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

Abstract

Abstract. We used the EMEP MSC-W (European Monitoring and Evaluation Programme Meteorological Synthesizing Centre – West) model version 4.34 coupled with WRF (Weather Research and Forecasting) model version 4.2.2 meteorology to undertake a present-day (2015) global and regional quantification of the concentrations, deposition, budgets, and lifetimes of atmospheric reactive N (Nr) and S (Sr) species. These are quantities that cannot be derived from measurements alone. In areas with high levels of reduced Nr (RDN = NH3+ NH4+), oxidized Nr (OXN = NOx+ HNO3+ HONO + N2O5 + NO3-+ “Other OXN” species), and oxidized Sr (OXS = SO2+ SO42-), RDN is predominantly in the form of NH3 (NH4+ typically <20 %), OXN has majority gaseous species composition, and OXS predominantly comprises SO42- except near major SO2 sources. Most continental regions are now “ammonia rich”, more so than previously, which indicates that, although reducing NH3 emissions will decrease the RDN concentration, decreasing these emissions will have little effect on mitigating secondary inorganic aerosol (SIA). South Asia is the most ammonia-rich region. Coastal areas around East Asia, northern Europe, and the north-eastern United States are “nitrate rich” where NH4NO3 formation is limited by NH3. These locations experience transport of OXN from the adjacent continent and/or direct shipping emissions of NOx, but NH3 concentrations are lower. The least populated continental areas and most marine areas are “sulfate rich”. Deposition of OXN (57.9 TgN yr−1, 51 %) and RDN (55.5 TgN yr−1, 49 %) contribute almost equally to total nitrogen deposition. OXS deposition is 50.5 TgS yr−1. Globally, wet and dry deposition contribute similarly to RDN deposition; for OXN and OXS, wet deposition contributes slightly more. Dry deposition of NH3 is the largest contributor to RDN deposition in most regions except for the Rest of Asia area and marine sectors where NH3 emissions are small and RDN deposition is mainly determined by the transport and rainout of NH4+ (rather than rainout of gaseous NH3). Thus, reductions in NH3 would efficiently reduce the deposition of RDN in most continental regions. The two largest contributors to OXN deposition in all regions are HNO3 and coarse NO3- (via both wet and dry deposition). The deposition of fine NO3- is only important over East Asia. The tropospheric burden of RDN is 0.75 TgN, of which NH3 and NH4+ comprise 32 % (0.24 TgN; lifetime of 1.6 d) and 68 % (0.51 TgN; lifetime of 8.9 d) respectively. The lifetime of RDN (4.9–5.2 d) is shorter than that of OXN (7.6–7.7 d), which is consistent with a total OXN burden (1.20 TgN) almost double that of RDN. The tropospheric burden of OXS is 0.78 TgS with a lifetime of 5.6–5.9 d. Total nitrate burden is 0.58 TgN with fine NO3- only constituting 10 % of this total, although fine NO3- dominates in eastern China, Europe, and eastern North America. It is important to account for contributions of coarse nitrate to global nitrate budgets. Lifetimes of RDN, OXN, and OXS species vary by a factor of 4 across different continental regions. In East Asia, lifetimes for RDN (2.9–3.0 d), OXN (3.9–4.5 d), and OXS (3.4–3.7 d) are short, whereas lifetimes in the Rest of Asia and Africa regions are about twice as long. South Asia is the largest net exporter of RDN (2.21 TgN yr−1, 29 % of its annual emission), followed by the Euro_Medi region. Despite having the largest RDN emissions and deposition, East Asia has only small net export and is therefore largely responsible for its own RDN pollution. Africa is the largest net exporter of OXN (1.92 TgN yr−1, 22 %), followed by Euro_Medi (1.61 TgN yr−1, 26 %). Considerable marine anthropogenic Nr and Sr pollution is revealed by the large net import of RDN, OXN, and OXS to these areas. Our work demonstrates the substantial regional variation in Nr and Sr budgets and the need for modelling to simulate the chemical and meteorological linkages underpinning atmospheric responses to precursor emissions.

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

“…195 In addition, the impacts of NH3 and SOx emissions reductions on NH4 + concentrations over North Africa are significantly greater than from NOx emissions reductions, indicating a dominance of (NH4)2SO4 within SIA in this region. This is consistent with the results reported by Ge et al (2022). They showed that large areas in North Africa are characterised by the SO4 2--rich chemical domain for SIA formation, which means that NH3 is predominantly taken up by SO4 2-, leaving no free NH3 to react with HNO3 to form NH4NO3.…”

“…All four regions are densely populated and have high Nr and Sr pollution as reported by Ge et al (2022).…”

“…Detailed global EMEP-WRF configurations used in this work are presented in Ge et al (2021bGe et al ( , 2022. In brief, the global domain has a horizontal resolution of 1° × 1° and 21 terrain-following vertical layers from the surface up to 100 hPa.…”

“…The simulated global baseline 2015 annual mean surface concentrations of Nr and Sr have been discussed in detail in Ge et al (2022). Here we analyse the sensitivities of the modelled surface concentrations to SIA precursor emissions reductions for RDN, OXN and OXS components.…”

“…In general, the oxidation rate decreases with decreased pH (Penkett et al, 1979;Maahs, 1983;Liang and Jacobson, 1999;Hattori et al, 2021), a process that is incorporated 320 into the EMEP MSC-W model (Simpson et al, 2012). As pointed out by Ge et al (2022), Europe and north-eastern China are much less ammonia-rich than India, which means that 20% and 40% NH3 emissions reductions are substantial enough to decrease the pH in the former two regions, leading to decreases in SO4 2production. However, given that SO2 levels increase and that there are still other effective oxidation pathways (e.g., OH, H2O2) which are independent of pH (McArdle and Hoffmann, 1983;Hoffmann, 1986;Seinfeld and Pandis, 2016), the decreases in SO4 2concentrations in Europe and north-325 eastern China due to NH3 emissions reductions are very small anyway.…”