中国东部地区的O_3污染形成机制及趋势的初步探讨（英文）

Ozone(O_3) pollution in eastern China: It's formation and a potential air quality problem in the region

本文探讨了中国地区在高气溶胶污染下臭氧形成的问题。过去对臭氧污染的讨论主要集中在臭氧前提物(NOx,CO和VOCs)的讨论。并根据其各自的排放量研究臭氧形成在本地区是由NOx还是VOCs控制的。然而,在中国高气溶胶频发的条件下,太阳辐射被强烈压抑,极大减少了臭氧形成的光化学过程。因此本文建议,除去臭氧形成的NOx及VOCs控制条件,还应有一个太阳光子控制条件。而中国由于高气溶胶污染,目前臭氧的形成应是受太阳光子控制条件的影响。因此压抑了臭氧的形成。然而,随着气溶胶污染的治理和改善,光化学活动的增强,臭氧污染会成为中国将来的严重问题,应加以重视。

Background, aim, and scope Currently, China is under going a rapid economic development, which results in a higher demand for energy, greater use of fossil fuels, and inevitably lead to more emission of pollutants into the atmosphere. As a result, China is experiencing heavy air pollution in the past two decades, with particle matter （PM） being one of the top pollutants. Measurements show that PM concentrations often exceed the new national ambient air quality standards of China （75 μg·m^-3 for 24 h average） in large cities in China. Aerosol particles have important impacts on photochemistry by scattering and absorbing solar and infrared radiation. In addition to aerosol particles, ozone is also a critical trace gas in the troposphere because it plays important roles in atmospheric chemistry, air quality, and climate change. Unlike other atmospheric pollutants, such as PM2.5; SO2, NOx, etc., tropospheric ozone is not directly emitted from the earth＇s surface. The tropospheric ozone is photo-chemically formed through a very complicated chemical process in atmosphere. The tropospheric ozone formation is influenced by its precursors （NOx, VOCs, CO）, and solar radiation. The effects of ozone precursors on ozone formation in eastern China have been intensive discussed. However, the photon effect on ozone formation has not been intensively discussed, especially under the heavy PM pollution conditions in eastern China. The aim of the paper is to discuss the limitation for ozone formation under heavy PM pollution in eastern China. Materials and methods This study used long-term measurement of ozone, and found that there was an increase trend of ozone in Shanghai. Based on the serous haze conditions in eastern China in the recent studies, the ozone formation under the high aerosol conditions was discussed. Results Although the aerosol pollution is a major concern for the air pollution control in China at present, the 03 concentrations increase in eastern China, showing that it may be a serious environmental problem in the future. The surface measurement in Shanghai shows the 03 concentrations increased rapidly from 2006 to 2007. The peak value in summer was 60 ppbv （.part per billion by volume mixing ratio, ppbv represents μL·m^3） in 2006, and increased to 100 ppbv in 2007. This ozone increase trend is consistent with a longer ozone measurement （2005--2011） by Shen and Wang （2012）. To better understand the 03 trend and formation in large cities, we need first to understand the tropospheric 03 formation. The ozone increase is related to several factors, including atmospheric transport, the precursors of ozone formation, and the photochemistry of ozone formation. In order to better understand the ozone trend, we illustrate some important issues related to ozone formation. In addition to the anthropogenic emissions of 03 precursors （NOx, CO, and VOCs）, the importance of solar radiation is emphasized in this study. At present, because of the extreme high aerosol pollution in China, the photochemical activities are greatly depressed. As a result, the tropospheric 03 formation in large cities of China is strongly affected by solar radiation, resulting in a strong photon-limitation of ozone formation in eastern China under heavy PM condition. Discussion Tropospheric ozone requires the following 3 critical factors, including （1） the solar radiation with wavelength smaller than 320 nm to produce OH radical, and 400 nm to photo-disassociate NO2 to produce atomic O; （2） emissions of CO and VOCs to generate HO2 and ROz; and （3） emissions of NO to produce NO2 and atomic O. This study proposes that under heavy haze condition, there is lack of photo-chemical activity, and 03 formation is photon-limit. In contrast, under non-haze condition, there is enough solar radiation for the photo-chemical activities to form 03. The 03 formation is either under VOCs limit or NOx limit, dependent on the emission ratios of NOx/VOCs. However, with the improving of the aerosol pollution in China, the photo-chemical activities will be enhanced, leading to potential high 03 concentrations in China. The potential high ozone formation is also dependent on the emission ratio of NOx/VOCs. With the rapidly economical development in China, the VOC emission increases, leading to the decrease of emission ratio of NOJVOCs. When the emission ratio of NOJVOCs changes to lower than 0.2, the ozone formation is switched from VOC-limited condition to NO,-limited condition. As a result, the case of the photon-limit for the 03 formation decreases, leading to the enhancement of photo- chemical activists and potentially high 03 concentration in large cities in eastern China. Conclusions The tropospheric ozone formation is discussed under the heavy aerosol pollution condition in China. This study suggests that under the heavy aerosol pollution condition, the photo-chemical activity is greatly depressed, and 03 formation is significantly decreased. In addition to the NO^-limit and VOCs- limit, this study suggests that the 03 formation in China is mainly under the photon-limit condition. Recommendations and perspectives With the improving of the aerosol pollution in China, the photo- chemical activities will enhance, leading to potential high 03 concentrations in the future. Attention should be paid for this serious environmental problem.