Implementation of a chemical background method for atmospheric OH measurements by laser-induced fluorescence: characterisation and observations from the UK and China
Implementation of a chemical background method for atmospheric OH measurements by laser-induced fluorescence: characterisation and observations from the UK and China
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DOI：
10.5194/amt-2019-487
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发表时间：
2020-01
影响因子：
3.8
通讯作者：
Robert Woodward-Massey;Eloise J. Slater;J. Alen;T. Ingham;D. Cryer;L. M. Stimpson;C. Ye;P. Seakins;L. Whalley;D. Heard
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文献类型：
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作者：
Robert Woodward-Massey;Eloise J. Slater;J. Alen;T. Ingham;D. Cryer;L. M. Stimpson;C. Ye;P. Seakins;L. Whalley;D. Heard
抽象。羟基（OH）和过氧氢（HO 2）自由基是理解大气化学的核心。由于它们的寿命很短，这些物种经常被用来测试模型预测的准确性及其潜在的化学机制。在森林环境中，激光诱导荧光-荧光分析的气体膨胀（LIF-FAGE）测量OH往往表现出与模型预测的实质性分歧，这表明在这样的环境中存在未知的OH源。然而，测量也可能受到仪器伪影的影响，这是由于存在干扰物质，使用通过调制激光激发波长（“OH波”）获得背景信号的传统方法无法区分这些干扰物质。干扰假设可以通过使用替代方法来测试，以确定OH背景信号，通过在环境空气采样之前添加化学清除剂（“OHchem”）。在这项工作中，利兹FAGE仪器进行了修改，包括这样一个系统，以方便测量的OHchem，其中丙烷被用来选择性地去除OH从环境空气中使用的入口预注射器（IPI）。详细表征了IPI系统，并且发现该系统没有降低仪器对OH的灵敏度（99%），而没有去除荧光池内形成的OH（<5%）。在有水蒸气存在的情况下对臭氧的光解干扰进行的测试表明，存在一种很小但可能很大的干扰，相当于OH浓度为1.44 ×105 molec。cm−3，典型大气条件下[O3] =50 ppbv，[H2O] = 1%。研究异戊二烯臭氧分解产物的潜在干扰的实验室实验确实产生了干扰信号，但当外推到环境臭氧和异戊二烯水平时，这些干扰信号可以忽略不计。NO3自由基的干扰也进行了测试，但发现在我们的系统中是微不足道的。利兹IPI模块部署在三个独立的现场密集发生在夏季在英国的沿海站点和夏季和冬季在中国北京的大城市，允许在广泛的化学和气象条件下的环境OH干扰的调查。环境OHchem测量与传统OHwave方法的比较显示出极好的一致性，OHwave与OHchem斜率为1.05-1.16，并且在日常基础上具有相同的行为，与实验室干扰试验一致。OH-wave与OHchem之差（“OHint”）与OHchem呈非线性关系，干扰上限为（5.0±1.4）×106 molec。cm−3，在测量的最高OHchem浓度（23×106 molec. cm-3），占总OH波信号的14%~ 21%。
Abstract. Hydroxyl (OH) and hydroperoxy (HO2) radicals are central to the understanding of atmospheric chemistry. Owing to their short lifetimes, these species are frequently used to test the accuracy of model predictions and their underlying chemical mechanisms. In forested environments, laser-induced fluorescence–fluorescence assay by gas expansion (LIF–FAGE) measurements of OH have often shown substantial disagreement with model predictions, suggesting the presence of unknown OH sources in such environments. However, it is also possible that the measurements have been affected by instrumental artefacts, due to the presence of interfering species that cannot be discriminated using the traditional method of obtaining background signals via modulation of the laser excitation wavelength (“OHwave”). The interference hypothesis can be tested by using an alternative method to determine the OH background signal, via the addition of a chemical scavenger prior to sampling of ambient air (“OHchem”). In this work, the Leeds FAGE instrument was modified to include such a system to facilitate measurements of OHchem, in which propane was used to selectively remove OH from ambient air using an inlet pre-injector (IPI). The IPI system was characterised in detail, and it was found that the system did not reduce the instrument sensitivity towards OH ( 99 %) without the removal of OH formed inside the fluorescence cell (< 5 %). Tests of the photolytic interference from ozone in the presence of water vapour revealed a small but potentially significant interference, equivalent to an OH concentration of ∼4×105 molec. cm−3 under typical atmospheric conditions of [O3] =50 ppbv and [H2O] =1 %. Laboratory experiments to investigate potential interferences from products of isoprene ozonolysis did result in interference signals, but these were negligible when extrapolated down to ambient ozone and isoprene levels. The interference from NO3 radicals was also tested but was found to be insignificant in our system. The Leeds IPI module was deployed during three separate field intensives that took place in summer at a coastal site in the UK and both in summer and winter in the megacity of Beijing, China, allowing for investigations of ambient OH interferences under a wide range of chemical and meteorological conditions. Comparisons of ambient OHchem measurements to the traditional OHwave method showed excellent agreement, with OHwave vs OHchem slopes of 1.05–1.16 and identical behaviour on a diel basis, consistent with laboratory interference tests. The difference between OHwave and OHchem (“OHint”) was found to scale non-linearly with OHchem, resulting in an upper limit interference of (5.0±1.4) ×106 molec. cm−3 at the very highest OHchem concentrations measured (23×106 molec. cm−3), accounting for ∼14 %–21 % of the total OHwave signal.