Investigation of potential interferences in the detection of atmospheric ROx radicals by laser-induced fluorescence under dark conditions
Investigation of potential interferences in the detection of atmospheric ROx radicals by laser-induced fluorescence under dark conditions
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DOI：
10.5194/amt-9-1431-2016
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发表时间：
2016-01-01
影响因子：
3.8
通讯作者：
Wahner, Andreas
中科院分区：
文献类型：
--
作者：
Fuchs, Hendrik;Tan, Zhaofeng;Wahner, Andreas
直接检测高活性的大气羟基自由基（OH）被广泛采用激光诱导荧光（LIF）仪器。该技术也适用于通过化学转化为OH间接测量HO 2和RO 2过氧自由基。它需要将环境空气采样到低压池中，在308 nm激光辐射激发后检测OH荧光。虽然空气在荧光池内的停留时间通常仅为毫秒级，但内部可能产生额外的OH，这将人为地增加测量的OH浓度。在这里，我们目前的实验研究调查潜在的干扰检测OH和过氧自由基的LIF仪器的Forschungszentrum Julich夜间条件。对于实验室实验，仪器的入口由含有一种或多种反应物的过量合成空气溢出。为了区分入口上游反应产生的OH和仪器内部产生的人工信号，应用了OH的化学滴定。在模拟室SAPHIR中进行额外的实验，其中通过开放路径差分光学吸收光谱仪（DOAS）的同时测量用作OH的参考以量化LIF仪器中的潜在伪影。实验包括调查潜在的干扰有关的硝酸根（NO3，N2 O 5），烯烃（乙烯，丙烯，1-丁烯，2，3-二甲基-2-丁烯，α-蒎烯，柠檬烯，异戊二烯）的臭氧分解，和丙酮的激光光解。研究丙酮的激光光解的实验在荧光池中产生OH信号，对于5 ppbv丙酮的混合比，该信号相当于0.05 x 10（6）cm（-3）OH。在大多数大气条件下，这种干扰可以忽略不计。在臭氧分解实验中，没有发现显着的干扰反应物的大气浓度。只有丙烯，α-蒎烯，柠檬烯，异戊二烯的反应物浓度，这是数量级高于在大气中，可以检测到人工OH。干扰的大小取决于臭氧分解反应的转化率。例如，当5.8 ppbv柠檬烯与600 ppbv臭氧反应时，观察到约1 x 10（6）cm（-3）的表观OH浓度。用硝酸根NO3的实验揭示了OH、HO 2和RO 2检测中的小干扰信号。实验参数上的不确定性指向通过在腔室壁处或在气体膨胀中的分子簇中的表面反应形成的人工OH。信号与NO3的存在成比例，给出每10 pptv NO3的1.1 x 10（5）cm（-3）OH、1 x 10（7）cm（-3）HO 2和1.7 x 10（7）cm（-3）RO 2的等效自由基浓度。
Direct detection of highly reactive, atmospheric hydroxyl radicals (OH) is widely accomplished by laser-induced fluorescence (LIF) instruments. The technique is also suitable for the indirect measurement of HO2 and RO2 peroxy radicals by chemical conversion to OH. It requires sampling of ambient air into a low-pressure cell, where OH fluorescence is detected after excitation by 308 nm laser radiation. Although the residence time of air inside the fluorescence cell is typically only on the order of milliseconds, there is potential that additional OH is internally produced, which would artificially increase the measured OH concentration. Here, we present experimental studies investigating potential interferences in the detection of OH and peroxy radicals for the LIF instruments of Forschungszentrum Julich for nighttime conditions. For laboratory experiments, the inlet of the instrument was over flowed by excess synthetic air containing one or more reactants. In order to distinguish between OH produced by reactions upstream of the inlet and artificial signals produced inside the instrument, a chemical titration for OH was applied. Additional experiments were performed in the simulation chamber SAPHIR where simultaneous measurements by an open-path differential optical absorption spectrometer (DOAS) served as reference for OH to quantify potential artifacts in the LIF instrument. Experiments included the investigation of potential interferences related to the nitrate radical (NO3, N2O5), related to the ozonolysis of alkenes (ethene, propene, 1-butene, 2,3-dimethyl-2-butene, alpha-pinene, limonene, isoprene), and the laser photolysis of acetone. Experiments studying the laser photolysis of acetone yield OH signals in the fluorescence cell, which are equivalent to 0.05 x 10(6) cm(-3) OH for a mixing ratio of 5 ppbv acetone. Under most atmospheric conditions, this interference is negligible. No significant interferences were found for atmospheric concentrations of reactants during ozonolysis experiments. Only for propene, alpha-pinene, limonene, and isoprene at reactant concentrations, which are orders of magnitude higher than in the atmosphere, could artificial OH be detected. The value of the interference depends on the turnover rate of the ozonolysis reaction. For example, an apparent OH concentration of approximately 1 x 10(6) cm(-3) is observed when 5.8 ppbv limonene reacts with 600 ppbv ozone. Experiments with the nitrate radical NO3 reveal a small interference signal in the OH, HO2, and RO2 detection. Dependencies on experimental parameters point to artificial OH formation by surface reactions at the chamber walls or in molecular clusters in the gas expansion. The signal scales with the presence of NO3 giving equivalent radical concentrations of 1.1 x 10(5) cm(-3) OH, 1 x 10(7) cm(-3) HO2, and 1.7 x 10(7) cm(-3) RO2 per 10 pptv NO3.