Long-term exposure to airborne fine particulate matter or PM2.5 is associated with an increase in the long-term risk of premature death that creates critical concerns for public health. This study uses twenty years (2002-2021) of daily remotely sensed data with multi-spatial resolution of 1 km to 3 km to examine the long-term spatiotemporal distribution of PM2.5 across Thailand. Good agreement is found between the in-situ measurements of PM2.5 and instantaneous estimates made from the satellite data with correlation coefficients of 0.51. Based on data analysis during the year 2002- 2021, the region with the highest yearly averaged concentration level of PM2.5 was a central region of Thailand (19.91 μg.m-3) followed by northern (19.11 μg.m-3), northeastern (18.92 μg.m-3), eastern (18.76 μg.m-3) and southern (16.16 μg.m-3) region, respectively. The period with the highest PM2.5 levels were during March and April with monthly averages 23.74 to 26.72 μg.m-3. For the 20-year record, monthly-mean PM2.5 concentration in northern Thailand showed statistically significant increase at the rate of 0.14 μg.m-3 month-1 in dry season, the same as in the northeastern (0.126 μg.m-3month-1), eastern (0.12 μg.m-3 month-1) and Central region (0.083 μg.m-3 month-1). While the southern region has a negative trend (-0.018 μg.m-3 month-1) which is different from other regions. The spatiotemporal variation and changing of PM2.5 concentrations were a result of both changing in meteorological factors and anthropogenic activities. Here, we discuss and present possible explanations for long-term spatiotemporal variation of PM2.5.

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