A new assessment of global and regional budgets, fluxes and lifetimes of atmospheric reactive N and S gases and aerosols

Abstract

<p>Air pollution has many effects on health and ecosystems. Of concern are high levels of reactive nitrogen (N<sub>r</sub>) and sulfur (S<sub>r</sub>) species. We use the EMEP MSC-W atmospheric chemistry and transport model driven by WRF meteorology (1º×1º resolution) to provide an updated evaluation of the global and regional concentrations, depositions, budgets, and lifetimes of reduced N<sub>r</sub> (RDN = NH<sub>3</sub> + NH<sub>4</sub><sup>+</sup>), oxidised N<sub>r</sub> (OXN = NO<sub>x</sub> + HNO<sub>3</sub> + HONO + N<sub>2</sub>O<sub>5</sub> + orgN + NO<sub>3</sub><sup>-</sup>) and oxidised S<sub>r</sub> (OXS = SO<sub>2</sub> + SO<sub>4</sub><sup>2-</sup>). Both HTAP (2010) and ECLIPSE<sub>E</sub> (ECLIPSE annual total with EDGAR monthly profile; 2010 and 2015) emissions inventories were used. Modelled surface concentrations and wet deposition are validated against measurements from 10 monitoring networks worldwide. Simulations of primary pollutants are somewhat sensitive to the choice of inventory in places where regional differences in emissions between the two inventories are apparent (e.g., East Asia), but much less so for secondary components. Comparisons between model and measurement demonstrate that the model captures well the overall spatial and seasonal variations of gas and particle N<sub>r</sub> and S<sub>r</sub> concentrations and their wet deposition in Europe, North America, Southeast Asia, and East Asia, although slightly less well in the latter region. The greater uniformity in spatial correlations than in biases suggests that the major driver of model-measurement discrepancies (aside from differing spatial representativeness and uncertainties in measurements) are shortcomings in absolute emissions rather than in modelling the atmospheric processes. Most populated regions are now NH<sub>3</sub>-rich with respect to secondary inorganic aerosol formation, and increasingly so as SO<sub>2</sub>and NO<sub>x</sub> emissions decline. Near-continent marine areas with major shipping are NO<sub>3</sub><sup>-</sup> rich. Global total deposition of RDN, OXN, and OXS in 2015 are 53.0 TgN, 55.3 TgN, and 49.6 TgS respectively. Dry deposition of NH<sub>3</sub> is the dominant form of RDN deposition in most continental regions, whereas in marine areas wet deposition of NH<sub>4</sub><sup>+</sup> (derived from particle NH<sub>4</sub><sup>+</sup> rather than rainout of NH<sub>3</sub>) contributes most. The dominant contributors to OXN deposition are wet and dry deposition of HNO<sub>3</sub> and coarse NO<sub>3</sub><sup>-</sup>. For OXS deposition, dry-deposited SO<sub>2</sub> and wet-deposited SO<sub>4</sub><sup>2-</sup> are the two largest contributors in all regions. The global lifetime of RDN (~4.2 days) is shorter than that of OXN (~6.7 days), consistent with a tropospheric OXN burden (1.04 TgN) almost double that of RDN (0.61 TgN). The tropospheric burden of OXS is 0.71 TgS with a global lifetime of ~5.3 days. Regional analyses show that South Asia and Europe are the two largest net exporters of RDN and OXN. Despite East Asia having the largest RDN emissions and deposition, the small net export shows this region is largely responsible for its own RDN pollution. Considerable marine N pollution is caused by large net export of RDN and OXN from continental areas. Our results reveal substantial regional variation in contributions of different components to N<sub>r</sub> and S<sub>r</sub> budgets and the need for modelling to reveal the chemical and meteorological linkages between emissions and atmospheric responses.</p>