Aerosol and Air Quality Research, 15: 2024–2036, 2015 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2014.12.0326 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 2026 Yu et al., Aerosol and Air Quality Research, 15: 2024–2036, 2015 Fig.