Abstract. Nitrogen oxides (NOx) and ammonia (NH3) from
anthropogenic and biogenic emissions are central contributors to particulate
matter (PM) concentrations worldwide. The response of PM to changes in the
emissions of both compounds is typically studied on a case-by-case basis,
owing in part to the complex thermodynamic interactions of these aerosol
precursors with other PM constituents. Here we present a simple but
thermodynamically consistent approach that expresses the chemical domains of
sensitivity of aerosol particulate matter to NH3 and HNO3
availability in terms of aerosol pH and liquid water content. From our
analysis, four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the “chemical regime” of PM sensitivity to NH3 and HNO3 availability is directly determined. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chemical transport models. With this extended understanding, aerosol pH and associated liquid water content naturally emerge as previously ignored state parameters that drive PM formation.