Fig. 3: Temporal trends in sulfate concentrations, aerosol pH, and aerosol aqueous pathways in selected urban areas in January from 2001 to 2019.

The bar charts represent sulfate concentrations (left axis). The dot-line charts represent aerosol pH (right axis). The pie charts represent the sulfate formation rate of four aqueous-phase pathways in the aerosol water, distinguished by color. Source data are provided as a Source Data file.