Shift in Aromatic Acids From Primary Emissions to Secondary Formation During Air Quality Improvement in the Pearl River Delta

Aromatic acids are key constituents of atmospheric organic aerosols (OA). However, the relative contributions of primary emissions and secondary formation to aromatic acids, and their evolution during air quality improvement, remain poorly constrained. Here, we present wintertime observations of five groups of aromatic acids in the Pearl River Delta during 2007–2020. The total concentrations increased from 104.8 ± 34.1 ng m⁻³ in 2007 to 233.7 ± 105.0 ng m⁻³ in 2013, and then declined to 78.3 ± 27.3 ng m⁻³ by 2020. Source apportionment shows that secondary aromatic acids increased significantly at a rate of 6% yr⁻¹ over the study period, raising their contribution to total aromatic acids from 22% to 48%. Although 1,2-phthalic acid is predominantly formed through secondary processes, a non-negligible contribution of 20%–30% from primary emissions was observed. Consequently, the widely applied tracer-based approach may systematically overestimate naphthalene-derived secondary organic aerosols (SOA). Despite substantial reductions in anthropogenic emissions, estimates of two representative anthropogenic SOA show that their concentrations remained largely unchanged. Our results suggest that enhanced regional oxidative capacity may increasingly offset the benefits of primary emission reductions, highlighting the growing importance of secondary formation under cleaner atmospheric conditions.