optical properties over Loess Plateau of Northwestern China[END_REF] similarly found that coarse particles dominated the volume-size distribution at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) on the CLP.

Small R effT values (0.33 µm) were found at the rural sites in eastern China, and relatively high aerosol volumes were observed there (0.18 µm 3 per µm 2 ). In the Yangtze River Delta (YRD) region, the R effF was large range for 0.16-0.17 µm, and the PV F values were 0.12-0.13 µm 3 per µm 2 . At the Mt. Longfeng background site in northeastern China, the total particle volume was low (0.08 µm 3 per µm 2 ), which is consistent with minimal anthropogenic influences and low aerosol loadings. Compared with the other sites, the urban areas had relatively low coarse-mode aerosol concentrations, but small particles were plentiful -the average R effT was 0.37 µm and total volume was high at 0.21 µm 3 per µm 2 . The average R effF of fine-mode particles at the urban sites was 0.16 µm with a PV F of 0.10 µm 3 per µm 2 while the R effC was 2.22 µm and PV C was 0.11 µm 3 per µm 2 .

The effective radii and PV F values showed strong relationships with population density and vehicle emissions at the urban sites. High volumes of fine-mode particles occurred at the northeastern urban site of Shenyang (R effT = 0.16 µm, PV F = 0.12 µm 3 per µm 2 ); at major cities in northern China, including Shijiazhuang (R effT = 0.16 µm, PV F = 0.12 µm 3 per µm 2 ) and Zhengzhou (R effT = 0.18 µm, PV F = 0.12 µm 3 per µm 2 ); at Chengdu, a city in the Sichuan Basin (R effT = 0.21 µm, PV F = 0.16 µm 3 per µm 2 ); and in the urban regions of Nanning (R effT = 0.18 µm, PV F = 0.13 µm 3 per µm 2 ) and Panyu (R effT = 0.16 µm, PV F = 0.10 µm 3 per µm 2 ) in southern China. Overall, these results show that the volumes of fine-mode particles increased at the urban sites where anthropogenic influences were most apparent. [START_REF] Cheng | Seasonal variation and difference of aerosol optical properties in columnar and surface atmospheres over Shanghai[END_REF] found different aerosol volume size distributions for dust and sea salt in Shanghai in eastern China, and they showed that their relative abundances varied with season and in response to local or long-range transport. [START_REF] Zhao | Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols[END_REF] reported the effect of sea salt aerosol on the aerosol absorption and radiative effects in the coastal region over northeastern China. The particles' hygroscopic growth, with different compositions observed in special climatic conditions, could especially affect aerosol microphysical properties with their geographically variable effects (Zhang et al., 2015;[START_REF] Sun | Highly time-and size-resolved characterization of submicron aerosol particles in Beijing using an Aerodyne Aerosol Mass Spectrometer[END_REF]. Like in the YRD region, hygroscopic growth of fine-mode particles could lead to larger a AOD and the scattering enhancing reported by Sun et al. (2018) and [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF]. [START_REF] Xia | Observational study of aerosol hygroscopic growth on scattering coefficient in Beijing: A case study in March of 2018[END_REF] observed the aerosol hygroscopic growth of the fine particle scattering coefficient in Beijing.

Spatial distributions of AOD and EAE

The spatial distributions of AOD 440 nm and EAE 440-870 nm are shown in Fig. 2. The AOD 440 nm increased from the remote and rural sites to the urban sites, and as one might expect, the remote sites were the least affected by particle emissions and had the lowest aerosol loadings. For example, the AOD 440 nm at the remote stations was low and had an average value of 0.12. The Lhasa and Shangri-La sites on the Tibetan Plateau had similar average AOD 440 nm values of 0.10. These phenomena are similar to the study of [START_REF] Li | Comprehensive Study of Optical, Physical, Chemical, and Radiative Properties of Total Columnar Atmospheric Aerosols over China: An Overview of Sun-Sky Radiometer Observation Network (SONET) Measurements[END_REF], who showed clean air conditions at Lhasa with AOD < 0.1. [START_REF] Cong | Elemental composition of aerosol in the Nam Co region, Tibetan Plateau, during summer monsoon season[END_REF][START_REF] Cong | Aerosol optical properties at Nam Co, a remote site in central Tibetan Plateau[END_REF]) also found a low AOD (0.05) at Nam Co, which was comparable to the background levels at other remote sites.

The AOD 440 nm values at the arid and semi-arid sites and those on or near the Loess Plateau ranged from 0.32 to 0.42, which is higher than at the remote sites. The high AOD 440 nm at Tazhong (0.60), which is near the deserts in northwestern China, was likely due to the large aerosol volume of 0.30 µm 3 per µm 2 (Sect. 3.1) caused by mineral dust. Indeed, arid and semi-arid regions in northwestern China are important sources of aeolian dust on a global scale [START_REF] Bi | Toward characterization of the aerosol optical properties over Loess Plateau of Northwestern China[END_REF]. [START_REF] Li | Analysis of aerosol sources and optical properties based on backward trajectory method over SACOL[END_REF] showed that the contribution of dust to the average AOD at SACOL near Lanzhou was 28.4 %. Other sites that showed large AOD 440 nm include regions with strong anthropogenic influences, such as Dengfeng (0.79) on the North China Plain, Huimin (0.83) in the YRD (0.83 to 0.87), and Huainan (0.91) on the Guanzhong Plain.

Compared with the sites just discussed, lower AOD 440 nm values were found at the Mt. Longfeng background station on the Northeast China Plain (0.34), the semi-arid rural site at Tongyu in northeastern China (0.23), and the clean Xiyong site in southern China (0.41). [START_REF] Zhu | Column-integrated aerosol optical and physical properties at a regional background atmosphere in North China Plain[END_REF] found a low AOD of 0.28 at the North China Plain regional background site. Che et al. (2009c) pointed out that the large AOD at Lin'an was likely affected by the high aerosol loadings in YRD region. Among the urban sites in China, large AOD4 40 nm values were found in the cities with strong influences of anthropogenic activities, such as the Northeast China Plain (Shenyang 0.89), North China Plain (Zhengzhou 0.99), central China (Wuhan 1.00) and Sichuan Basin (Chengdu 1.17); the average value for these sites was 0.79. Lower AOD 440 nm values, that is < 0.50, occurred at remote sites in northwestern China, including Ürümqi (0.42) and Yinchuan (0.37); these sites are less affected by industrial activities and the population densities are lower compared with the sites in northern or eastern China.

It is worth noting that the particle emissions in or around the urban sites could lead to large optical extinctions due to hygroscopic aerosol growth, especially in summer when the relative humidity is often high. In a related study, Zhang et al. (2018) found a large AOD of 1.10 at Wuhan in central China that was linked to secondary aerosol formation under the high summertime temperatures. [START_REF] Li | Observed aerosol optical depth and angstrom exponent in urban area of Nanjing, China[END_REF] similarly concluded that high temperatures and humidity promoted the formation of fine particles and led to hygroscopic aerosol growth at Nanjing. [START_REF] Qin | A campaign for investigating aerosol optical properties during winter hazes over Shijiazhuang, China[END_REF] observed a high AOD 500 nm of 1.04 at Shijiazhuang and related this to the hygroscopic growth of aerosol fine-mode particles during polluted days. Clear spatial variability in EAE values over China is evident in Fig. 3, and at the remote sites the average EAEs were 1.03. The EAE at Lhasa (0.77) was lower than at Akedala (EAE = 1.13), which is in an arid region of central Asia, or at Shangri-La (EAE = 1.19) in Tibet. The average coarse-mode average effective radius (R effC ) at Lhasa was 2.26 µm and the fractional volume was 0.04 µm 3 per µm 2 , this result suggests the major components of the large mineral dust particles in aerosol populations over that region. The smaller sphericity fraction (∼ 42.70) and lower FMF (0.66) at Lhasa indicates the presence of non-spherical aerosol coarse particles compared with the spherical fine particles in the urban sites.

At arid and semi-arid sites in China, the average EAE value (0.71) was relatively low and the FMF also was low (0.58). The EAE was extremely low at Tazhong (0.25), which is in the Taklamakan Desert in the Xinjiang Uygur Autonomous Region of northwestern China and the sphericity fraction (12.87) and FMF (0.35) there were lower compared with most of the other sites. This finding indicates a strong contribution of large particles in this desert region consistent Large EAEs (1.23) were found at the sites in eastern China, and the FMFs also were large (0.89) at those sites. This result can be attributed to the strong impacts of anthropogenic in the more urbanized eastern part of the country. On the other hand, large EAE values also occurred at the clean sites in northeastern China, including Mt. Longfeng (1.38), where the sphericity fraction was 58.5 and the FMF 0.90. This shows that small particles can have stronger effects in these areas relative to some other regions of China. The EAE at Lin'an was larger than that at Shangdianzi in the North China Plain or Mt. Longfeng in Northeastern China for most months according to data from Che et al. (2009c). At the urban sites, large EAEs were found at sites in southern China, including Nanning (EAE = 1.36, sphericity fraction = 70.12, FMF = 0.95), Panyu (EAE = 1.43, sphericity fraction = 75.55, FMF = 0.93) and Zhuzilin (EAE = 1.45, sphericity fraction = 55.51, FMF = 0.94). This is likely because the large populations and widespread vehicle ownership in those cities led to the dominance of fine-mode particles throughout the year. [START_REF] Cheng | Seasonal variation and difference of aerosol optical properties in columnar and surface atmospheres over Shanghai[END_REF] found a unimodal distribution of EAE centred in 1.1-1.6 with the occurrence frequency about 72 %, which indicated an abundance of fine primary particles at Shanghai in eastern China. At the urban Nanjing site, which is in eastern central China, small particles were dominant, and the annual average EAE was 1.21 ± 0.28 [START_REF] Li | Observed aerosol optical depth and angstrom exponent in urban area of Nanjing, China[END_REF].

Spatial distribution of aerosol single-scattering albedo

The spatial distribution of SSA at 440 nm of the 50 CARSNET stations is shown in Fig. 4. As a frame of reference, Eck et al. (2005) reported that that SSA 440 nm from the AERONET retrievals were 0.82 to 0.98 globally. We note that SSA 440 nm values in this range reflect slightly to strongly absorbing aerosols, and these particles originate from multitude sources [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF]. The SSA 440 nm values decreased from remote and rural to the urban sites and from west to east, which means that there were higher percentages of absorbing particles at the urban and eastern stations. The average SSA 440 nm at the remote sites was about 0.91, which is indicative of particles with moderate absorption.

The absorbing aerosols at the remote sites were more likely mineral dust particles because those sites are less likely to be affected by carbonaceous particles, which also are absorbing but mainly produced by anthropogenic activities.

The SSA 440 nm values for the arid and semi-arid sites were 0.89. The relatively high SSA at Tazhong (0.92) was prob-ably due to slightly absorbing, coarse mode dust particles (EAE = 0.25).

A study by [START_REF] Bi | Toward characterization of the aerosol optical properties over Loess Plateau of Northwestern China[END_REF] showed that SSAs increased slightly with wavelength when dust was present at the SACOL site. Moderately absorbing particles were found in our study on or near the Chinese Loess Plateau where the SSA 440 nm values were typically 0.88 to 0.89. Eck et al. (2005) concluded that the spectral SSA demonstrated effects of dust at Yulin because the SSA increased for wavelengths from 440 to 675 nm. At the rural sites in eastern China, large SSA 440 nm values mainly occurred at sites in the YRD affected anthropogenic influences; these include Tonglu (0.93), Xiaoshan (0.93), Xiyong (0.94). [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF] found the slightly absorbing particles came from industrial activity and anthropogenic sources at YRD region with the SSA 440 nm between 0.91 and 0.94.

The average value of SSA 440 nm at the urban sites was 0.90, which indicates that particles with moderate absorption dominated the aerosol populations. [START_REF] Cheng | Seasonal variation and difference of aerosol optical properties in columnar and surface atmospheres over Shanghai[END_REF] reported a seasonal range of SSA from 0.88 to 0.91 at Shanghai, with higher values in autumn and winter compared with spring and summer. Lower SSA 440 nm values occurred at the urban sites and industrial regions in northeastern China, such as Shenyang (0.84), Anshan (0.89), Fuhsun (0.84), which indicates that the particles were more strongly absorbing in that region. On the other hand, higher SSA 440 nm values were found at urban sites in southern China, including Nanning (0.92), Panyu (0.90) and Zhuzilin (0.96), and this indicates that the particles at those sites were slightly or weakly absorbing.

Moreover, we found that the SSA 440 nm spatial distribution reflected the percentages of absorbing aerosols at the urban sites both in northern and eastern China. The reports of [START_REF] Dubovik | A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements[END_REF], [START_REF] Dubovik | Variability of absorption and optical properties of key aerosol types observed in worldwide locations[END_REF][START_REF] Dubovik | Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust[END_REF] showed that SSA values vary with both particle size and composition, and [START_REF] Su | Regional transport of anthropogenic pollution and dust aerosols in spring to Tianjin -A coastal megacity in China[END_REF] used the variations in SSA with wavelength to indicate the presence of brown carbon aerosols at Tianjin, a coastal megacity in China. [START_REF] Qin | A campaign for investigating aerosol optical properties during winter hazes over Shijiazhuang, China[END_REF] suggested that the small SSAs found at Shijiazhuang indicated the presence of fine-mode absorbing particles, such as brown carbon. [START_REF] Zhuang | Optical properties and radiative forcing of urban aerosols in Nanjing over China[END_REF] reported that the SSA at the Nanjing urban site ranged from 0.90 to 0.95, and the aerosol was more absorbing in autumn, possibly due to the biomass burning emission in the YRD. As evident in the results presented in Sect. 3.1, one can see that the R effT , R effF and R effC between northeastern and southern China was very similar. For example, at Shenyang, a megacity in northeastern China, the effective radii of total, fine-and coarse-mode particles were 0.31, 0.16, 2.23 µm and the corresponding volumes were 0.22, 0.12, 0.10 µm 3 per µm 2 , respectively. At Hangzhou in the YRD region, the R effT , R effF and R effC were 0.30, 0.17, and 2.21 µm with volumes of about 0.22, 0.12, and 0.10 µm 3 per µm 2 , respectively. Therefore, the different SSA 440 nm distributions in the two regions may be attributed to the special aerosol composition related to the urban in- dustrial background of northeastern China (lower SSA 440 nm ) and more anthropogenic sources in eastern China (higher SSA 440 nm ).

Dust aerosols with light-absorbing properties occur more frequently in spring in northeastern China than in more southern regions [START_REF] Zhao | Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols[END_REF]. Anthropogenic emissions from seasonal biomass burning and residential heating are two other main factors that affect aerosol composition between the two regions [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF]. There was high percentage of absorbing aerosols at the northeastern sites, especially in winter, more than likely caused by emissions of carbonaceous aerosol from residential heating [START_REF] Zhao | Temporal variability of the visibility, particulate matter mass concentration and aerosol optical properties over an urban site in Northeast China[END_REF]. Climatic conditions are also the main factors affecting the absorption characteristics of aerosols in different regions of northern and southern China. The increased light scattering could well be due to the particles hygroscopic growth demonstrated in other studies. For example, [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF] found that AODs and SSAs both increased with relative humidity at Guangdong in the Pearl River Delta (PRD) region, which suggests that condensational growth can affect aerosol optical properties.

Spatial distributions of absorption aerosol optical depth (AAOD)

The spatial distribution of AAOD at 440 nm, shown in Fig. 5, indicates that, overall, the AAOD 440 nm values increased from north to south and from remote and rural to urban sites. Lower AAOD 440 nm values were found at the remote stations, where the average value was 0.01. The AAOD 440 nm at Akedala, a remote site in northwestern China, was 0.02, and that was higher than at Shangri-La or Lhasa (0.01), both of which are on the Tibetan Plateau. The low AAOD 440 nm values throughout that region indicate that the aerosol population was not strongly absorbing. Compared with these three sites, the average AAOD 440 nm values at the arid and semiarid sites were higher (0.03); for example, an AAOD 440 nm of 0.05 was found at Tazhong, which is adjacent to the desert, and that indicates that the aerosol particles were more absorbing. As discussed in Sect. 3.2 and 3.3, dust aerosols likely make a significant contribution to aerosol light absorption in the areas impacted by the desert areas.

The low AAOD 440 nm found at Xilinhot (0.02) was probably due to the low aerosol loadings (AOD 440 nm = 0.21) in this region. The AAOD 440 nm values at the Mt. Gaolan and Yulin rural sites, which are on or around the CLP, were about 0.04 and 0.03, respectively, and the particles were moderately absorbing (SSA = 0.89). The large AAOD 440 nm at Datong (0.09) can be explained by the high AOD 440 nm (0.58) there. Indeed, large AAOD 440 nm values were found at rural sites in eastern China, where there were high AODs and low SSAs, as noted in Sect. 3.2 and 3.3. Of these sites, Dengfeng (AOD 440 nm = 0.08) and Huimin (AOD 440 nm = 0.08) are located on the North China Plain, while Huainan (AOD 440 nm = 0.10) is on the Guanzhong Plain. Lower AAOD 440 nm values, from 0.02 to 0.03, occurred at Tongyu (0.03), which is in a semi-arid region in northeastern China, at the Mt. Longfeng (0.03) regional background site on the Northeast China Plain, at the Yushe rural site in northern China (0.03), and at the clean Xiyong site in the PRD (0.02).

Several urban sites showed AAOD 440 nm values greater than 0.10: these include Fushun (0.11) and Shenyang (0.14) in northeastern China, Lanzhou (0.10) in northwestern China, and Nanjing (0.10) and Wuhan (0.11) ern and central China. Lower AAOD 440 nm values occurred in other urban areas, such as Yinchuan (AAOD 440 nm = 0.02, AOD 440 nm = 0.37) in the northwest and Zhuzilin (AAOD 440 nm = 0.03, AOD 440 nm = 0.66) in the PRD; both of these sites had relatively low AOD 440 values, indicating weaker anthropogenic influences compared with metropolitan regions in some other areas. We note that there are significant uncertainties in relating aerosol absorbing properties to particle types, such as black carbon, organic matter, and mineral dust (Russell et al., 2010;[START_REF] Giles | An analysis of AERONET aerosol absorption properties and classifications representative of aerosol source regions[END_REF]. Nonetheless, the information presented here on the spatial distribution of AAODs over China may be useful for further investigations into the relationships between light absorption and particle type (Liu et al., 2018;Schuster et al., 2016a, b).

Spatial distribution of direct aerosol radiative effect at the Earth's surface and top of the atmosphere

The spatial distributions of the DAREs calculated for both the bottom and top of the atmosphere are shown in Fig. 6. Overall, the DARE-BOAs increased from northwest to southeast and from rural to urban sites, consistent with impacts from the densely populated regions around the sites. The average DARE-BOA at the remote sites was -24.40 W m -2 , and, in comparison, a higher DARE-BOA (-33.65 W m -2 ) occurred at Akedala, which occurred in a remote region of northwestern China. The AOD 440 nm at Akedala was relatively low (0.17 region (-126.17 W m -2 ), and Nanjing (-143.38 W m -2 ) and Wuhan (-171.80 W m -2 ) in central China. These results indicate that anthropogenic aerosols can cause significant direct radiative effects at urban sites.

The DARE-TOAs increased from north to south and from rural to urban sites, and the average DARE-TOA for the remote stations was low, about -4.79 W m -2 (Fig. 7). The DARE-TOAs at Lhasa and Shangri-La were -5.04 and -8.93 W m -2 , respectively. A notably small DARE-TOA was found at Akedala (-0.42 W m -2 ), indicating that the effects of the aerosol on the temperature of Earth-atmosphere system there would be weak. The average DARE-TOA at the arid and semi-arid sites was -10.17 W m -2 . The large DARE-TOA found at Tazhong (-23.49 W m -2 ) could represent the larger contribution of slightly absorbing mineral aerosols (SSA = 0.92) and a large AOD (0.60); this indicates more cooling at the surface through the absorption and scattering solar radiation compared with the less impacted sites. This is consistent with the results for Tazhong discussed in Sect. 3.1, which showed high volumes of coarse-mode particles with large radii.

The average DARE-TOA at rural sites on the Chinese Loess Plateau or nearby was about -14.56 W m -2 . Although the SSA 440 nm were close between Mt. Gaolan and Yulin, about 0.89, the TOAs were quite different (Mt. Gaolan -20.87 W m -2 ; Yulin -9.09 W m -2 ), which could be due to the different AOD 440 nm , about 0.36 and 0.32, respectively. In rural eastern China, the DARE-TOA was about -32.40 W m -2 , and, to put this in context, [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF] found DARE-TOAs of -40 W m -2 at rural sites in the YRD region, which is indicative of a relatively strong cooling effect. Low DARE-TOAs were found at the Mt.

Longfeng rural site in northeastern China (DARE-TOA = -11.34, AOD 440 nm = 0.34, SSA = 0.89) and at the Tongyu semi-arid site in northeastern China (DARE-TOA = -8.87, AOD 440 nm = 0.23, SSA = 0.88), where the aerosol loadings were relatively low and the absorption was moderate.

At the urban sites at central and eastern China, the average DARE-TOA values were about -30.05 W m -2 . Higher DARE-TOAs occurred at Anshan on the Northeast China Plain (-39.66 W m -2 ), Chengdu in the Sichuan Basin (-52.21 W m -2 ), Hangzhou in the YRD (-40.16 W m -2 ), Jiaozuo (-39.35 W m -2 ) and Zhengzhou (-46.18 W m -2 ) on the North China Plain, and Zhuzilin (-40.15 W m -2 ) in the PRD region. The high DARE-TOA values at these urban sites imply relatively strong cooling effects due to higher aerosol loadings in the atmosphere.

Spatial distributions of aerosol mixing properties

The spatial distribution of aerosol mixing properties (Fig. 8) was obtained by using the SSA 440 nm , FMF, and EAE results to classify the particles based on size and absorbing properties. In previous studies by Zheng et al. (2017) and [START_REF] Mai | Column-integrated aerosol optical properties of coarse-and fine-mode particles over the Pearl River Delta region in China[END_REF], the particles in this study were grouped into eight types as shown in Table 2. Moreover, the FMF has been provided to give the particle size information in the group of the particles.

At the remote Akedala and Lhasa sites (FMF = 0.70-0.78 and SSA 440 nm = 0.85), the percentages of mixed absorbing particles (Type V) were 35 %-40 %, while at Shangri-la (FMF = 0.76, SSA 440 nm = 0.84) the percentage was slightly lower, 24.62 %. The characteristics of the particles at these remote, high-altitude sites were probably affected by the respectively. The proportion of coarse-mode particles with strongly absorbing properties in Group VII and coarse-mode particles with weakly absorbing properties in Group VIII at the rural sites in eastern China was < 11 %. These patterns indicated that there are differences between the eastern region and northwestern China because in the east coarse-mode particles only make a minor contribution to aerosol absorption. The percentage of fine-mode particles with weakly absorbing properties in Type IV and mixed absorbing particles in Type V combined to be about ∼ 50 % at the eastern sites. This result suggests that mixed aerosols originated from a variety of sources and that many of the sites were affected by anthropogenic emissions from megacities upwind.

The fine-mode particles with absorbing properties in Types I, II, III, and V at most of the urban sites accounted for 50 % to 90 %. The percentages of these four particle types combined were especially large in eastern China; for example, at Panyu, particle Types I-IV composed 90.83 % of the total and the FMF there was 0.90-0.94, while at Zhuzilin, the percentage of Types I-IV was 92.55 % and the FMF was 0.92-0.94. These results are another indication that fine-mode particles are important for light absorption in ur-ban areas. In contrast, the Lanzhou and Ürümqi urban sites were less affected by absorbing fine particles because the percentages of Type I-IV particles were only 19.73 % and 18.36 %, respectively. The mixed absorbing Type V particles accounted for large percentages of the total at Lanzhou (48.80 %, EAE = 0.88, SSA = 0.82, FMF = 0.73) and at Ürümqi (59.39 %, EAE = 0.94, SSA = 0.84, FMF = 0.75). Different from the other urban sites, these patterns show that larger particles had significant contributions to the aerosol absorption at these two northwestern sites.

Conclusions

Aerosol microphysical and its optical properties obtained from the ground-based sun photometer deployed at 50 CARSNET stations were used to begin the development of their climatology characteristics and to investigate potential aerosol-climate effects over vast area of China. Direct aerosol radiative effects (DAREs) at the bottom and at the top of the atmosphere were calculated, and eight types of aerosols were classified based on the particle size and ab- sorbing properties. The annual mean values of the R effT decreased from the arid and semi-arid sites (0.55 µm) to the urban sites (0.37 µm). The aerosol volumes increased from the remote sites (0.05 µm 3 per µm 2 ) to the urban sites (0.21 µm 3 per µm 2 ). The volumes of coarse-mode particles were larger than those for the fine mode at the remote and arid and semiarid sites -this can be explained by the greater relative abundances of mineral dust compared with pollution-derived particles at those sites. At the urban sites, where anthropogenic influences were relatively strong, the proportion of fine-mode particles increased gradually with aerosol volume.

The AOD 440 nm progressively increased from the remote sites (0.12), to the arid and semi-arid sites (0.32), to rural sites in eastern China (0.70), and finally to the urban sites (0.79), which were the ones most strongly affected by anthropogenic activities. The average EAE 440-870 nm values at the arid and semi-arid sites were relatively low (0.71), which indicates an important contribution of larger particles to the aerosol extinction in those regions. The consistently large EAE 440-870 nm values at the urban sites (> 1.20) and the high FMFs that those sites (0.88) are evidence that fine-mode particles are prevalent throughout year. The average SSA 440 nm values at the remote, rural, and urban sites were relatively similar, averaging about 0.89, and this indicates the particles were moderately absorbing.

Overall, dust aerosols with light-absorbing properties (in spring) and emissions from biomass burning and residential heating during the colder months were the main factors that led to spatial differences in the percentages of absorbing aerosols over China. The AAOD 440 nm values increased from the remote sites (0.01), to the arid and semi-arid sites (0.03), to the rural sites of eastern China (0.05), and finally to the urban sites (0.07). High AAOD 440 nm values were caused by light-absorbing dust aerosols at the rural sites and by the strong anthropogenic emissions in the metropolitan areas. The spatial patterns in the absorbing aerosols were not only affected by the chemical composition of the aerosol but also by physical effects imposed by topography, weather, and climate.

The average DARE-BOA values were -24.40 W m -2 at the remote sites, -56.43 W m -2 at the arid and semi-arid sites, -74.67 W m -2 at the sites on the CLP or nearby, -85.25 W m -2 at the rural sites in eastern China, and -103.28 W m -2 at the urban sites. The larger DARE-BOA values at the urban sites imply stronger cooling effects from anthropogenic emissions compared with those from mineral dust at the remote sites or those near the desert. Moreover, larger DARE-TOAs also occurred at the urban sites (-30.05 W m -2 ), which indicates strong cooling effects due to the large aerosol extinctions between the Earthatmosphere system displayed by the moderate to strong light absorption. Mixed-absorbing particles were the most abundant aerosol type in the remote and rural sites on or near the Chinese Loess Plateau and in eastern China. Mineral dust particles with moderate to strong absorbing properties were dominant at the arid and semi-arid sites, while absorbing fine-mode particles accounted for 50 % to 90 % of the aerosol at most urban sites.

The results of this study have considerable value for ground-truthing satellite observations and for validating aerosol models. Moreover, the results have also provided significant information on aerosol optical and radiative properties for different types of sites covering a broad expanse of China. These results also are a major step towards developing climatology for aerosol microphysical and optical properties for China and even East Asia.

Figure 1 .

 1 Figure 1. Annual spatial distribution of aerosol volume-size distributions at the CARSNET sites.

  r max r min (ln rln r V ) 2 dV (r) d ln r d ln r r max r min dV (r) d ln r d ln r .

Figure 2 .

 2 Figure 2. Annual spatial distribution of aerosol optical depth (AOD) at 440 nm at the CARSNET sites.

Figure 3 .

 3 Figure 3. Annual spatial distribution of extinction Ångström exponent (AE) 440-870 nm at the CARSNET sites.

Figure 4 .

 4 Figure 4. Annual spatial distribution of fine-mode fraction at the CARSNET sites.

Figure 5 .

 5 Figure 5. Annual spatial distribution of the single-scattering albedo (SSA) at 440 nm at the CARSNET sites.

  ) and the SSA moderate (0.90). The moderate absorption of aerosol could lead to more strong surface cooling effects with little higher DARE-BOA than the other remote sites. The DARE-BOAs for Lhasa and Shangri-La were -22.13 and -17.43 W m -2 , re-spectively. These results indicate weaker surface cooling effects at the remote sites relative to other regions because the aerosol loadings were relatively low, as indicated by AOD 440 nm values of < 0.20.The average DARE-BOTs at the arid and semi-arid sites in China were about -56.43 W m -2 , and those high DARE-BOAs can be explained by the moderately absorbing particles (SSA = 0.89) and large AOD 440 nm values (0.32) compared with the remote sites. A large DARE-BOA (-91.20 W m -2 ) occurred at the Tazhong site near the northwestern deserts, and there the high AOD (0.60) and the slight absorption of mineral dust (SSA = 0.92) imply substantial surface cooling. The average DARE-BOA for rural sites on the Chinese Loess Plateau or surrounding area was -74.67 W m -2 , which also implies cooling at the surface.Several rural sites in northern and eastern China had large DARE-BOA values; these include Huimin (-111.58 W m -2 ), Dengfeng (-104.78 W m -2 ), and Huainan (-129.17 W m -2 ), and at those sites the AODs were high, from 0.80 to 0.90, and the SSAs were ∼ 0.89. These results show stronger surface cooling effects at sites influenced by anthropogenic emissions compared with the remote sites or those near the deserts. The large negative DARE-BOA values (-103.28 W m -2 ) at the urban sites indicate that the combination of high AOD 440 nm values (0.79) and moderate SSAs (0.90) can cause significant surface cooling. Indeed, anthropogenic emissions presumably led to the high DARE-BOAs at urban sites, including Shenyang (-144.88 W m -2 ) and Fushun (-116.91 W m -2 ) on the Northeast China Plain, Xi'an on the Guanzhong Plain (-132.55 W m -2 ), Chengdu in the Sichuan Basin (-110.42 W m -2 ), Lanzhou in the western

Figure 6 .

 6 Figure 6. Annual spatial distribution of absorption aerosol optical depth (AAOD) at 440 nm at the CARSNET sites.

Figure 7 .

 7 Figure 7. Annual spatial distribution of direct aerosol radiative effect at the bottom of the atmosphere at the CARSNET sites.

Figure 8 .

 8 Figure 8. Annual spatial distribution of direct aerosol radiative effect at the top of the atmosphere at the CARSNET sites.

Figure 9 .

 9 Figure 9. Annual spatial distribution of the aerosol type classification of types I-VII at the CARSNET sites.

Table 1 .

 1 Aerosol type classification based on the optical properties.

	Type EAE	SSA	Absorbing properties and particle size
	I	EAE > 1.20	SSA 440 nm ≤ 0.85	fine-mode particles with highly absorbing properties
	II	EAE > 1.20	0.85 ≤ SSA 440 nm < 0.90 fine-mode particles with moderately absorbing properties
	III	EAE > 1.20	0.90 ≤ SSA 440 nm < 0.95 fine-mode particles with slightly absorbing properties
	IV	EAE > 1.20	SSA 440 nm > 0.95	fine-mode particles with weakly absorbing properties
	V	0.60 ≤ EAE < 1.20 SSA 440 nm ≤ 0.95	mixed absorbing particles
	VI	0.60 ≤ EAE < 1.20 SSA 440 nm > 0.95	mixed slightly absorbing particles
	VII	EAE ≤ 0.60	SSA 440 nm ≤ 0.95	coarse-mode particles with strongly absorbing properties (mainly dust)
	VIII	AE ≤ 0.60	SSA 440 nm > 0.95	coarse-mode particles with weakly absorbing properties

  with large volume of the coarse-mode particles (0.27 µm 3 per µm 2 ) noted in Sect. 3.1. The average EAE reached 0.93 at the rural sites near the CLP, and the average value of FMF for those sites was 0.73.Eck et al. (2005) found especially low EAE values in March and April (0.3 and 0.4, respectively) at Yulin, China, where the dust aerosol dominated the optical column.

www.atmos-chem-phys.net/19/11843/2019/ Atmos. Chem. Phys., 19, 11843-11864, 2019

  www.atmos-chem-phys.net/19/11843/2019/ Atmos. Chem. Phys., 19, 11843-11864, 2019

Table A2 .

 A2 Annual data for aerosol microphysical properties and optical and direct radiative parameters. No. Site a ReffT a ReffF a ReffC a VolT a VolF a VolC a AODT b EAE a FMF a SSAT a Image

	1 2 3 Arid and semi-arid sites (six sites) Akedala 47.12 Lhasa 29.67 Shangri-La 28.02 4 Dunhuang 40.15 5 Ejina 41.95 6 Minqin 38.63 7 Tazhong 39.00 8 Xilinhot 43.95 9 Tongyu 44.42 Rural sites on (or near) the Chinese Loess Plateau (three sites) 87.97 562.0 55 km west of Fuhai county, Xinjiang Province, and 250-300 km southeast of Kazakhstan 91.13 3663.0 In the centre of Lhasa, Qinghai-Tibetan Plateau. 99.73 3583.0 12 km northeast of Shangri-La county, Diqing area, Yunnan Province 94.68 1139.0 1.5 km northeast of Dunhuang, Gansu Province, near the Kumtag Desert in China 101.07 940.5 West of Inner Mongolia, near Mongolia, and Badanjilin Desert 103.08 1367.0 In Minqin county, east of the Tenggeli desert and north of the Badanjilin Desert, Gansu Province 83.67 1099.4 In the middle of Taklamakan Desert, Xinjiang Province 116.12 1003.0 5 km southeast of Xilinhot, near Hunshandake, Inner Mongolia Province, 122.87 151.0 In Tonyu, west of Jilin Province 10 Mt. Gaolan 36.00 103.85 2161.6 5 km north of Lanzhou in Gansu Province 11 Yulin 38.43 109.20 1135.0 10 km north of Yulin in Shaanxi Province 12 Datong 40.10 113.33 1067.3 Within 9 km of Datong but within an area of rapid urbanization, Shanxi Province Rural sites in eastern China (15 sites) 13 Changde 29.17 111.70 565.0 18 km northwest of Changde, Hunan Province 14 Dongtan 31.52 121.96 10.0 On Chongmin Island, 30km east of Shanghai 15 Chun'an 29.61 119.05 171.4 151 km southwest of Hangzhou, Zhejiang Province 16 Huimin 37.48 117.53 11.7 100 km northeast of Jinan, Shandong Province 17 Lin'an 30.30 119.73 138.6 150 km northeast of Shanghai and 50 km west of Hangzhou, Zhejiang Province 18 Mt.Longfeng 44.73 127.60 330.5 In Wuchang county, 175 km northeast of Harbin, Heilongjiang Province 19 Fuyang 30.07 119.95 17.0 44.1 km southwest of Hangzhou, Zhejiang Province 20 Shangdianzi 40.65 117.12 293.0 In Miyun county, 150 km northeast of Beijing 21 Yushe 37.07 112.98 1041.5 1.5 km east of Yushe, Shanxi Province 22 Dengfeng Remote sites (3 sites) 1 Akedala 0.36 0.14 2.45 0.06 0.02 0.04 0.17 1.13 0.81 0.90 0.0117 1.4540 947 2010-2017 437 2012-2017 325 2013-2017 2030 2012-2017 1970 2013-2017 481 2013-2017 1279 2013-2017 1464 2013-2017 817 2010-2011 769 2015-2016 716 2010-2016 914 2014-2017 344 2013-2016 986 2012-2016 1286 2011-2015 2243 2009-2017 1834 2011-2015 1515 2012-2016 710 2014-2015 1520 2014-2017 1479 2013-2017 0.02 -33.65 a BOA 2 Lhasa 0.64 0.13 2.26 0.05 0.01 0.04 0.10 0.77 0.66 0.90 0.0106 1.5541 0.01 -22.13 3 Shangri-La 0.39 0.14 2.33 0.03 0.01 0.02 0.10 1.19 0.85 0.93 0.0086 1.4626 0.01 -17.43 Average 0.47 0.14 2.35 0.05 0.01 0.03 0.12 1.03 0.77 0.91 0.0103 1.4902 0.01 -24.40 Arid and semi-arid sites (6 sites) 4 Dunhuang 0.62 0.14 1.52 0.15 0.02 0.13 0.33 0.48 0.44 0.88 0.0103 1.5491 0.04 -63.61 5 Ejina 0.56 0.14 1.78 0.11 0.02 0.09 0.24 0.64 0.52 0.89 0.0116 1.5265 0.03 -47.66 6 Minqin 0.56 0.13 1.87 0.13 0.02 0.11 0.30 0.68 0.59 0.86 0.0145 1.5430 0.04 -59.83 7 Tazhong 0.71 0.14 1.38 0.30 0.03 0.27 0.60 0.25 0.35 0.92 0.0054 1.5257 0.05 -91.20 -23.49 a TOA -0.42 -5.04 -8.93 -4.79 -8.96 -7.20 -5.01 8 Xilinhot 0.48 0.13 2.45 0.08 0.02 0.05 0.21 1.03 0.78 0.89 0.0139 1.5183 0.02 -37.14 -7.47 9 Tongyu 0.39 0.13 2.36 0.07 0.02 0.05 0.23 1.16 0.82 0.88 0.0179 1.5377 0.03 -39.13 -8.87 Average 0.55 0.14 1.89 0.14 0.02 0.12 0.32 0.71 0.58 0.89 0.0123 1.5334 0.03 -56.43 -10.17 Rural sites on the Chinese Loess Plateau or nearby (3 sites) 10 Mt. Gaolan 0.58 0.14 2.03 0.16 0.03 0.13 0.36 0.81 0.64 0.89 0.0108 1.5154 0.04 -59.36 -20.87 11 Yulin 0.53 0.15 2.05 0.11 0.03 0.08 0.32 0.84 0.72 0.89 0.0122 1.5070 0.03 -56.81 -9.09 12 Datong 0.35 0.13 2.15 0.19 0.09 0.10 0.58 1.15 0.83 0.86 0.0171 1.4905 0.09 -107.86 -13.71 Average 0.49 0.14 2.08 0.15 0.05 0.10 0.42 0.93 0.73 0.88 0.0134 1.5043 0.05 -74.67 -14.56 Rural sites in eastern China (15 sites) 13 Changde 0.32 0.16 2.18 0.14 0.07 0.07 0.58 1.15 0.88 0.93 0.0101 1.4619 0.04 -75.33 -31.44 14 Dongtan 0.37 0.16 2.12 0.17 0.08 0.09 0.62 1.21 0.86 0.93 0.0080 1.4624 0.04 -79.41 -33.18 15 Chun'an 0.30 0.18 2.30 0.19 0.12 0.08 0.81 1.22 0.92 0.94 0.0066 1.4095 0.04 -86.49 -46.48 16 Huimin 0.36 0.15 2.07 0.22 0.10 0.12 0.83 1.14 0.86 0.89 0.0147 1.4852 0.08 -111.58 -25.49 17 Lin'an 0.29 0.17 2.24 0.21 0.12 0.09 0.87 1.29 0.91 0.93 0.0089 1.4172 0.06 -93.09 -41.73 18 Mt.Longfeng 0.28 0.15 2.44 0.08 0.04 0.04 0.34 1.38 0.90 0.89 0.0165 1.4647 0.03 -51.17 -11.34 19 Fuyang 0.29 0.17 2.28 0.21 0.13 0.09 0.89 1.31 0.92 0.94 0.0070 1.4147 0.05 -91.69 -42.29 20 Shangdianzi 0.40 0.15 2.33 0.12 0.05 0.07 0.43 1.17 0.86 0.89 0.0148 1.4840 0.04 -59.99 -20.58 21 Yushe 0.41 0.15 2.18 0.14 0.06 0.08 0.50 1.07 0.84 0.92 0.0090 1.4878 0.03 -66.72 -25.99 22 Dengfeng 0.39 0.15 2.03 0.23 0.09 0.13 0.79 1.02 0.83 0.89 0.0131 1.4782 0.08 -104.78 -35.84 23 Huainan 0.30 0.17 2.25 0.21 0.13 0.08 0.91 1.17 0.92 0.88 0.0166 1.4308 0.10 -129.17 -24.44 24 Jiande 0.29 0.17 2.18 0.20 0.12 0.08 0.84 1.34 0.91 0.92 0.0099 1.4085 0.06 -91.06 -40.07 25 Tonglu 0.29 0.17 2.20 0.20 0.12 0.08 0.83 1.31 0.91 0.93 0.0091 1.4269 0.06 -89.82 -41.28 26 Xiaoshan 0.28 0.17 2.24 0.22 0.13 0.09 0.87 1.35 0.91 0.93 0.0082 1.4134 0.06 -95.23 -40.39 27 Xiyong 0.33 0.16 2.43 0.11 0.06 0.05 0.41 1.32 0.89 0.94 0.0074 1.4072 0.02 -53.18 -25.45 Average 0.33 0.16 2.23 0.18 0.09 0.08 0.70 1.23 0.89 0.92 0.0107 1.4435 0.05 -85.25 -32.40 Urban sites (23 sites) 28 Anshan 0.36 0.17 2.24 0.26 0.12 0.14 0.94 1.12 0.86 0.89 0.0158 1.4759 0.10 -117.99 -39.66 29 Beijing-Nanjiao 0.45 0.15 2.33 0.19 0.07 0.12 0.65 1.12 0.84 0.92 0.0100 1.4939 0.05 -82.06 -29.43 30 Beijing-CAMS 0.50 0.16 2.37 0.19 0.07 0.12 0.65 1.12 0.79 0.90 0.0115 1.5108 0.05 -72.66 -29.10 31 Chengdu 0.34 0.21 2.26 0.26 0.16 0.10 1.17 1.12 0.92 0.97 0.0033 1.4116 0.04 -110.42 -52.21 34.46 a Real a AAOD 32 Dalian 0.35 0.16 2.24 0.16 0.08 0.09 0.62 1.22 0.87 0.93 0.0095 1.4584 0.04 -75.50 -37.42
	33	Fushun	0.38	0.17	2.34	0.22	0.09	0.12	0.80	1.12	0.87	0.84	0.0244 1.4954	0.11 -116.91 -19.59
	34	Hangzhou	0.30	0.17	2.21	0.22	0.12	0.10	0.87	1.30	0.90	0.91	0.0109 1.4337	0.07	-31.57 -40.16
	35	Hefei	0.29	0.15	2.37	0.18	0.10	0.08	0.69	1.28	0.90	0.85	0.0195 1.4253	0.10 -105.83 -19.22
	36	Jiaozuo	0.35	0.16	2.17	0.20	0.10	0.10	0.76	1.14	0.88	0.91	0.0105 1.4722	0.05	-92.29 -39.35
	37	Lanzhou	0.54	0.14	2.04	0.28	0.06	0.22	0.66	0.81	0.66	0.83	0.0197 1.5193	0.10 -126.17 -13.81
	38	Nanjing	0.33	0.16	2.16	0.25	0.12	0.12	0.94	1.13	0.88	0.88	0.0154 1.4446	0.10 -143.38 -28.29
	39	Nanning	0.30	0.18	2.53	0.20	0.13	0.06	0.97	1.36	0.95	0.92	0.0107 1.4272	0.07 -121.92 -33.35
	40	Panyu	0.26	0.16	2.29	0.16	0.10	0.06	0.69	1.43	0.93	0.90	0.0137 1.4155	0.07	-96.03 -26.56
	41	Shanghai	0.40	0.15	1.93	0.19	0.08	0.11	0.68	1.10	0.84	0.88	0.0142 1.4814	0.07 -106.89 -24.34
	42	Shenyang	0.31	0.16	2.23	0.22	0.12	0.10	0.89	1.20	0.90	0.84	0.0253 1.4589	0.14 -144.88 -15.02
	43	Tianjin	0.42	0.16	2.26	0.23	0.10	0.13	0.83	1.11	0.86	0.89	0.0134 1.4957	0.07 -108.09 -33.26
	44	Ürümqi	0.48	0.14	2.14	0.15	0.04	0.10	0.42	0.93	0.75	0.85	0.0192 1.5371	0.05	-70.55 -11.74
	45	Xi'an	0.37	0.16	1.85	0.26	0.11	0.15	0.98	0.98	0.82	0.88	0.0150 1.4888	0.10 -132.55 -35.93
	46	Yinchuan	0.38	0.14	2.02	0.11	0.04	0.07	0.37	1.12	0.81	0.94	0.0054 1.4930	0.02	-48.67 -21.89
	47	Zhengzhou	0.43	0.18	2.22	0.28	0.12	0.16	0.99	1.10	0.86	0.95	0.0045 1.4626	0.04 -101.10 -46.18
	48	Shijiazhuang	0.40	0.16	2.28	0.26	0.12	0.14	0.95	1.09	0.87	0.88	0.0154 1.4754	0.09 -125.05 -33.66
	49	Wuhan	0.34	0.17	2.22	0.22	0.12	0.10	1.00	1.16	0.91	0.88	0.0196 1.4779	0.11 -171.80 -20.40
	50	Zhuzilin	0.27	0.17	2.45	0.15	0.09	0.05	0.66	1.45	0.94	0.96	0.0049 1.4438	0.03	-73.16 -40.65
		Average	0.37	0.16	2.22	0.21	0.10	0.11	0.79	1.15	0.86	0.90	0.0136 1.4695	0.07 -103.28 -30.05

a Optical parameters at a wavelength of 440 nm. b Ångström exponents between 440 and 870 nm. www.atmos-chem-phys.net/19/11843/2019/ Atmos. Chem. Phys., 19, 11843-11864, 2019

Atmos.Chem. Phys., 19, 11843-11864, 2019 www.atmos-chem-phys.net/19/11843/2019/

www.atmos-chem-phys.net/19/11843/2019/ Atmos. Chem. Phys., 19, 11843-11864, 2019

Acknowledgements. This work was supported by grants from the National Science Fund for Distinguished Young Scholars (41825011), the National Key R&D Program Pilot Projects of China (2016YFA0601901), National Natural Science Foundation of China (41590874), the CAMS Basis Research Project (2017Z011), the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 262254, AERONET-Europe ACTRIS-2 program, and the European Union's Horizon 2020 research and innovation programme under grant agreement no. 654109. Financial support. This research has been supported by the National Science Fund for Distinguished Young Scholars (grant no. 41825011), the National Key R & D Program Pilot Projects of China (grant no. 2016YFA0601901), the National Natural Science Foundation of China (grant no. 41590874), the European Union's Horizon 2020 research and innovation programme (grant no. 654109), and the European Union Seventh Framework Programme (FP7/2007-2013).

Review statement. This paper was edited by Xiaohong Liu and reviewed by three anonymous referees.

Data availability. The detailed data used in the study have been deposited in the figshare database [START_REF] Che | Datasets for "Spatial distribution of aerosol microphysical and optical properties and direct radiative effect from the China Aerosol Remote Sensing Network[END_REF], https://doi.org/10.6084/m9.figshare.9731339.v2).

Author contributions. All authors contributed to shaping the ideas and reviewing the paper. HC, XX, and XZ designed and implemented the research and prepared the manuscript. HC, HZ, YW, and HW contributed to analysis of the CARSNET dataset. HC, XX, JZ, OD, BNH, PG, and ECA contributed to the CARSNET data retrieval. HC, BQ, WG, HY, RZ, LY, JC, YZ, KG, and XZ carried out the CARSNET observations. OD, BNH, PG, and ECA provided constructive comments on this research.

Competing interests. The authors declare that they have no conflict of interest.


Amr El-Demerdash

Ludmila Ermolenko

Emmanuelle Gros

Pascal Retailleau

T B Nguyen

Anne Gauvin-Bialecki

Ali Al-Mourabit
email: ali.almourabit@cnrs.fr

Binh Nguyen

Short-Cut Bio-Inspired Synthesis of Tricyclic Guanidinic Motifs of Crambescidins and Batzelladines Marine Alkaloids

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Polycyclic guanidine alkaloids from marine origin are known since the eighties . [1] They are recognized for displaying a vast scope of bioactivities, [2] including nanomolar cytotoxicity against several tumor cell lines and antiviral activity. [3] More than 100 metabolites have been recorded with varied guanidinic cyclic systems. [4] About 40 members of this family have been synthesized to date. Many of the accomplished total syntheses within this family were performed in the groups of Snider and Murphy and Overman. [5] The characteristic structural feature of this class of guanidine alkaloids is the presence of a fused tricyclic guanidine core (Figure 1). We became interested in their biomechanistic analysis since we have isolated several new members from a Monanchora n. sp. marine sponge. Since the enantioselective total synthesis of (-)ptilomycalin by Lary E. Overman in 1995 [6] and for many years, their complex structures have presented remarkable challenges for synthetic chemists. [7a] Although synthetic accesses to some biologically and structurally important molecules have been achieved, the majority of the reported syntheses relied on rather long multi-step sequences. [7b,c] Practical synthesis for a possible application in medicinal chemistry is still desirable to explore the extremely interesting biological activities of this class of compounds. A more concise, economically, and ecologically viable access to these motifs would be obviously highly desirable, especially for large-scale synthesis. To progress in a possible solution to this question, our tactic to approach the synthesis of the common tricyclic guanidine core was centered on multicomponent reaction to construct the pyrrolidine ring. Our approach was inspired by the strategy applied to the elegant and concise biomimetic synthesis of tropinone that had been disclosed in 1917 by Robinson [8] and further studied by Schöepf. [9] Although the chemoselectivity and the stereoselectivity of the one pot multistep reactions remain hazardous, this way of conceiving the synthesis constitutes a remarkable efficient multi-bond construction combined with multicomponent reactions in a single process. After the first paper of Robinson, the method was then declined in other elegant syntheses with different reagents like succinaldehyde with primary amine described by John M. McIntosh [10] or even symmetrical double Michael acceptors. [11]

Results and Discussions

The original Robinson method used for the bicyclic tropinone consists of multicomponent reaction from fairly simple starting material including succinaldehyde, methylamine and acetonedicarboxylic acid or even acetone. The synthesis featured a bioinspired reaction as biosynthetic pathway involves the similar functionalities. Our objective was the multicomponent reaction directly involving the guanidine or one of its protected derivatives and affording polycyclic structural core element (Figure 2). Our neutral process targets the aminocarbinol structural element of crambescidins, while the reductive version targets the guanidinic tricycle of betzelladines. As shown in Figure 2, we envisioned a four-component condensation between a guanidine surrogate, a succinaldehyde and two methyl ketone derivatives. Such combination would generate disubstituted pyrrolidine core with 2,5disubstituted side chains readily cyclized to provide the final polycyclic guanidine core. Depending on the oxidation degree of the carbon atoms derived from ketone functions in the final, the cyclization could follow the nonredox self-assembling affording the two aminocarbinol groups of the crambescidin 359 [12] or a reductive amination pathway giving the tricyclic element of the merobatzelladine B. [13] Because succinaldehyde is not a readily available and bench-stable reagent, we choose to use as its stable and inexpensive precursor 2,5-dimethoxytetrahydrofuran. The result of our short strategy mirrors the earlier proposal by Murphy [14] and further adapted by Stonik. [15] In the later hypothesis, guanidine undergoes double intramolecular Michael additions on plausible polyketide-type precursors. The elegant proposal requiring the prior preliminary preparation of the Michael double acceptor chain was tested by Murphy with success. [14] Figure

Multicomponent strategy to fused guanidinic pyrrolidine rings

To regioselectivity activate the methyl group of the ketones, their acylacetic acid analogues are employed. In the first instance, we started with two identical ketones, 2,5dimethoxytetrahydrofuranand and guanidine. Actually, our laboratory is working on the development of simple, general, and scalable methods towards the synthesis of diverse structures of the guanidine alkaloids. One of the most important aspects behind this chemistry is the choice of guanidine surrogates. [16] Because free guanidine is highly basic with unpredictable and uncontrollable reactivities, and thus difficult to handle, it is rarely used in guanidine derivative synthesis. Murphy et al. used the free guanidine which reacted successfully with a the beforehand prepared bis-unsaturated ketones. [17] For our objective of multicomponent reaction, we made a screening of free guanidine and other protected derivatives. After some unfruitful attempts and detours with different protocols, we identified 2-aminopyrimidine as a successful protected guanidine surrogate in a model reaction with inexpensive acetoacetic acid. The reaction conducted in water at 40° C provides condensed product 1-3 (Scheme 1) in a four component decarboxylative condensation in moderate yields of 25-62% respectively.

Scheme 1. Four component regioselective synthesis of 1-3

The pyrrolidine product was identified as an equimolar mixture of cis and trans diastereoisomers. The moderate yield is understandable based on the easy autodecarboxylation of acetoacetic acid under acidic and heating conditions into unreactive acetic acid. Effort to better control acidity (using buffer solutions) and to lower temperature did not lead to any clear improvement. Indeed, using buffer solutions means adding other components to the reaction mixtures, which probably affected the reaction in the undesirable directions. Lower the reaction temperature led to longer reaction times. Indeed, when this reaction was operated at 60° C it afforded only the cis diastereomer in 19 % yield when R is a methyl group. At this stage, our efforts were focused on removing trimethine group of the 2-aminopyrimidine moiety to recover the free guanidine and its intrinsic reactivity for the rest of the cyclisation to reach the desired tricycle. Classical treatment with hydrazine did not lead to any useful transformation. While the ketones were attacked by hydrazine to provide the corresponding hydrazone, 2-aminopyrimidine moiety remained unchanged despite prolonged heating. We wondered if the symmetry of this aromatic ring was responsible for its inertness. Indeed, all known examples of hydrazine or hydroxylamine promoted deprotection were described with non-symmetrical 2aminopyrimidine. [18] Thus, a desymmetrization of 2-aminopyrimidine moiety would be necessary for the deprotection. After some unsuccessful alkylation trials even with the strongest alkylating agents such as methyl triflate or trimethyloxonium trifluoroborate, we found serendipitously that under acidic conditions of HBF4 resulting from hydrolysis of trimethyloxonium trifluoroborate, [19] the intra-cyclic nitrogen of the 2-aminopyrimidine moiety attacks the ketone function quantitatively leading to the hemiaminal 4, where its structure was determined by extensive 2D-NMR experiments and confirmed X-ray diffraction of one crystalized cis diasterisomer (Scheme 2). The driving force of this cyclization is obviously the functional group proximity. This cyclization polarizes the 2-aminopyrimidine moiety, thus makes it more reactive towards nucleophiles, thus prone to deprotection by hydrazine. Methyl hydrazine was chosen as deprotecting agent as it is more nucleophile, thus more reactive than unsubstituted hydrazine for this kind of deprotection reaction. Unlike hydrazine, which is commercialized as hydrate complex, methylhydrazine is readily available in anhydrous form. Its lower boiling point (87 °C) compared to unsubstituted hydrazine (bp 114 °C) as well as the lower boiling point of its by-product (Nmethylpyrazole bp 127 °C vs pyrazole bp 186-188 °C). Interestingly, the excess methylhydrazine as well as the resulting Nmethylpyrazole were simply removable in vacuo during the concentration under vacuo. Along with the removal of trimethine moiety, the remaining ketone function was also transformed into methylhydrazone 5 (Scheme 2). Gratifyingly, purification of this product over LH-20 using water as eluent provided the full tricyclic guanidine aminocarbinol diastereisomeric core 6a+b as a result of hydrazone hydrolysis and subsequent direct intramolecular nonredox cyclization. The two diastereomers 6a+b bearing aminocarbinol groups of the crambescidin 359 (Figure 2) were obtained. The moderate yield (25% over 3 steps) is the consequent of non-optimized purification process of the polar compounds. Final reduction of this mixture with sodium borohydride in protic mixture of H2O: MeOH or MeOH: EtOH 1:1 afforded the tricyclic diastereoisomers 7a+b as equimolar mixture of meso (syn form) and C2-symmetry (anti form) geometry. The structures and stereochemistries were confirmed by extensive NMR experiments. Only half set of 1 HNMR signals were observed for each of these compounds. In order to validate our approach for the synthesis of natural products like the merobatzelladine B (Figure 1), we explored the same reaction cascade on the pyrrolidine product 2 to provide the target compounds 11a and 11b in comparable yields as for methyl derivatives. All the structures 8-11 were confirmed by spectroscopic data including HRMS and integrated 2D-NMR experiments (Scheme 3). 1 HNMR spectra that differ from those of merobatzelladine B suggests that 11a and 11b are diastereomers. Although the spectroscopic data suggested the planar structure of merobatzelladine B, [13] the obtained diastereoisomers 11a and 11b are symmetrical (meso or C2) and the 1 H and 13 C NMR chemical shifts showed notable differences. Because of the symmetry, the 1 H and 13 C NMR spectra contained half signals (Table 1). NOESY experiments for both compounds displayed a key correlation between H-6 and H-8 (Scheme 3). The relative configurations differentiating the meso or the C2-symmetry 11a/11b was established based on the values reported for similar compounds in the literature. Close analogues having a 4C carbons side chains (instead of 5C for our compounds) were synthesized by Nagasawa and co-workers. [20] The selected data for comparison are summarized in Table 1. With the comparison of 13 It should be noted that, although the configuration of both synthesized tricyclic guanidines 11a and 11b differ from the natural merobatzelladine B, both of the products share the relative configurations of the two tricyclic guanidinic cores of batzelladine F as corrected by Overman [21] (Figure 1). These results suggest that the biosynthesis of this family of metabolites is stereodivergent and depends on the phylogeny of the producing marine sponge.

Conclusion

In conclusion, a convenient bio-inspired short model synthesis of tricyclic guanidine-containing natural products featuring a fourcomponent reaction between 2,5-dimethoxytetrahydrofuran, 2aminopyrimidine and two molecules of acylacetic acids had been developed with an overall yield 2-5%. The chosen models 10a+b and 11a+b corresponds to the core structure for crambescidin 359, batzelladine F and merobatzelladine B. Currently we would like to optimize the reaction conditions for improved chemo and stereoselectivity, as well as to extend the multicomponent approach to non-symmetrical cases. Modified acylacetic acids terminally substituted by a hydroxy group could be employed to provide directly the pentacyclic guanidinic core of crambescidin 800 and crambescidin 359 (Figure 1). Although our strategy still requires improvement in terms of stereoselectivity control and yield we anticipate that this unique design using the 2-aminopyrimidine as a guanidine equivalent for regioselective access to polycyclic guanidinic natural products could be applicable to other derivatives to be isolated in the near future.

Experimental Section

Detailed experimental procedures, spectroscopic data including 1 H and 13 C NMR spectra of new compounds (PDF) are available free of charge on publications website.

Accession Codes

supplementary crystallographic data for this paper. These data can be obtained free of charge by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 1 .

 1 Figure 1. Examples of complex guanidine alkaloids with various relative configuration of the guanidinic tricycle

  C NMR of syn tricyclic guanidine 11a and anti 11b, the similarities of the carbons C-5 to C-10 have become clear. The angular carbons C-6 and C-8 clearly show the differences between the chemical shifts of the syn and anti-configurations.

Table 1 .

 1

	1 H, compared to previously reported analogues 13 C NMR (CD3OD) spectral data for compounds 11a and 11b [20]
		11a		Litt	.20	11b		Litt.	20
		meso		meso	C2 symmetry	C2 symmetry
	No	δH, (J in Hz)	δC, Type	δC, Type	δH, (J in Hz)	δC, Type	δC, Type
	1	0.93	14.4	-		0.93	14.6	-
	2	1.36	23.7	-		1.36	23.9	-
	3	1.36	32.9	-		1.36	33.0	-
	4	1.42	26.1	-		1.38	26.2	-
	5	1.57	36.0	35.6		1.57	37.0	36.6
	6	3.43	51.7	51.5		3.51	53.1	53.1
	7	1.25/2.27	35.0	34.8		1.36/2.32	34.4	34.4

Acknowledgements

Financial support from CNRS is gratefully acknowledged. The authors thank the ANR (POMARE project, 2011-EBIM-006-01). A. El-' P w f by E yp Government (Ministry of Higher Education), they are gratefully acknowledged. Special thanks for J-F. Gallard for technical assistance and NMR measurements. Remerciement Robert Dodd

Graphical Abstract

We report a convenient short synthesis of tricyclic guanidine-containing natural products model featuring four-component reaction between 2,5-dimethoxytetrahydrofuran, 2-aminopyrimidine and two acylacetic or benzoylacetic acids. A synthetic route combining the Robinson-Schöepf bioinspired strategy and biomechanistic analysis of the crambescidin and batzelladine alkaloids family. An application of this strategy to the synthesis of two unnatural stereoisomers of merobatzelladine B is described from the protected guanidine in the form of aminopyrimidine.