| 摘要: |
| 2013年6月10 — 25日在河北保定市固城镇运用大流量采样器进行每3小时1次PM2.5样品采集，对其进行有机碳（OC）、元素碳（EC）、水溶性有机碳（WSOC）、水溶性有机氮（WSON）、水溶性总氮（WSTN）、吸湿增长因子、吸光度以及无机离子分析，探讨其浓度、组成、吸湿性能与吸光性的变化特征。结果表明：采样期间固城镇PM2.5中WSON 平均浓度为5.0 ± 4.0 μg∙m-3，最高浓度达15 μg∙m-3；污染期WSON为6.9 ± 3.9 μg∙m-3，是清洁期的四倍。整个采样期间WSON与、和呈强线性相关（R2>0.89），污染天阳阴离子当量比值F = 1.01，清洁天F = 1.45，表明污染期颗粒物酸性增强有利于气态有机胺等WSON通过酸碱中和转移到颗粒相。不同相对湿度下水溶性组分的吸湿增长因子（Gf）测量结果显示：[WSOC+WSON]/离子的比值越大，吸湿增长因子越小，表明与无机离子相比，水溶性有机物吸湿性能较低。固城夏季大气PM2.5中WSOC在365 nm波长下质量吸收效率（MAE）均值为0.52 m2∙g-1，表明WSOC对PM2.5整体消光效应具有重要贡献。 |
| 关键词: PM2.5 水溶性离子 组成 吸湿性 质量吸收效率MAE |
| DOI：10.7515/JEE201601006 |
| CSTR： |
| 分类号: |
| 基金项目:国家杰出青年科学基金项目（41325014） |
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| Composition, hygroscopicity and light absorption of water-soluble fraction of PM2.5 at a rural site near Beijing |
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HAN Yanni, WANG Gehui1,2
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. University of Chinese Academy of Sciences, Beijing 100049, China
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| Abstract: |
| Background, aim, and scope Haze episodes have frequently occurred in North China Plain (NCP) due to rapid increases in vehicle numbers and fossil fuel consumption. Beijing is the largest megacity in NCP and has experienced severe air pollution. Previous studies indicate that the transportation of fine particulates from NCP is an important source to haze formation in Beijing. Many researches about PM2.5 have focused on Beijing urban aerosols, while the information about the physical-chemical properties of aerosols from Beijing surrounding regions especially the rural areas is very limited. Hygroscopicity is one of the key factors affecting the behavior of an aerosol in the atmosphere, because by water vapor uptake aerosol may significantly alter its physical-chemical properties such as light scattering and absorbing, transportation, gas-particle phase partitioning and aqueous reaction. This paper aims to investigate the chemical composition, hygroscopic growth factor and optical absorption efficiency of fine particles at Gucheng, a rural site with a distance of about 100 km southwest to Beijing. Materials and methods 121 PM2.5 samples were collected during June 10th—25th 2013 at Gucheng, Hebei Province by using high-volume sampler (1.13 m3∙min-1) with a 3 h interval. The samples were measured for element carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC) and water-soluble organic nitrogen (WSON), inorganic ions, hygroscopic growth factor (Gf) and optical mass absorption efficiency (MAE) at 365 nm light wavelength. Results During sampling period PM2.5 ranged from 5.0 μg∙m-3 to 372 μg∙m-3 with an average of 135 μg∙m-3, which was about two times the national air quality secondary standard (GB 3095—2012, 75 μg∙m-3). During the sampling period, NO3-,SO42- and NH4+ were the dominant inorganic ions, accounting for 29%, 30%, 14% of the total inorganic ions, respectively. The average concentration of WSON was 5.0 ± 4.0 μg∙m-3 with a maximum of 15 μg∙m-3 during the whole campaign and four times higher in polluted periods (6.9 ± 3.9 μg∙m-3) than in the clean periods (1.7 ± 0.9 μg∙m-3). In the whole sampling period WSON well correlated with NH4+, SO42- and NO3- (R2 > 0.89), and enahced with an increase in the equivalent ratio of cations to anions from 1.01 in the polluted periods to 1.45 in the clean periods, suggesting that acidity of PM2.5 was favorable for the gas-to-particle partitioning of WSON species such as low molecular weight amines. Hygroscopic growth factors (Gf) of the water-soluble fraction of the PM2.5 samples were measured by hygroscopic tandem differential mobility analyzer (HTDMA). The results showed that Gf negatively correlated with the mass ratio of (WSOC + WSON) to (inorganic ions), indicating that water-soluble organic compounds were less hygroscopic in comparison with inorganic ions. Mass absorption efficiency (MAE) of WSOC at the Gucheng site was 0.52 m2∙g-1, higher than that in the United States, South Korea and other regions. Discussion By comparison the concentrations of NO3-, SO42-, NH4+, and PM2.5 from the Gucheng site with those from other cities, we found that Cl- and K+ at Gucheng site mainly derived from biomass burning. A comparison of MAE values measured in this study with those in the literature suggests that the values of MAE of WSOC are generally greater in cities than in rural regions and higher in winter than summer. Moreover, the Gucheng MAE values, together with others reported, showed that MAE was higher for Chinese aerosols those for any other countries, suggesting the importance of anthropogenic WSOC in China. Conclusions High-volume PM2.5 samples were collected during June 10th—25th 2013 at Gucheng, a rural site near Beijing, and determined for chemical composition including inorganic ions, organic carbon, elemental carbon, water-soluble organic carbon and water-soluble organic nitrogen, hygroscopic growth factor, and light absorption to investigate the sources, formation mechanisms, hygroscopicity and optical properties of PM2.5 in NCP. During the whole period NO3-,SO42- and NH4+ were dominant inorganic ions, and acidity of aerosols was stronger in pollution period than clean period which was favorable for the gas-to-particle partitioning of low molecular weight amines. Hygroscopicity analysis showed that Gf of the samples was largely determined by its water-soluble inorganic fraction. MAE was higher in Gucheng than in other cities because of the consumption of coal in winter. Recommendations and perspectives In current work sources, hygroscopic and optical properties of PM2.5 were investigated in rural area near Beijing. Water-soluble organic compounds constitute an important fraction of the rural fine particles, and have been found to be light absorbing in UV-visible wavelength range. Details in molecular compositions of the water-soluble organic compounds are needed for explaining the haze formation in China. |
| Key words: water-soluble matter composition hygroscopicity mass absorption efficiency (MAE) |
