Chemical and optical properties of carbonaceous aerosols in Chiang Mai, Thailand, during the 2024 El Niño event

This study characterizes the chemical and optical properties of carbonaceous aerosols in urban Chiang Mai, Thailand, during an intensive monitoring period in March 2024. The campaign coincided with a strong El Niño event, resulting in extreme high temperatures and severe drought conditions. The mean PM 2.5 concentration was 64.3 µg/m³, exceeding the WHO 24-hour guideline (15 µg/m³) by approximately 4.3 times. High OC/EC ratios (mean 6.0) and a strong correlation with CO ( R = 0.71, 924.5 ppb) indicated a strong influence of biomass burning on particulate air quality, while urban baseline emissions also contributed. Furthermore, severe nighttime atmospheric stagnation trapped pollutants, exacerbating high-concentration episodes. Source apportionment via an Aethalometer model revealed that biomass burning-derived black carbon (BC bb ) drove the overall variability during these episodes. Notably, brown carbon (BrC) significantly enhanced shortwave light absorption at 370 nm, accounting for a campaign mean contribution of 35.1% and reaching an hourly maximum of 50.5% during the high-concentration episode. The strong correlation ( R = 0.99) between BrC absorption and BC bb mass concentration, despite shared optical input data, further reflects the dominant influence of biomass burning on shortwave light absorption. These results highlight how anomalous meteorological conditions—co-occurring with the regional 2024 El Niño background—coincided with intensified biomass burning and enhanced light absorption, providing insights into aerosol optics under extreme dry season weather. The findings emphasize the imperative need for integrated strategies to mitigate the escalating synergy between climate change and regional air quality. Size-resolved black carbon characteristics, chemical composition, and brown carbon light absorption in the Chiang Mai basin during the 2024 El Niño event.