Abstract
Abstract. Ecosystem productivity is strongly modulated by the atmospheric deposition of inorganic reactive nitrogen (the sum of ammonium and nitrate). The individual contributions of ammonium and nitrate vary considerably over space and time, giving rise to complex patterns of nitrogen deposition. In the absence of rain, much of this complexity is driven by the large difference between the dry deposition velocity of nitrogen-containing molecules in the gas or condensed phase. Here we quantify how aerosol liquid water and acidity, through their impact on gas–particle partitioning, modulate the deposition velocity of total NH3 and total HNO3 individually while simultaneously affecting the dry deposition of inorganic reactive nitrogen. Four regimes of deposition velocity emerge: (i) HNO3 – fast, NH3 – slow, (ii) HNO3 – slow, NH3 – fast, (iii) HNO3 – fast, NH3 – fast, and (iv) HNO3 – slow, NH3 – slow. Conditions that favor partitioning of species to the aerosol phase strongly reduce the local deposition of reactive nitrogen species and promote their accumulation in the boundary layer and potential for long-range transport. Application of this framework to select locations around the world reveals fundamentally important insights: the dry deposition of total ammonia displays little sensitivity to pH and liquid water variations, except under conditions of extreme acidity and/or low aerosol liquid water content. The dry deposition of total nitric acid, on the other hand, is quite variable, with maximum deposition velocities (close to gas deposition rates) found in the eastern United States and minimum velocities in northern Europe and China. In the latter case, the low deposition velocity leads to up to 10-fold increases in PM2.5 nitrate aerosol, thus contributing to the high PM2.5 levels observed during haze episodes. In this light, aerosol pH and associated liquid water content can be considered to be control parameters that drive dry deposition flux and can accelerate the accumulation of aerosol contributing to intense haze events throughout the globe.
Highlights
Human civilization and a high standard of living fundamentally depend on a sustainable food and energy supply (NAS, 2016)
Given that aerosol nitrate is often a significant constituent of PM in regions where the aerosol exhibits a mildly acidic pH (e.g., Cabauw, China), this constitutes an important and overlooked positive feedback between acidity, nitric acid production, ammonium, and PM buildup that is a consequence of aerosol thermodynamic partitioning and deposition velocity
Our analysis identifies four deposition velocity regimes: (i) HNO3 – fast, NH3 – slow, (ii) HNO3 – slow, NH3 – fast, (iii) HNO3 – fast, NH3 – fast, and (iv) HNO3 – slow, NH3 – slow
Summary
Human civilization and a high standard of living fundamentally depend on a sustainable food and energy supply (NAS, 2016) Both can be linked to the availability of reactive nitrogen (Nr) species in agricultural soils, which are key nutrients required by the biosphere for growth and crop production. Given that species in the gas phase generally have a different atmospheric residence time than those in the aerosol phase (Seinfeld and Pandis, 2016), the degree of gas–particle partitioning can directly impact the atmospheric residence time of Nr species, with important implications for particulate matter levels and dry deposition (e.g., Pinder et al, 2007, 2008). It is important to consider how aerosol acidity and liquid water content, through their effect on gas–particle partitioning, can affect the deposition fluxes of Nr species. We demonstrate the power of this new framework with observational data to understand the relevant “chemical regimes” that apply to select places around the world and to understand the implications that changes in acidity have on deposition flux and PM levels
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