Formation mechanisms of organic nitrates and their impacts on air quality in Hong Kong

Abstract

Organic nitrates are important constituents of reactive nitrogen species (NOy) in the atmosphere. Alkyl nitrates (RONO2) and peroxyacetyl nitrate (PAN), as two major organic nitrates, are both formed as byproducts in the process of ozone (O3) formation, through reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx). Due to relatively long lifetimes, they act as temporary reservoirs of nitrogen, hence regulating nitrogen cycling as well as O3 formation. Besides, PAN is also important in photochemical smog and causes human eye irritation and affects vegetation growth at high levels. Considering the impact of organic nitrates on atmospheric chemistry on local, regional and even global scale, research on their temporal variations, sources and formation mechanism, as well as their impacts on local O3 production was conducted in this thesis. Collection of whole air samples was carried out from 2002 to 2016 in autumn in Hong Kong. During this period, C1-C4 RONO2 all experienced dramatic increase (p < 0.05), while 2-BuONO2 had the highest increasing rate (1.33 ± 0.28 pptv/yr). Propane (C3H8), one of the parent hydrocarbons, also showed remarkable upward trend (p < 0.05), which partially explained the increase of n/i-PrONO2, while other VOC precursors including parent hydrocarbons and other VOCs remained unchanged or even decreased. Furthermore, the significant decrease of both NO and NO2 and increase in atmospheric oxidative capacity fueled the photochemical formation and contributed to the elevation in C1-C4 RONO2. Subsequently, sources of RONO2 were identified and quantified for the past 15 years in autumn. Photochemical formation always contributed the most to ambient RONO2 in Hong Kong, followed by biomass burning and oceanic emissions. According to the temporal variations of source contributions, secondarily formed RONO2 kept increasing and contributed to elevated ambient RONO2. Biomass burning source also caused the elevation in RONO2 in past years, especially during 2002-2013. Apart from the long-term variations in Hong Kong, the spatial distributions of RONO2 in the Pearl River Delta (PRD) region were further studied using the data collected in 2018. More intensive RONO2 pollution in the western PRD and higher RONO2 levels in Hong Kong when northwesterly wind dominated confirmed the contribution of regional transport from inland PRD to Hong Kong. In addition, photochemical formation of C1-C5 RONO2 was explored in suburban Hong Kong (TC site) as well as over the South China Sea (WSI site) during an intensive sampling campaign in summer/autumn 2013. It was found that propane and n-butane were significantly lower (p < 0.05) at WSI, while C3-C4 RONO2 were comparable at two sites, which was possibly attributed to higher formation efficiency of RONO2 at the offshore site. This study for the first time applied relative incremental reactivity (RIR) to evaluate RONO2-precursor relationships. Contrary to the consistent VOCs-controlled regime at TC, RONO2 production at WSI switched from VOC-limited regime during O3 episodes to VOCs and NOx co-limited regime during non-episodes when NOx level was low. At both sites, higher NOx on O3 episodes was generally accompanied with regional transport from PRD region. Unlike C2-C5 RONO2, C1 RONO2 formation was always positively contributed by NO2, thus the importance of reaction between alkylperoxy radical (RO2) and nitrogen dioxide (NO2) in its production was addressed. Furthermore, prominent contributions of parent hydrocarbons to C4-C5 RONO2 were confirmed, whereas the production of C1-C3 RONO2 was more sensitive to other VOCs, such as aromatics and carbonyls, which accounted for ~40-90 % of C1-C3 RONO2 productions. This was the combined effect of the decomposition of larger molecules and the regulation of OH abundance caused by other VOCs. It was noteworthy that other VOCs stimulated hydroxyl (OH) production in NOx-rich environment, but suppressed OH production in NOx-lean environment. As another group of organic nitrates, formation mechanism of PAN during a sampling campaign conducted in autumn 2016 was studied in suburban Hong Kong. Ambient PAN averaged at 0.63 ± 0.05 ppbv, peaked at 7.30 ppbv. Higher PAN/O3 ratio was captured on O3 episodes than on non-episodes, due to the fact that PAN production was more efficient than O3 when there was an elevation of precursors (i.e. VOCs and NOx). Model simulations revealed that oxidations of acetaldehyde, methylglyoxal (MGLY) and other oxygenated VOCs (OVOCs), and radical cycling were the main production pathways to peroxyacetyl (PA) radical, hence PAN. Furthermore, RIR approach was applied to evaluate PAN-precursor relationships. PAN formation in Hong Kong was controlled by both VOCs and NO2. Among all VOC species, carbonyls were the largest contributor to PAN production, followed by aromatics and biogenic VOCs (BVOCs). They contributed to PAN through direct oxidation and/or decomposition. In addition, active VOCs including carbonyls, aromatics, BVOCs and alkenes/alkynes facilitated PAN production by stimulating OH production in such a NOx-rich environment. Apart from primary emissions, carbonyls were also generated from secondary formation of first-generation precursors, in which xylenes contributed the most to PAN production. This study for the first time quantified the impact of PAN formation on local O3 production in this region. PAN production decreased NO2, OH and hydroperoxyl (HO2) levels and increased NO level, further suppressing O3 reaction cycle. Each production / loss pathway was weakened and net O3 production rate was reduced by ~36 % due to PAN photochemistry during the study period. O3 abundance was reduced by 2.84 ppbv per ppbv of PAN formation. To sum up, this study advanced our knowledge of long-term variations and spatial distributions of RONO2, the impact factors, origins and formation mechanism of RONO2 in coastal and maritime environments, and the atmospheric fate of PAN and its influence on local O3 production in Hong Kong. This project filled research gaps in the area of organic nitrates, and made remarkable contributions to the scientific communities.