太原冬季PM2.5中有机碳和元素碳的变化特征

2005年12月—2006年2月在太原市区持续观测了气溶胶细粒子PM_2.5, 并应用Sunset碳分析仪进行了有机碳 (organic carbon, OC) 和元素碳 (elemental carbon, EC) 的测定。结果表明:太原冬季PM_2.5, OC和EC浓度均较高, 其中PM_2.5日平均浓度变化范围为25.4~419.0 μg/m3, 日平均浓度为193.4±102.3 μg/m3, OC平均浓度为28.9±14.8 μg/m3, EC平均浓度为4.8±2.2 μg/m3, OC/EC平均比值是7.0±3.9, 即太原市冬季PM_2.5和碳气溶胶污染严重。OC在PM _2.5中占18.6%, EC占2.9%, 这表明碳气溶胶是太原大气细粒子污染控制的关键组分。在太原市冬季, 采暖燃烧的煤是OC和EC的主要贡献源, 造成OC大大高于EC, 从而使OC/EC比值增大。各种气象条件对PM_2.5, OC, EC和OC/EC比值的变化都有不同程度的影响, 特别是大雾天气、相对湿度、风速和降雪是影响碳气溶胶浓度变化的重要因素。

Characteristics of Organic Carbon and Elemental Carbon in PM2.5 During Winter in Taiyuan

Taiyuan is a city characterized by coal-combustion pollution. Aerosol is one of the main pollutants in Taiyuan. The primary objectives of the study are to examine the temporal variations of PM_2.5, OC, EC concentrations with OC/EC ratio during winter in Taiyuan, and to find and understand the correlations among PM_2.5, OC, EC, OC/EC ratio and meteorological factors. Continuous observation of PM_2.5 is conducted in Taiyuan during high pollution seasons from December 18, 2005 to February 3, 2006. PM_2.5 samples are obtained from the rooftop of Shanxi Meteorological Sciences Institute. PM_2.5 samples are collected every day using TEOM series 1400a ambient particulate monitor-ACCU. The samples are collected on 47 mm Whatman quartz microfiber filters. Continuous meteorological data are obtained from Shanxi Meteorological Observatory. EC and OC in PM_2.5 are determined by Sunset carbon analyzer.The study shows that the concentrations of PM_2.5, OC and EC are high during winter in Taiyuan. The daily concentration of PM_2.5 varies from 25.4 to 419.0 μg/m3 with the average of 193.4±102.3 μg/m3. According to the standard of daily average value of PM_2.5 (65 μg/m3) published by US EPA in 1997, 89% of daily average PM_2.5 values exceeds the US air quality standard during winter, which shows fine particles pollution is serious during winter in Taiyuan. The average OC concentration is 28.9±14.8 μg/m3, while EC is 4.8±2.2 μg/m3. The highest values/lowest values of OC and EC are 3.3, 1.1, respectively. High variability of OC concentrations may be due to the contributions of different emission sources. The OC and EC levels at Taiyuan, especially OC, are higher than other urban cities, reflecting more severe carbonaceous pollution in Taiyuan. OC and EC account for 18.6% and 2.9% of PM_2.5, respectively, which indicates that carbonaceous aerosols are key components for controlling fine particles pollution in Taiyuan. Coal combustion from residential heating during winter is the major emission source of OC and EC, and the emission of OC increases largely relative to EC during winter. The most OC/EC ratios exceed 2.0 and average OC/EC ratio is 7.0±3.9 in winter. Higher OC/EC ratios are found during heating seasons with increased primary emission sources like coal combustion. The OC-EC correlation is low during winter in Taiyuan city, pointing to the complex of emission sources. The meteorological conditions have significant effects on the ambient concentrations of PM_2.5, OC, EC and OC/EC ratio. Fog, the relative humidity, wind speed and snows are major factors that influence the concentration variation of carbonaceous aerosols. Positive correlation exists between PM_2.5, OC and OC/EC ratio with relative humidity, meanwhile negative correlation exists between PM_2.5, OC, EC and OC/EC ratio with the visibility and wind speed.