Lu Quanyang, Murphy Benjamin N, Qin Momei, Adams Peter J, Zhao Yunliang, Pye Havala O T, Efstathiou Christos, Allen Chris, Robinson Allen L
Center of Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA.
Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, USA.
Atmos Chem Phys. 2020 Apr 14;20(7):4313-4332. doi: 10.5194/acp-20-4313-2020.
We describe simulations using an updated version of the Community Multiscale Air Quality model version 5.3 (CMAQ v5.3) to investigate the contribution of intermediate-volatility organic compounds (IVOCs) to secondary organic aerosol (SOA) formation in southern California during the CalNex study. We first derive a model-ready parameterization for SOA formation from IVOC emissions from mobile sources. To account for SOA formation from both diesel and gasoline sources, the parameterization has six lumped precursor species that resolve both volatility and molecular structure (aromatic versus aliphatic). We also implement new mobile-source emission profiles that quantify all IVOCs based on direct measurements. The profiles have been released in SPECIATE 5.0. By incorporating both comprehensive mobile-source emission profiles for semivolatile organic compounds (SVOCs) and IVOCs and experimentally constrained SOA yields, this CMAQ configuration best represents the contribution of mobile sources to urban and regional ambient organic aerosol (OA). In the Los Angeles region, gasoline sources emit 4 times more non-methane organic gases (NMOGs) than diesel sources, but diesel emits roughly 3 times more IVOCs on an absolute basis. The revised model predicts all mobile sources (including on- and off-road gasoline, aircraft, and on- and off-road diesel) contribute ~ 1 μgm to the daily peak SOA concentration in Pasadena. This represents a ~ 70% increase in predicted daily peak SOA formation compared to the base version of CMAQ. Therefore, IVOCs in mobile-source emissions contribute almost as much SOA as traditional precursors such as single-ring aromatics. However, accounting for these emissions in CMAQ does not reproduce measurements of either ambient SOA or IVOCs. To investigate the potential contribution of other IVOC sources, we performed two exploratory simulations with varying amounts of IVOC emissions from nonmobile sources. To close the mass balance of primary hydrocarbon IVOCs, IVOCs would need to account for 12% of NMOG emissions from nonmobile sources (or equivalently 30.7 t d in the Los Angeles-Pasadena region), a value that is well within the reported range of IVOC content from volatile chemical products. To close the SOA mass balance and also explain the mildly oxygenated IVOCs in Pasadena, an additional 14.8% of nonmobile-source NMOG emissions would need to be IVOCs (assuming SOA yields from the mobile IVOCs apply to nonmobile IVOCs). However, an IVOC-to-NMOG ratio of 26.8% (or equivalently 68.5 t d in the Los Angeles-Pasadena region) for nonmobile sources is likely unrealistically high. Our results highlight the important contribution of IVOCs to SOA production in the Los Angeles region but underscore that other uncertainties must be addressed (multigenerational aging, aqueous chemistry and vapor wall losses) to close the SOA mass balance. This research also highlights the effectiveness of regulations to reduce mobile-source emissions, which have in turn increased the relative importance of other sources, such as volatile chemical products.
我们描述了使用社区多尺度空气质量模型第5.3版（CMAQ v5.3）的更新版本进行的模拟，以研究中挥发性有机化合物（IVOCs）对加州空气流域研究（CalNex）期间南加州二次有机气溶胶（SOA）形成的贡献。我们首先从移动源的IVOC排放中推导出一个适用于模型的SOA形成参数化方法。为了考虑柴油和汽油源的SOA形成，该参数化方法有六种集总前体物种，它们既能解析挥发性又能解析分子结构（芳香族与脂肪族）。我们还实施了新的移动源排放清单，该清单基于直接测量对所有IVOCs进行了量化。这些清单已在SPECIATE 5.0中发布。通过纳入半挥发性有机化合物（SVOCs）和IVOCs的综合移动源排放清单以及实验约束的SOA产率，这种CMAQ配置最能代表移动源对城市和区域环境有机气溶胶（OA）的贡献。在洛杉矶地区，汽油源排放的非甲烷有机气体（NMOGs）比柴油源多4倍，但柴油在绝对量上排放的IVOCs大约多3倍。修订后的模型预测，所有移动源（包括道路上和非道路上的汽油、飞机以及道路上和非道路上的柴油）对帕萨迪纳的每日峰值SOA浓度贡献约为1μg/m³。这比CMAQ的基础版本预测的每日峰值SOA形成量增加了约70%。因此，移动源排放中的IVOCs对SOA的贡献几乎与单环芳烃等传统前体相当。然而，在CMAQ中考虑这些排放并不能重现环境SOA或IVOCs的测量结果。为了研究其他IVOC源的潜在贡献，我们进行了两次探索性模拟，其中非移动源的IVOC排放量各不相同。为了闭合一次烃类IVOCs的质量平衡，IVOCs需要占非移动源NMOG排放量的12%（或相当于洛杉矶 - 帕萨迪纳地区30.7吨/天），这个值完全在挥发性化学产品报告的IVOC含量范围内。为了闭合SOA质量平衡并解释帕萨迪纳中轻度氧化的IVOCs，非移动源NMOG排放量还需要额外的14.8%是IVOCs（假设移动IVOCs的SOA产率适用于非移动IVOCs）。然而，非移动源26.8%（或相当于洛杉矶 - 帕萨迪纳地区68.5吨/天）的IVOC与NMOG的比例可能高得不太现实。我们的结果突出了IVOCs对洛杉矶地区SOA生成的重要贡献，但强调必须解决其他不确定性因素（多代老化、水相化学和气壁损失）以闭合SOA质量平衡。这项研究还突出了减少移动源排放法规的有效性，这些法规反过来增加了其他源（如挥发性化学产品）的相对重要性。
