Investigation of Aerosol Optical Depth (AOD) and Ångström Exponent Over

Investigation of Aerosol Optical Depth (AOD) and Ångström Exponent over the Desert Region of Northwestern China Based on Measurements from the China Aerosol Remote Sensing Network (CARSNET)

Jie Yu1,2, Huizheng Che2,3*, Quanliang Chen1, Xiangao Xia4,5, Hujia Zhao6, Hong Wang2, Yaqiang Wang2, Xiaoye Zhang2, Guangyu Shi7

1 Plateau Atmospheric and Environment Key Laboratory of Sichuan Province, College of Atmospheric Sciences, Chengdu University of Information Technology, Chengdu 610225, China 2 Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition, Chinese Academy of Meteorological Sciences, CMA, Beijing 100081, China 3 Jiangsu Collaborative Innovation Center of Climate Change, Nanjing, 210093, China 4 Laboratory for Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 5 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China 6 Institute of Atmospheric Environment, China Meteorological Administration, Shenyang 110016, China 7 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China

ABSTRACT

Aerosols at ten sites in northwestern China are classified in this study: (1) by using the aerosol optical depth (AOD), the Ångström exponent (α) and the Ångström exponent difference (δα); and (2) by using the total means of AOD440nm and α. The seasonal variations of the AOD and α show that the maximum AODs occur in spring, except at Urumqi and Lanzhou. The seasonal mean α values are lower than 0.80 in all four seasons at Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, and Jiuquan, but higher than 0.80 in winter at Urumqi, Lanzhou and Yinchuan. The first classification method shows that coarse mode particles are found at all ten sites, but that fine mode growth only happens at Urumqi, Lanzhou, and Yinchuan. The relationship between AOD440nm and α show that α smaller than 0.80 decrease with increasing AOD440nm at all ten sites.

Aerosols are classified into four types (Type I–IV) according to the total mean τ440 ( 440 ) and total mean Ångström exponent ( ) of each site. Aerosols with a τ440 smaller than  440 , but greater than or equal to  (τ440 <  440 ; α ≥  ) are classified as Type I; aerosols with τ440 ≥  440 and α ≥  are Type II; those with τ440 <  440 and α <  are Type III; and those with τ440 ≥  440 and α <  are Type IV. The second aerosol classification method shows that Type I and Type III aerosols are the most common at all ten sites. Type II aerosols are the least at Tazhong and Hotan, but are the most common at Urumqi, Lanzhou, and Yinchuan. On the contrary, Type IV aerosols are the most common at Tazhong and Hotan, but are the least common at Urumqi, Lanzhou and Yinchuan.

Keywords: Aerosol optical depth (AOD); Ångström exponent (α); Aerosol classification; Northwestern China.

INTRODUCTION affect the earth–atmosphere radiative balance (Ackerman and Toon, 1981). They also serve as cloud condensation nuclei, Atmospheric aerosols play an important role in global thereby affecting the size distribution of cloud droplets in and regional climate change (Charlson et al., 1992). Through an indirect way (Twomey et al., 1984). Despite many aerosol scatting or absorbing solar radiation, aerosol particles directly studies, aerosol concentrations and optical properties remain one of the largest sources of uncertainty in current assessments and predictions of global climate change (Hansen et al., 2000). The aerosol optical depth (AOD) and * Corresponding author. Ångström exponent (α) are two basic optical parameters Tel.: 86-10-58993116; Fax: 86-10-62176414 necessary for climate change research (Breon et al., 2002). E-mail address: [email protected] There are numerous studies on aerosol optical properties of

Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 2025 several regions, including North China, the northeast, the sunphotometer measurements to investigate the detailed northwest, the Yangtze River Region and the South of China aerosol optical properties through aerosol classification by (Xia et al., 2005; Garland et al., 2008; Che et al., 2009a; different methods at ten sites in northwestern China. This Pan et al., 2010; Wang et al., 2010a; Che et al., 2011; Liu research will help to validate the satellite observations and et al., 2011; Zhao et al., 2013; Che et al., 2014; Tao et al., improve estimations of the effect of East Asian dust aerosols 2014). on global and regional climate change. This paper includes Dust aerosols are the mainly important aerosol particles site distribution, instruments and data first. Then, seasonal in East Asia. Dust aerosols from arid and semi-arid regions variation of AOD440 and the Ångström exponent are analyzed. are transported thousands of kilometers away from their Aerosol classification by the AOD670, Ångström exponent original source regions (Gong et al., 2003). The global dust and Ångström exponent difference and aerosol classification –1 emission is estimated about 1500 Tg yr (Tegen and Fung, by total mean AOD440 and total mean Ångström exponent 1995). Nearly 800 Tg yr–1 of the dust emission is emitted into are discussed. Finally, the summary and the discussion are the atmosphere each year, half of which is deposited close to conducted. the source and adjacent regions, while the rest is transported to the remote Pacific Ocean (Zhang, 2001). Thus, it is necessary SITE DISTRIBUTION, INSTRUMENTS AND DATA to study the optical properties, and the temporal and spatial variability of dust aerosols in order to accurately estimate Site Distribution the influence of dust aerosols on global and regional climate Sun-photometers (CE318, Cimel Electronique, France) change (Wang et al., 2006, 2010b). are installed at ten observation sites in northwestern China: The arid and semi-arid regions in northwestern China Tazhong, Hotan, Hami, Urumqi, Ejina, Dunhuang, Minqin, are major sources of dust aerosols in East Asia (Zhang et Jiuquan, Lanzhou, and Yinchuan, as shown in Table 1 and al., 2003). Many researchers have studied the characteristics Fig. 1. Tazhong, Hotan, Hami, Urumqi are in the Xinjiang of dust aerosols in East Asia in recent years. Alfaro et al. Province, Ejina is in the Inner Mongolia Province, Dunhuang, (2003) observed aerosol optical characteristics in spring Minqin, Jiuquan, Lanzhou are in the Gansu Province and 2002 at the ACE-Asia supersite (Aerosol Characterization Yinchuan is in the Ningxia Province. Among the ten sites, Experiments, Zhenbeitai, China) and found that dust optical Tazhong and Hotan are located in the Taklamakan Desert, characteristics measured during dust storms were one of the largest sand deserts in the world. The desert representative of pure dust emitted from the northwestern covers an area of 270,000 km2, and includes of the Tarim high desert sources. Xia et al. (2005) showed that AOD over Basin, which is 1000 km long and 400 km wide (Huang et North China in spring was dominated by contributions from al., 2009). It is regarded as one of the largest sources of Asian dust over western China. Dust aerosols not only enhance Aeolian dust aerosol (Mikami et al., 2006). Hami, Ejina, the aerosol loading but also reduce light absorption. Cheng Dunhuang, Minqin, and Jiuquan are all located in arid and et al. (2006) indicated that high AOD corresponded to dust semi-arid regions over northwestern China, where aerosols event occurrence, while the Ångström exponent decreased are dominated by dust aerosol particles. However, Urumqi, with increasing AOD to zero or negative values, when very Lanzhou and Yinchuan are located in the center of cities, dusty events occurred in the Hunshan Dake desert. Huang where aerosols mainly come from fine particle pollution. et al. (2009) determined that both shortwave and longwave The observation times at ten sites are from 2002 to 2012 radiative forcing of dust aerosols played an important role in and the details of the valid data are listed in Table 1. the radiative energy budget, at both the top of the atmosphere and the surface. Xia and Zong (2009) demonstrated that Instruments and Data Earth’s system was cooled in the shortwave but warmed in The China Aerosol Remote Sensing Network (CARSNET) the longwave by Taklamakan dust aerosols. These studies are is a ground-based aerosol monitoring network, established essential to improve understanding of the essential properties by the China Meteorological Administration in 2002. and variations of dust aerosols in East Asia. CARSNET is a routine operation network for the study of The objective of this research is to use the ground-based aerosol optical property over the different areas in China

Table 1. Details for each site. Site (city) Location Altitude Running time Valid data Tazhong (Xinjiang) 83°40′E, 39°00′N 1099.3 m 2004.01–2012.04 60381 Hotan (Xinjiang) 79°56′E, 37°08′N 1374.6 m 2002.05–2005.03 21429 Hami (Xinjiang) 93°31′E, 42°49′N 737.2 m 2002.04–2005.03 22604 Urumqi (Xinjiang) 87°37′E, 43°47′N 935 m 2002.04–2012.04 48435 Ejina (Inner Mongolia) 101°04′E, 41°57′N 940.5 m 2002.05–2012.04 66456 Dunhuang (Gansu) 94°41′E, 40°09′N 1140 m 2002.06–2011.12 63171 Minqin (Gansu) 103°05′E, 38°38′N 1367 m 2004.02–2012.04 41122 Jiuquan (Gansu) 98°29′E, 39°46′N 1477.2 m 2002.04–2005.03 14079 Lanzhou (Gansu) 103°53′E, 36°03′N 1517.2 m 2002.07–2012.04 61947 Yinchuan (Ningxia) 106°13′E, 38°29′N 1111.4 m 2002.05–2004.08 9416

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Fig. 1. Geographic distribution of the sun-photometers at the ten sites in northwestern China (Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, Jiuquan, Urumqi, Lanzhou, and Yinchuan). and for the validation of satellite aerosol retrievals (Che et quality and reliability. al., 2009b). The instrument used by CARSNET is automatic The valid data at the ten sites are calculated by the Cimel sun and sky scanning radiometer (Cimel Electronique ASTPwin software (Cimel Ltd. Co.) for the Level 1.0 AOD Cimel-318), the same instrument as the Aerosol Robotic (raw result without cloud screening), the Level 1.5 AOD Network (AERONET). The CE-318 sun-photometer has a (cloud-screened AOD based on the work of Smirnov et al., 1.2° full field-of-view and eight channels: four observation (2000)) and the Ångström exponent between 440 and 870 channels at 440 nm, 670 nm, 870 nm and 1020 nm; three nm. The aerosol optical depth at wavelength λ (AODλ ≡ τλ) 870 nm polarization channels; and a 940 nm water vapor represents the extinction of radiation of wavelength λ. The channel (Holben et al., 1998). Measurements at 440 nm, Ångström exponent α represents the slope of the wavelength 670 nm, 870 nm and 1020 nm are used to retrieve the dependence of the AOD in logarithmic coordinates AOD, and measurements at 940 nm are used to obtain the (Angstrom, 1929): α(λ1, λ2) = –ln(τλ2/τλ1)/ln(λ2/λ1). Four total precipitable water content in cm (Holben et al., 1998). seasons are defined as: spring (March to May), summer The total uncertainty in optical depth is about 0.01–0.02 (June to August), autumn (September to November) and (Eck et al., 1999). winter (December to February) to investigate the seasonal The inter-comparison calibration protocol for the variation of aerosol optical properties. CARSNET field instruments were as follows: (a) only raw data collected from 02:00 to 6:00 AM (GMT+8:00) on RESULTS AND ANALYSIS clean and clear days were used, (b) the AODs at 500 nm on calibration days had to be less than 0.20 and without major Seasonal Variation of AOD at 440 nm and the Angström fluctuations, (c) the time intervals between the measurements Exponent made with two masters and the instruments to be calibrated Fig. 2 illustrates the seasonal variation of the AOD and had to be less than 10 s. The AODs obtained from un- Ångström exponent at ten sites in northwestern China. The calibrated instruments differed by 4.5% to 15.3% compared mean AOD values at Tazhong and Hotan in spring and with those measured by reference instruments. After the summer (more than 0.50) are higher than those in autumn calibration with the master instruments, however, the daily and winter. Tazhong and Hotan are both located in the average AODs differed by < 1.5% relative to the master Taklamakan Desert, and dust events are very frequent during measurements (Che et al., 2009b). According to Holben et spring and early summer, causing large aerosol loading in al. (1998), yearly calibrations of the field instruments ensured the atmosphere over the region (Xue et al., 2009). In contrast, the accuracy of the CARSNET measurements, and therefore, there are few dust events during autumn and winter. Fine the AODs from the 10 stations in this study were of high particles (such as black carbon) from anthropogenic

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Fig. 2. Seasonal box plots of the AOD and Ångstrom exponent at the ten sites (The extreme “” means the maximum and minimum value; the “×” means 99% and 1% percentile value; the“□” means the mean value).

2028 Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 activities add to the aerosol burden (Li et al., 2010), but the In these coordinates, the aerosol particles are further classified anthropogenic aerosols are mainly transported from outside by representing their AODs in different colors. A cloud sources. Compared with mineral dust in spring and summer, contamination or an increase in coarse aerosols of the AOD the anthropogenic loading during autumn and winter is larger than 90% will be located at α~δα~0. However, much less. The seasonal variation of AOD at Hami, Ejina, hydration moves in the opposite direction with respect to Dunhuang, Minqin, and Jiuquan is similar. The mean AOD cloud contamination, leading to a growth in both the size values are lower than 0.50 and the maximum values occur in and the fine mode extinction fractions (η). Therefore, it allows spring at these five rural sites, reflecting the effect of floating for easy identification between fine mode growth and coarse or blowing dust events (Wang et al., 2005). However, the mode particle contamination. maximum mean AODs occur in winter at Urumqi and Gobbi et al. (2007) performed Mie calculations of the Lanzhou; in spring at Yinchuan. The higher AODs occur in aerosol spectral extinction for fine mode particles (Rf) of spring at the three urban sites, corresponding to frequent 0.05, 0.1, 0.15, 0.2, 0.3 and 0.5 µm, for coarse mode particles local or regional dust events in northwestern China. The (Rc) of 0.75, 1, 2, and 4 µm, and combined them to provide maximum mean AODs at Urumqi and Lanzhou may be the η values of 1, 10, 30, 50, 70, 90 and 99%. Both the fine caused by the worse atmospheric diffusion conditions. Cao and coarse mode particles are assumed to have a log- et al. (2013) pointed out that the pollution emissions are normal size distribution, respectively. According to Gobbi very high in Urumqi and Lanzhou during winter season et al. (2007), the maximum Rf and η indeterminations are because of the coal combustion for warm-keeping. The of ± 25% and ± 10% for refractive index varying between m seasonal variation of AOD at Yinchuan is similar to those = 1.33–0.00i (water droplets) and m = 1.53– 0.003i (mineral five rural sites. Because Yinchuan is located between the Gobi dust) for a given point (α, δα). And the graphical approach desert in the north and the Loess Plateau in the southeast, dust for refractive index of m = 1.40–0.001i is robust enough to particles are possibly transported when either northerlies or provide an operational classification of most common aerosol south-easterlies are prevalent (Kim et al., 2004). typologies (e.g., Dubovik et al., 2002) on the basis of standard Eck et al. (2005) showed that an Ångström exponent less photometric observations. than 0.80 indicates coarse mode aerosol dominance. The mean The above aerosol classification method is applied at the Ångström exponent values at Tazhong and Hotan are less ten sites in northwestern China in this study to analyze the than 0.60 throughout the year (Fig. 2), suggesting that coarse aerosol optical properties using the instantaneous mode aerosol strongly dominates the AOD at the two desert observations. Fig. 3 shows simulations of the classification sites. The seasonal variation of the Ångström exponent at of the aerosol properties at the ten sites as a function of the Tazhong and Hotan shows characteristically low values in Ångström exponent α(440, 870) and the difference δα = spring and summer and higher values in autumn and winter, α(440, 675) – α(675, 870), for bimodal lognormal size indicating that the aerosol particles are larger in spring and distributions with a refractive index of 1.40–0.001i. summer than in autumn and winter. The mean Ångström The further classification of AODs in different colors is exponent values at the rural sites of Hami, Ejina, Dunhuang, showed as follows: the black hollow circle, 0.15 < AOD ≤ Minqin and Jiuquan are lower than 0.80 in four seasons, 0.30; the red hollow circle, 0.30 < AOD ≤ 0.40; the green indicating that coarse mode aerosol dominates the AOD. The hollow circle, 0.40 < AOD ≤ 0.70; the blue solid dot, 0.70 seasonal variation of the Ångström exponent at these seven < AOD ≤ 1.00, the cyan solid dot, 1.00 < AOD ≤ 1.50, the sites is similar to the results of Che et al. (2013) at Tazhong. magenta solid dot, 1.50 < AOD ≤ 2.00 and the yellow solid However, the mean Ångström exponent values are larger than dot, 2.00 < AOD ≤ 3.00. The black solid lines are each for 0.80 year-round at Urumqi, and larger than 0.80 in summer, a fixed size of the fine mode, Rf , and the dashed blue lines autumn and winter at Lanzhou and Yinchuan (smaller than indicate a fixed fraction contribution (η) of the fine mode 0.80 in spring), suggesting that fine particles mostly contribute to the total AOD at 670 nm. In many cases the higher AODs to the composition of aerosol at the three urban sites. are associated with coarse mode particles (α~δα~0, η < 30%) due to dust events at Tazhong and Hotan: the two sites Aerosol Classification by the AOD at 670 nm, Ångström are located in the Taklamakan Desert, where the aerosols are Exponent and Ångström Exponent Difference dominated by dust coarse mode particles. The higher AODs Kaufman (1993) showed that negative values of the are mainly clustered at the slope of α and δα about 0, Ångström exponent difference, δα, equal to α(440, 613) indicating that the effects of coarse mode particles at Hami, –α(440, 1003), indicate the dominance of fine mode aerosols, Ejina, Dunhuang, Minqin, and Jiuquan are prominent. It while positive differences indicate the effect of two separate suggests that the five rural sites are impacted by dust aerosols particle modes. Gobbi et al. (2007) built on this concept from northwestern China. Ejina has “typical pollution” with and proposed a new aerosol classification method, to track 1.00 > AOD > 0.70 (blue dots) that corresponds to a fine mixtures of pollution aerosol with dust, to distinguish fraction of 99% > η > 70% moving along the black line of aerosol growth from cloud contamination, and to observe 0.15 µm, different from the other four rural sites. It may be aerosol humidification. The method defines the Ångström associated with cloud contamination (Gobbi et al., 2007). exponent difference δα = α(440, 675) –α(675, 870) as a Fine mode growth is evident at Urumqi and Lanzhou, but less measure of the Ångström exponent curvature with respect so at Yinchuan. The observed data at Urumqi and Lanzhou to wavelength, λ: dα/dλ. The δα vs. α (440, 870) space is (AOD > 0.70) are mainly clustered in the fine mode growth plotted as the framework for analyzing aerosol properties. wing (α < 1.50, δα < 0). The extension of the two urban

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Fig. 3. Ångström exponent difference, α = α(440, 670) – α(670, 870), as a function of the 440–870 nm Ångström exponent and AOD for ten CARSNET stations (from top): Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, Jiuquan, Urumqi, Lanzhou, and Yinchuan. Only cloud-screened data with AOD670 > 0.15 were used.

2030 Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 sites measures to higher AODs happen perpendicularly to (comprising 35.89% of all aerosol found there) and Hotan the black line, into larger size of the fine mode and fine (32.03%), but Type II aerosols are the least common fraction mainly between 60% and 90%. At the same time, (Tazhong: 0.87%, Hotan: 2.08%) at all ten sites. This suggests coarse particles such as mineral dust (α~δα~0, η < 30%), that there are more dust events and fewer anthropogenic add their signal to the pollution signature in Urumqi and activities in and around Tazhong and Hotan than any of the Lanzhou. Hence, it can be inferred that the high extinction other eight sites. This pattern is similar to those observed in at Urumqi and Lanzhou could be linked to a hygroscopic Sapporo, Japan (Aoki and Fujiyoshi, 2003). Type II and Type and/or coagulation growth from aging of the fine mode IV aerosols are medium common at Hami (9.06% and aerosols, and dust storm events. The results are similar to 23.57%), Ejina (10.03% and 20.96%), Dunhuang (5.47% and those observed in Beijing and Kanpur, India (Gobbi et al., 27.74%), Minqin (11.44% and 24.14%), and Jiuquan 2007). However, the effect of the coarse mode particles and (8.74% and 23.43%). This indicates that the aerosols at the the fine mode growth are less evident at Yinchuan, whose five rural sites are affected by both dust events and mean size is smaller than the other two urban sites. It suggests anthropogenic activities. This result is similar to those that anthropogenic activities are frequent at Urumqi and found in Tsukuba (Aoki and Fujiyoshi, 2003). Type IV Lanzhou, and infrequent at Yinchuan. aerosols are less common and Type II aerosols are more common at Urumqi, Lanzhou, and Yinchuan. This suggests Aerosol Classification by Total Mean AOD at 440 nm and that the effect of the dust events is smaller and anthropogenic Total Mean Ångström Exponent activities are more frequent. The result is similar to those Fig. 4 shows the relationship between AOD at 440 nm calculated in Tokyo, Japan (Aoki and Fujiyoshi, 2003). (AOD440) and Ångström exponent at ten sites. The vertical Fig. 5 shows the frequency distributions of the four types of and horizontal blue dashed lines represent the total mean aerosols at ten sites. Type I and Type IV aerosols have AOD and Ångström exponent, respectively. The plots show similar seasonal variations at Tazhong and Hotan. Type I the mean AOD(τ440) (the red solid circles) with the standard aerosols are the most common in later autumn and winter, deviations (red error bars) at different Ångström exponents but Type IV aerosols are common in spring and summer. It of –0.5–0.0, 0.0–0.5, 0.5–1.0, 1.0–1.5 and 1.5–2.0. The mean suggests that the atmosphere is clean in later autumn and AODs and standard deviations are also listed in Table 2. winter but dust particles are dominant in spring and summer at The relationship between the AOD and Ångström exponent Tazhong and Hotan due to the contribution of the frequent shows that Ångström exponents of smaller than 0.80 decrease dust events. Type II aerosols appear infrequently at both with increasing AOD440 at all ten sites, indicating that Tazhong and Hotan with frequency occurrence less than 15%, coarse particles, e.g., mineral dust, are responsible for large which suggests the anthropogenic activities contribute few AOD. Aerosols are classified into four types (Type I–IV) effect at the two sites. Type I aerosols are common in later according to the total mean τ440 ( 440 ) and total mean autumn and winter at Ejina, Dunhuang, Minqin and Jiuquan, Ångström exponent ( ) of each site, as in Table 3. Type I but in summer at Hami. The difference between Hami and the aerosols are defined as those with τ440 smaller than the total other four sites may be caused by relative large precipitation mean τ440, and α larger than the total mean α (i.e., τ440 < in June and July (Tu and Kong, 2014). Type IV aerosols are

 440 ; α ≥  ). These conditions correspond to relatively common in spring at the five sites, and the frequency of small particles with small optical depths, and capture a appearance is about 25–60%. This indicates the influence large range of the basic background aerosols. Type II aerosols of dust events is dominant at these sites as well as Tazhong are characterized by a τ440 larger than the total mean τ440, and Hotan. Type II aerosols also appear less than 22% at and an α larger than the total mean α (τ440 ≥  440 ; α ≥  ): the five sites except for Hami ~40% in January. This indicates i.e., relatively small particles with a large optical depth, that anthropogenic aerosol emissions and subsequent gas-to- representing the emission of anthropogenic aerosol and the particle processes likely lead to the high mean AOD values corresponding gas-to-particles conversion processes. Type in winter at Hami (Tu and Mu, 2010). Type I aerosols are III aerosols have a τ440 smaller than the total mean τ440, and common in summer at three urban sites of Urumqi, Lanzhou, an α smaller than the total mean α, (τ440 <  440 ; α <  ): and Yinchuan with the occurrence about 20–55%, which i.e. large particles with a small optical depth. This is related suggests good air qualities at three sites in summer season. to local characteristics. Type IV aerosols have a τ440 larger Type II aerosols are obviously common (> 48%) during than the total mean of τ440, and an α smaller than the total November to January at Urumqi and Lanzhou, which mean of α, (τ440 ≥  440 ; α <  ): corresponding to large suggests air pollutions from anthropogenic activities (Li et particles with a large optical depth, they are often associated al., 2005; Cao et al., 2013). Type IV aerosols are 20%– with local dust or dust events. Note that this type classification 50% in spring at three urban sites which are mainly caused does not necessarily correspond to species classification of by the dust events (Tao et al., 2009). The seasonal variations individual aerosols: these types are instead useful for of Type III aerosols are different, which suggests local characterizing atmospheric conditions (Aoki and Fujiyoshi, characteristics are different at each of ten sites. 2003). From the percentage ratios of the four types in Table 3, we see that Type I and Type III aerosols are the most SUMMARY AND DISCUSSION common at all ten sites. They represent the condition of background aerosols and the effect of the local characteristics, The maximum mean AOD values occur in spring at respectively. Type IV aerosols are most common at Tazhong Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin, Jiuquan,

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Fig. 4. Relationship between the AOD at 440 nm and the Ångström exponent at ten sites.

2032 Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 and Yinchuan, reflecting the effect of dust events, floating are higher in spring and winter at Urumqi and in spring, dust and blowing dust events occurring in arid and semi-arid autumn and winter at Lanzhou, which reflects the hybrid regions over northwestern China. However, the mean AODs effect of the dust events and anthropogenic activities.

Table 2. The mean and standard deviation (St. dev.) of AOD at different Ångström exponents of –0.5–0.0, 0.0–0.5, 0.5– 1.0, 1.0–1.5 and 1.5–2.0 at the ten study sites. Tazhong Hotan Hami Ejina Dunhuang Minqin Jiuquan Urumqi Lanzhou Yinchuan –0.5–0 1.138 0.795 0.992 0.306 0.776 0.748 1.302 0.582 1.380 1.500 St. dev. 0.399 0.516 0.556 0.339 0.474 0.528 0.425 0.593 0.256 0.200 0–0.5 0.538 0.555 0.296 0.276 0.359 0.411 0.340 0.447 0.872 0.678 St. dev. 0.347 0.389 0.216 0.248 0.278 0.273 0.258 0.323 0.389 0.396 0.5–1.0 0.188 0.335 0.183 0.184 0.195 0.321 0.217 0.343 0.668 0.427 St. dev. 0.086 0.130 0.093 0.115 0.101 0.214 0.112 0.249 0.280 0.205 1.0–1.5 0.177 0.412 0.190 0.184 0.195 0.306 0.292 0.376 0.824 0.487 St. dev. 0.083 0.134 0.140 0.155 0.110 0.223 0.167 0.287 0.344 0.213 1.5–2.0 0.139 - 0.230 0.244 0.114 0.219 0.403 0.352 0.690 0.537 St. dev. 0.094 - 0.201 0.284 0.045 0.151 0.279 0.288 0.232 0.127

Table 3. Percentage of each aerosol type at the ten study sites. Tazhong Hotan Hami Ejina Dunhuang Minqin Jiuquan Urumqi Lanzhou Yinchuan Type I 33.46 33.06 36.32 37.69 37.81 29.97 33.51 34.90 30.12 34.49 Type II 0.87 2.08 9.06 10.03 5.47 11.44 8.74 16.57 26.51 21.37 Type III 29.79 32.83 31.05 31.33 28.98 34.46 34.31 31.85 27.90 28.23 Type IV 35.89 32.03 23.57 20.96 27.74 24.14 23.43 16.69 15.46 15.91

Fig. 5. Monthly frequency of appearance τ440 and α for each type (Type I–Type IV) at ten sites.

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Fig. 5. (continued).

The mean Ångström exponent values are lower than 0.80 at about 0 at all ten sites, which indicates the presence of coarse Tazhong, Hotan, Hami, Ejina, Dunhuang, Minqin and mode particles. Only at Urumqi, Lanzhou, and Yinchuan, Jiuquan, which suggests coarse mode aerosols dominate is fine particles growth found. the AOD at these seven sites. However, the mean Ångström Aerosol classification by total mean AOD at 440 nm and exponent values are larger than 0.80 year-round at Urumqi, total mean Ångström exponent shows Type I aerosols are larger than 0.80 in summer, autumn and winter at Lanzhou dominant in later autumn and winter seasons at rural sites and Yinchuan, which indicates fine particles are the of Tazhong, Hotan, Ejina, Dunhuang, Minqin and Jiuquan, primary contributors to aerosols at the three urban sites. but in summer at urban sites of Urumqi, Lanzhou, and Aerosol classification by the AOD at 670 nm, Ångström Yinchuan. Type II aerosols are mostly less than 20% at exponent and Ångström exponent difference show that the rural sites but larger than 48% at Urumqi and Lanzou in higher AODs are mainly clustered at the range of α and δα later autumn and winter. Type III aerosols show different

2034 Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 seasonal variation characteristics due to the different local Profiles and Optical Depth Using Lidar Measurement over characteristics. Type IV aerosols are dominant at all ten Lanzhou, China since 2005–2008. J. Quant. Spectrosc. sites in spring season because of the effect of dust events. Radiat. Transfer 122: 150–154. Combining two kinds of aerosol classification methods, Charlson, R.J., Schwartz, S.E., Hales, J.M., Cess, R.D., we can easily understand the aerosol characteristics at the Coakley, J.A., Hansen, J.E. and Hofmann, D.J. (1992). ten sites of Northwestern China. From the second aerosol Climate Forcing by Anthropogenic Aerosols. Science classification method, we can know the dominant aerosol 255: 423–430. types at each site. Type IV aerosols are common at Tazhong, Che, H., Yang, Z., Zhang, X., Zhu, C., Ma, Q., Zhou, H. Hotan, Hami, Ejina, Dunhuang, Minqin and Jiuquan, which is and Wang, P. (2009a). Study on the Aerosol Optical associated with many high AOD cases caused by coarse Properties and Their Relationship with Aerosol Chemical mode particles (α~δα~0, η < 30%). However, Type II aerosols Compositions over Three Regional Background Stations are not common at above sites, which is associated with in China. Atmos. Environ. 43: 1093–1099. few fine mode growth because of less frequent anthropogenic Che, H., Zhang, X., Chen, H., Damiri, B., Goloub, P., Li, activities. At three urban sites of Urumqi, Lanzhou and Z., Zhang, X., Wei, Y., Zhou, H., Dong, F., Li, D. and Yinchuan, Type II aerosols are common and the fine particles Zhou, T. (2009b). Instrument Calibration and Aerosol growth phenomena are obvious due to the frequent Optical Depth Validation of the China Aerosol Remote anthropogenic activities. Moreover, Type IV aerosols are Sensing Network. J. Geophys. Res. 114: D03206, doi: also common especially in spring season caused by coarse 10.1029/2008JD011030. mode particles. Che, H., Wang, Y. and Sun, J. (2011). Aerosol Optical Properties at Mt. Waliguan Observatory, China. Atmos. ACKNOWLEDGMENTS Environ. 45: 6004–6009. Che, H., Wang, Y., Sun, J., Zhang, X., Zhang, X. and Guo, This work is financially supported by grants from the J. (2013). Variation of Aerosol Optical Properties over National Key Project of Basic Research (2014CB441201), the Taklimakan Desert in China. Aerosol Air Qual. Res. the Project (41375153) supported by NSFC, the project of 13: 777–785. Institute of Atmospheric Environment Programme (2014IAE- Che, H., Shi, G., Zhao, H., Nakajima, T., Khatri, P., CMA05), the Strategic Priority Research Programme of the Takamura, T., Wang, H., Wang, Y., Sun, J. and Zhang, Chinese Academy of Sciences (XDA05100301), the X. (2014). Aerosol Optical Properties Retrieved from a CAMS Basis Research Project (2014R17) and the Climate Prede Sky Radiometer over an Urban Site of Beijing, Change Special Fund of CMA (CCSF201504). The authors China. J. Meteorolog. 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