Abstract. Heterogeneous reactions occurring at the surface of atmospheric aerosol particles regulate the production and lifetime of a wide array of atmospheric gases. Aerosol surface area plays a critical role in setting the rate of heterogeneous reactions in the atmosphere. Despite the central role for aerosol surface area, there are few assessments of the accuracy of aerosol surface area concentrations in regional and global models. In this study, we compare aerosol surface area concentrations in the EPA’s Community Multiscale Air Quality (CMAQ) model with commensurate observations from the 2011 NASA flight-based DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality) campaign. The study region includes the Baltimore and Washington, DC metropolitan area. Dry aerosol surface area was measured aboard the NASA P-3B aircraft using an Ultra-High Sensitivity Aerosol Spectrometer (UHSAS). We show that modeled and measured dry aerosol surface area, S_a,mod and S_a,meas respectively, are modestly correlated (r² = 0.52) and on average agree to within a factor of two (S_a,mod/S_a,meas = 0.44) over the course of the 13 research flights. We show that S_a,mod/S_a,meas does not depend strongly on photochemical age or the concentration of secondary biogenic aerosol, suggesting that the condensation of low-volatility gas-phase compounds does not strongly affect model-measurement agreement. In comparison, there is strong agreement between measured and modeled aerosol number concentration (N_mod/N_meas = 0.87, r² = 0.63). The persistent underestimate of S_a in the model, combined with strong agreement in modeled and measured aerosol number concentrations, suggests that model representation of the size distribution of primary emissions or secondary aerosol formed at the early stages of oxidation may contribute to the observed differences. For reactions occurring on small particles, the rate of heterogeneous reactions is a linear function of both S_a and the reactive uptake coefficient (γ). To assess the importance of uncertainty in modeled S_a for the representation of heterogeneous reactions in models, we compare both the mean and the variance in S_a,mod/S_a,meas to that in γ(N₂O₅)_mod/γ(N₂O₅)_meas. We find that the uncertainty in model representation of heterogeneous reactions is primarily driven by uncertainty in the parametrization of reactive uptake coefficients, although the discrepancy between S_a,mod and S_a,meas is not insignificant. Our analysis suggests that model improvements to aerosol surface area concentrations, in addition to more accurate parameterizations of heterogeneous kinetics, will advance the representation of heterogeneous chemistry in regional models.

中文翻译：

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基于观测的模型气溶胶表面积约束：对非均相化学的影响

摘要。大气气溶胶颗粒表面发生的非均相反应调节着各种大气气体的产生和寿命。气溶胶表面积在设定大气中异质反应的速率方面起着关键作用。尽管气溶胶表面积具有核心作用，但在区域和全球模型中很少评估气溶胶表面积浓度的准确性。在这项研究中，我们将 EPA 的社区多尺度空气质量 (CMAQ) 模型中的气溶胶表面积浓度与 2011 年 NASA 基于飞行的 DISCOVER-AQ（从 COlumn 和垂直解析的与空气质量相关的观测中获取表面条件信息）的相应观测值进行比较） 活动。研究区域包括巴尔的摩和华盛顿特区大都市区。使用超高灵敏度气溶胶光谱仪 (UHSAS) 在 NASA P-3B 飞机上测量干气溶胶表面积。我们展示了模拟和测量的干气溶胶表面积，S _{a ,mod}和Sa _{, meas}分别是适度相关的 ( r ² = 0.52) 并且在 13 项研究的过程中平均同意在两倍以内 ( S _{a ,mod} / Sa _{, meas} = 0.44)航班。我们证明了S _{a ,mod} / S _{a ,meas}不强烈依赖于光化学年龄或二次生物气溶胶的浓度，这表明低挥发性气相化合物的冷凝不会强烈影响模型测量一致性。相比之下，测量和模拟的气溶胶数浓度之间存在很强的一致性（N _mod / N _meas = 0.87，r ² = 0.63）。对S _a的持续低估在模型中，结合模拟和测量的气溶胶数量浓度的强烈一致性，表明在氧化早期阶段形成的初级排放物或次级气溶胶的尺寸分布的模型表示可能有助于观察到的差异。对于发生在小颗粒上的反应，非均相反应的速率是S _a和反应吸收系数 (γ) 的线性函数。为了评估建模S _a中不确定性对模型中异质反应表示的重要性，我们将Sa _,mod / Sa _{,meas中的均值和方差与}γ(N ₂ O₅ ) _mod /γ(N ₂ O ₅ )_测量。我们发现，非均相反应模型表示的不确定性主要是由反应吸收系数参数化的不确定性驱动的，尽管S _{a ,mod}和S _{a ,meas}之间的差异并非不显着。我们的分析表明，对气溶胶表面积浓度的模型改进，以及更准确的多相动力学参数化，将促进多相化学在区域模型中的表现。