Global sensitivities of reactive N and S gas and particle concentrations and deposition to precursor emissions reductions

Abstract. The reduction of fine particles (PM_2.5) and reactive N (N_r) and S (S_r) species is a key objective for air pollution control policies because of their major adverse effects on human health, ecosystem diversity, and climate. The sensitivity of global and regional N_r, S_r, and PM_2.5 to 20 % and 40 % individual and collective reductions in anthropogenic emissions of NH₃, NO_x, and SO_x (with respect to a 2015 baseline) is investigated using the EMEP MSC-W atmospheric chemistry transport model with WRF meteorology. Regional comparisons reveal that the individual emissions reduction has multiple co-benefits and small disbenefits on different species, and those effects are highly geographically variable. Reductions in NH₃ emissions are effective at decreasing NH₃ concentrations and deposition but much less so for NH₄⁺. A 40 % NH₃ emissions reduction decreases regional average NH₃ concentrations by 47–49 %, while sensitivities of NH₄⁺ concentrations decrease in the order Euro_Medi (Europe and Mediterranean, 18 %), East Asia (15 %), North America (12 %), and South Asia (4 %), reflecting the increasing regional ammonia-richness. A disbenefit is the increased SO₂ concentrations in these regions (10–16 % for 40 % NH₃ emissions reductions) because reduced NH₃ levels decrease SO₂ deposition by altering atmospheric acidity. The 40 % NO_x emissions reductions decrease NO_x concentrations in East Asia by 45 %, Euro_Medi and North America by ~38 %, and South Asia by 22 %, whilst decreases in fine NO₃^- are regionally reversed, which is related to enhanced O₃ levels in East Asia (and also, but by less, in Euro_Medi), and decreased O₃ levels in South Asia (and also, but by less, in North America). Consequently, the oxidation of NO_x to NO₃^- and of SO₂ to SO₄^2- is enhanced in East Asia but decreased in South Asia, which in East Asia causes a more effective decrease in NO_x and SO₂ but a less effective decrease in NO₃^- and even an increase in SO₄^2-; in South Asia it causes a less effective decrease in NO_x and an increase in SO₂ but a more effective decrease in NO₃^- and SO₄^2-. For regional policy making, it is thus important to reduce NH₃, NO_x and SO_x emissions together and/or go for stronger reductions to minimise such adverse effects in East Asia and Euro_Medi. Reductions in SO_x emissions are slightly more effective for SO₂ than SO₄^2-. A disbenefit is that SO_x emissions reductions increase NH₃ total deposition and ecosystem eutrophication (~12 % increase for 40 % emissions reduction). PM_2.5 mitigation in South Asia is most sensitive to 40 % SO_x reduction (3.10 μg m^-3, 10 %) and least sensitive to NH₃ reduction (0.29 μg m^-3, 1 %), which is because South Asia is so ammonia-rich that reducing NH₃ has little impact. The most effective measure for North America is reducing NO_x emissions with an 8 % (0.63 μg m^-3) decrease in PM_2.5 in response to a 40 % reduction. In Euro_Medi, the sensitivities of PM_2.5 to 40 % individual emissions reductions range 5–8 % (0.55–0.82 μg m^-3). In the UK and Scandinavia PM_2.5 is more sensitive to NH₃, in central Europe it is more sensitive to NO_x, while in the Mediterranean it is more sensitive to SO_x. In East Asia, reductions in SO_x, NO_x and NH₃ emissions are almost equally effective with PM_2.5 sensitivities to 40 % reductions of 7–8 % (1.89–2.33 μg m^-3). Due to the varying contributions of SIA, PM_2.5 sensitivities to 40 % collective reductions in all 3 precursors decrease in the order East Asia (20 %), Euro_Medi and North America (17 %), South Asia (13 %). The geographically-varying non-linear chemical responses of N_r, S_r, and PM_2.5 to emissions reductions revealed by this work show the importance of both prioritising emissions strategies in different regions and combining several precursor reductions together to maximise the policy effectiveness.