Seasonal variation and size distribution of aromatic acids in urban aerosols in Beijing, China

Aromatic acids are an integral component of organic acids in the atmosphere, contributing to the formation of climate-altering brown carbon (BrC). To better understand the sources and formation processes of aromatic acids, we collected size-segregated particulate matter samples in urban Beijing from April 2017 to January 2018, which were analyzed using solvent-extraction followed by gas chromatography/mass spectrometry. Phthalic acid (o-PhA) had the greatest average annual concentration, followed by terephthalic acid (p-PhA), 4-hydroxybenzoic acid (4-OHBA), dehydroabietic acid (DA), syringic acid (SA), 3-hydroxybenzoic acid (3-OHBA), isophthalic acid (m-PhA), and vanillic acid (VA). We identified distinct seasonal variations in aromatic acids, with o-PhA peaking in summer due to photochemical activity, while p-PhA and 4-OHBA elevated in autumn and summer, respectively, influenced by open waste and biomass burning. Wintertime variations in all aromatic acids were driven by complex meteorology and increased anthropogenic emissions, including rural biomass burning for cooking and heating. Particle size distribution of aromatic acids was affected by seasonal agricultural activities and dust storms, multiple emission sources, and formation mechanisms. The o-PhA has predominantly bimodal distribution, with diverse sources and complex formation mechanisms including gas- and aqueous-phase chemistry. The applicability of o-PhA as a tracer for specific secondary organic aerosols has been questioned due to its potential primary sources. The 3-OHBA, 4-OHBA, VA, SA, and DA exhibited bimodal or trimodal patterns during haze and non-haze periods across different particle size ranges. The seasonal variation in VA/SA and VA/4-OHBA ratios demonstrated the complexity of biomass burning types, influenced by season, particle size, meteorological conditions, and combustion sources.