Polycyclic aromatic hydrocarbons (PAHs) and secondary organic aerosol (SOA) versus primary carbonaceous aerosols, 2006-2008

We use the chemical transport model GEOS-
Chem to evaluate the hypothesis that atmospheric polycyclic
aromatic hydrocarbons (PAHs) are trapped in secondary
organic aerosol (SOA) as it forms. We test the ability of three
different partitioning configurations within the model to
reproduce observed total concentrations in the midlatitudes
and the Arctic as well as midlatitude gas-particle phase
distributions. The configurations tested are (1) the GEOS-
Chem default configuration, which uses instantaneous
equilibrium partitioning to divide PAHs among the gas
phase, a primary organic matter (OM) phase (absorptive),
and a black carbon (BC) phase (adsorptive), (2) an SOA configuration in which PAHs are trapped in SOA when emitted and slowly evaporate from SOA thereafter, and (3) a configuration in which PAHs are trapped in primary OM/BC upon emission and subsequently slowly evaporate. We also test the influence of changing the fraction of PAHs available for particle-phase oxidation. Trapping PAHs in SOA particles upon formation and protecting against particle-phase oxidation (2) better simulates observed remote concentrations compared to our default configuration (1). However, simulating adsorptive partitioning to BC is required to reproduce the magnitude and seasonal pattern of gas−particle phase distributions. Thus, the last configuration (3) results in the best agreement between observed and simulated concentration/phase distribution data. The importance of BC rather than SOA to PAH transport is consistent with strong observational evidence that PAHs and BC are coemitted.

本研究采用化学传输模型GEOS-Chem，验证大气多环芳烃（polycyclic aromatic hydrocarbons, PAHs）会在二次有机气溶胶（secondary organic aerosol, SOA）生成过程中被捕获这一假说。我们测试了模型中三种不同分配配置的性能，以复现中纬度与北极地区的观测总浓度，以及中纬度区域的气-粒相分布特征。所测试的配置如下：（1）GEOS-Chem默认配置：采用瞬时平衡分配法，将PAHs分配至气相、一次有机物质（organic matter, OM）吸收相以及黑碳（black carbon, BC）吸附相；（2）SOA配置：PAHs在排放时被捕获于SOA中，并随后从其中缓慢蒸发；（3）PAHs在排放时被捕获于一次OM/BC中，随后缓慢蒸发的配置。我们还测试了可参与颗粒相氧化的PAHs占比变化对模拟结果的影响。相较于默认配置（1），在SOA生成时捕获PAHs并使其免受颗粒相氧化的配置（2），能够更精准地模拟偏远地区的观测浓度。然而，若要复现气-粒相分布的幅值与季节模式，则必须模拟PAHs向BC的吸附分配过程。因此，第三种配置（3）可实现观测与模拟的浓度/相分布数据之间的最优匹配。BC而非SOA对PAH传输具有关键意义这一结论，与PAHs与BC共排放的大量观测证据相一致。