A quantitative study on the influencing pathways of aerosol acidity: the non-neglectable role of non-ideality

Aerosol acidity plays a crucial role in atmospheric processes, impacting both environmental and human health. However, previous studies have not comprehensively quantified the pathways through which meteorological parameters and chemical components influence pH changes, particularly overlooking the role of non-ideality (γH⁺). In this study, we investigate the seasonal variation in aerosol acidity in an urban area of the Pearl River Delta, China, as a case to quantify the contributions of various pathways. We find that H⁺ concentration is the primary pathway for pH changes, contributing 64.2–81.3 %, followed by non-ideality, which accounts for 18.4–26.7 % and surpasses aerosol liquid water content (ALWC). Temperature affects pH mainly through H⁺ concentration variation, while relative humidity influences pH primarily via ALWC. Among chemical components, sulfate and ammonium (NH₄⁺) impact pH through H⁺, while nitrate's influence is mainly driven by non-ideality. Excluding non-ideality shifts the fraction of NH₄⁺ by as large as 48.5 %, underscoring the importance of considering non-ideality in gas-particle partitioning models. Lastly, ionic strength and nitrate fraction significantly influence the activity coefficient, highlighting the need to consider non-ideality when examining aerosol acidity processes.