Trends and Variability in Aerosol Optical Depth over North China from MODIS C6 Aerosol Products during 2001–2016

This study analyzed the variability and trend in aerosol optical depth (AOD) over North China using the latest MODIS/Terra C6 merged Dark Target/Deep Blue AOD monthly data at 550 nm from 2001 to 2016. The spatial distribution of the annual mean AOD was generally characterized by two prominent high-value centers located in the industrially and economically developed areas of the North China Plain and East China, and the dust aerosol-dominated areas of southern Xinjiang. The seasonally averaged AOD reached its maximum in spring (0.430 ± 0.049), followed by summer (0.356 ± 0.035) and winter (0.282 ± 0.039), with the minimum occurring in autumn (0.219 ± 0.022). There were notable long-term annual trends in AOD in different regions over North China during 2001–2016: a decreasing AOD trend was found in Qinghai Tibet (−0.015 ± 0.010/decade), Northwest China (−0.059 ± 0.013/decade at 99% confidence level), and the North China Plain (−0.007 ± 0.021/decade), but a positive increasing trend was identified in northern Xinjiang (0.01 ± 0.006/decade), southern Xinjiang (0.002 ± 0.013/decade), East China (0.053 ± 0.042/decade), and Northeast China (0.016 ± 0.029/decade). Seasonal patterns in the AOD regional long-term trend were evident. The AODs in spring over all the study regions, except East China, exhibited a decreasing trend, with the maximum trend value observed in Northwest China (−0.099 ± 0.029/decade at 99% confidence level); whereas AODs in autumn, except in Northwest China, showed an increasing trend, with the maximum trend value occurring in East China (0.073 ± 0.038/decade). Geographically, we also examined the annual and seasonal spatial patterns of AOD trends over North China. The annual spatial trends in AOD revealed a dominance of positive trends in most regions over the whole of North China from 2001 to 2016, but especially in East and Northeast China (AOD trend value of about 0.16/decade); whereas a negative trend was observed over northern Inner Mongolia (AOD trend value of about −0.12/decade). In addition, seasonal spatial trend analyses indicated that a continual clear upward trend occurred in East China in the autumn and winter seasons during the study period, with the maximum average increase occurring in winter (about 0.20/decade)