Field observations reveal biomass burning and cooking as the significant sources of PM_2.5-bound organic acids in Central China

Organic acids (OAs), crucial components of fine particulate matter (PM_2.5), not only influence cloud condensation nuclei but also promote particle nucleation and growth. Elucidating the chemical composition and key sources of atmospheric OAs is therefore crucial, particularly in Central China, a region with agricultural and cooking practices. This study investigated the chemical characteristics and sources of PM_2.5-bound OAs (C₉-C₃₂ monocarboxylic acids, C₄-C₁₀ dicarboxylic acids, 22 aromatic acids) in urban Changsha from August 2021 to January 2022. Results show that the quantified total OAs concentrations continuously increased from 362.2 ng/m³ in August to 894.1 ng/m³ in January. Chemically, hexadecanoic acid exhibited the highest average concentration (14.5% of quantified OAs), followed by p-Terephthalic acid (10.2%), octadecanoic acid (9.6%), succinic acid (9.1%), and azelaic acid (7.9%). Source apportionment revealed vehicle emissions as the predominant contributor (32.2% on average). Notably, biomass burning (24.8%) and cooking emissions (20.2%) also made substantial contributions. Particularly during October and November, biomass burning emission contributions surged to 32.2%, attributable to intensified open straw burning in October and residential firewood heating in surrounding suburban/rural areas in November. Even in August, with minimal agricultural burning and residential heating, biomass burning still contributed over 20%. A significant correlation (R² = 0.90) exists between biomass burning and cooking emissions tracers in August, which features prevalent charbroiling cooking and concurrently explains the marked contribution of cooking emissions (25.0% on average). This study provides critical insights into the molecular composition and potential sources of OAs in Central China, highlighting the importance of biomass burning and cooking emissions in developing air-quality management strategies for this region.