Global sensitivities of reactive N and S gas and particle concentrations and deposition to precursor emissions reductions
Authors: Ge Yao , Vieno Massimo , Stevenson David S. , Wind Peter , Heal Mathew R. 
Published: October 2022

Abstract. The reduction of fine particles (PM2.5) and reactive N (Nr) and S (Sr) 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 Nr, Sr, and PM2.5 to 20 % and 40 % individual and collective reductions in anthropogenic emissions of NH3, NOx, and SOx (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 % NOx emissions reductions decrease NOx 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 NOx to NO3- and of SO2 to SO42- is enhanced in East Asia but decreased in South Asia, which in East Asia causes a more effective decrease in NOx and SO2 but a less effective decrease in NO3- and even an increase in SO42-; in South Asia it causes a less effective decrease in NOx and an increase in SO2 but a more effective decrease in NO3- and SO42-. For regional policy making, it is thus important to reduce NH3, NOx and SOx emissions together and/or go for stronger reductions to minimise such adverse effects in East Asia and Euro_Medi. Reductions in SOx emissions are slightly more effective for SO2 than SO42-. A disbenefit is that SOx emissions reductions increase NH3 total deposition and ecosystem eutrophication (~12 % increase for 40 % emissions reduction). PM2.5 mitigation in South Asia is most sensitive to 40 % SOx 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 NOx emissions with an 8 % (0.63 μg m-3) decrease in PM2.5 in response to a 40 % reduction. In Euro_Medi, the sensitivities of PM2.5 to 40 % individual emissions reductions range 5–8 % (0.55–0.82 μg m-3). In the UK and Scandinavia PM2.5 is more sensitive to NH3, in central Europe it is more sensitive to NOx, while in the Mediterranean it is more sensitive to SOx. In East Asia, reductions in SOx, NOx and NH3 emissions are almost equally effective with PM2.5 sensitivities to 40 % reductions of 7–8 % (1.89–2.33 μg m-3). Due to the varying contributions of SIA, PM2.5 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 Nr, Sr, and PM2.5 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.