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) plays a vital role in adverse effects on air quality, climate systems, and human public health. Large-scale OBB, including forest fires and crop residue burning, is detected in Southeast Asia (SEA), a region with agrarian countries. The characteristics of OBB have been widely studied in SEA; however, the daytime and nighttime variations in fire and the effects of fire production remain limited. Particulate matter (PM) is released in significant amounts, burying open biomass during the episode. This study uses 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 hot spots occur more often at night than during the day. The VIIRS fire detection data provides better response to small fires and better mapping of extensive fire perimeters. PM1.0–0.5 showed the highest mass concentration among particle sizes. Moreover, the fire hotpots are the highest correlated with PM0.5-0.1 during daytime and PM1.0–0.5 during nighttime. The large OBB in Chiang Mai significantly contributes to ambient PM. This study offers crucial insights into particulate pollution from biomass burning.