Figure 2
The conceptual thermodynamical framework with the "coordinates" being pH and aerosol liquid water content (ALWC). Aerosols are sensitive to HCl, HNO3, and NH3 concentrations. The black, blue, and red lines define the "characteristic pH" where chemical regimes are separated by a pre-defined threshold value of ε = 10%. Above pH′, pH″, and pH‴ aerosols are sensitive to the variation in precursors like HCl, HNO3, and NH3, and below which aerosols are deemed insensitive to variations in precursors. The six chemical regimes of (a) PM1 and (b) PM2.5 aerosols are shown in different color shades, with the observational data points colored by mass concentrations (µg m−3) of PM1 and PM2.5 monitored during the Winter Fog Experiment (WiFEx) field campaign of 2017–18.
