典型城市黑碳气溶胶光谱吸收效率的变化特征及其影响机制

Mass absorption efficiency of black carbon (BC) aerosol (MAEBC, defined as the ratio of absorption coefficient to BC mass concentration) is one of the key parameters in estimating direct radiative forcing due to BC. It is closely related to the conversion factor when determining BC concentration from optical measurement as well. Thus, it’s important to quantify the value of MAEBC and study its change mechanism to reduce the uncertainty in assessing the effect of BC on climate. Moreover, haze occurred frequently in recent years and has become the primary atmospheric environmental issue in China. As the major light-absorbing aerosol component, BC plays an important role in haze formation and development. Thus, it’s meaningful to study MAEBC in the high BC emission urban areas to promote the understanding of the effect of BC on regional climate change and its role during haze periods. In this study, the spectral aerosol absorption coefficients determined using the filter-based optical method (Model AE33 Aethalometer, Magee Scientific, US) will be inter-compared to those determined using the photoacoustic technology (PASS-3, DMT, US), proposing a suitable wavelength-dependent correction factor for the absorption coefficients determined by optical method. On this basis, aerosol samples including PM1 and PM2.5 will be collected at a typical urban site in eastern China (e.g., Beijing). The optical (SootSanTM Model OT21 Transmissometer, Magee Scientific, US) and thermal-optical (DRI Model 2001 TOR carbon analyzer, Atmoslytic Inc., US) methods are employed to determine the absorption coefficient and BC concentration, respectively for each sample, which are used to quantify the value of MAEBC. Variation in MAEBC will be discussed. Thermal denuding will be performed to each sample to reduce the impact of volatile aerosol components on MAEBC. Combining the analysis on chemical components, and the mixing state and size distribution of BC determined by a single particle soot photometer (SP2, DMT, US), the change mechanism of MAEBC will be further studied. The optical closure study will also performed to MAEBC using the theoretical model. The discrepancy in theoretical and measured MAEBC will be discussed.

黑碳（BC）气溶胶光谱吸收效率（MAEBC）既是估算BC直接辐射强迫的关键参数，又与光学法测量BC浓度的换算因子密切相关，对其准确定量并研究其变化机制对降低BC气候效应的不确定性具有重要意义。近年来，雾霾成为影响我国大气环境的首要问题，作为最主要的吸光成分，BC对雾霾的形成和发展有至关重要的作用，研究BC排放较强的典型城市地区MAEBC的变化及影响机制对进一步认识BC的区域气候效应及其对雾霾的贡献具有现实意义。本研究拟用光声法和光学法对比测量气溶胶光谱吸收系数，提出针对光学法的合理订正因子，并在我国典型城市采集气溶胶样品，利用光学法和热光法分别测量样品的吸收系数和BC浓度，并对吸收系数进行修正，获得典型城市MAEBC及其变化特征。基于化学组分的分析和BC混合、粒径等的同步测量，结合BC热剥离的方法，探讨影响环境大气MAEBC的主要因子和机理，结合理论模型，开展MAEBC光学闭合研究。

黑碳是大气气溶胶吸收太阳辐射最主要的成分，对全球增温有重要贡献，并能通过改变大气稳定度加重霾污染。黑碳气溶胶对太阳辐射的吸收不仅决定于其在大气中的绝对含量，也受到其质量吸收效率MAEEC的影响。然而受到测量方法和气溶胶理化性质变化的影响，针对环境大气中MAEEC的定量还存在很大的不确定性。本项目首先通过多种气溶胶光学吸收测量仪器的比对研究，给出了适用黑碳仪测量获取气溶胶吸收系数bap的订正因子。在此基础上，准确定量了不同季节采集的北京城市大气PM1和PM2.5样品对应的bap，结合对样品中元素碳EC含量的定量分析，获得方法上可比的MAEEC。结果表明，MAEEC存在明显的季节差异，春季最高，均值（±标准差）达4.59±1.05 m2 g−1（880nm），约为冬季均值（3.24±1.01 m2 g−1）的1.4倍，夏、秋季均值略高于冬季，分别为3.67±0.72 m2 g−1和3.52±0.67 m2 g−1。春季较高的MAEEC与这个季节更多受到生物质燃烧的影响有关。研究发现，除了夏季，其他季节MAEEC与NO3−/EC质量比均存在较好的相关性，表明NO3−的相对含量可能是影响黑碳气溶胶混合程度，进而造成MAEEC变化的主要因素。夏季MAEEC与NO3−/EC质量比相关性较差，可能与这个季节NO3−主要由均相反应生成有关，该途径生成的NO3−更多与EC外混，对MAEEC的影响较小。其他季节，尤其秋冬季，NO3−的光化学均相生成较弱，非均相反应（如N2O5水解）可能是NO3−的主要生成途径，该反应主要发生在已经存在的颗粒（如含EC颗粒）表面，容易增加EC的包裹程度，进而增强MAEEC。研究也发现，PM1的MAEEC平均约为PM2.5的MAEEC的1.1倍，可能与大粒径段（1–2.5μm）黑碳气溶胶较大的核粒径以及较低的包裹程度有关。基于单颗粒黑碳光度计对黑碳浓度、粒径和包裹厚度的测量，结合核壳结构的Mie理论，计算了880nm 的bap，发现与黑碳仪测量的bap有很好的一致性，在此基础上分析了粒径和混合对MAEEC变化的相对贡献，结果表明，MAEEC的变化主要来源于黑碳气溶胶混合态的变化，污染过程中包裹可以造成MAEEC增强2倍以上。本研究提高了对污染环境大气中MAEEC及其主控因素的认识，对降低黑碳气溶胶辐射强迫估算不确定性有科学价值。