我国大气中挥发性有机物的分布特征

Characteristics of volatile organic compounds in China

大气中挥发性有机物(VOCs)是臭氧和二次有机气溶胶形成的关键前体物之一,研究表明烷烃、烯烃、芳香烃是我国大气VOCs的重要组分。在不同区域,城市地区烷烃含量最高,而偏远地区芳香烃为含量最丰富的VOCs。VOCs浓度日间变化多呈双峰分布趋势,峰值多出现在早晨与傍晚的上下班高峰期。目前对我国臭氧污染事件的研究均表明芳香烃和烯烃是对臭氧生成贡献最大的化合物。VOCs源解析中广泛运用的模型包括CMB、PMF和PCA/APCS,各模型均存在优点和局限性。比较各地VOCs源解析结果,发现交通排放源和工业排放源为我国VOCs的主要人为来源。VOCs的跨区域传输决定与周边地区的合作将是未来空气治理中的发展方向。

Background, aim, and scope Volatile organic compounds （VOCs） are a series of atmospheric pollutants that are characterized as high volatility, and strong environmental impact. Some of VOCs are smelly, toxic and carcinogenic, which do harm to human health. In addition, Most of VOCs are key precursors of tropospheric ozone and secondary organic aerosol. In recent years, the concentrations of ground level ozone are increasing in many megacities of China, especially in Beijing, Pearl River Delta （PRD） and Yangtze River Delta （YRD） region. Consequently, identifying the distributions and profiles of VOCs is in urgent need for environmental management. In contrast to our country, USA and Europe have conducted more comprehensive studies on VOCs, and most information we use are referenced from these foreign countries, thus it＇s necessary to establish our own test method and database. Materials and methods This paper reviews studies about VOCs in China, of which thedistribution, sources apportionment, and effects on ozone and secondary organic aerosol （SOA） formation were discussed in detail. Results In China, the concentrations of VOCs are mainly affected by anthropogenic sources, such as combustion processes （utilizing fossil fuels, petroleum refining, and storage）, distribution of petroleum products, solvent use and other industrial processes. Alkanes, alkenes and aromatics are the most abundant VOCs in China, and concentrations and composition of VOCs vary with sites. In China, ground-level ozone production was limited by the concentrations of VOCs in most eastern urban areas, and limited by NOx in western areas. Industry and traffic emissions are the top two emission sources of VOCs in China. Discussion In general, concentrations of VOCs is higher in megacities, and it is found that VOCs level in Beijing, Shanghai, Guangzhou is higher than other cities, while composition of VOCs in remote sites is mainly influenced by long range atmospheric transport and biogenic emission. The concentration of atmospheric VOCs generally presented significant temporal variations because of emission strength and photochemical reactions. Levels of VOCs in urban areas were mainly controlled by emission sources, while in suburban the diffusion rate of atmospheric boundary layer is the main factor, which results in the VOCs level higher in winter than that in summer in most cases. In the remote sites, VOCs mainly come from biogenic source and air pollutant transportation. For diurnal variations, it＇s common to see bimodal distributions of atmospheric concentrations, concentration peaks are highly correlated with the traffic flow, and concentrations mainly peak at 8：00 LST （local standard time, LST） and a second one from 16：00 to 19：00 LST. Under the effect of free radical, ozone, atmospheric VOCs could be oxidized to carbonyls, and carbonyls are one of the most important radical sources, especially for the wintertime in polluted urban environments. Formaldehyde, acetaldehyde and acetone were found to be the most abundant OVOCs in China, and the lowest concentration levels were found in Xiamen and Qinghai. It was reported that formaldehyde/acetaldehyde （C1/C2） ratio usually varied from 1 to 2 at urban areas and was about 10 at forest areas; therefore, the C1/C2 ratio could be used as a measure of a biogenic source of formaldehyde, while acetaldehyde/propionaldehyde （C2/C3） ratio was often used as effective indicators of anthropogenic carbonyls. In recent years, increasing ground-level ozone concentrations were observed in both the background and urban sites. As key ozone precursors, VOCs are the most important chemicals contributing to high ozone production rates in Pearl River Delta and Beijing-Tianjin-Hebei region, where ozone formation is sensitive to VOCs. It is found that ozone in the lower troposphere over Beijing had a strong positive trend （2% per year） during the period 1995 to 2005. As VOCs-sensitive chemistry has been found to be most likely to occur in urban sites of China, it is critical to distinguish the contribution of individual VOCs to ambient ozone formation for efficient emission control. The ozone formation potential （OFP） is a widely used method for evaluating the maximum ozone formation capacity, and aromatics and alkenes take the most part of OFP in China. Principal component analysis （PCA）, chemical mass balance （CMB）, and positive matrix factorization （PMF） are widely used source apportionment tools in the world, each of them have advantages and shortages. PMF results are not affected by uncertainties in emission profiles, making it the most popular method among these three tools. Conclusions Nowadays, most studies on VOCs are concentrated in megacities such as Beijing, Shanghai and Guangzhou in China, and most of researches were focused on factors like wise source apportionment, VOCs compositions, spatial- temporal variation etc. Recommendations and perspectives Although studies on OVOCs were rarely today, there will be more related studies coming up because of their important role in ozone formation. By targeting the high OFP-contributing species rather than the high emission-contributing species, reactivity-based control was more efficient than the emission-based approach in alleviating ozone pollution.