天津市区夏季碳气溶胶特征及改进的二次有机碳估算方法

Characteristics of Carbon Aerosols in Summer in Tianjin Urban Area and Improved Estimation Method for Secondary Organic Carbon

    摘要
  • 摘要: 为更加准确地估算环境受体PM2.5中SOC（二次有机碳）的质量浓度，于2015年6-8月利用在线监测仪器同步采集小时分辨率的PM2.5及OC（有机碳）和EC（元素碳）样品数据，分析碳气溶胶的变化特征，并尝试运用改进的EC示踪法估算ρ（SOC）.结果表明：天津市区夏季ρ（PM2.5）为（70.9±46.0）μg/m3，ρ（OC）和ρ（EC）分别为（7.6±3.1）（2.2±1.5）μg/m3，占ρ（PM2.5）的11.8%±4.6%和3.1%±1.4%，OC/EC（质量浓度之比，下同）的平均值为4.0±2.0.ρ（OC）与ρ（EC）之间的Pearson相关系数（R）仅为0.66，说明OC和EC的来源较为复杂，SOC的产生可能是重要影响因素.ρ（NO2）与OC/EC呈显著负相关（R=-0.47，P < 0.01），并且OC/EC（4.0）相对较低，说明天津市区机动车可能对碳气溶胶具有重要影响.ρ（SO2）与ρ（OC）、ρ（EC）的相关性较低（R均为0.33，P均小于0.01），说明天津市区碳气溶胶可能受燃煤源的影响较低.改进的EC示踪法主要是利用O3和CO、EC作为光化学反应和一次源排放的指标，并结合ρ（OC）、ρ（EC）和OC/EC的变化特征，逐步筛选一次排放源主导的时间段的ρ（OC）和ρ（EC）数据，然后利用最小二乘法拟合获得ρ（OC）和ρ（EC）的线性方程，最后进行ρ（SOC）和ρ（POC）（POC为一次有机碳）的估算.天津市区夏季ρ（SOC）的平均值为（2.5±2.0）μg/m3，分别占ρ（OC）和ρ（PM2.5）的28.8%±15.0%和3.7%±3.6%；ρ（POC）的平均值为（5.2±1.7）μg/m3，分别占ρ（OC）和ρ（PM2.5）的71.2%±15.0%和8.1%±5.2%，说明天津市区夏季有机碳的主要来源是一次排放源.研究显示，相比于EC示踪法，改进的EC示踪法估算的ρ（SOC）明显降低，ρ（POC）明显升高.AT（大气温度）对ρ（SOC）的影响较为显著，而WS（风速）对ρ（POC）的影响较为显著.

     

    Abstract: To investigate the characteristics of carbon aerosols in summer in the Tianjin urban area, ambient PM2.5, organic carbon (OC) and elemental carbon (EC) were collected at 1-hour resolution by on-line monitoring instruments from June to August 2015; and an improved EC-tracer method was used to estimate the contents of second organic carbon (SOC). The results indicated that the average concentration of PM2.5 was (70.9±46.0) μg/m3 in summer in the Tianjin urban area. The concentrations of OC and EC were (7.6±3.1) and (2.2±1.5) μg/m3, respectively, which accounted for 11.8%±4.6% and 3.1%±1.4% of PM2.5 mass, respectively. The average ratio of OC/EC was 4.0±2.0. The Pearson correlation coefficient between OC and EC was only 0.66, which might indicate that the sources of OC and EC were complex, and that the production of SOC might be an important factor. NO2 and OC/EC showed a significantly negative correlation (R=-0.47, P < 0.01), and the ratio of OC/EC (4.0) was relatively low, suggesting that vehicles probably had an important influence on the carbon aerosols in the Tianjin urban area. The correlations between SO2, OC and EC were low (R=0.33, P < 0.01, respectively), implying that the carbon aerosols might be less affected by coal sources. In the improved estimation method, O3, CO, and EC were used as indicators of photochemical activity and primary source emission, respectively, considering the variation characteristics of OC, EC concentrations and OC/EC; and then the OC and EC datasets derived mainly from primary emissions were gradually screened. The linear equations of OC and EC were fitted using the least squares method to estimate the concentrations of SOC. The average concentration of SOC was (2.5±2.0) μg/m3 in summer in the Tianjin urban area, which accounted for 28.8%±15.0% and 3.7%±3.6% of OC and PM2.5; and that of POC was up to (5.2±1.7) μg/m3 accounting for 71.2%±15.0% and 8.1%±5.2% of OC and PM2.5, respectively, indicating that the organic carbon mainly originated from primary sources. Compared to the unimproved EC-tracer method, the concentrations of SOC estimated by the improved EC-tracer method decreased significantly, and those of POC increased significantly. The effect of the atmospheric temperature was more significantly on SOC, while that of wind speed was more significantly on POC.

     

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