Exploring the sensitivity of atmospheric nitrate concentrations to nitric acid uptake rate using the Met Office's Unified Model

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-sulphate-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 Wm−2 in the “fast” simulation and −0.07 Wm−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 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.

Below-cloud scavenging of aerosol by rain: a review of numerical modelling approaches and sensitivity simulations with mineral dust in the Met Office's Unified Model

“…Mapping the binned emissions to the UKCAmode dust scheme requires a degree of pragmatism and trial and error. In previous test bed simulations, an optimal mapping emerged, wherein Bin 2 + 1/2 Bin 3 was emitted to the accumulation mode, while 1/2 Bin 3 + Bin 4 + Bin 5 were emitted to the coarse mode (Jones et al, 2021). This mapping is subject to change given ongoing improvements to the dust scheme.…”

“…To answer the KQs, 20-year integrations are performed with the Met Office's Unified Model (UM) in a climate configuration, where the sole variable is the formulation of BCS applied to mineral dust aerosol. The UM represents aerosol using the double-moment Global Model of Aerosol Processes (GLOMAP-mode) model, which is coupled to the United Kingdom Chemistry and Aerosol (UKCA) model in the UM and cumulatively denoted UKCA-mode (Mann et al, 2010;Mulcahy et al, 2018;Jones et al, 2021). Whilst UKCA-mode has in-built functionality to represent mineral dust in two insoluble modes representing accumulation-sized and coarse-sized (diameters of d p ≥ 1 µm) particles, this scheme has never been the default option in the Met Office Global Atmosphere science configuration -which forms the physical atmosphere in the UK's Earth system model -owing to the lack of fidelity between simulations and observations, with UKCA-mode dust exhibiting dust concentrations that are too high away from source regions.…”

Radiative impact of improved global parameterisations of oceanic dry deposition of ozone and lightning-generated NOx

“…Szopa et al (2021) report that there is a relatively small negative contribution to ERF through formation of nitrate aerosols. Recently, a nitrate scheme was incorporated in UKCA (Jones et al, 2021), and this should be tested in the future to examine the impact of nitrate aerosol from lightning on radiation. Although our model does not include a nitrate aerosol scheme, the LNO x changes would impact aerosol through perturbations to background tropospheric oxidants, for example increases in aerosol abundances due to faster oxidation rates of sulfur to sulfate as LNO x is increased (Murray, 2016).…”

Response to Reviewers on acp-2021-400 by Anthony Jones

“…GA7.1's default GLOMAP-mode configuration includes the aerosols sulphate (SO4), black carbon (BC), organic matter (OM), and sea-salt (SS), with species in each mode treated as an internal mixture. Mineral dust is represented in GA7.1 by the CLASSIC 6 bin scheme described by Woodward et al (2001). Aerosol processes in GLOMAP-mode include binary homogeneous SO4 nucleation, mode-merging, cloud processing, condensational uptake of H2SO4 gas and secondary organic vapour from UKCA, and coagulation within and between modes.…”