西安市冬季重污染天PM_(2.5)高分辨率及其中碳气溶胶污染特征分析

Analysis of the high resolution variation of PM_(2.5) and its carbonacious components at Xi'an during high pollution period in winter

为了研究西安市冬季重污染天PM2.5及其中碳气溶胶的变化特征，在2013年1月1日至2013年2月28日大气污染严重的天气进行24 h连续的PM2.5样品采集，再通过Model-4型全自动半连续式在线光/热法大气气溶胶OC/EC分析仪分析得出OC、EC的连续质量浓度值。结果表明，西安市PM2.5质量浓度冬季重污染天日循环变化规律明显，均大致呈现双峰模式，白天和夜间各有一个高峰。2月份的每个PM2.5质量浓度值高峰和低峰的出现均比1月份晚2-3 h，夜晚的高峰值比1月份低，PM2.5质量浓度比1月份上升得慢，下降得快。气象条件能对PM2.5质量浓度产生较强的影响。2月份PM2.5质量浓度值整体比1月份低，但在2月10日出现突越（499μg·m-3），这与春节假期人为活动变化有关。OC/PM2.5、EC/PM2.5、TCA/PM2.5日变化幅度都较小，这说明OC、EC、TCA的来源比较一致；OC/EC值的平均值为6.63，表明西安冬季重污染期PM2.5中的一次来源主要为燃煤排放。PM2.5、OC、EC、TCA和OC/EC的值较2010年都有明显的上升，但OC/PM2.5、EC/PM2.5、TCA/PM2.5的值却是下降的，这说明近年来PM2.5及碳气溶胶的控制措施效果不明显，碳气溶胶二次来源增加，PM2.5的排放来源变得更加复杂。OC和TCA日循环变化呈现出明显的双峰特征；EC的变化趋势不明显。一天中OC/EC值多数时候处于较高水平，且受早晚车流量高峰的影响不明显，说明西安冬季重污染期间碳气溶胶受光化学反应转化的二次来源影响比较大。OC、EC的线性相关性比较好，且白天相对夜晚好，说明西安市冬季夜间燃煤采暖增加了碳气溶胶来源的复杂性。

The purpose of this study is to investigate the variation characteristics of PM2.5 and its carbonaceous components at Xi’an during winter high pollution period. Hourly PM2.5 samples were collected from January 1 to February 28 in 2013.Then the sapmples were weighted and the OC and EC were analyzed using a Model-4 Semi-Continuous OC/EC Field Analyzer. The result indicated that the diurnal variation of PM2.5 concentrations was significant with twin-peak model pattern, one of which was in daytime and another was in nighttime. The presences of the peak of Every PM2.5 concentration in February were later 2-3 hours than in January. The peak value of nighttime in February was lower than in January . PM2.5 concentration in February rose more slowly and fell more fast than in January .And PM2.5 concentration was impacted strongly by the weather conditions.The PM2.5 concentrations were normally lower in February than in January, except a peak value （499 μg·m-3） was observed on February 10th, which associated with decreased human activities during the Spring Festival in February. The daily variations of OC/PM2.5, EC/PM2.5 and TCA/PM2.5 were not obvious, it suggested that the sources of TCA, OC and EC were consistent. The average value of OC/EC was 6.63, it showed that the main source of PM2.5 was coal combustion during the high pollution period in winter. The values of PM2.5, OC, EC, TCA and OC/EC increased significantly Comparison with the values of 2010, yet the descreasing trend was found for the vlaues of OC/PM2.5, EC/PM2.5 and TCA/PM2.5. This indicates that the control effects of PM2.5 and carbon aerosol were not obvious in recent years and the PM2.5 emission sources become more complicated. The diurnal variation of OC and TCA was significant with twin-peak model pattern, while the diurnal variation of EC was not obvious. Most of the time, the OC/EC ratio values were at a high level in a day and the influence of the traffic peaks in the morning and evening on the values was not significant, which indicates that more organic carbonacious aerosols were secondary formed through photochemical reaction during the winter high pollution days at Xi’an. The good correlation of OC and EC was observed, and the correlation of daytime was better than nighttime, it revealed the nighttime coal combustion for heating increased the complexity of the source of carbonacious aerosols.