Abstract

Constraining the climate impact of particulate brown carbon (BrC) will require identification of formation mechanisms and isolation of its different components to allow for source apportionment. For fresh combustion aerosols, the light absorption characteristics and the Absorption Ångstrom Exponent (AAE) are principally controlled by the combustion conditions in which the particles formed and evolved. We investigated the influence of combustion temperatures on the BrC or black carbon (BC) emission characteristics for a miniCAST soot generator (propane fuel) and a modern heavy-duty diesel engine (petroleum diesel and two renewable diesel fuels). Changes in the AAE, mass spectral signatures, and thermal-optical characteristics were studied. We show that changing operating parameters to gradually reduce the combustion temperatures in these two fundamentally different combustion devices result in a regression from BC dominated to BrC dominated particle emissions. The regression toward BrC was associated with: (1) an increasing mass fraction of particulate non-refractory polycyclic aromatic hydrocarbons (PAHs), (2) an increasing fraction of refractory organic carbon, (3) more curved soot nanostructures and shorter fringe lengths, and (4) increased signal from (refractory) large carbon fragments in IR laser-vaporization aerosol mass spectra. Based on these results we argue that fresh BrC dominated combustion aerosols are attributed to primary emissions from low temperature combustion, highlighting the influence of refractory constituents and soot nanostructure. Higher temperatures favor the growth of conjugated polyaromatic structures in the soot, a progression hypothesized to control the evolution from BrC to BC character of the emitted aerosols.

摘要

限制颗粒状褐碳 (BrC) 对气候的影响将需要确定形成机制并隔离其不同成分，以便进行来源分配。对于新鲜的燃烧气溶胶，光吸收特性和吸收埃指数 (AAE) 主要由颗粒形成和释放的燃烧条件控制。我们研究了燃烧温度对 miniCAST 烟灰发生器（丙烷燃料）和现代重型柴油发动机（石油柴油和两种可再生柴油燃料）的 BrC 或黑碳 (BC) 排放特性的影响。研究了 AAE、质谱特征和热光特性的变化。我们表明，改变运行参数以逐渐降低这两种根本不同的燃烧装置中的燃烧温度会导致从 BC 主导到 BrC 主导的颗粒排放的回归。向 BrC 的回归与：（1）颗粒非难熔多环芳烃（PAH）的质量分数增加，（2）难熔有机碳的比例增加，（3）更多弯曲的烟灰纳米结构和更短的条纹长度， (4) 红外激光汽化气溶胶质谱中（难熔的）大碳碎片的信号增加。基于这些结果，我们认为新鲜 BrC 主导的燃烧气溶胶归因于低温燃烧的初级排放，突出了耐火成分和烟尘纳米结构的影响。