Abstract
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.
Highlights
Gas-phase ammonia (NH3(g), hereafter “nitric acid (HNO3) and ammonia (NH3)”) is one of the most important atmospheric alkaline species and contributor to atmospheric fine particle mass (Seinfeld and Pandis, 2016)
Ammonia reacts with sulfuric and nitric acids to form ammonium sulfate/bisulfate and nitrate aerosol that globally constitute an important fraction of ambient PM2.5 mass (Kanakidou et al, 2005; Sardar et al, 2005; Zhang et al, 2007)
The conceptual framework explicitly considers acidity, aerosol liquid water content, and temperature as the main parameters controlling secondary inorganic particulate matter (PM) sensitivity, and it identifies four policy-relevant regimes: (i) NH3 dominated, (ii) HNO3 dominated, (iii) both NH3 and HNO3, and (iv) a previously unidentified domain where neither NH3 nor HNO3 are important for PM formation
Summary
Gas-phase ammonia (NH3(g), hereafter “NH3”) is one of the most important atmospheric alkaline species and contributor to atmospheric fine particle mass (Seinfeld and Pandis, 2016). Reductions in ammonium sulfate due to SO2 reductions can be balanced, at least in part, by ammonium nitrate formation (e.g., West et al, 1999; Heald et al, 2012; Karydis et al, 2016; Vasilakos et al, 2018) This behavior arises because nitrate may remain in the gas phase as HNO3 when insufficient amounts of total ammonia (i.e., gas + aerosol) or nonvolatile cations (NVCs) from dust and sea salt exist to “neutralize” aerosol sulfate (i.e., completely consume any free sulfuric acid or bisulfate salts). We present such a framework, and demonstrate it with observational data, to understand the chemical regimes associated with aerosol sensitivity to changes in ammonia and nitrate availability
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