Observed and modeled changes in boundary-layer and surface-level actinic flux due to wildfire smoke plumes in the California Central Valley in summer 2018

Wildfire smoke is increasingly degrading air quality across the U.S. via the emission and transport of pollutants. Smoke’s direct role as a pollutant is well-documented; however, smoke also affects pollutant concentration indirectly by changing the shortwave actinic flux necessary for photochemical reactions. We compute smoke-driven changes in surface-level and boundary-layer downwelling actinic flux (F↓) at 550 nm and 380 nm (NO2 photolysis peak) along a 2018 Western wildfire experiment for Cloud chemistry, Aerosol absorption, and Nitrogen (WE-CAN) research flight through the California Central Valley. The onboard HIAPER Airborne Radiation Package (HARP)–Actinic Flux instrument measured F↓. To assess changes in F↓ relative to smoke-free conditions and at altitudes not sampled by the aircraft, we calculate F↓ under assumed background and observed smoke conditions using the U.S. National Science Foundation (NSF) National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet and Visible (TUV) radiation model. Under smoke-impacted conditions, modeled F↓ minorly underestimates HARP observations; the average modeled-to-measured ratio is 0.93 at 550 nm and 0.89 at 380 nm. Relative to modeled background conditions, observed (modeled) smoke-impacted F↓ at 380 nm decreased by 24% (38%), 15% (24%), and 8% (18%) at 0-0.5 km, 0.5-1 km, and 1-1.5 km, respectively. At the ground, smoke decreased modeled F↓ at 380 nm by 43%—likely an upper bound, as the modeled values slightly underestimate observations. As wildfire seasons grow more severe with climate change, understanding smoke’s combined impact on actinic flux and concentrations of VOCs and nitrogen species is essential for constraining future air quality.

野火烟雾通过污染物的排放与传输，正日益加剧美国全境的空气质量恶化。作为污染物的烟雾，其直接作用已有充分文献记载；但烟雾还会通过改变光化学反应所需的短波光化通量（actinic flux），间接影响污染物浓度。我们针对2018年美国西部野火实验中，穿越加利福尼亚中央谷地的云化学、气溶胶吸收与氮（WE-CAN）研究飞行任务，计算了550 nm与380 nm（二氧化氮光解峰值波长）处的地表及边界层下行光化通量（F↓）的烟雾驱动变化。HIAPER机载辐射套件（HARP）-光化通量测量仪器对F↓进行了实地观测。为评估无烟条件下以及飞机未采样高度处的F↓变化，我们借助美国国家科学基金会（National Science Foundation, NSF）下属美国国家大气研究中心（National Center for Atmospheric Research, NCAR）的对流层紫外与可见光（Tropospheric Ultraviolet and Visible, TUV）辐射模型，分别在假设的背景条件与实际观测到的烟雾条件下计算了F↓。在受烟雾影响的条件下，模拟得到的F↓略低于HARP的观测值；550 nm处的模拟值与观测值平均比值为0.93，380 nm处为0.89。相较于模拟得到的背景条件，观测到的（模拟得到的）受烟雾影响的380 nm处F↓在0~0.5 km、0.5~1 km及1~1.5 km高度层分别下降了24%（38%）、15%（24%）与8%（18%）。在地面处，烟雾使模拟得到的380 nm处F↓下降了43%——这一结果或为上限，因模拟值略低于观测值。随着气候变化导致野火季愈发严峻，厘清烟雾对光化通量以及挥发性有机物（Volatile Organic Compounds, VOCs）与氮物种浓度的综合影响，对约束未来空气质量预测至关重要。