| 摘要: |
| 外来输入是城市大气颗粒物的主要来源之一，外来源气溶胶理化特征研究是理解其对城市大气颗粒物影响的前提和基础。本研究分析了昆明市冬季典型天气下主城区上风向大气PM2.5（空气动力学直径小于2.5 µm）的化学组分（包括元素、水溶性离子和碳组分）特征，结合气象要素筛选，获得了昆明市外来输入气溶胶的化学组成。昆明市外来源气溶胶PM2.5中碳组分、水溶性离子和地壳组分质量百分比分别为39.9%±3.3%、24.4%±9.0%和23.7%±9.2%；外来输入气团主要来自昆明市东北方向和西南方向，后向轨迹分析显示西南方向输入气团主要来自云南西南部的普洱、红河，而东北方向输入的气团主要来自云南省工业相对集中的曲靖等地。不同风向输入的气溶胶PM2.5的质量浓度和化学组成存在着明显的差异：东北方向输入的气溶胶质量浓度平均为(29.3±7.8) µg∙m−3，高于西南方向输入的气溶胶质量浓度（平均(24.1±6.0) µg∙m−3）约21%；前者SO42-、NO3-和NH4+平均含量是后者的两倍左右；东北方向Ca/Ti和K/Ti也明显高于西南方向。外来源气溶胶碳和离子组分显示长距离传输“老化”气溶胶的特征，元素的比值反映了气溶胶源的特征，可以用来指示气溶胶的来源。 |
| 关键词: 上风向 OC/EC 水溶性离子 元素 |
| DOI：10.7515/JEE202019 |
| CSTR：32259.14.JEE202019 |
| 分类号: |
| 基金项目:陕西省重点产业创新链项目（2018ZDCXL-SF-02-06）；国家自然科学基金项目（4161101160） |
| 英文基金项目:Shaanxi Key Industry Innovation Chain Project (2018ZDCXL-SF-02-06); National Natural Science Foundation of China (4161101160) |
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| Chemical composition of long-range transport aerosol in winter in Kunming |
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GUO Xiao, WU Feng, SHI Zhengtao, LIU Suixin, LI Shaoshi
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1. College of Life Sciences, Yunnan Normal University, Kunming 650500, China
2. Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
3. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
4. Faculty of Geography, Yunnan Normal University, Kunming 650500, China
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| Abstract: |
| Background, aim, and scope Input of external pollutants contribute to urban atmospheric particulate matter. The chemical characteristics of external aerosols is needed to estimate their contribution to urban atmospheric particulate matter. Our object is to obtain the chemical composition characteristics of external aerosols in winter in Kunming, capital of Yunnan province of China. Materials and methods The 24 hours PM2.5 (particulate matters with aerodynamic diameters less than 2.5 µm) samples were collected from 1 November to 6 December, 2017, at Chenggong sampling site (24º52ʹ02ʺN, 102º51ʹ14ʺE), 30 km upwind area of Kunming. Samples were analyzed to determine mass, water-soluble ions, inorganic elements and carbonaceous aerosol using MC5 electronic microbalance, ion chromatography, energy-dispersive X-ray fluorescence spectroscopy and thermal/optical reflectance (TOR) carbon analyzer, respectively. The chemical characteristics of external aerosols were obtained by screening all data to exclude those likely impacted by anthropogenic emissions from local and regional areas based on the meteorological parameters. Sources of external aerosols were identified by backward trajectory analysis. Results PM2.5 mass concentration of the external aerosols during winter in Kunming was on average (25.2±6.5) µg·m−3. The average mass concentration of PM2.5 from northeast direction was (29.3±7.8) µg·m−3, about 21% higher than those from southwest direction ((24.1±6.0) µg·m−3). Organic carbon (OC) in PM2.5 was 2.7—6.4 µg·m−3, accounting for 13.3%—19.4% of PM2.5 mass. Elemental carbon (EC) was 0.6—1.8 µg·m−3, about 3.7%—5.4% of PM2.5 mass. The ratio of OC / EC was 3.1—5.1. Total water-soluble inorganic ions was 2.8—12.5 µg·m−3, accounting for 12.7%—41.3% of PM2.5. SO42- was the most abundant anion with the average mass concentration of (3.2±1.9) µg·m−3, followed by NO3-, which concentrated average (0.8±0.4) µg·m−3, accounting for 3.1% of PM2.5 mass. Cl- in most aerosol samples was below the minimum detection limit. The main cation is NH4+, of which the average mass concentrations was (1.1±0.8) µg·m−3, accounting for 4.4% of PM2.5 mass. Ca2+ was (0.6±0.2) µg·m−3, accounting for 2.5% of PM2.5 mass. The average mass concentration of K+ was (0.2±0.1) µg·m−3, accounting for 0.9% of PM2.5 mass. Anions and cations show significant correlation (R2=0.98), and the ratio of cations to anions was about 1.1. The K+/K ratio was 0.4—0.9, which is significantly higher than their ratio in the crustal source (less than 0.1), and close to those (close to 1) of the biomass burning aerosol. The average contents of SO42-, NO3- and NH4+ in aerosol from the northeast direction were 19.0%±1.5%, 5.0%±2.1% and 7.5%±2.2%, which are about two times of those in aerosol from the southwest direction. Ti, one of the major crustal elements, was (0.03±0.02) µg·m−3 in mass, accounting for 0.13% of PM2.5 mass. Discussion Back-trajectory analysis showed that air masses from the southwest direction mainly originated from Pu’er and Honghe in southwestern Yunnan, while the northeast air masses originated from Qujing area to the northeast Kunming. The char-EC/soot-EC ratio was 2.4—15.6 (average 5.8) and the OC/EC was 3.1—5.1 (average 4.1), indicating that EC is mainly from biomass burning. OC showed good correlation with EC (R2=0.83) indicating a homogeneous source shared by them. SOC was estimated as 0.3—2.0 µg·m−3, accounting for 23.2% of PM2.5 mass, indicating that aerosol has experienced a long-term atmospheric aging process. The K+/K ratio is 0.4—0.9, indicating that biomass burning is major source of K+. Cl- in most samples was lower than the detection limit, also indicating that the aerosol is “aged”. Fe had a good correlation with Ti during the whole sampling periods and their enrichment factors relative to the surface soil of Yunnan is close to 1, indicating that it originated from the soil. Both Ca and K showed good relation with Ti in aerosol samples with the same wind direction, reflecting the characteristics of sources. The Ca/Ti and K/Ti ratios of aerosols from different directions are significantly different, and can be used as a tracer for aerosols source. Conclusions The mass percentages of carbon components, water-soluble ions and crustal components in external aerosols in winter in Kunming were 39.9%±3.3%, 24.4%±9.0% and 23.7%±9.2%, respectively. External aerosols in winter in Kunming mainly originated from Pu’er, Honghe area in southwest Yunnan Province and Qujing area to the northeast Kunming. Mass concentration of PM2.5 from the northeast direction of Kunming was (29.3±7.8) µg·m−3, about 21% higher than those from southwest Yunnan ((24.1±6.0) µg·m−3). The average content of SO42-, NO3- and NH4+ in former was about twice that of the latter; Ca/Ti and K/Ti in the former are also obvious higher than the latter. Carbon and ionic components of long-range transport aerosols showed the characteristics of long-distance “aged” aerosols. The ratio of Ca/Ti and K/Ti reflects the characteristics of aerosol sources in different directions and can be used as tracer for aerosols source. Recommendations and perspectives The chemical composition of external aerosols in winter in Kunming is obtained, these data can be used for estimating the contribution of external aerosols to the atmospheric PM2.5 in Kunming City, and then for understanding the sources of atmospheric particulate matter in Kunming City. |
| Key words: upwind OC/EC water-soluble ions element |
