Validation of MERRA-2 AOD and Seasonal Variation in Phimai, Thailand, and Factors That Affect AOD over Indochina Peninsula

Abstract:

Biomass burning (BB) is active during the dry season (December―April) over Indochina peninsula (ICP), thereby it worsens the air quality. We conducted validation of the Modern-Era Retrospective analysis for Research and Applications, Version 2(MERRA-2) aerosol optical depth (AOD) using sky radiometer at the SKYNET Phimai site (15.18°N, 102.56°E) and clarify causes of seasonal variation from 2009 to 2019. Besides, we analyzed factors that influence AOD over ICP using the validated MERRA-2 AOD from 2003 to 2019. The correlation coefficients (R) were 0.89(0.88) and 0.94(0.92) for daily and monthly average (dry season) comparisons, respectively, and MERRA-2 AOD was ~15% underestimated. Results show that the major cause of seasonal variation in Phimai was organic carbon (OC) AOD, which was dominant during the dry season. Additionally, the ratio of sulfate AOD to total AOD was high throughout the year. Analysis by MODIS active fires and trajectory analysis by Meteorological Data Explorer (METEX) indicated that BB over ICP dominates OC AOD, and sulfate aerosol was transported from China. As well as AOD over ICP was affected by these aerosols, especially, the ratio of sulfate AOD to total AOD was exceeded 0.5 over northeast ICP in most months, indicating the strong impact by sulfate transport from China. Thus, MERRA-2 can simulate AOD within 15% under the active BB season, and its high spatial-temporal AOD potentially reproduced the influences of OC aerosol originated BB and sulfate aerosol transported from China.