Natural and anthropogenic contributions to the variability of aerosol optical depth in arid and semi-arid areas 后印本

摘要: Aerosol optical depth (AOD) is an important indicator for evaluating atmospheric pollution. Xinjiang Uygur Autonomous Region, China and its surrounding areas including Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, and Tajikistan are well known as the typical arid and semi-arid areas mainly affected by dust aerosols, but the researches of different aerosol types, particularly the anthropogenic emissions are not enough. Moreover, the applicability of Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis AOD data remains unclear and the long-term trends of AOD in the study area have not been thoroughly investigated. Therefore, the accuracy of MERRA-2 reanalysis AOD data was evaluated against Aerosol Robotic Network (AERONET) observations. Subsequently, the contributions of five aerosol types (dust, sulfate, organic carbon, black carbon, and sea salt) derived from MERRA-2 AOD data from 2012 to 2022 were qualitatively and quantitatively explored. Results showed that AOD based on MERRA-2 reanalysis exhibited reasonable agreement with 19 AERONET sites (R2=0.49; bias=–0.07), confirming its regional reliability. The overall trends of MERRA-2 AOD revealed that aerosol concentrations in the Taklimakan Desert showed a significant increase (Theil-Sen slope ≥0.005; standardized test statistic (Z) ≥1.96) and higher variability (standard deviation (SD)>0.05), mainly owing to strong influence of natural disturbances, such as dust transport events. Conversely, low AOD variability (SD<0.01) appeared in the high-altitude and sparsely populated areas, such as eastern Tajikistan. The contributions of five aerosol types showed that dust AOD was the highest (0.120), followed by sulfate (0.067) and organic carbon (0.022), whereas black carbon and sea salt contributed relatively little, with values of 0.006 and 0.003, respectively. Spatially, dust AOD exhibited significant variation and gradually decreased from south to north, with high values mainly concentrated in the Taklamakan and Karakum deserts, whereas northern Kazakhstan was weakly affected by dust aerosols (AOD<0.060), but showed relatively high sulfate aerosol loading (AOD>0.100), mainly associated with anthropogenic emissions. From the perspective of land use types, spring dust aerosols made the largest contribution under the influence of dust sources of arid areas, with the highest contributions observed in barren land (75.28%; AOD=0.255), followed by cropland (55.21%; AOD=0.128), grassland (53.45%; AOD=0.150), and forestland (34.17%; AOD=0.068). Although greatly affected by dust and organic carbon at four seasons, sulfate remained the dominant aerosol type in forestland, driven by long-distance transport of industrial emissions, with percentages exceeding 46.00% (AOD>0.080). These findings indicate that aerosol variability in Xinjiang Uygur Autonomous Region, China and its surrounding areas is jointly controlled by natural dust emissions, anthropogenic sulfate inputs, and land-use-dependent seasonal processes, highlighting the importance of distinguishing natural and anthropogenic aerosol contributions in regional air quality and climate assessments.