Li Rui, Palm Brett B, Ortega Amber M, Hlywiak James, Hu Weiwei, Peng Zhe, Day Douglas A, Knote Christoph, Brune William H, de Gouw Joost A, Jimenez Jose L
†Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, United States.
⊥Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania 16802, United States.
J Phys Chem A. 2015 May 14;119(19):4418-32. doi: 10.1021/jp509534k. Epub 2015 Apr 6.
Oxidation flow reactors (OFRs) containing low-pressure mercury (Hg) lamps that emit UV light at both 185 and 254 nm ("OFR185") to generate OH radicals and O3 are used in many areas of atmospheric science and in pollution control devices. The widely used potential aerosol mass (PAM) OFR was designed for studies on the formation and oxidation of secondary organic aerosols (SOA), allowing for a wide range of oxidant exposures and short experiment duration with reduced wall loss effects. Although fundamental photochemical and kinetic data applicable to these reactors are available, the radical chemistry and its sensitivities have not been modeled in detail before; thus, experimental verification of our understanding of this chemistry has been very limited. To better understand the chemistry in the OFR185, a model has been developed to simulate the formation, recycling, and destruction of radicals and to allow the quantification of OH exposure (OHexp) in the reactor and its sensitivities. The model outputs of OHexp were evaluated against laboratory calibration experiments by estimating OHexp from trace gas removal and were shown to agree within a factor of 2. A sensitivity study was performed to characterize the dependence of the OHexp, HO2/OH ratio, and O3 and H2O2 output concentrations on reactor parameters. OHexp is strongly affected by the UV photon flux, absolute humidity, reactor residence time, and the OH reactivity (OHR) of the sampled air, and more weakly by pressure and temperature. OHexp can be strongly suppressed by high OHR, especially under low UV light conditions. A OHexp estimation equation as a function of easily measurable quantities was shown to reproduce model results within 10% (average absolute value of the relative errors) over the whole operating range of the reactor. OHexp from the estimation equation was compared with measurements in several field campaigns and shows agreement within a factor of 3. The improved understanding of the OFR185 and quantification of OHexp resulting from this work further establish the usefulness of such reactors for research studies, especially where quantifying the oxidation exposure is important.
氧化流动反应器（OFRs）包含能发射185纳米和254纳米紫外线的低压汞（Hg）灯（“OFR185”），用于产生羟基自由基和臭氧，在大气科学的许多领域以及污染控制设备中都有应用。广泛使用的潜在气溶胶质量（PAM）OFR是为研究二次有机气溶胶（SOA）的形成和氧化而设计的，它能实现广泛的氧化剂暴露范围，且实验持续时间短，壁面损失效应小。尽管有适用于这些反应器的基础光化学和动力学数据，但自由基化学及其敏感性此前尚未得到详细建模；因此，我们对这种化学过程理解的实验验证非常有限。为了更好地理解OFR185中的化学过程，已开发出一个模型来模拟自由基的形成、循环和破坏，并对反应器中的羟基暴露（OHexp）及其敏感性进行量化。通过从痕量气体去除量估算OHexp，将OHexp的模型输出与实验室校准实验进行了评估，结果显示二者在2倍的范围内相符。进行了一项敏感性研究，以表征OHexp、HO2/OH比率以及臭氧和过氧化氢输出浓度对反应器参数的依赖性。OHexp受紫外线光子通量、绝对湿度、反应器停留时间以及采样空气的羟基反应性（OHR）的强烈影响，而受压力和温度的影响较弱。高OHR会强烈抑制OHexp，尤其是在低紫外线条件下。一个作为易于测量量的函数的OHexp估算方程在反应器的整个运行范围内能以10%（相对误差的平均绝对值）的精度重现模型结果。将估算方程得出的OHexp与几次野外实验中的测量值进行了比较，结果显示二者在3倍的范围内相符。这项工作对OFR185的更好理解以及对OHexp的量化，进一步确立了此类反应器在研究中的实用性，特别是在量化氧化暴露很重要的情况下。
