Measurement Report: Quantitative Analysis of Aerosol Acidity and Its Driving Factors in Guanzhong Area, Northwest China

Abstract. Aerosol acidity significantly affects atmospheric chemistry and human health, yet its driving factors remain controversial. This study systematically examined a year-long characteristics of water-soluble inorganic ions in PM_2.5 in the semi-arid Guanzhong Plain, Northwest China, and quantitatively analyzed its acidity and driving factors. The annual mean pH of PM_2.5 was 3.8 ± 1.0 (winter > spring > summer > autumn). As pollution increased, aerosol pH shifted from the acidic range (2–5) on clean days to a near-neutral range (3–6) on polluted days. Sensitivity tests and driver analysis revealed that atmospheric temperature (22.3 %–33.8 %), NH_x (gas NH₃+NH₄⁺, 11.4 %–44.9 %), and SO₄^2- (8.5 %–10.8 %) were common key factors influencing pH across all seasons. Among these, temperature played a dominant role in seasonal variations, while NH_x was the primary contributor during autumn and winter. Notably, Ca²⁺ emerged as a unique driver specific to spring, the relative standard deviation (RSD) was 8.8 %. Relative humidity (RH) exhibited a distinctive non-linear regulatory effect, i.e., aerosol pH initially decreases and subsequently increases with rising RH (inflection point occurred at 60 %–85 %). This phenomenon is primarily attributed to an abrupt change in liquid water content triggered by the deliquescence of hygroscopic components. Our results enhance the understanding of the contributions of various factors to aerosol pH and offers a new perspective for developing PM_2.5 control strategies in the semi-arid and ammonia-rich region.