Enrichment of Organic Acids in Fine Particles Over a Megacity in South China

Organic acids are abundant aerosol components that impact the hygroscopicity and multiphase processes of particulate matter, yet their changing compositions and sources are poorly understood. In this study, filter-based PM_2.5 samples were collected concurrently at three sites (urban, rural, and coastal) during the summer and winter of 2018 in the megacity Guangzhou, south China, to determine organic acids including C₉–C₃₂ fatty acids, C₄–C₁₀ dicarboxylic acids, and aromatic acids. On average, the total concentrations of the speciated organic acids were higher in winter than in summer, and were the highest at the urban site. Compared to observations at the same sites in winter 2009, average concentrations of organic acids in winter 2018 increased by 35.0%, 66.0%, and −3.2% at the urban, rural and coastal sites, while their fractions in organic matter (OM) increased by approximately 261.5%, 266.7%, and 61.1%, and their fractions in PM_2.5 increased by approximately 318.2%, 308.7%, and 221.1%, respectively. Source apportionment of organic acids revealed that cooking emission and vehicle exhaust were the major sources at the urban site, plant wax and vehicle exhaust were the two largest sources at the rural site with substantial contributions from biomass burning in winter, while vehicle exhaust and ship emission were the two major sources at the coastal site. Less contributions from secondary production were retrieved in winter 2018 than in winter 2009, and the enrichment of organic acids in PM_2.5 or OM was attributed to their elevated primary emissions from vehicles and ships, cooking, and plant wax.