气象条件对2013—2015年冬季关中地区空气质量的影响

Influence of meteorological conditions on the wintertime air quality in the Guanzhong basin during 2013 to 2015

大气污染是由过度的人为源排放和不利于大气污染扩散的气象条件共同作用的结果,通过探讨天气形势及气象要素与细颗粒物(PM_(2.5))的关系,分析气象条件对空气质量的影响,从而为关中地区的环境治理提供有利的参考依据。本文利用NCEP-FNL再分析资料、环境监测资料及有关的降水数据,对2013—?2015年冬季关中地区空气质量的演变趋势及其与气象条件的关系进行了研究。结果表明:2013—?2015年冬季关中地区平均PM_(2.5)浓度分别为159.5μg?m^( -3),74.2μg?m^( -3),101.8μg?m^( -3),呈现出先明显下降然后又回升的趋势。对2013—?2015年冬季关中地区天气形势的分类分析表明:这三年关中冬季的有利天气形势呈先上升后下降的趋势,能基本解释同期关中地区PM2.5的演变趋势。有利天气形势不但会使当天的PM2.5降低,还会使第二天的PM_(2.5)维持在较低的水平。因此,有利天气形势天数的增加会使冬季的整体PM_(2.5)浓度下降;反之,有利天气形势的减少或不利天气形势的持续则会使PM_(2.5)浓度飙升到很高的水平。此外,风速与每天的PM_(2.5)浓度呈显著的负相关,可以解释同一天气形势下日平均PM_(2.5)浓度的变化。降水对污染物有明显的湿清除作用,但降水并不是造成2014年关中地区PM_(2.5)浓度偏低的主要原因。

Background, aim, and scope Rapid industrialization and urbanization have caused severe air pollution in the Guanzhong basin （GZB）, as refected in the severe and persistent haze with extremely high fne particulate matter （PM2.5） frequently engulfs the GZB, particularly during wintertime. Numerous studies have demonstrated that the air pollution is fundamentally caused by excessive anthropogenic emissions and unfavorable synoptic situations. The meteorological conditions play a key role in the formation, transformation, difusion, transport, and removal of the air pollutants in the atmosphere. Recent advances in understanding the role of meteorological conditions in the air pollution formation in China have mainly concentrated on the regions of Beijing-Tianjin-Hebei, the Pearl River Delta, and the Yangtze River Delta. However, only a few studies have focused on the impact of the large-scale atmospheric circulation pattern on the air pollution events in the GZB. The large-scale synoptic pattern has been found to generally steer the transport and dispersion of air pollutants in the GZB, providing favorable （unfavorable） conditions for pollutants dispersion （accumulation）. The purpose of the present study attempts to investigate evolutions of the air quality and its relationship with meteorological conditions in the GZB during the wintertime of 2013 to 2015. Materials and methods Using the NCEP-FNL reanalysis data, through analyzing the synoptic conditions at 850 hPa and the surface level, the large-scale synoptic situations influencing the GZB have been categorized into favorable （“southwest-trough” and “southeast-high”） and unfavorable conditions （“north-low”, “southeast-trough”, “southeast-high”, “transition”）. Together with air quality measurements, we have further explored the evolution characteristics of the air quality and its relationship with the synoptic situations in the GZB during the wintertime from 2013 to 2015. The 24-h precipitation data during this period has also been used to fnd out its impact on the air quality during the study period. Results The observed average PM2.5 concentrations in GZB during the wintertime of 2013 to 2015 are 159.5 μg ？ m？3, 74.2 μg ？ m？3, and 101.8 μg ？ m？3, respectively. The classifcation of the synoptic conditions in the GZB from 2013 to 2015 shows that favorable synoptic conditions have substantially increased in 2014 and slightly decreased in 2015 compared to those in 2013, which is generally consistent with the variation of PM2.5 concentrations in the GZB. In addition, low-level basin-wide average wind speeds and the daily PM2.5 concentrations exhibit a good negative correlation, which explains the variation of the daily PM2.5 concentrations under the control of the same synoptic situation. Analyses have also indicated that the wind speed has potentials to dominate the air quality irregardless of synoptic situations in the GZB. Precipitation generally efciently washouts air pollutants in the atmosphere, but does not constitutes the main factor to lower the PM2.5 level in GZB in 2014. This is mainly due to the small daily precipitation amount and the fewer precipitation days with more favorable synoptic situations during the wintertime of 2014.Discussion Favorable synoptic conditions effectively ventilate the GZB and substantially decrease the PM2.5 level, improving the air quality in GZB. However, under unfavorable situations, persistent and widespread haze with extremely high levels of PM2.5 frequently engulfs GZB. The occurrence of extreme pollution episodes is primarily caused by the air pollutants accumulation from the previous days and the extreme low wind speed. The wind speed is the main factor to determine how much air pollutants could be dispersed or accumulated, which in turn afect the concentration of the PM2.5 level in the GZB. Conclusions Synoptic situations signifcantly afect the air pollution in the GZB during the wintertime of 2013 to 2015. Generally favorable synoptic conditions result in good air quality in the GZB and vice versa. The classifcation of the synoptic situations in the GZB from 2013 to 2015 has shown that the evolution of favorable synoptic conditions is generally consistent with the variation of PM2.5 concentrations in the GZB. In addition, the persistent unfavorable synoptic conditions lead to rocketing of the PM2.5 level, while the increase of favorable synoptic conditions decrease the PM2.5 level overall. The meteorological elements, such as the wind speed, can substantially afect the air quality in GZB. Since the precipitation over the GZB in the wintertime is overall small （smaller than 20 mm during the most of the years）, and the precipitation days are generally less in the year with more favorable synoptic situations, the wet deposition process is not the main reason for the low PM2.5 level in the winter time of 2014. Recommendations and perspectives Since the classifcation of the synoptic situations are mainly made at 850 hPa that infuence the GZB, possible uncertainties may exist in the current results. Future study may include synoptic analysis on more levels and at more times. More quantitative studies are necessary in the future study to improve the understanding on the relationship between meteorological conditions and air quality and provide guidance for the air quality forecasting warning.