Contrasting seasonal pattern between ground-based PM2.5 and MODIS satellite-based aerosol optical depth (AOD) at an urban site in Hanoi, Vietnam

This study investigated the seasonal variations of the ground-based PM2.5 concentration measured at an urban site and Terra MODIS satellite-based aerosol optical depth (AOD) in Hanoi, Vietnam, during the year 2016. The findings showed the large differences in terms of the seasonal variation of both PM2.5 concentration and columnar AOD. Clearly, the ground-based PM2.5 concentrations decreased during the summer period and highly increased during the late fall and winter periods. The Terra MODIS AOD values also decreased in summer, however, significantly increased in spring. Interestingly, the increasing trend of AOD which in contrast to the decreasing trend of PM2.5 concentration was found during the spring period, and vice versa for the winter period. The seasonal analysis of meteorological factors including relative humidity (RH), precipitation, and boundary layer height (BLH) as well as regional fire activities expressed that the increased columnar AOD in spring was largely influenced by the hygroscopic growth of aerosol induced by the increased RH, the larger BLH causing the more effective transport and mixing of atmospheric particles to higher altitudes, and especially the intensive fire activities in the Southeast Asian region during the spring period, whereas the significantly increased surface PM2.5 concentration in winter was partly attributed to the impact from the highly industrialized and polluted source region in the Pearl River Delta of China. Different from the spring and winter periods, the abundant precipitation amount and the dominance of maritime air masses arrived in Hanoi in summer were the main reasons for the low AOD and surface PM2.5 concentration during this period.