Secondary organic aerosol formation in biomass-burning plumes: theoretical analysis of lab studies and ambient plumes
Secondary organic aerosol formation in biomass-burning plumes: theoretical analysis of lab studies and ambient plumes
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DOI：
10.5194/acp-17-5459-2017
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发表时间：
2017-04-28
影响因子：
6.3
通讯作者：
Pierce, Jeffrey R.
中科院分区：
文献类型：
--
作者：
Bian, Qijing;Jathar, Shantanu H.;Pierce, Jeffrey R.
二次有机气溶胶（SOA）在实验室和实地研究中已被证明在生物质燃烧排放物中形成。然而，在质量增强的研究中存在显著的差异，这可能是由于燃料、火灾条件、稀释和/或实验室实验和观察的限制的差异。本研究的重点是了解在实验室烟雾室实验和环境羽流中影响生物质燃烧SOA形成的过程。在传统SOA前体的实验室研究中，蒸汽壁损失已被证明是抑制SOA形成的重要因素；然而，尚未研究蒸汽壁损失对生物质燃烧SOA的影响。我们使用包含挥发性和氧化化学表征的气溶胶微物理模型来估计在FLAME III烟雾室研究中观察到的蒸汽壁损失对SOA形成的影响。我们基于化学和壁面损失的基本情况假设进行的模拟预测，当模拟汽壁损失时，所有实验的平均OA质量增强（最终与初始OA质量之比，修正了颗粒相壁面损失）为1.8，大致与FLAME III期间观察到的平均增强相匹配。当蒸汽壁损失关闭时，平均OA增强增加到3以上，这意味着蒸汽壁损失减少了明显的SOA形成。我们发现，在壁面损失、传质系数和化学机制的关键模型假设的不确定性范围内，由于汽壁损失导致的明显SOA形成的减少是强劲的。然后，我们将类似的关于SOA形成化学和物理的假设应用于排放到大气中的烟雾。在环境羽流中，羽流稀释率影响气相和颗粒相之间的有机分配，这可能影响SOA形成的可能性以及SOA形成的速度。我们在气溶胶微物理模型中加入高斯色散，以估计不同环境羽流条件下SOA形成的变化（例如，火灾规模、排放质量通量、大气稳定性）。小型火灾产生的烟雾，如典型的规定燃烧，迅速稀释，这促使颗粒相的有机蒸汽蒸发，导致更有效的SOA形成。大型火灾的排放物，如强烈的野火，稀释缓慢，抑制OA蒸发和随后在近场形成SOA。我们还证明，计算环境羽流中OA增强的不同方法可能导致关于SOA形成的不同结论。使用惰性示踪剂（如黑碳或CO）计算的OA质量增强比约为1，传统上被解释为很少或没有SOA形成；然而，我们表明SOA的形成可能在很大程度上促成了这些羽流的质量。对比实验室和羽流的结果，两者之间OA增强的不一致可能部分归因于室壁和羽流稀释的影响。我们的研究结果强调，实验室和现场实验的重点是燃料和火灾条件也需要考虑羽流稀释或蒸汽损失的影响。
Secondary organic aerosol (SOA) has been shown to form in biomass-burning emissions in laboratory and field studies. However, there is significant variability among studies in mass enhancement, which could be due to differences in fuels, fire conditions, dilution, and/or limitations of laboratory experiments and observations. This study focuses on understanding processes affecting biomass-burning SOA formation in laboratory smog-chamber experiments and in ambient plumes. Vapor wall losses have been demonstrated to be an important factor that can suppress SOA formation in laboratory studies of traditional SOA precursors; however, impacts of vapor wall losses on biomass-burning SOA have not yet been investigated. We use an aerosol-microphysical model that includes representations of volatility and oxidation chemistry to estimate the influence of vapor wall loss on SOA formation observed in the FLAME III smogchamber studies. Our simulations with base-case assumptions for chemistry and wall loss predict a mean OA mass enhancement (the ratio of final to initial OA mass, corrected for particle-phase wall losses) of 1.8 across all experiments when vapor wall losses are modeled, roughly matching the mean observed enhancement during FLAME III. The mean OA enhancement increases to over 3 when vapor wall losses are turned off, implying that vapor wall losses reduce the apparent SOA formation. We find that this decrease in the apparent SOA formation due to vapor wall losses is robust across the ranges of uncertainties in the key model assumptions for wall-loss and mass-transfer coefficients and chemical mechanisms.We then apply similar assumptions regarding SOA formation chemistry and physics to smoke emitted into the atmosphere. In ambient plumes, the plume dilution rate impacts the organic partitioning between the gas and particle phases, which may impact the potential for SOA to form as well as the rate of SOA formation. We add Gaussian dispersion to our aerosol-microphysical model to estimate how SOA formation may vary under different ambient-plume conditions (e.g., fire size, emission mass flux, atmospheric stability). Smoke from small fires, such as typical prescribed burns, dilutes rapidly, which drives evaporation of organic vapor from the particle phase, leading to more effective SOA formation. Emissions from large fires, such as intense wildfires, dilute slowly, suppressing OA evaporation and subsequent SOA formation in the near field. We also demonstrate that different approaches to the calculation of OA enhancement in ambient plumes can lead to different conclusions regarding SOA formation. OA mass enhancement ratios of around 1 calculated using an inert tracer, such as black carbon or CO, have traditionally been interpreted as exhibiting little or no SOA formation; however, we show that SOA formation may have greatly contributed to the mass in these plumes.In comparison of laboratory and plume results, the possible inconsistency of OA enhancement between them could be in part attributed to the effect of chamber walls and plume dilution. Our results highlight that laboratory and field experiments that focus on the fuel and fire conditions also need to consider the effects of plume dilution or vapor losses to walls.