Assessment of Aerosol Optical Depth Measurements from MODIS and AERONET in Thailand, from 2004 to 2023

Air pollution adversely affects human health and the environment. The issue has been raised globally for a long time. Thailand has experienced severe air pollution for decades, particularly in Bangkok, the capital, likely attributable to highly congested traffic, and Chiang Mai, Thailand's second largest city, has been ranked the world's most polluted city several times. To forecast air quality, a sophisticated method like machine learning can be used; however, predefined precision levels and observational parameters must be acquired before being injected into the simulations. This study examines Aerosol Optical Depth (AOD) in Thailand, utilising data from ten AERONET sites and MODIS over a 20-year period, 2004-2023. We evaluate correlations between two observations considering various criteria, including MODIS observational satellites, AOD retrieval algorithms, and the spatiotemporal window sizes utilised in the comparison. The correlations are high in all scenarios (0.72-0.93). Aqua satellite observations have higher correlations than Terra's. The AODs from the Dark Target algorithm exhibit markedly higher correlations than those from the Deep Blue algorithm (0.07-0.17), but the Deep Blue algorithm can retrieve more reliable pixels. Spatiotemporal variation indicates that correlations are more related to observation period duration than spatial window size variations. AERONET measurements indicate seasonal variations in AODs across Thailand, except in the southern region, which exhibits low and stable AOD levels. During the summer, AERONET measurements indicate significantly high AOD levels and considerable daytime variations. This coincides with MODIS observations revealing that northern Thailand has encountered markedly high AOD over a wide area during the summer for the past 20 years. Our investigation revealed that wildfires are probably the primary source of the severe pollution.  We analyse time-series AOD across two cities using decompositions. Over the last 20 years, the AOD over Bangkok has increased by ~0.1 while remaining stable over Chiang Mai.