2.1 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.

2.2 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 (Holben et al., 1998; Che et al., 2009a; Tao et al., 2014a). The reference instruments for CARSNET were periodically calibrated at Izaña, Tenerife, Spain, located 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 CE-318 sun photometers (440, 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 (Holben et al., 1998; Dubovik and King, 2000). 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, 2015). 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).

2.3 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 Dubovik et al. (2002, 2006). 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 Dubovik et al. (2006). The sphericity fraction retrieved from the inversions is defined as spherical particles/(spheroidal particles + spherical particles) (Giles et al., 2011).

Dubovik et al. (2002, 2006) 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 coarse-mode (R_effC) aerosols, the effective radii are calculated by the following equation:

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 Whitey (1978), Shettle and Fenn (1979), and Remer and Kaufman (1998):

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:

Then the standard deviation is calculated from the volume median radius:

The volume concentration (µm³ per µm²) is speculated by the following equation:

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 Dubovik et al. (2002, 2006). The AAOD and AAE for wavelength λ were calculated as follows:

The total AODs' uncertainty was 0.01 to 0.02 according to Eck et al. (1999). The accuracy of SSA retrieved from AOD_440 nm > 0.50 with a solar zenith angle of > 50 was 0.03 (Dubovik et al., 2002). The accuracy of the particle volume size distribution was 15 %–25 % between 0.1 µm $\le r\le \mathrm{7.0}$ 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 (García et al., 2008, 2012). 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:

where F and F⁰ 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 (Nakajima and Tanaka, 1988; Stamnes et al., 1988). 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 (García et al., 2008). García et al. (2008) pointed out that the error for the observed solar radiation at the surface in global was $+\mathrm{2.1}\pm \mathrm{3.0}$ % for an overestimation of about $+\mathrm{9}\pm \mathrm{12}$ Wm⁻². The data used in preparing the figures for the present paper have been made available as an Appendix.

3.1 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.

Table 1Aerosol type classification based on the optical properties.

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The average (arithmetic mean) R_effT at the remote sites was about 0.47 µm with the volume about 0.05 µm³ per µm² (Table 1). A large R_effT (0.64 µm) was found at Lhasa, and the total aerosol volume there was 0.05 µm³ per µm². These results are consistent with those reports by Li et al. (2018), 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 µm³ per µm². 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 µm³ per µm², while the average coarse-mode effective radii (R_effC) was 2.35 µm and the coarse-mode fractional volume (PV_C) was 0.03 µm³ per µm². 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 Cong et al. (2009) 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 µm³ per µm²), 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 µm³ per µm². Large PV_C values were found at the arid and semi-arid sites (0.05–0.27 µm³ per µm²). The arithmetic mean R_effT (0.49 µm) at the rural sites on or near the CLP had total aerosol volumes (0.15 µm³ per µm²) 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. Bi et al. (2011) 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 µm³ per µm²). 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 µm³ per µm². At the Mt. Longfeng background site in northeastern China, the total particle volume was low (0.08 µm³ per µm²), 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 µm³ per µm². The average R_effF of fine-mode particles at the urban sites was 0.16 µm with a PV_F of 0.10 µm³ per µm² while the R_effC was 2.22 µm and PV_C was 0.11 µm³ per µm².

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 µm³ per µm²); at major cities in northern China, including Shijiazhuang (R_effT=0.16 µm, PV_F=0.12 µm³ per µm²) and Zhengzhou (R_effT=0.18 µm, PV_F=0.12 µm³ per µm²); at Chengdu, a city in the Sichuan Basin (R_effT=0.21 µm, PV_F=0.16 µm³ per µm²); and in the urban regions of Nanning (R_effT=0.18 µm, PV_F=0.13 µm³ per µm²) and Panyu (R_effT=0.16 µm, PV_F=0.10 µm³ per µm²) 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.

Cheng et al. (2015) 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. Zhao et al. (2018) 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; Sun et al., 2010). 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 Che et al. (2018). Xia et al. (2019) observed the aerosol hygroscopic growth of the fine particle scattering coefficient in Beijing.

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

3.2 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 Li et al. (2018), who showed clean air conditions at Lhasa with AOD < 0.1. Cong et al. (2007, 2009) 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 µm³ per µm² (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 (Bi et al., 2011). Li and Zhang (2012) 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). Zhu et al. (2014) 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. Li et al. (2015) similarly concluded that high temperatures and humidity promoted the formation of fine particles and led to hygroscopic aerosol growth at Nanjing. Qin et al. (2017) 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.

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

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 µm³ per µm², 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 with large volume of the coarse-mode particles (0.27 µm³ per µm²) 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.

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. Cheng et al. (2015) 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 (Li et al., 2015).

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

3.3 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 (Che et al., 2018). 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 probably due to slightly absorbing, coarse mode dust particles (EAE =0.25).

A study by Bi et al. (2011) 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). Che et al. (2018) 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. Cheng et al. (2015) 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 Dubovik and King (2000), Dubovik et al. (2002, 2006) showed that SSA values vary with both particle size and composition, and Su et al. (2017) used the variations in SSA with wavelength to indicate the presence of brown carbon aerosols at Tianjin, a coastal megacity in China. Qin et al. (2017) suggested that the small SSAs found at Shijiazhuang indicated the presence of fine-mode absorbing particles, such as brown carbon. Zhuang et al. (2014) 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 µm³ per µm², 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 µm³ per µm², respectively. Therefore, the different SSA_440 nm distributions in the two regions may be attributed to the special aerosol composition related to the urban industrial 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 (Zhao et al., 2018). Anthropogenic emissions from seasonal biomass burning and residential heating are two other main factors that affect aerosol composition between the two regions (Che et al., 2018). 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 (Zhao et al., 2015). 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, Mai et al. (2018) 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.

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

3.4 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 semi-arid 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) in eastern 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 AOD₄₄₀ 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; Giles et al., 2012). 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).

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

3.5 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) 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, respectively. 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 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.

Figure 7Annual spatial distribution of direct aerosol radiative effect at the bottom of the atmosphere at the CARSNET 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, Che et al. (2018) 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 $=-\mathrm{11.34}$, AOD_440 nm=0.34, SSA =0.89) and at the Tongyu semi-arid site in northeastern China (DARE-TOA $=-\mathrm{8.87}$, 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.

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

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

3.6 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 Che et al. (2018), 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 rugged topography, which would promote particle mixing. The proportion of coarse-mode particles (mainly dust)with moderate to strong absorption (Group VII) was highest at the arid and semi-arid sites. The percent abundances of Group VII particles were 57.90 % at Dunhuang (AE =0.26, SSA_440 nm=0.85, FMF =0.43) and 58.52 % at Tazhong (AE =0.20, SSA _440 nm=0.87, FMF =0.37), respectively. Mixed absorbing particles (Type V) and strongly absorbing dust particles (Group VII) accounted for 30 % to 70 % of the aerosol in the rural sites on or near the CLP. The percentages of mixed absorbing particles (Type V) at Mt. Gaolan, Yulin, and Datong were 31.98 %, 45.22 %, and 29.04 %, respectively, and the average FMFs at those sites ranged from 0.70 to 0.76.

The proportions of the coarse-mode aerosols with strongly absorbing properties in Group VII was about 35.23 % at Mt. Gaolan and 21.21 % at Yulin, which was mainly dust particles, with the FMFs at those sites being 0.43 and 0.48, 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 urban 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.