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. Understanding long-term variations in aerosol loading isessential for evaluating the health and climate effects of airborneparticulates as well as the effectiveness of pollution control policies.The expected satellite lifetime is about 10 to 15 years. Therefore, to study thevariations of atmospheric constituents over longer periods informationfrom different satellites must be utilized. Here we introduce a method to construct a combined annual and seasonal longtime series of AOD at 550 nm using the Along-Track Scanning Radiometers(ATSR: ATSR-2 and AATSR combined) and the MODerate resolution ImagingSpectroradiometer on Terra (MODIS/Terra), which together cover the 1995–2017 period. Thelong-term (1995–2017) combined AOD time series are presented for all ofmainland China, for southeastern (SE) China and for 10 selected regions inChina. Linear regression was applied to the combined AOD time seriesconstructed for individual L3 (1 ∘ ×  1 ∘ ) pixels toestimate the AOD tendencies for two periods: 1995–2006 (P1) and 2011–2017(P2), with respect to the changes in the emission reduction policies in China. During P1, the annually averaged AOD increased by 0.006 (or 2 % of theAOD averaged over the corresponding period) per year across all of mainlandChina, reflecting increasing emissions due to rapid economic development. InSE China, the annual AOD positive tendency in 1995–2006 was 0.014 (3 %)per year, reaching maxima (0.020, or 4 %, per year) in Shanghai and thePearl River Delta regions. After 2011, during P2, AOD tendencies reversedacross most of China with the annually averaged AOD decreasing by −0.015 ( −6  %)per year in response to the effective reduction of the anthropogenic emissions ofprimary aerosols, SO2 and NOx . The strongest AOD decreases were observedin the Chengdu ( −0.045 , or −8  %, per year) and Zhengzhou ( −0.046 , or −9  %, per year) areas, while over the North China Plain and coastal areasthe AOD decrease was lower than −0.03 (approximately −6  %) per year. Inthe less populated areas the AOD decrease was small. The AOD tendency varied by both season and region. The increase in theannually averaged AOD during P1 was mainly due to an increase in summer andautumn in SE China (0.020, or 4 %, and 0.016, or 4 %, per year,respectively), while during winter and spring the AOD actually decreasedover most of China. The AOD negative tendencies during the2011–2017 period were larger in summer than in other seasons over the whole of China(ca.  −0.021 , or −7  %, per year) and over SE China (ca. −0.048 , or −9  %, per year). The long-term AOD variations presented here show a gradual decrease in theAOD after 2011 with an average reduction of 30 %–50 % between 2011 and2017. The effect is more visible in the highly populated and industrializedregions in SE China, as expected.

atmospheric chemistry and physics

Spatial and seasonal variations of aerosols over China from two decades of multi-satellite observations – Part 2: AOD time series for 1995–2017 combined from ATSR ADV and MODIS C6.1 and AOD tendency estimations