Study of nitrous acid (HONO) formation mechanism and its impact on photochemistry in Hong Kong

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Study of nitrous acid (HONO) formation mechanism and its impact on photochemistry in Hong Kong


Author: Wu, Jueqi
Title: Study of nitrous acid (HONO) formation mechanism and its impact on photochemistry in Hong Kong
Degree: M.Phil.
Year: 2014
Subject: Gases -- Measurement
Air -- Pollution -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: v, 110 leaves : color illustrations ; 30 cm
Language: English
InnoPac Record:
Abstract: Nitrous acid (HONO) is an important trace gas in the atmosphere. It is a key precursor of the hydroxyl radical (OH) which is one of the most significant oxidants in the atmosphere. Therefore, HONO plays an important role in atmospheric chemistry and formation of air pollution. There has been a growing amount of research in the recent decade on the HONO sources and its environmental impacts, but the daytime formation mechanism of HONO still remains under debate.
Hong Kong is a coastal city in southern China which suffers from severe photochemical air pollution. Due to the lack of field measurements, little is known about the abundances, sources, and atmospheric consequences of HONO in Hong Kong. In this study, the first continuous measurements of ambient HONO in Hong Kong, as well as measurements of other related gases and aerosols were conducted by researchers in the Hong Kong Polytechnic University. In order to better interpret the observed HONO data, a box model coupled with an explicit Master Chemical Mechanism (MCM, version 3.1) is applied to a strong photochemical episode during the comprehensive field campaign. The model is constrained by real-time observation data, including O3, CO, NO, NO2, VOCs, etc., which helps to improve the model performance. Based on the numerical simulations, daytime potential HONO sources and the impact of HONO on photochemistry are investigated. The major findings include: (1) High daytime HONO concentrations (over 1 ppbV) were frequently measured, which is among the highest HONO level in the existing records. (2) Nighttime HONO accumulation is mainly attributed to NO{208} heterogeneous reaction on ground surfaces, while the high daytime HONO level cannot be explained by the already known gas-phase reaction (NO+OH). Only a significant extra daytime source(s) (Pextra) of 1.12 pptV/s would sustain the observed HONO levels. Correlation analysis further indicates that a heterogeneous reaction of NO{208} on aerosol appears to be a significant source of daytime HONO at the site, which is different from the results in many other studies. (3) HONO photolysis is found to be an important OH source throughout the day. If we only considered the homogenous source (NO+OH) in the MCM model, as is the case in most current air quality models, the OH concentration would be underestimated by up to 20%. This study has proved that there exists strong unknown HONO source(s) in the daytime at our study site. Moreover, modeling results suggest that a comprehensive HONO formation mechanism should be coupled into the chemical mechanism so as to get a better model performance of OH radical and other related species. Since this is the very first piece of effort on HONO study in Hong Kong, more focus should be put on this topic in the future, in order to obtain a whole picture of HONO in Hong Kong.

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