Surface particulate matter (PM2.5, i.e., particulate matter with a median diameter of 2.5 µm or less) is one of the main air pollutants that has a significant negative effect on human health. Exposure to PM2.5 has been associated with many negative health outcomes including acute cardiovascular morbidity and mortality, respiratory mortality, hypertension, lung cancer, etc, and can also reduce people's life expectancy (e.g., Brook et al., 2010; L. Miller & Xu, 2018; Pope et al., 2009). While the anthropogenic PM2.5 concentrations are on the decline in the United States (U.S.) due to pollution controls implemented by the Environmental Protection Agency (EPA) and the daily average standard of 35 µg/m 3 is never met unless the concentrations are high (100 s of µg/m 3) due to smoke from fires or dust events. In parts of Asia, while de-seasonalized surface PM2.5 values are decreasing by about 2-8 µg/m 3 per year, health impacts continue to be profound due to high pollution levels (Liang et al., 2020; Zhai et al., 2019). Shi et al. (2018) report that between 1999 and 2014, there was a net 38% increase in premature deaths in Asia due to PM2.5 exposure. While monitoring network in the U.S. is dense in urban regions, there are gaps in some rural areas, especially in the central plains and mid-west (www.airnow.gov, accessed November 17, 2020). Elsewhere in the world, the coverage is especially sparse. The network of ground monitors across the world though is spreading with the use of low cost sensors (openaq.org, accessed November 17, 2020). Until the quality of the data gathered from these low cost sensors is assured, attempts to use satellite derived aerosol optical depth (AOD) to estimate surface PM2.5 that can alleviate the spatial coverage problem associated with ground measurements will continue (R. M. Hoff & Christopher 2009; van Donkelaar et al., 2010). Many methods have been developed to estimate surface PM2.5 from AOD (Chu et al., 2016), such as the simple linear regression methods (Engel