Modelling of global atmospheric reactive nitrogen and sulfur and their mitigation strategies
The reduction of atmospheric reactive N (Nr) and S (Sr) species is a key objective for air quality control policies because they contribute to the formation of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm), which have significant effects on human health and climate, and their deposition affect ecosystem biodiversity. As a range of emission sources and atmospheric chemical and physical processes contribute to Nr and Sr concentrations, atmospheric chemistry transport models (ACTMs) are an essential tool to identify important processes controlling their impacts and effective mitigation. Here, the EMEP MSC-W ACTM coupled with WRF meteorology at 1º×1º resolution is used. The chemical climate for Nr and Sr pollution has changed dramatically in the past two decades, it is therefore necessary to update our understanding of global Nr and Sr chemistry and investigate their mitigation under current atmospheric state. Thus, the aims of the thesis are (i) to evaluate the global model simulation of reduced N (RDN), oxidised N (OXN), and oxidised S (OXS) against ambient measurements; (ii) to analyse current global and regional budgets, fluxes, and lifetimes of RDN, OXN, and OXS, quantifying important processes controlling their regional distributions in the atmosphere; and (iii) to quantify the sensitivities of emissions reductions for the mitigation of PM2.5, Nr and Sr species. Firstly, the evaluation of the EMEP MSC-W model is conducted both spatially and temporally, covering 10 monitoring networks worldwide measuring surface concentrations and wet deposition. Model simulations for 2010 compared use of both HTAP and ECLIPSEE (ECLIPSE annual total with EDGAR monthly profile) emissions inventories; those for 2015 used ECLIPSEE only. 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., China), but much less so for secondary components. Comparisons of 2010 and 2015 annual concentrations between model and measurement demonstrate that the model captures well the overall spatial and seasonal variations of the major inorganic pollutants NH₃, NO₂, SO₂, HNO₃, NH₄⁺, NO₃⁻, SO₄²⁻ , and their wet deposition in East Asia, Southeast Asia, Europe, and North America. The model shows better correlations with annual average measurements for networks in Southeast Asia (Mean R for 7 species: /R₇ = 0.73), Europe ( /R₇ = 0.67) and North America ( /R₇ = 0.63) than in East Asia ( /R₅ = 0.35) (data for 2015), which suggests potential issues with the measurements in the latter network. The evaluation of wet deposition shows that the greatest consistency is in North America (R: 0.75-0.82), followed by Southeast Asia (R: 0.51-0.68), Europe (R: 0.61-0.64), and East Asia (R: 0.13-0.59). Model-measurement bias varies between species in different networks. The greater uniformity in spatial correlations than in biases as revealed in this work suggests that the major driver of model-measurement discrepancies (aside from their differing spatial representativeness and uncertainties in measurements) are shortcomings in absolute emissions rather than in modelling the atmospheric processes. Secondly, the EMEP-WRF model is used to undertake a present-day (2015) global and regional quantification of Nr and Sr concentrations, fluxes, and lifetimes, which are quantities that cannot be derived from measurements alone. In areas with high levels of RDN (NH3 + NH4+), OXN (NOx + HNO3 + HONO + N2O5 + NO3- + Other OXN species), and 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 in areas 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 little effect on mitigating secondary inorganic aerosol (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. Dry deposition of NH3 is the largest contributor to RDN deposition in most continental regions except for remote areas where NH3 emissions are small and RDN deposition is mainly determined by transport of NH4+. The two largest contributors to OXN deposition in all regions are HNO3 and coarse NO3- (via both wet and dry deposition). The tropospheric lifetime of NH3 (1.6 days) is much shorter than that of NH4+ (8.9 days), consistent with a global NH4+ burden (68% of total RDN burden) almost double that of NH3 (32%). Fine NO3- only constitutes 10% of global nitrate burden, albeit fine NO3- dominates in eastern China, Europe, and eastern North America. It is therefore important to account for contributions of coarse nitrate to tropospheric nitrate budgets. Lifetimes of RDN, OXN, and OXS species vary by a factor of 4 across continental regions with East and Southeast Asia generally having the shortest lifetimes. South Asia is the largest net exporter of RDN (2.21 TgN yr-1, 29% of its annual emission) and OXS (1.62 TgS yr-1, 37%). Africa is the largest net exporter of OXN (1.92 TgN yr-1, 22%). 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. Finally, the sensitivity of Nr, Sr, and PM2.5 to 20% and 40% individual and collective reductions in anthropogenic emissions of NH3, NOx, and SOx relative to the 2015 baseline is investigated. 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. A 40% NH3 emission reduction decreases regional average NH3 concentrations by 47-49%, but only decreases NH4+ by 18% in Euro_Medi, 15% in East Asia, 12% in North America, and 4% in South Asia. This order follows the regional ammonia-richness. A disbenefit is the increased SO2 concentrations in these regions (10-16% for 40% reductions) because reduced NH3 levels decrease SO2 deposition through altering atmospheric acidity. A 40% NOx emission reduction reduces NOx concentrations in East Asia by 45%, Euro_Medi and North America by ~38%, and South Asia by 22%, whilst the regional order is reversed for fine NO3-, 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 causes a less effective decrease in NO3- and even an increase in SO42- in East Asia, but quite the opposite in South Asia. For regional policy making, it is thus vital to reduce three precursors together to minimise such adverse effects. A 40% SOx emission reduction is slightly more effective in reducing SO2 (42-45%) than SO42- (34-38%), whilst the disbenefit is that it yields ~12% increase in NH3 total deposition in the four regions which further threatens ecosystem diversity. This work also highlights important messages for policy-makers concerning the mitigation of PM2.5. More emissions controls focusing on NH3 and NOx are necessary for regions with better air quality such as northern Europe and eastern North America. In East Asia, the three individual reductions are equally effective, whilst in South Asia only SOx reduction is currently effective. The geographically-varying non-one-to-one proportionality of 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. In summary, the comprehensive model evaluation, as the opening study of this thesis, supports the application of this model framework for analysis of current Nr and Sr budgets (second study) which then provides theoretical explanations for responses of Nr and Sr to potential emission controls (third study). By demonstrating the model’s capability of simulating global atmospheric chemistry and transport and presenting an update of current atmospheric state in terms of particle formation, this thesis provides useful suggestions for global and regional policymakers to mitigate PM2.5 pollution and reduce N and S deposition.