Temporal evolution of aerosols and their extreme events in polluted Asian regions during Terra's 20-year observations

Abstract

Aerosol pollution is an acute environmental issue in developing countries. Asia has been experiencing rapid changes in anthropogenic aerosols during the past two decades due to fast growth in population and economy. It is still an open question how aerosol loadings, represented by aerosol optical depth (AOD), have evolved in this century, particularly during the past decade when China and India implemented a clean air act aiming to improve air quality. Based on Terra aerosol retrievals and aerosol reanalysis, a change point of AOD trend is detected at 2010 in East China versus a persistent increasing AOD trend in the Indian subcontinent with no detectable change point from 2000 to 2019. In East China, positive AOD trend (+0.11 ± 0.022 decade−1) is confirmed from 2000 to 2010 (hereinafter the former period) yet negative trend (−0.26 ± 0.027 decade−1) is identified from 2011 to 2019 (hereinafter the later period). In the Indian subcontinent, persistent positive trend (+0.04 ± 0.001) is detected from 2000 to 2019 (hereinafter the whole period). All of these trends are attributed mainly to changes in sulfate aerosols. Further analysis of the aerosol pollution extreme events (APEE; defined as daily AOD over the long-term local 90th AOD percentile) manifest a positive trend (+0.16 ± 0.029 decade−1) of the APEEs' magnitude in East China during the former period yet a negative trend (−0.11 ± 0.020 decade−1) during the latter period; the Indian subcontinent demonstrates a positive trend (+0.02 ± 0.004 decade−1) during the whole period due to increasing sulfate aerosols. The APEEs have become more frequent (+3.5 ± 0.53 day month−1 decade−1) in East China during the former period yet less frequent (−3.6 ± 0.39 day month−1 decade−1) during the latter period; in the Indian subcontinent, more frequent APEEs (+1.1 ± 0.25 day month−1 decade−1) have been detected during the whole period. Consistent with the AOD trends, clear-sky radiation in East China shows a negative trend at the surface (−3.2 ± 0.67 W m−2 decade−1), a positive trend in the atmosphere (+1.4 ± 0.68 decade−1), and a negative trend at the top of the atmosphere (−1.8 ± 0.43 decade−1) during the former period, respectively; opposite trends with much larger magnitude are seen during the latter period. In the Indian subcontinent, the clear-sky radiation trends during the whole period are −1.4 ± 0.38, +1.7 ± 0.31, and + 0.5 ± 0.16 W m−2 decade−1 at the surface, in the atmosphere, and at the top of the atmosphere, respectively. Comparison of radiation trends at clear-sky and all-sky conditions suggests that absorbing aerosols dominate the radiation budget in the atmosphere and the aerosol reanalysis of the Modern-Era Retrospective Analysis for Research ans Applications version 2 (MERRA-2) might overestimate the radiation response to clouds. This study provides an up-to-date analysis of the long-term trends in aerosols and their extreme events and radiation in two of the world's heavily polluted regions and the results have important implications for assessment of the environmental and climatic impacts of the ongoing clean air acts in Asia.