Abstract
Abstract. Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium–sulfate–nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly developed nitrate scheme in which first-order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ=0.193) and slow (γ=0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 W m−2 in the fast simulation and −0.07 W m−2 in the slow simulation. In general, the fast simulation exhibits better spatial correlation with observed nitrate concentrations, while the slow simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions, suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a novel mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step change in aerosol modelling capability in the UK across weather and climate timescales.
Highlights
Air pollution poses a significant hazard to human health and to the environment worldwide
Most models assume that NH4NO3 concentrations reach thermodynamic equilibrium instantaneously without considering kinetic limitations on the condensation of HNO3 or NH3 onto existing aerosol particles, as is considered here. This is the first study to investigate the sensitivity of NH4NO3 concentrations to the HNO3 uptake coefficient and provide an efficient method for reducing NO3 concentration biases in global climate models (GCMs)
The INSTANT simulation is near-indistinguishable from FAST using these metrics (Table S5) – suggesting that NH4NO3 concentrations in FAST reach thermodynamic equilibrium near instantaneously – and INSTANT is omitted from further analysis
Summary
Air pollution poses a significant hazard to human health and to the environment worldwide. In 2016, 90 % of the global population was exposed to pollutant concentrations in excess of World Health Organisation (WHO)-defined safe levels, resulting in ∼ 7 million premature deaths (WHO, 2020). Solid or liquid particulate matter (PM) is a significant component of air pollution, and particles with diameters less than 2.5 μm (i.e. PM2.5) are harmful to human health. Lelieveld et al (2015) estimate PM2.5-related global mortality to be 3.3 million deaths per year in 2010, far greater than the second deadliest air pollutant, ozone (O3, 142 thousand deaths per year). Sources of air pollution differ with region; in northern Africa and the Middle East, the predominant source is nat-
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