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

摘要

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).