Sources and pH Regulate Redox-Active Metal Solubility in Urban PM_2.5 in Northwest China

Redox-active metals are key toxic components of ambient PM_2.5 that may pose significant health risks through oxidative stress, yet their environmental behavior and bioavailability under varying atmospheric conditions remain insufficiently understood. This study investigated the concentrations, sources, and water solubility of three major redox-active metals (Fe, Cu, Mn) in PM_2.5 collected in urban Xi'an before and during the COVID-19 pandemic restrictions. Results showed that Fe was the most abundant metal but exhibited low solubility (<10%), while Cu and Mn showed higher solubility, with average values exceeding 40% across all periods except the winter during the pandemic restrictions. Source apportionment indicated that dust was the important Fe source, whereas Cu and Mn mainly originated from traffic-related and combustion emissions. During pandemic restrictions, total metal concentrations increased by 20%–35%, yet metal solubility significantly decreased, particularly in winter, coinciding with a significant rise in aerosol pH (from ∼3 to 5.38). Multiple linear regression analysis suggested that pH was the main factor affecting the variations of Cu and Mn solubility, while Fe solubility was more strongly associated with emission sources. Notably, the results suggest that combustion sources exhibited a dual role by directly emitting more soluble metal species and indirectly enhancing solubility via acid precursor emissions that lowered aerosol pH. These findings highlight that, beyond emission reductions, aerosol pH regulation is also critical for mitigating the bioavailability of toxic metals. Therefore, effective air quality management in urban environments should integrate source control with aerosol acidity regulation to reduce the bioavailability of PM_2.5-bound redox-active metals.