Numerical studies on haze and photochemical smog pollution in China : toward a better understanding of their formation mechanisms

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Numerical studies on haze and photochemical smog pollution in China : toward a better understanding of their formation mechanisms

 

Author: Zhang, Li
Title: Numerical studies on haze and photochemical smog pollution in China : toward a better understanding of their formation mechanisms
Degree: Ph.D.
Year: 2017
Subject: Smog -- China.
Air -- Pollution -- China.
Photochemical smog -- China.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xviii, 192 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2952778
URI: http://theses.lib.polyu.edu.hk/handle/200/8911
Abstract: As a result of extremely fast economic developments since the late 1970's, severe air pollution problems have emerged in major cities/city clusters in China. Among them are the Northern China Plain (NCP) and the Pearl River Delta region and Hong Kong (PRD-HK) which have suffered from severe haze pollution and photochemical smog pollution, respectively, and thus have been of great concern to the public and scientific community. The NCP is frequently assaulted by severe haze events especially in winter, while in the PRD-HK area, smog pollution is one of the major air quality problems. Although many modeling efforts have been conducted to study the formation mechanisms, varying from regional transport/local formation under unfavorable meteorological conditions to gaseous/aqueous chemistry, few have comprehensively investigated the role that heterogeneous chemistry plays in the haze pollution over the NCP or unveiled the potential effects of nitrous acid (HONO), an important source of OH radical, on the smog pollution in the PRD-HK. In this present study, heterogeneous processes were incorporated into a state-of-the-art chemical transport model (WRF-Chem) and four typical winter haze events over the NCP during January of 2010-2014, including an unprecedentedly severe one in January 2013, were selected and investigated by using the revised model. Simulations of these haze events indicated that the strong local accumulation of pollutants within the stable and shallow PBL and continuous regional transport under weak southerly winds were the major causes of the winter haze pollution over the NCP. The added heterogeneous processes further enhanced the regional averaged PM2.5 over the NCP by 8.0 g m⁻³ (6.4%), 7.6 g m⁻³ (5.6%), 11.3 g m⁻³ (7.6%), and 2.3 g m⁻³ (2.5%) during the 2010, 2012, 2013, and 2014 episode, respectively. During the severest 2013 episode, the climate anomalies characterized with extremely weak winds, high humidity, and strong vertical temperature inversion within the PBL resulted in stronger accumulation and more intensive heterogeneous formations of secondary aerosols over the NCP, making this haze event much worse than other three historical episodes. Our results showed that under humid conditions, heterogeneous reactions led to strong secondary formations of aerosols and noticeably contributed to the haze pollution, indicating the needs for considering heterogeneous chemistry in chemical transport models to better simulate the haze pollution in China. We also concluded that, due to the special location and topography, Beijing was naturally susceptible to heavy haze in winter, making it tough to control the air pollution in this megacity. Much more stringent control strategies must be taken over the NCP as well as a regional effort over the entire eastern China to alleviate the haze pollution in Beijing and the NCP.
To investigate the effects of HONO on smog pollution over the PRD-HK, comprehensive sources of HONO, including the heterogeneous formations on the ocean, urban, and vegetation surfaces, traffic emissions, new gaseous formation pathways, and recently proposed biological sources, were parameterized into WRF-Chem. The updated model was applied to study a severe multiple-day smog episode in August 2011 over PRD-HK. Our simulations suggested that the heterogeneous source of HONO from ground, ocean, and aerosol surfaces were the major contributor (~54%) to the observed HONO concentration in Hong Kong, followed by the biological source (~29%) and gaseous sources (~9%). The new HONO sources noticeably affect the regional oxidation capacity with an increase of 10-20% in OH radical and significantly enhanced the smog pollution with increases of 8-15%, 20-30%, and 10-15% in O3, nitrate, and total PM2.5, respectively, over the PRD-HK. The results clearly show that HONO plays an important role in photochemical smog pollution over the PRD-HK region and underscores the needs of implementations of comprehensive HONO formations into regional air quality models before utilizing them for refining ozone-control or PM2.5-control policies in the future.

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