Assessment of Urban Size-Fractionated PM Down to PM0.1 Influenced by Daytime and Nighttime Open Biomass Fires in Chiang Mai, Northern Thailand

Open biomass burning (OBB) adversely affects air quality, climate systems, and public health. Large-scale OBB, including forest fires and crop residue burning, is prevalent in Southeast Asia (SEA), a region with agrarian countries. The characteristics of OBB have been widely studied in SEA; however, the understanding of daytime and nighttime variations in fire activity and the effects of fire production remains limited. A significant amount of particulate matter (PM) is released into the atmosphere during OBB episodes. This study employs the Visible Infrared Imaging Radiometer Suite (VIIRS) to detect active fires during daytime and nighttime from OBB in Chiang Mai, Thailand, during March–April 2020, and investigates the mass concentration of size-specific PM down to PM0.1. The results showed that hotspots occur more often at night than during the day. The VIIRS fire detection data provided a better response to small fires and improved mapping of extensive fire perimeters. PM0.5–1.0 showed the highest mass concentration among particle sizes. Moreover, fire hotpots show the highest correlations with PM0.1–0.5 during the daytime and PM0.5–1.0 during the nighttime, and the large OBB in Chiang Mai significantly contributes to ambient PM. Overall, this study offers crucial insights into particulate pollution from biomass burning.