棕色碳气溶胶来源、性质、测量与排放估算

随着对碳气溶胶吸光性认识的提高,近年来吸光有机碳——BrC(brown carbon,棕色碳)的吸光问题成为继BC(black carbon,黑碳)之后国际大气环境领域的新热点. 基于已有的研究报道,将BrC大体分为焦油类物质、类腐殖质(HULIS)和其他吸光性有机气溶胶三大类,其来源包括一次排放和二次生成2种. 由于BrC缺少BC所具有的类石墨烯结构,致使颗粒间较为分散,加之含氧官能团比重较高,因而在水及有机溶剂中均有较强的可溶性. BrC的光学性质通常借助AAE(ngstrm吸收波长指数)、MAE(质量吸收效率)、RI(折射率)及SSA(单次散射反照率)来表示,其中由于BrC分子结构中缺少sp2杂化成分,形成了区别于BC的典型特征,即AAE>1(而对于BC,其AAE=1)). 虽然已有借助于光学法、热光法、化学法和质谱法进行BrC测定的报道,但目前没有公认的标准测定方法和参考物质,测定结果实际依赖于选定的测定方式. 在排放估算研究方面,BrC远落后于BC,致使有些排放估算方法多以相伴的BC排放量作为参照. 建议今后对BrC研究应主要面向气候影响、生成机理、测定方法、排放因子与控制策略等领域来展开. 

Origin, Properties, Measurement and Emission Estimation of Brown Carbon Aerosols

As understanding of the light absorption of carbonaceous aerosols increases, brown carbon (BrC, i.e., light-absorbing organic carbon), another species of light-absorbing carbon in addition to black carbon (BC), has been highlighted in recent years and has become a new hotspot in the research field of atmospheric environment internationally. However, research on BrC in China is not common until now. Based on existing literature, BrC can be roughly divided into three categories, i.e., tarry materials, humic-like substances (HULIS) and other light-absorbing organic aerosols, and can originate from two pathways, i.e., primary emission and secondary formation. In addition, BrC is in a weakly-bound aggregate morphology because of the shortage of graphitic structure, and is usually highly soluble because of the richness of oxygen content. The optical properties of BrC are often characterized by ngstrm absorption exponent (AAE), mass absorption efficiency (MAE), refractive index (RI) and single scattering albedo (SSA), with a distinctive feature of AAE>1, in contrast to BC (for BC, AAE=1), due to the deficiency of sp2 hybrid orbital structure in BrC. Although BrC is by some researchers quantified through methods based on the optical, thermal/optical, chemical and mass-spectrometric principles, there are no widely-accepted standard methods and reference materials for BrC quantification, which makes BrC measurements operationally-dependent. Given that the history of BC emission estimation is longer than that of BrC's, some researchers have tried estimating BrC emissions by referencing concurrently released BC emissions. Future research on BrC is proposed towards BrC's climate effects, formation mechanisms, emission factors, measurement methods and emissions abatement.