A new assessment of global and regional budgets, fluxes and lifetimes of atmospheric reactive N and S gases and aerosols
Authors: Ge Yao , Vieno Massimo , Stevenson David , Wind Peter , Heal Mathew 
Published: March 2022

Abstract. We used the EMEP MSC-W model version 4.34 coupled with WRF 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+), oxidised Nr (OXN = NOx + HNO3 + HONO + N2O5 + NO3- + Other OXN species), and oxidised 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’, and more so than previously, which indicates that whilst reducing NH3 emissions will decrease RDN concentration it will have small effect on mitigating SIA. South Asia is the most ammonia-rich region. Coastal areas around East Asia, northern Europe, and 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 Rest of Asia and marine areas where NH3 emissions are small and RDN deposition is mainly determined by transport and rainout of NH4+ (rather than rainout of gaseous NH3). Reductions in NH3 would thus 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 = 1.6 days) and 68 % (0.51 TgN; lifetime = 8.9 days), respectively. The lifetime of RDN (4.9–5.2 days) is shorter than that of OXN (7.6–7.7 days), 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 days. 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 days), OXN (3.9–4.5 days), and OXS (3.4–3.7 days) are short, whereas lifetimes in Rest of Asia and Africa 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 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.