川西平原棕碳气溶胶吸光特性及来源解析研究

Brown carbon (BrC), a vital component of organic aerosol, has a strong absorption toward the near UV. The climate and environmental effects of BrC have been a recent explosion of interest. The influence factors of BrC light-absorption property are extremely complicated. Furthermore, due to the different sources and atmospheric conditions, the concentrations and chemical components of BrC and its light-absorption property exerted a significant discrepancy. The city cluster in the Western Sichuan Plain is one of the “Three economic zones and ten urban clusters” key regions that urgently required to improve air quality. The air pollution in this district is heavy and the concentration of carbonaceous aerosol is high. The present study will choose a representative sampling site in the Western Sichuan Plain (Mianyang), and will operate a field observation in typical seasons by using a set of high volume cascade impactor sampler. The absorbance of water soluble/insoluble BrC will be measured, then the light-absorption parameters will be obtained to characterize BrC light-absorption property as well as its size distribution and seasonal variations. The correlations between the light-absorption parameters of BrC with specific organic tracers and chemical components of particle will be examined, combining with PMF reception model, the primary/secondary source apportionment of BrC in different seasons will be analyzed. Finally, the Mie theory will be applied to get the contribution of BrC to the total absorbance of aerosol. This study will help to enrich understandings of BrC sources and light-absorption properties in the Western Sichuan Plain. Meantime, the results will provide basic data and parameters to further study the regional climate and environmental effects of BrC.

棕碳（BrC）是有机气溶胶重要组成，作为对近紫外光具有强烈吸收的碳质气溶胶，其气候环境效应近年来受到广泛关注。BrC吸光性影响因素复杂，不同地区因来源与大气环境条件的差异，BrC的含量、化学组成与光学性质也会有很大不同。川西平原城市群是我国大气污染重点防控的“三区十群”之一，区域空气污染严重、碳质气溶胶浓度高。本研究拟在川西平原的绵阳市选择代表性站点，通过典型季节大流量分级滤膜采样，测定水溶性和非水溶性BrC的吸光度并计算其吸光参数，认识BrC吸光特性及其粒径分布和季节变化特征，探讨BrC吸光参数与颗粒物化学组成、有机示踪物之间的关系，同时结合PMF受体模型，解析BrC在不同季节的一次与二次来源贡献。最后通过Mie氏理论计算，明确BrC对颗粒物总体光吸收的贡献。项目研究有助于丰富对川西平原BrC吸光特性、来源等方面的认识，同时可为深入研究区域BrC气候环境效应提供基础数据和参数。

2016-2017年秋冬季，在川西平原地区城郊采集了24小时PM2.5膜样品。分析了这一地区秋冬季颗粒物无机/有机化学组成和水溶性/甲醇溶解性棕碳（BrC）吸光度。PM2.5日均浓度为90.0±8.3μgm−3，采样期间有69%的天数PM2.5日均浓度大于75μgm−3。有机质是颗粒物中含量最高的组分（27.8±2.3μgm−3, 32.4±1.8%），其次为硝酸盐（14.2±1.8μgm−3, 15.3±0.9%）。二次组分的增加是PM2.5质量变化的主要原因，其中二次无机离子的增加大于二次有机碳质组分的增加。25种PAHs的总浓度日均值为268.1±22.4ngm−3。苯并a芘日均浓度为21.2±2.2ngm−3，超标21倍（GB3095-2012，1ngm−3），超标率为100%。燃煤源和生物质燃烧源是川西平原地区PAHs的主要来源，贡献比例分别是50.1%和31.0%。颗粒物水溶性/甲醇溶解性（WSOC/MSOC）BrC吸光系数（Abs365，Water和Abs365，MeoH）日均值分别为19.2±1.6Mm-1和34.2±3.3Mm-1。WSOC和MSOC溶液的吸收埃指数平均为6.8±0.1和6.8±0.04。随着WSOC和OC浓度的增加，Abs365，Water和Abs365，MeoH均显著上升（P＜0.01）。与WSOC相比，MSOC中BrC吸光物质的吸光能力更强（MAE365，MeOH=2.4±0.1m2g-1）。Abs365，Water和Abs365，MeoH与有机示踪物（Picene和Levoglucosan）呈显著线性正相关关系（P＜0.01）。在不同波长范围，BrC对EC吸光系数的贡献比例f（300-2500nm）＜f（300-700nm）＜f（300-400nm）。在300-400 nm波长范围内，fWSOC/EC和fMSOC/EC分别为47±5%和79±7%。与EC相比，近地面BrC在近紫外波长范围内具有强烈吸光特性。PAHs总吸光系数AbsPAHs平均为0.13Mm-1。随着空气质量的恶化，高环数的PAHs吸光系数增加明显。PAHs对MSOCBrC（300-700nm）吸光比例的贡献为2.7%。项目研究结果有助于厘清川西平原地区颗粒物碳质/无机/PAHs组成变化特征及BrC吸光特性、来源等，同时为深入研究区域气候环境效应提供基础参数。