Exchanges of energy and mass between the surrounding air and plant surfaces occur below, within, and above a forest's vegetative canopy. The canopy also can lead to vertical gradients in light, trace gases, oxidant availability, turbulent mixing, and properties and concentrations of organic aerosol (OA). In this study, a high-resolution time-of-flight aerosol mass spectrometer was used to measure non-refractory submicron aerosol composition and concentration above (30 m) and below (6 m) a forest canopy in a mixed deciduous forest at the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport tower in northern Michigan during the summer of 2016. Three OA factors are resolved using positive matrix factorization: more-oxidized oxygenated organic aerosol (MO-OOA), isoprene-epoxydiol-derived organic aerosol (IEPOX-OA), and 91Fac (a factor characterized with a distinct fragment ion at $m/z$ 91) from both the above- and the below-canopy inlets. MO-OOA was most strongly associated with long-range transport from more polluted regions to the south, while IEPOX-OA and 91Fac were associated with shorter-range transport and local oxidation chemistry. Overall vertical similarity in aerosol composition, degrees of oxidation, and diurnal profiles between the two inlets was observed throughout the campaign, which implies that rapid in-canopy transport of aerosols is efficient enough to cause relatively consistent vertical distributions of aerosols at this scale. However, four distinct vertical gradient episodes are identified for OA, with vertical concentration differences (above-canopy minus below-canopy concentrations) in total OA of up to 0.8 µg m⁻³, a value that is 42 % of the campaign average OA concentration of 1.9 µg m⁻³. The magnitude of these differences correlated with concurrent vertical differences in either sulfate aerosol or ozone. These differences are likely driven by a combination of long-range transport mechanisms, canopy-scale mixing, and local chemistry. These results emphasize the importance of including vertical and horizontal transport mechanisms when interpreting trace gas and aerosol data in forested environments.

中文翻译：

---

混合落叶林冠层上方和下方运输驱动的气溶胶差异

周围空气和植物表面之间的能量和质量交换发生在森林植被冠层的下方、内部和上方。冠层还可能导致光、痕量气体、氧化剂可用性、湍流混合以及有机气溶胶 (OA) 的性质和浓度的垂直梯度。在这项研究中，高分辨率飞行时间气溶胶质谱仪被用于测量非耐火亚微米气溶胶成分和浓度高于 (30 m) 和低于 (6 m) 计划的混交落叶林中的森林冠层用于氧化剂研究：2016 年夏季密歇根州北部的光化学、排放和运输塔。使用正矩阵分解解决了三个 OA 因素：更多氧化的含氧有机气溶胶 (MO-OOA)、异戊二烯-环氧二醇衍生的有机气溶胶(IEPOX-OA),$米/z$ 91) 来自顶篷上方和下方的入口。MO-OOA 与从污染较严重地区到南部的远距离传输最密切相关，而 IEPOX-OA 和 91Fac 与短距离传输和局部氧化化学相关。在整个运动过程中观察到两个入口之间的气溶胶成分、氧化程度和昼夜剖面的总体垂直相似性，这意味着气溶胶的快速冠层内传输足以有效地导致该尺度上相对一致的气溶胶垂直分布。然而，针对 OA 确定了四个不同的垂直梯度事件，总 OA 的垂直浓度差异（冠层上方减去冠层下方的浓度）高达 0.8  µg m ^-3，该值是活动平均 OA 浓度 1.9 µg m ^{-3 的}42%  。这些差异的幅度与硫酸盐气溶胶或臭氧的同时垂直差异相关。这些差异可能是由远程传输机制、冠层混合和局部化学作用共同驱动的。这些结果强调了在解释森林环境中的痕量气体和气溶胶数据时包括垂直和水平传输机制的重要性。