Long–term Trends in PM_2.5 Chemical Composition and Its Impact on Aerosol Properties: Field Observations from 2007 to 2020 in Pearl River Delta, South China

Abstract. Long–term data of PM_2.5 chemical composition provide essential information for evaluating the effectiveness of air pollution control measures and understanding the evolving mechanisms of secondary species formation in the real atmosphere. This study presented field measurements of PM_2.5 and its chemical composition at a regional background site in the Pearl River Delta (PRD) from 2007 to 2020. PM_2.5 concentration declined significantly from 87.1 ± 15.5 μg m⁻³ to 34.0 ± 11.3 μg m⁻³ (–4.0 μg m^–3 yr^–1). The proportion of secondary species increased from 57 % to 73 % with the improvement in air quality. Among these species, sulfate (SO₄^2–) showed a sharp decline, while nitrate (NO₃^–) exhibited a moderate decrease. Consequently, the proportion of NO₃^– in 2020 doubled relative to 2007. In addition, we further found that SO₄^2– reduction (–10 % yr^–1) lagged behind SO₂ reduction (–13 % yr^–1), while NO₃⁻ reduction (–6 % yr^–1) outpaced that of NO₂ (–3 % yr^–1). These contrasting trends were associated with an increase in sulfur oxidation rate (SOR) and a decrease in nitrogen oxidation rate (NOR). Changes in PM_2.5 chemical composition also influenced aerosol physicochemical properties, such as aerosol pH (0.06 yr^–1), aerosol liquid water content (ALWC, –1.1 μg m^–3 yr^–1), and the light extinction coefficient (b_ext, –21.44 Mm^–1 yr^–1). Given important roles of aerosol acidity and ALWC in the heterogeneous reactions, these changes may further inhibit the formation of secondary species in the atmosphere, particularly SOA.