An investigation into seasonal and regional aerosol characteristics in East Asia using model-predicted and remotely-sensed aerosol properties

Abstract

In this study, the spatio-temporal and seasonal distributions of EOS/Terra Moderate Resolution Imaging Spectroradiometer (MODIS)-derived aerosol optical depth (AOD) over East Asia were analyzed in conjunction with US EPA Models-3/CMAQ v4.3 modeling. In this study, two MODIS AOD products (&tau;_MODIS: &tau;_M-BAER and &tau;_NASA) retrieved through a modified Bremen Aerosol Retrieval (M-BAER) algorithm and NASA collection 5 (C005) algorithm were compared with the AOD (&tau;_CMAQ) that was calculated from the US EPA Models-3/CMAQ model simulations. In general, the CMAQ-predicted AOD values captured the spatial and temporal variations of the two MODIS AOD products over East Asia reasonably well. Since &tau;_MODIS cannot provide information on the aerosol chemical composition in the atmosphere, different aerosol formation characteristics in different regions and different seasons in East Asia cannot be described or identified by &tau;_MODIS itself. Therefore, the seasonally and regionally varying aerosol formation and distribution characteristics were investigated by the US EPA Models-3/CMAQ v4.3 model simulations. The contribution of each particulate chemical species to &tau;_MODIS and &tau;_CMAQ showed strong spatial, temporal and seasonal variations. For example, during the summer episode, &tau;_MODIS and &tau;_CMAQ were mainly raised due to high concentrations of (NH₄)₂SO₄ over Chinese urban and industrial centers and secondary organic aerosols (SOAs) over the southern parts of China, whereas during the late fall and winter episodes, &tau;_MODIS and &tau;_CMAQ were higher due largely to high levels of NH₄NO₃ formed over the urban and industrial centers, as well as in areas with high NH₃ emissions. &tau;_CMAQ was in general larger than &tau;_MODIS during the year, except for spring. The high biases (&tau;_CMAQ&gt;&tau;_MODIS) may be due to the excessive formation of both (NH₄)₂SO₄ (summer episode) and NH₄NO₃ (fall and winter episodes) over China, possibly from the use of overestimated values for NH₃ emissions in the CMAQ modeling. According to CMAQ modeling, particulate NH₄NO₃ made a 14% (summer) to 54% (winter) contribution to &sigma;_ext and &tau;_CMAQ. Therefore, the importance of NH₄NO₃ in estimating τ should not be ignored, particularly in studies of the East Asian air quality. In addition, the accuracy of &tau;_M-BAER and &tau;_NASA was evaluated by a comparison with the AOD (&tau;_AERONET) from the AERONET sites in East Asia. Both &tau;_M-BAER and &tau;_NASA showed a strong correlation with &tau;_AERONET around the 1:1 line (<I>R</I>=0.79), indicating promising potential for the application of both the M-BAER and NASA aerosol retrieval algorithms to satellite-based air quality monitoring studies in East Asia.