Abstract. Understanding long-term trends in aerosol loading is essential for evaluating the health and climate effects of airborne particulates as well as the effectiveness of pollution control policies. Here we introduce a method to construct a combined annual and seasonal AOD long time series using the Along-Track Scanning Radiometers (ATSR: ATSR-2 and AATSR) and MODerate resolution Imaging Spectroradiometer Terra (MODIS/Terra), which together cover the period of 1995–2017. The long-term (1995–2017) annual and seasonal combined AOD time series are presented for the all of mainland China, for southeastern (SE) China and for 10 selected regions in China and analyzed to reveal the AOD tendencies during the last 23 years. Linear regression has been applied to individual L3 (1°×1°) pixels of the annual and seasonal combined AOD time series to estimate the AOD tendencies for three periods: 1995–2006 (P1) and 2011–2017 (P2), as regarding the changes in the emission reduction policies, and the whole period 1995–2017 (WP), when combined AOD time series is available. Positive tendencies of annual AOD (0.006, or 2 % of AOD, per year) prevailed across all of mainland China before 2006 due to emission increases induced by rapid economic development. In southeastern China, the annual AOD positive tendency in 1995–2006 was 0.014, or 3 % of AOD, per year in SE China, reaching maxima (0.020, or 4 % of AOD, per year) in Shanghai and the Pearl River Delta regions. Negative AOD tendencies (−0.015, or −6 % of AOD, per year) were identified across most of China after 2011 in conjunction with effective emission reduction in anthropogenic primary aerosols, SO2 and NOx (Jin et al., 2016, van der A et al., 2017). The strongest AOD decrease is observed in Chengdu (−0.045, or −8 % of AOD, per year) and Zhengzhou (−0.046, or −9 % of AOD, per year) areas, while over the North China plane and coastal areas the AOD decrease was lower than −0.03, or ca. −6 % of AOD, per year. In the less populated areas, the AOD decrease was small. The AOD tendencies for the whole period 1995–2017 were much less pronounced compared to P1 and P2. The reason for that is that positive AOD tendency has been observed at the beginning of WP (in P1) and negative AOD tendency has been observed at the end of WP (in P2), which partly cancel each other during 1995–2017. In the WP, AOD was slightly increasing over the Beijing-Tianjin-Hebei area (0.008, or 1.3 % of AOD, per year) and the Pearl River Delta (0.004, or 0.6 % of AOD, per year). A slightly negative AOD tendency (−0.004, or −0.7 % per year) was observed in the Chengdu and Zhengzhou areas. Seasonal patterns in the AOD regional long-term trend are evident. The contribution of seasonal AOD tendencies in annual tendencies was not equal along the year. While the annual AOD tendency was positive in 1995–2006, the AOD tendencies in winter and spring were slightly negative (ca. −0.002, or −1 % of AOD, per year) over the most of China during that period. AOD tendencies were positive in summer (0.008, or 2 % of AOD, per year) and autumn (0.006, or 6 % of AOD, per year) over all mainland China and SE China (0.020, or 4 % of AOD, per year and 0.016, or 4 % of AOD, per year in summer and autumn, respectively). The AOD negative tendencies in 2011–2017 were higher compared to other seasons in summer over China (ca. −0.021, or −7 % of AOD, per year) and over SE China (ca. −0.048, or −9 % of AOD, per year). The results obtained in the current study show that the effect of the changes in the emission regulations policy in China during 1995–2017 is evident in AOD gradual decrease after 2011. The effect is more visible in the highly populated and industrialized regions in SE China.
Coupling the Modified Linear Spectral Mixture Analysis and Pixel-Swapping Methods for Improving Subpixel Water Mapping: Application to the Pearl River Delta, China
