Asymmetric trends in sulfates, nitrates, and ammonium in PM2.5 of Chengdu: Insights from five years of hourly observations

Sulfate, nitrate, and ammonium (SNA) are critical components of PM_2.5, posing significant health risks and playing a vital role in air pollution formation in megacities. Long-term monitoring and high-time resolution temporal data of these pollutants are essential for identifying potential sources as well as evaluating and planning control measures. This study analyzes a five-year real-time dataset of PM_2.5 and SNA in Chengdu, southwest China, with a focus on the effects of human activities, particularly during holidays, on air pollution. We examined annual and seasonal trends, weekend and holiday effects, meteorological influences, and regional sources of these pollutants using Generalized Additive Model, Conditional Probability Function, and Concentration Weighted Trajectory methods. The results indicate a 12.34 % reduction in PM_2.5 concentrations from 2017 to 2021, with limited improvement compared to other megacities. Holiday periods, such as Labor Day and Dragon Boat Festival, were associated with significant concentration spikes in PM_2.5, sulfate, nitrate, and ammonium due to increased human activities and biomass burning. Dewpoint temperature was identified as the most significant meteorological factor influencing SNA formation with a nonlinear effect. These findings highlight the need for targeted pollution control measures during holiday periods to mitigate short-term pollution spikes.