Evolution Characteristics of Aerosol Chemical Components and Water Content During Sandstorms in the Fenwei Plain
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摘要: 为研究沙尘过程对下游城市地区空气质量的影响，本文基于2021年3月10日—4月6日气溶胶化学组分和气象要素的在线观测数据，结合MODIS卫星遥感AOD (Aerosol Optical Depth)数据、MERRA 2再分析数据和环境六要素数据，探讨了汾渭平原两次沙尘过程和一次扬尘过程中气溶胶化学组分的演变特征，使用ISORRPIA Ⅱ模式计算了气溶胶含水量和pH，分析了其变化特征. 结果表明：① 沙尘1(2021年3月16日00:00—21日07:00)、扬尘(3月21日08:00—28日14:00)和沙尘2(3月28日15:00—31日21:00)期间，西安市PM10平均浓度分别为309.5、168.3和472.2 μg/m3，分别是沙尘前(3月10日00:00—15日22:00)的2.7、1.5和4.1倍. ②两次沙尘和扬尘均起源于内蒙古自治区阿拉善盟以及甘肃省酒泉市—张掖市—金昌市一带，但由于传输路径存在显著差异，导致对地面PM2.5浓度的影响不同，沙尘1、扬尘和沙尘2期间PM2.5浓度分别是沙尘前的1.2、1.1和2.5倍. ③沙尘和扬尘过程中PM2.5中水溶性离子浓度占比均减小，沙尘1、扬尘和沙尘2中水溶性离子占比分别为45.8%、37.9%和14.8%. 在不同阶段PM2.5中水溶性离子占比最高的均为NO3−，范围为24.6%(沙尘2)~38.7%(沙尘前)；其次是NH4+，占比在19.1%(沙尘前)~22.3%(沙尘2)之间. 沙尘过程对SO2转化生成SO42−的影响要弱于对NO2转化生成NO3−的影响. ④沙尘1过程中POC (一次有机碳)占比最高为49.0%，但在扬尘和沙尘2过程中POC占比均较低，分别为42.3%和41.2%，略低于沙尘前. ⑤沙尘1、扬尘和沙尘2过程中气溶胶含水量均显著降低，分别为沙尘前的29.9%、43.7%和6.2%. ⑥沙尘过程使得水溶性离子中阳离子含量增加，使得气溶胶碱性增强. 沙尘1和沙尘2过程中pH分别为6.7和6.5，高于沙尘前(6.2). 研究显示，沙尘传输对汾渭平原城市大气中PM2.5化学组分影响较大，显著降低了气溶胶的含水量，增加了气溶胶的pH，导致气溶胶的酸碱性变化，从而影响气溶胶化学组分的生成机制.Abstract: Repeatedly occurring sandstorms represent a significant concern in northern China because of their related hazards since particulate matter has harmful impacts on air quality in leeward cities. From March to April 2021, two sandstorms and one dust event (SDS) occurred in the Fenwei Plain. The spatial-temporal variation of aerosol chemical components was studied using simultaneous on-line ambient ions monitor, OC/EC online analyzer, weather and environmental data. The spatial distribution and transport of the sandstorms was represented by the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6.1 (C6.1) aerosol optical depth (AOD) product sand Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA 2) reanalysis datasets. Additionally, the ISORROPIA Ⅱ model was used to calculate aerosol water content and pH. The results showed that during sandstorm 1 (00:00, March 16th-07:00, March 21st), dust (08:00, March 21st-14:00, March 28th) and sandstorm 2 (15:00, March 28 th-21:00, March 31st), the average PM10 concentration in Xi´an City reached 309.5, 168.3 and 472.2 μg/m3, respectively, which was 2.7, 1.5 and 4.1 times higher than that before the sandstorm (00:00, March 10th-22:00, March 15th). The SDS events originated from Alxa League in Inner Mongolia and Jiuquan-Zhangye-Jinchang in Gansu Province, and experienced different transport pathways before reaching the Fenwei Plain, therefor the contribution of SDS to the surface PM2.5 concentrations showed significant difference. PM2.5 concentration increased by 2.5 times during sandstorm 2 compared with that before sandstorm, followed by the concentration increase in sandstorm 1 (1.2 times) and dust event (1.1 times). The content of water-soluble ions in PM2.5 decreased during the SDS events, accounting for 45.8%, 37.9% and 14.8% of PM2.5 in sandstorm 1, dust and sandstorm 2, respectively. Among the water-soluble ions, NO3− had the largest proportion in all SDS events, accounting for 24.6% (sandstorm 2)-38.7% (before sandstorm), followed by NH4+, accounting for 19.1% (before sandstorm)-22.3% (sandstorm 2). The effect of the sandstorm process on the conversion of SO2 to SO42− was weaker than that of NO2 to NO3−. The proportion of POC (Primary Organic Carbon) in PM2.5 was the highest in sandstorm 1, reaching 49.0%, and decreased to 42.3% and 41.2% in the dust and sandstorm 2, respectively, slightly lower than before the sandstorm. During sandstorm 1, dust and sandstorm 2, the water content of aerosols decreased by 29.9%, 43.7% and 6.2%, significantly, compared with those before sandstorm. The SDS process increased the concentration of water-soluble cations, resulting in an increase of aerosol alkalinity. The pH values of sandstorm 1 and sandstorm 2 were 6.7 and 6.5, respectively, which were higher than the pH (6.2) before the sandstorm. In summary, the impact of the SDS events on the PM2.5 chemical composition in the Fenwei Plain was significant, leading to a significant decrease in aerosol water content, a increase in pH, a change in acidity/basicity, and even a change of the mechanism of aerosol chemical composition formation.
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Key words:
- sandstorm /
- Fenwei Plain /
- water-soluble ions /
- carbonaceous aerosol /
- aerosol water content /
- pH
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图 1 观测期间离子电荷平衡关系以及不同阶段OC与EC浓度的散点图
Figure 1. The fit relationship between AE and CE concentrations during the measurement period and the comparison of OC and EC concentrations in PM2.5 at different stages
图 2 观测期间西安市气象要素和PM浓度的时间演变特征
Figure 2. Time series of meteorological variables and mass concentration of PM during the measurement period in Xi′an City
图 3 西安市观测期间不同阶段AOD的空间分布特征
Figure 3. Spatial distribution of AOD at different stages during measurements in Xi′an City
图 4 西安市观测期间逐日24 h后向轨迹
注：10~31代表3月10—31日的后向轨迹，1~5代表4月1—5日的后向轨迹.
Figure 4. Daily backward trajectory cluster analysis (24 h) during the measurement period during measurements in Xi′an City
图 5 西安市不同阶段下MERRA 2地面沙尘浓度空间分布特征
Figure 5. Spatial distribution of surface dust concentrations based on MERRA 2 data at different stages during measurements in Xi′an City
图 6 观测期间不同阶段PM2.5中化学组分浓度和占比
Figure 6. Chemical composition mass concentrations and proportions of PM2.5 at different stages
图 7 观测期间不同阶段PM2.5中POC和SOC的浓度及在OC中占比
Figure 7. POC and SOC mass concentrations and ratios distributions of OC in PM2.5 at different stages
图 8 观测期间不同阶段下PM2.5中气溶胶含水量和pH
Figure 8. Aerosol water content and pH values in PM2.5 at different stages
表 1 西安市观测期间不同阶段大气污染物浓度和气象要素汇总
Table 1. Summary of the atmospheric pollutants and meteorological variables at different stages during measurements in Xi′an City
项目 沙尘前 沙尘1 扬尘 沙尘2 沙尘后 SO2浓度/(μg/m3) 9.2 8.1 9.7 10.2 7.3 NO2浓度/(μg/m3) 52.8 40.1 59.8 39.3 33.0 O3浓度/(μg/m3) 27.8 29.8 35.5 57.6 34.8 CO浓度/(mg/m3) 1.0 0.7 0.7 0.7 0.6 PM10浓度/(μg/m3) 114.1 309.5 168.3 472.2 35.9 PM2.5浓度/(μg/m3) 56.0 66.3 58.9 138.0 22.8 PM2.5/PM10 0.45 0.24 0.30 0.30 0.52 AE 0.79 0.36 0.23 0.18 0.26 CE 0.95 0.50 0.43 0.46 0.37 NO3−/SO42− 2.31 1.76 3.35 2.19 2.13 NOR 0.31 0.17 0.11 0.11 0.18 SOR 0.46 0.33 0.16 0.17 0.24 温度/℃ 13.4 11.9 15.6 17.8 11.6 风速/(m/s) 0.6 0.7 0.6 0.8 0.5 RH/% 75.5 62.3 49.6 51.5 88.0 能见度/km 5.9 11.0 14.1 9.6 14.1 
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