1.School of Environmental Science and Engineering, Nanjing University of Information Science &2.Technology
汾渭平原因其封闭的地形条件以及煤炭为主的能源结构，大气污染问题一直存在，并于2018年被列入大气污染防控的重点区域。文章利用2015年以来PM10、PM2.5、SO2、NO2、CO、O3质量浓度的观测数据和空气质量指数（Air Quality Index, AQI），分析了汾渭平原AQI及大气污染物质量浓度的时空分布特征；使用多元线性回归模型研究了气象条件对冬季PM2.5和夏季O3浓度日最大8h滑动平均值（MDA8_O3）日变化和年际变化的影响。研究发现，汾渭平原的空气质量在2015~2017年间逐年变差，在2018~2019年有所好转，污染较重的城市为西安、渭南、咸阳、临汾、运城、三门峡、洛阳，集中在汾河平原与渭河平原交界处。汾渭平原的首要大气污染物多为PM2.5、PM10或O3，三者占比之和约90%。重污染时期主要集中在天气条件不利及污染物排放量大的冬季供暖期，但夏季O3浓度的升高趋势使得汾渭平原夏季污染情况越来越严重。影响汾渭平原冬季PM2.5浓度和夏季MDA8_O3日变化最主要的气象要素都是2米气温（T2M），相对贡献分别是45.5%、35.3%，都表现为正相关；第二主要的气象要素都是2米相对湿度（RH2M），相对贡献分别是41.5%（正相关）、25.4%（负相关）。影响汾渭平原冬季PM2.5浓度年际变化最主要的2个气象要素及其相对贡献分别为T2M（43.6%）、RH2M（31.9%），且都呈正相关，2015~2019年汾渭平原冬季气象条件的变化会导致PM2.5浓度上升，部分削弱了人为减排导致的下降趋势（-8.3 μg m-3 yr -1）。影响汾渭平原夏季MDA8_O3年际变化最主要的2个气象要素及其相对贡献分别为T2M（71.7%，正相关）和850 hPa风速（WS850，16.3%，负相关）。2015~2019年汾渭平原夏季气象条件的变化导致O3污染呈上升趋势（1.2 μg m-3 yr -1），但O3污染的总上升趋势（8.7 μg m-3 yr -1）中，人为排放变化的贡献更大（7.5 μg m-3 yr -1）。本研究表明，汾渭平原大气污染形势严峻，其颗粒物污染问题尚未解决，还面临着新的臭氧污染的挑战，汾渭平原内的11个地级市分属陕西、山西、河南三省管辖，三省交界处又是重污染区域，所以需要三省联合防治防控，协同改善汾渭平原的空气质量。
Due to the special terrain conditions and coal-based energy structure, air pollution in the Fenwei Plain has been a serious issue. In 2018, the Fenwei Plain was listed as a key area for air pollution prevention and control. This study used the observed concentrations of PM10, PM2.5, SO2, NO2, CO, O3 over 2015–2019 and the Air Quality Index (AQI) to analyze the temporal and spatial distributions of AQI and mass concentrations of pollutants in the Fenwei Plain. We applied the multiple linear regression model to identify the meteorological conditions that influenced the daily and interannual variations of PM2.5 in winter and the maximum daily average 8-hour O3 (MDA8_O3) in summer in the Fenwei Plain. We found that air quality of the Fenwei Plain deteriorated year by year from 2015 to 2017 but improved from 2018 to 2019. The most polluted cities were Xi’an, Weinan, Xianyang, Linfen, Yuncheng, Sanmenxia, Luoyang, which were located in the junction of the Fenhe plain and the Weihe plain. The primary air pollutants in the Fenwei Plain were PM2.5, PM10 or O3, which accounted for about 90% of the polluted days. Severe pollution occurred mainly in the winter heating period when the weather conditions were unfavorable and the emissions of pollutants were large. In summer, concentrations of O3 in the Fenwei Plain increased over the past years. The most important meteorological parameter for daily variations of both PM2.5 in winter and MDA8_O3 in summer was 2-meter air temperature (T2M), with relative contributions of 45.5% and 35.3%, respectively. T2M was positively correlated with PM2.5 in winter and MDA8_O3 in summer. The second important meteorological parameter was 2-meter relative humidity (RH2M) for both PM2.5 in winter and MDA8_O3 in summer, with relative contributions of 41.5% (positive correlation) and 25.4% (negative correlation), respectively. With respect to interannual variations in PM2.5 in winter, the two most dominant meteorological parameters were T2M (43.6%) and RH2M (31.9%), which were both positively correlated with concentrations. Changes in meteorological conditions in winter over 2015–2019 had an effect of increasing PM2.5, which offset to some extent the decreases in emissions. With respect to interannual variations in summertime MDA8_O3, the two most dominant meteorological parameters were T2M (71.7%, positive correlation) and wind speed at 850 hPa (WS850, 16.3%, negative correlation). Changes in meteorological conditions in summer over 2015–2019 had an effect of increasing O3 (1.2 μg m-3 yr -1), which was a smaller effect compared to the increases in O3 (7.5 μg m-3 yr -1) caused by changes in anthropogenic emission. Our results indicate that air pollution in the Fenwei Plain is severe. Which the particulate pollution has not yet been resolved yet, it now also faces new challenges of ozone pollution. Considering that the Fenwei Plain is under influenced by Shaanxi, Shanxi, and Henan, it is necessary for the three provinces to joint prevention and control to improve the air quality in the Fenwei Plain.