Photochemical formation of alkyl nitrates and their impacts on ozone production in Hong Kong

Abstract

Alkyl nitrates (RONO₂) are important constituents of organic nitrates and atmospheric odd nitrogen. The widely recognized sources of RONO₂ include emissions from equatorial oceans, biomass burning and photochemical formation. Since RONO₂ stabilize the alkyl and nitrate groups in the molecules, they generally pose non-negligible impact on atmospheric photochemistry in the processes of their formation and degradation. To understand the spatiotemporal patterns of RONO₂ and their parent hydrocarbons in Hong Kong, the formation routes of RONO₂ and the impacts of RONO₂ photochemistry on ozone (O₃) production, whole air samples were collected over a wide range of environments in Hong Kong through the first sampling in 2001 to the latest sampling in 2014. Based on the concentrations of speciated C₁-C₅ RONO₂, the photochemical formation, evolution and degradation of RONO₂, as well as their impacts on O₃ production were studied in this thesis. Results indicated that the C₁-C₅ RONO₂ increased significantly in Hong Kong during the past 15 years (2001-2014), which was mainly attributable to the increased abundances of parent hydrocarbons and enhanced oxidative capacity of the atmosphere (e.g. increased O₃). The spatially disproportional distributions of the parent hydrocarbons and regional transport from inland Pearl River Delta led to higher levels of RONO₂ in northwestern Hong Kong than in east of Hong Kong. With regard to observed changes in parent hydrocarbons of RONO₂, this work mainly focused on the evaluation of a program implemented by Hong Kong Environmental Protection Department for the purpose of reducing the emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx) from liquefied petroleum gas (LPG)-fueled vehicles. Overall, this program effectively decreased the emissions of VOCs from LPG-fueled vehicles, including the parent hydrocarbons of RONO₂ (such as propane and n-butane). Due to the concurrent emission reduction of NOx, the atmospheric oxidative capacity (represented by O₃, OH and HO₂) slightly increased after the program. In the highly vehicle-populated roadside environment, C₂-C₄ RONO₂ decreased considerably as a result of the substantial reductions of parent hydrocarbons. However, RONO₂ at the roadside sites were still not obviously lower than at the other locations in Hong Kong. This work for the first time updated an advanced protocol in a photochemical box model describing atmospheric processes of speciated RONO₂, based on master chemical mechanism (a near-explicit chemical mechanism). With the appropriate settings of branching ratios and dry deposition velocities, the model well reproduced the observed RONO₂. A branching ratio of 0.0003 was suggested as the most appropriate value for CH₃O₂ reacting with NO to form CH₃ONO₂, with an estimated uncertainty of less than 50%. The first application of the model at a coastal site in Hong Kong confirmed that the gas-phase formations of RONO₂ were dominated by the pathway of alkylperoxy radicals (RO₂) reacting with nitric oxide (NO) for C₂-C₅ RONO₂. However, the association between methoxy radicals (CH₃O) and nitrogen dioxide (NO₂) made considerable contribution to CH₃ONO₂. RONO₂ formation led to the reductions of O₃ production at this coastal site, due to the extraction of oxidative radicals from the atmosphere. As a temporary nitrogen reservoir, C₁-C₅ RONO₂ constituted approximately 4% of the total nitrogen as estimated by the model. Regional transport and meso-scale circulation were important factors elevating RONO₂ levels at the mountainous site where the parent hydrocarbons were much less abundant than at a coupled urban site. In addition, the oxidative capacity of the atmosphere at the mountainous site was stronger than that at the urban site, as identified by the photochemical box model incorporating master chemical mechanism (PBM-MCM). As a result, the oxidation efficiencies of parent hydrocarbons at the mountainous site were higher than at the urban site, thus increasing the production of RO₂ radicals and the production of RONO₂. This led to comparable or even higher concentrations of some RONO₂ at the mountainous site, but not for all the RONO₂ species. Furthermore, the relationships between RONO₂ production and their precursors were comprehensively studied with the aid of the PBM-MCM model. The isopleths of RONO₂ production clearly demonstrated the VOC-limited and NOx-limited regime in controlling RONO₂ formation. The dividing ratio of TVOC/NOx between NOx-limited and VOC-limited regime for C₂-C₄ RONO₂ shifted from 8.7/1 ppbv/ppbv to 10.0/1 ppbv/ppbv from the urban environment to the mountainous environment. However, the ratio was decreased to 3.1/1 and 2.4/1 ppbv/ppbv for the formation of CH₃ONO₂ at the urban and mountainous site, respectively. This discrepancy was mainly caused by the pathway of CH₃O reacting with NO₂ that contributed significantly to the production of CH₃ONO₂ under high NOx. RONO₂ formation led to the reduction of O₃ at both the urban and the mountainous sites. Moreover, different mechanisms were found in terms of the impacts of RONO₂ degradation on O₃ production, including NO₂ stimulating, NO₂ suppressing and radicals stimulating. In addition, model simulations revealed the greater impacts of organic nitrates formed from the precursors of higher O₃ formation potentials (e.g. alkenes, isoprene and aromatics) on O₃ production, which needs further study in the future. In summary, this work addressed knowledge gaps in RONO₂ research and made contributions to the scientific community, including the development of the RONO₂ module in the PBM-MCM model, the verification of RONO₂ formation pathways, the first isopleths showing the RONO₂-VOC-NOx relationships, and the unprecedented finding of the mechanisms of RONO₂ degradation influencing O₃ formation, including NO₂ stimulation, NO₂ suppression and RO stimulation.