Exploring the vertical profile of atmospheric organic aerosol: comparing 17 aircraft field campaigns with a global model
Exploring the vertical profile of atmospheric organic aerosol: comparing 17 aircraft field campaigns with a global model
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DOI：
10.5194/acp-11-12673-2011
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发表时间：
2011-01-01
影响因子：
6.3
通讯作者：
Dunlea, E. J.
中科院分区：
文献类型：
--
作者：
Heald, C. L.;Coe, H.;Dunlea, E. J.
全球有机气溶胶（OA）收支具有高度不确定性，以往研究表明，模型大大低估了观测到的浓度。这些研究中很少有考察有机气溶胶垂直分布的。此外，许多模型与测量的比较是针对单个外场观测活动使用不同模型进行的。我们综合了2001 - 2009年17次飞机外场观测活动中的有机气溶胶测量数据，并利用这些观测结果对一个GEOS - Chem模型模拟进行一致性评估。这个广泛的数据集中包含了偏远地区、受污染地区以及受火灾影响地区的情况。观测到的平均有机气溶胶浓度范围为0.2 - 8.2微克/立方米，占非难熔气溶胶的15% - 70%。标准的GEOS - Chem模拟重现了观测到的垂直廓线，尽管在17次外场观测活动中的13次观测值被低估（观测值与模拟值的中位数比率范围为0.4 - 4.2），在人为源区域模型偏差最大。然而，该模型在这些受人为影响的区域最能捕捉到观测到的变化性（R² = 0.18 - 0.57），但在偏远地区或受火灾影响地区效果不佳。在11次外场观测活动中，模型偏差随着相对湿度的增加而增大，这可能表明缺失了水相二次有机气溶胶（SOA）的生成过程。然而，水相二次有机气溶胶的模型模拟显示在对流层中层（2 - 6千米）有明显特征，而这里所考察的观测结果并不支持这一点。斯普拉克兰等人（2011年）建议增加一个类似于100太克/年的人为控制的二次有机气溶胶源来缩小测量与模型之间的差距，我们将其作为人为二次有机气溶胶添加进去。这消除了在源附近模型的低估情况，但导致在一些区域的高空以及偏远地区出现高估，这表明要么有机气溶胶存在额外的汇，要么在较低温度下存在挥发性更高的气溶胶。敏感性模拟表明，有机物在非均相或气相氧化过程中的裂解可能是模型中有机气溶胶一个重要的（缺失的）汇，分别使全球二次有机气溶胶负荷降低15%和47%。当模拟的人为控制的二次有机气溶胶增加到类似于100太克/年，并伴有气相裂解过程或有机气溶胶分配的温度依赖性降低（通过将汽化焓从42千焦/摩尔降低到25千焦/摩尔）时，与观测结果的吻合度最佳。这些结果表明，模型可能需要额外的源和汇来捕捉观测到的有机气溶胶浓度。
The global organic aerosol (OA) budget is highly uncertain and past studies suggest that models substantially underestimate observed concentrations. Few of these studies have examined the vertical distribution of OA. Furthermore, many model-measurement comparisons have been performed with different models for single field campaigns. We synthesize organic aerosol measurements from 17 aircraft campaigns from 2001-2009 and use these observations to consistently evaluate a GEOS-Chem model simulation. Remote, polluted and fire-influenced conditions are all represented in this extensive dataset. Mean observed OA concentrations range from 0.2-8.2 mu g sm(-3) and make up 15 to 70% of non-refractory aerosol. The standard GEOS-Chem simulation reproduces the observed vertical profile, although observations are underestimated in 13 of the 17 field campaigns (the median observed to simulated ratio ranges from 0.4 to 4.2), with the largest model bias in anthropogenic regions. However, the model is best able to capture the observed variability in these anthropogenicallyinfluenced regions (R-2 = 0.18-0.57), but has little skill in remote or fire-influenced regions. The model bias increases as a function of relative humidity for 11 of the campaigns, possibly indicative of missing aqueous phase SOA production. However, model simulations of aqueous phase SOA suggest a pronounced signature in the midtroposphere (2-6 km) which is not supported in the observations examined here. Spracklen et al. (2011) suggest adding similar to 100 Tg yr(-1) source of anthropogenically-controlled SOA to close the measurement-model gap, which we add as anthropogenic SOA. This eliminates the model underestimate near source, but leads to overestimates aloft in a few regions and in remote regions, suggesting either additional sinks of OA or higher volatility aerosol at colder temperatures. Sensitivity simulations indicate that fragmentation of organics upon either heterogeneous or gas-phase oxidation could be an important (missing) sink of OA in models, reducing the global SOA burden by 15% and 47% respectively. The best agreement with observations is obtained when the simulated anthropogenically-controlled SOA is increased to similar to 100 Tg yr(-1) accompanied by either a gas-phase fragmentation process or a reduction in the temperature dependence of the organic aerosol partitioning (by decreasing the enthalpy of vaporization from 42 kJ mol(-1) to 25 kJ mol(-1)). These results illustrate that models may require both additional sources and additional sinks to capture the observed concentrations of organic aerosol.