Simulation of dinitrogen pentoxide and nitryl chloride in China with the WRF-chem model : sources, abundance and roles in the formation of secondary air pollutants

Abstract

Dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂) are important nitrogen reservoirs and have significant impacts on the reactive nitrogen budget, atmospheric oxidative capacity and the formation of secondary air pollutants. Previous studies have reported elevated concentrations of N₂O₅ and ClNO₂ at several observation sites in North America, west Europe and China. However, the abundance of the two nitrogen species on a regional scale in China has not been quantified, and their impact on the photochemical and haze pollution has not been evaluated in China. The first part of the thesis is the application of meteorological analysis and a dispersion model to support the analysis of N₂O₅ and ClNO₂ measured in three field campaigns in China. The second and the main component of the thesis is the incorporation of N₂O₅ and ClNO₂ processes into a widely-used chemical transport model, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The third constituent of the thesis is the validation and application of the updated WRF-Chem model in southern China in winter of 2013 and in China in summer of 2014. The fourth part of the thesis is the application of an improved WRF-Chem model with reactive nitrogen chemistry to evaluate the effect of the nitrogen chemistry on the O₃ sensitivity to its precursors. Meteorological analysis and a dispersion model, the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), were adopted to identify the source regions and the history of the air masses that arrived at sampling sites during three measurement campaigns of N₂O₅ and ClNO₂, which were conducted at Mt. TMS in Hong Kong in winter 2013, at Wangdu and Mt. Tai in northern China in summer 2014. The results suggested that the HYSPLIT could identify the source regions and air mass histroy affecting the observations, determine the fractions of the air masses from different altitudes above ground level, and explain the variations of the total reactive nitrogen (NOy), N₂O₅ and ClNO₂ concentrations during these campaigns, including the common variations and the variation under extreme synoptic conditions. For the investigation of the N₂O₅ and ClNO₂ chemistry, WRF-Chem model was further developed in this study to incorporate pre-requisite relevant chemistry that was not well-represented in the model, including an aerosol thermodynamic module, ISORROPOIA II, the heterogeneous uptake of N₂O₅ on aerosol surface, the subsequent production of ClNO2, and the gaseous chlorine chemistry initialited by the chlorine radical. The revised WRF-Chem model was then validated by the N₂O₅ and ClNO2 concentrations measured in the TMS campaign in winter of 2013. The updated model is capable of reproducing the concentration level and temporal pattern of the observed N2O5 and ClNO2 but overestimates N₂O₅ uptake and ClNO₂ production. The validated WRF-Chem model was applied in the Hong Kong - Pearl River Delta (HK-PRD) region during the period of TMS campaign in which the photochemical pollution was severe in southern China. The model results suggested that under average meteorological conditions, elevated levels of ClNO₂ (>0.25 ppb within the boundary layer) were present in the south-western PRD instead of over HK area. The addition of N2O5 heterogeneous uptake and ClNO₂ production reduced the NO and NO2 levels by as much as 1.93 ppb (~7.4%) and 4.73 ppb (~16.2%), respectively, increased the total nitrate and ozone concentrations by up to 13.45 µg m-3 (~ 57.4%) and 7.23 ppb (~16.3%), respectively, in the boundary layer. Sensitivity tests showed that the simulated chloride and ClNO₂ concentrations were highly sensitive to the emission of chlorine species. The study suggests the need to measure the vertical profiles of N₂O₅ and ClNO₂ under various meteorological conditions, to consider the chemistry of N₂O₅ and ClNO₂ in the chemical transport model, and to develop an updated chlorine emission inventory over China. An updated chlorine emission inventory, Reactive Chlorine Emission Inventory - China (RCEI-China), has been proposed in this study. The RCEI-China dataset was then applied in the WRF-Chem implemented with detailed N2O5 and ClNO₂ processes to simulate the spatial and temporal distribution of chloride, N₂O₅, and ClNO₂, and to investigate their roles in secondary pollution in China in the period of Wangdu campaign in which O₃ and particulate matter were at elevated levels in northern China. The results demonstrated elevated N₂O₅ and ClNO₂ concentrations in China, with the higher mixing ratios over the city clusters and their adjacent regions. The vertical distribution of ClNO₂ appeared to affect the temporal variation of ClNO₂ at the surface. The heterogeneous uptake of N₂O₅ on aerosol (without ClNO₂ production) reduced O₃ concentration and significantly increased the formation of total nitrate. The further production of ClNO₂ substantially enhanced the ozone production across China and slightly decreased the total nitrate. Tropospheric ozone pollution has been a major environmental issue in the last decades throughout the world. The control of such persistent problem demands the comprehensive understanding of the ozone formation mechanism and the sensitivity of ozone to its precursors, nitrogen oxides (NOx) and volatile organic compound (VOC). Recent research proposed the 'new' sources of nitrous acid (HONO) and the 'new' processes of heterogeneous uptake of N₂O₅ and the production of ClNO which can significantly affects the budget of radicals and NOx, both of which determine the formation of ozone and the relationship of ozone to the precursors. However, these 'new' chemistry of NOy were not considered in the previous studies on the ozone sensitivity to NOx and VOC. A revised WRF-Chem model, incorporated with the 'new' nitrogen chemistry, was adopted to simulate the impacts of nitrogen chemistry on the prediction of ozone sensitivity regime in China in summer when photochemical ozone pollution is severe. The results showed that the nitrogen chemistry significantly increased the level of ROx radicals and reduced the concentration of NOx. The spatial patterns of the ozone sensitivity regime simulated by original and revised WRF-Chem were noticeably different, and ~40% of the area in which ozone formation is influenced by anthropogenic emissions changed the regime because of the nitrogen chemistry, mostly from VOC-sensitive to mixed-sensitive and from NOx-sensitive to mixed-sensitive. Our simulations indicated that the nitrogen chemistry strongly changes the ozone isopleths in major cities in China, and implied different strategy in controlling O₃ pollution. This study suggested the need to consider the new NOy chemistry in evaluating the efficacy of current and near-future policy. Overall, this thesis investigates the sources and abundance of N₂O₅ and ClNO₂ and the roles of N₂O₅ and ClNO₂ chemistry in the photochemical and haze pollution. The methods and models developed in this study can be applied to other regions, and the findings of this study can be used as a scientific basis to formulate air pollution control policy.