Aerosol optical depth over the Tibetan Plateau and its relation toaerosols over the Taklimakan DesertXiangao Xia,1Pucai Wang,1Yuesi Wang,1Zhanqing Li,2Jinyuan Xin,1Jing Liu,1,3andHongbin Chen1Received 11 June 2008; revised 10 July 2008; accepted 21 July 2008; published 19 August 2008.[1] The Multiangle Imaging SpectroRadiometer (MISR)aerosol optical depths (AODs) over the Tibetan Plateau (TP)are compared with ground-based remote sensing data. Theresult shows that 27 out of 32 MISR AODs fall within theexpected uncertainty, i.e., 0.05 or 20%  AOD. The meanbias and the root mean square error between ground andsatellite AODs are 0.01 and 0.03, respectively. The 7-yearMISR AOD data are used to study seasonal and inter-annualvariations of AOD over the TP. The results show distinctseasonal variation, with seasonal AOD being 0.27, 0.25,0.13 and 0.11 from spring to winter. AOD over the TP isclosely related to that over the Taklimakan desert insummer; however poor correlation is observed in spring.Higher AOD in spring and summer over the TP meritsfurther study, including its causes and implications forclimate and environment.Citation: Xia, X., P. Wang,Y. Wang, Z. Li, J. Xin, J. Liu, and H. Chen (2008), Aerosoloptical depth over the Tibetan Plateau and its relation to aerosolsover the Taklimakan Desert, Geophys. Res. Lett., 35, L16804,doi:10.1029/2008GL034981.1. Introduction[2] The Tibetan Plateau (TP) is a vast and elevatedplateau in East Asia that extends over the area of 27° –45° N, 70° –105° E and its mean elevation is higher than4000 m above sea level. The influence of the TP uponatmospheric circulation and climate through its mechanicalas well as thermal forcing is early recognized [Yeh et al.,1957; Wu et al., 2007]. The elevated heating of the TP to theatmosphere plays a fundamental role in the formation andmaintenance of the Asian monsoon system [Wu et al., 2007].Much attention has been paid to atmospheric chemistry overthe TP since discoveries of Tibetan ozone valley [Zhou andLuo, 1994; Zou, 1996] and an ozone mini-hole over the TP[Bian et al., 2006]. Active upwelling motion over the TP isan important factor contributing to the ozone minimum.Large adiabatic expansion of air mass along with theupwelling motion is also favorable for particle formation[Tobo et al., 2007]. The heterogeneous chemistry on aerosolsurfaces is an important mechanism influencing ozone;more importantly, aerosol has widely been recognized asan important climate-forcing agent via its direct and indirecteffects [Wang et al., 2001].[3] In-situ measurements and ground-based remote sens-ing of aerosol have been carried out over the TP. Elementalcompositions of aerosol at sites such as, for example, inUdaoliang, Lasha, and Gongga, were measured [Zhang etal., 2000]. The average dust concentration at these sites was82 mgm3, which was about 30% and 48% of that over theChinese deserts and over the Loess Plateau, respectively.The concentrations of S and Pb in Mount Qomolangmaregion (28.19° N, 86.83° E) are 91.64 ng m3and 2.93 ngm3, respectively, indicating this region is rarely influencedby human activities and long-range transportation [Zhang etal., 2001]. Measurements of vertical profiles of aerosolsconcentration using a balloon-borne optical particle counterat Lasha showed number concentration of sub-micron sizeaerosols with radii of 0.15 0.16 was 0.7–0.8 particlescm3near the tropopause region (130– 70 hPa) [Tobo et al.,2007]. Short-term ground-based remote sensing aerosoloptical depth i n Lasha, showed that the daily averageaerosol optical depth (AOD) at 500 nm was less than 0.2during summer of 1998 [Bai et al., 2000]. AOD at 600 nmmeasured at Dangxiong (30.50° N, 91.10° E) during Mayand June 1998 ranged from 0.02 to 0.12, with mean valuesbeing about 0.08 [Zhang et al., 2000]. The Cloud-AerosolLidar and Infrared Pathfinder Satellite Observations (CAL-IPSO) satellite data, such as column-averaged volumedepolarization and total volume color ratios, suggested thatin summer frequent dust events occurred over remotenorthwestern Tibet where surface observation was verylimited due to high elevation and harsh climate [Huang etal., 2007]. Surface observation showed the annual days ofdust storms were about 20 over the west Tibet region [Bai etal., 2006].[4] Satellite remote sensing is able to observe large,inaccessible high plateau regions. The objective of thisstudy is to present seasonal and inter-annual variations ofaerosol optical depth (AOD) using the Multiangle ImagingSpectroRadiometer (MISR) aerosol retrievals from May2000 to May 2007. While spatio-temporal characteristicsof aerosol over the TP were derived from StratosphericAerosol and Gas Experiment II (SAGE II) data [Li and Yu,2001] and the Moderate Resolution Imaging Spectroradi-ometer data [Jin, 2006], current study differs in severalways. First, ground-based remote sensing data over the TPare used to validate the MISR aerosol retrievals at the firsttime. Second, only satellite aerosol retrievals over the TPare analyzed (27 ° –39° N, 76° –105° E and elevation>3000 m). Third, the potential influence of dust stormsGEOPHYSICAL RESEARCH LETTERS, VOL. 35, L16804, doi:10.1029/2008GL034981, 2008ClickHereforFullArticle1Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing, China.2Department of Atmospheric and Oceanic Science and Earth SystemScience Interdisciplinary Center, University of Maryland, College Park,Maryland, USA.3College of Environment Science and Engineering, Nanjing Universityof Information Science and Technology, Nanjing, China.Copyright 2008 by the American Geophysical Union.0094-8276/08/2008GL034981$05.00L16804 1of5originated in Taklimakan Desert on AOD over the TP isdiscussed.2. Data[5] Ground-based remote sensing aerosol data employedin this study were acquired by the LED (light-emittingdiode) hazemeter at two Chinese Ecological ResearchNetwork (CERN) sites over the TP since August 2004.One is Lhasa (91.33° E, 29.67° N) and the other is Haibei(101.32° E, 37.45° N). The AODs at four wavelengths (405,500, 650 and 880 nm) are derived from hazemeters measure-ments from 10:00 to 14:00 local time. The hazemeter resultsare generally in good agreement with the CIMEL resultswith discrepancies on the order of 2% to 6% [Xin et al.,2007].[6] MISR is a push-broom camera instrument measuringthe same point on Earth at 9 different along-track viewangles (with 4 forward, one nadir and 4 aft cameras), and