Biomass Burning Greatly Enhances the Concentration of Fine Carbonaceous Aerosols at an Urban Area in Upper Northern Thailand: Evidence From the Radiocarbon‐Based Source Apportionment on Size‐Resolved Aerosols.

To study the role of biomass burning (BB) in air pollution at upper‐northern Thailand, the source apportionment of size‐resolved carbonaceous aerosols from Chiang Mai was carried out based on the radiocarbon (14C) analysis. The fraction of modern carbon (F14C) was generally decreased with particle size increasing and with the highest and lowest values of 0.90 ± 0.04 and 0.61 ± 0.04, respectively. Elemental carbon, regardless of emission sources, and BB‐derived organic carbon (OCbb) showed unimodal size distribution patterns with peaks at 0.43–0.65 μm. Fossil‐fuel derived‐OC (OCf) displayed a bimodal mode with the major peak at 2.1–10 μm, and the minor one at 0.43–0.65 μm. The biogenic secondary organic aerosols (BSOA) showed a typical fine‐mode unimodal size distribution pattern during the high BB (HBB) season, and a bimodal mode during the low BB season. The BSOA concentration increased by 189% ± 80% due to the interaction with open BB plums during HBB season, which was quantified by a 14C‐involved random forest model. Besides, the concentration of biogenic primary organic aerosols also showed a significant increment during the HBB season, especially in sub‐microns. Our results highlight the critical importance of controlling open fires to reduce air pollutants and the potential exposure risk. Plain Language Summary: Large‐scale biomass burning (BB) activities take place annually in Southeast Asia. BB can emit large amounts of carbonaceous aerosols (CA) thus affecting human health and causing climate change. The particle size, directly affects how deep the particle can penetrate the human body, can be significantly changed by BB. In this study, BB‐derived CA contributed the most to the total concentration of CA. The concentration of BB‐derived CA in PM10 (particles with an aerodynamic diameter of less than 10 μm) was 12 times higher in high BB (HBB) season than in low BB (LBB) season. More specifically, BB‐derived CA in fine (PM1.1) and coarse (PM1.1–10) particles increased by roughly 13 and 5 times in HBB season compared to LBB season, respectively. Besides, BB enhanced the concentration of biogenic secondary organic aerosols by 180%, wherein 65% of the increment was allocated in PM1.1. Our results indicated that BB can not only emit particles or precursors of aerosol particles but also promote the generation of biogenic aerosols. And BB significantly enhanced the loading of CA, particularly in fine particles. Key Points: Fossil‐derived carbonaceous aerosols (CA) showed a bimodal mode size distribution patternNon‐fossil‐derived CA showed a typical fine‐mode unimodal size distribution patternBiomass burning greatly enhanced the production of biogenic secondary organic aerosols particularly in fine aerosols [ABSTRACT FROM AUTHOR]