Author: Dai, Jianing
Title: Numerical studies of the impact of oceanic emissions on maritime ozone in a tropical region of east Asia
Advisors: Wang, Tao (CEE)
Degree: Ph.D.
Year: 2021
Subject: Air -- Pollution
Ships -- Fuel
Combustion gases -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xx, 141 pages : color illustrations
Language: English
Abstract: Ozone (O3) is an important greenhouse gas that contributes directly to global warming and has an adverse impact on human health and vegetation. The formation of O3 is influenced by the emission of precursors, such as oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs), the level of radicals for oxidizing the precursors, and the meteorological conditions that affect the emissions and chemical processes of the precursors and atmospheric transport patterns. While it has been known that urban and industrial regions of China have experienced severe O3 pollution, the long-term O3 trend and its drivers are still poorly understood. To better understand the factors affecting O3 pollution, it is necessary to investigate reactive nitrogen species, which have also not been well understood. Specifically, nitryl chloride (ClNO2) and nitrous acid (HONO) can recycle NOx and produce hydroxyl (OH•) and chlorine (Cl•) radicals after their photolysis and thereby affect the reactive nitrogen budget, atmospheric oxidative capacity, and the formation of secondary air pollutants. Previous studies have mostly focused on the formation of these reactive nitrogen species through the land-based emissions and their subsequent impact on haze and photochemical O3 in polluted land area. However, the impact of marine emissions, such as sea-salt emission and ship emissions, on the formation of ClNO2 and HONO and their subsequent impact on secondary pollution have rarely been evaluated. The first part of this thesis analyzes the long-term O3 trend in the lower troposphere in Hong Kong and the factors affecting O3 variations. The second part is the validation of the model performance of N2O5, ClNO2, and particulate chloride by a Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) in early autumn of 2018 and the evaluation of the impact of sea-salt chloride on the formation of ClNO2 and O3 in a coastal area of south China. The third part is application of a further improved WRF-Chem model with updated reactive nitrogen chemistry to evaluate the impact of ship emissions on the O3 and fine particulate matter (PM2.5) in the summer of 2017 in Asia. The major results are summarized below. The long-term (26 years) O3 and CO measurements at a background site in Hong Kong (Hok Tsui; HT) were analyzed. An overall increase was found in the O3 level (0.40 ppbv year-1 (yr-1)). We also found a sizeable O3 increase (~20% per decade) in the air from the marine area. Results from modeling showed that recent weather conditions had reduced maritime O3, counteracting the impact of growing Southeast Asia's emissions. The ozone trend in another background site (Tap Mun; TM) in Hong Kong was also evaluated to support the analysis of ozone trends. For the TM site, we found an overall increase rate of O3 by 0.17 ppbv yr-1 during 1999-2019. The significant increase in O3 trend showed in different air masses but not displayed in different seasons, indicating the contribution of long-range transport of O3 and its precursors at TM site. The number of O3 exceedance days and the trend of maximum daily 8-h average (MD8A) O3 during these days increased in both sites. The result indicated that extensive ozone pollution still occurred in south China, and more measures are needed to mitigate the ozone pollution in this region. In analyzing the vertical distribution of O3 measured by ozonesonde, we found that the average O3 mixing ratio during 1994-2019 increased by 0.59 ppbv yr-1, 0.41 ppbv yr-1, and 0.29 ppbv yr-1 at 0.5 km, 3km, and 6km, respectively. These results reveal an increasing trend in the lower tropospheric O3 on the South China coast. The interannual O3 variations were potentially affected by the El Nino event, especially at the upper layers. Large springtime O3 increases were also found in South Asia air masses by 0.35 ppbv yr-1 and 0.24 ppbv yr-1 below 6km and at the surface, potentially affected by the biomass burning emissions in the region.
We also investigated the impact of sea-salt chloride on O3 formation by a revised WRF-Chem model with up-to-date chloride chemistry. This model was applied to examine the effects of sea-salt chloride through the heterogenous reactions on O3 formation during two O3 episodes in early autumn (September 2017 and 2018) over the Hong Kong-Pearl River Delta and surrounding maritime regions. During the maritime inflow phase, the fine particulate Cl- from sea salt emissions penetrated deep inland and was depleted by up to 40% through N2O5 heterogeneous reaction. This depletion of fine particulate Cl-further leads to elevated ClNO2 mixing ratios (up to 0.6 ppbv) produced at night. During continental winds, the oceanic particulate Cl- in the coastal areas was depleted by 18-33% by the heterogeneous reaction of N2O5, leading to an increase in ClNO2 mixing ratio up to 0.8 ppbv in the residual layer (~300 m). The production of ClNO2 from sea-salt chloride increased the O3 mixing ratios by up to 2.0 ppbv (4%) over the inland areas during marine winds and up to 3.8 ppbv (5.5%) and 6.5 ppbv (7.6%) over the South China Sea. This study highlights the considerable impact of reactive nitrogen on O3 formation through the heterogeneous reaction in coastal regions. The WRF-Chem model was also applied to evaluate the impact of emissions from international shipping on the formation of O3 and PM2.5 during summer in Asia. We found that the ship-derived HONO and ClNO2 increased ROx radicals' concentration approximately two to three times in the marine boundary layer. The enhanced radicals then increased the O3 and PM2.5 concentrations in marine areas, with the ship contributions rising from 9% to 21% and from 7% to 10%, respectively. The most considerable ROx enhancement was simulated over the remote ocean with the ship contribution increasing from 29% to 50%, which led to increases in ship-contributed O3 and PM2.5 from 21% to 38% and 13% to 19%, respectively. In coastal cities, the enhanced levels of radicals also increased the maximum ship contribution to O3 and averaged PM2.5 concentrations from 5% to 11% and from 4% to 8% to 4% to 12%, respectively. These findings indicate that modeling studies without considering HONO and ClNO2 can significantly underestimate ship emissions' impact on radicals and secondary pollutants. Therefore, it is important that these nitrogen compounds be included in future models to evaluate the effects of ship emissions on air quality. Overall, this thesis investigates the long-term trend of O3 in the lower troposphere in South Asia and illustrates the increase in summertime O3 due to marine emissions. We also evaluate the impact of oceanic source of ClNO2 and HONO on the formation of O3. It underscores the importance of reactive nitrogen chemistry in assessing the effects of marine emissions. The findings can be used as a scientific basis to formulate control policies for ship emissions.
Rights: All rights reserved
Access: open access

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