Mixing ratios of the criteria air contaminant nitrogen dioxide (NO₂) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O₃ chemiluminescence (CL). In this work, the performance of a photolytic NO₂ converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO₂ and NO_x (= NO + NO₂) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO₂ photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 s⁻¹. A maximum 96% conversion of NO₂ to NO over a large dynamic range was achieved at a residence time of (1.5 ± 0.3) s, independent of relative humidity. Interferences from odd nitrogen (NO_y) species such as peroxyacyl nitrates (PAN; RC(O)O₂NO₂), alkyl nitrates (AN; RONO₂), nitrous acid (HONO), and nitric acid (HNO₃) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified as follows: direct photolysis, foremost of HONO at a rate constant of 6% that of NO₂; thermal decomposition, primarily of PAN; surface promoted photochemistry; and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connecting tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO₂ in ambient air.

Implications

A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO2) in ambient air by NO-O3 chemiluminescence (CL) was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of nitrous acid, which occurred at a rate constant 6% that of NO2, thermal decomposition of PAN and N2O5, surface promoted chemistry involving nitric acid, and secondary chemistry involving NO in the tubing connecting the converter and CL analyzer. These interferences are predicted to occur in all NO2 P-CL systems but can be avoided by appropriate thermal management and operating at high flow rates.

中文翻译：

---

用于环境空气监测的光解二氧化氮转换器中的潜在干扰：原型评估

通常通过使用光解转化器还原成一氧化氮（NO），然后进行NO-O ₃化学发光（CL）来量化标准空气污染物二氧化氮（NO ₂）的混合比。在这项工作中，评估了最初设计用于连续发射监测和在395 nm处发光的光解NO ₂转化器原型的性能。通过蓝二极管激光腔衰荡光谱法（CRDS）监测进入和离开转化器的NO ₂和NO _x的混合比（＝ NO + NO ₂）。NO ₂通过测量转化为NO的速率作为转化器停留时间的函数来确定光解频率，发现其为4.2s ^-1。在（1.5±0.3）s的停留时间下，无论相对湿度如何，在较大的动态范围内，NO ₂到NO的最大转化率为96％。来自奇异氮（NO _y）物种的干扰，例如过氧酰硝酸盐（PAN; RC（O）O ₂ NO ₂），硝酸烷基酯（AN; RONO ₂），亚硝酸（HONO）和硝酸（HNO _3）通过在最佳工作范围之外（即在更高的压力和更长的停留时间下）操作原型转换器来评估干扰）。识别出产生伪影和干扰的四种机理如下：直接光解，最重要的是HONO，其速率常数为NO ₂的6％; 热分解，主要是PAN 表面促进的光化学；和连接管中的第二化学反应。这些干扰可能在当前使用的所有光解转化器中都有一定程度的存在，但很少评估或报告。有关改善光解转化器性能的建议包括：在较低的电解槽压力和较高的流速下运行；进行热管理，理想的结果是使光解裂解池的温度与环境条件相匹配；并尽量减小连接管的长度。如果实施得当，则在测量周围空气中的NO ₂时，这些干扰可以忽略不计。

含义

表征了一种新型的近紫外光解转化器，用于通过NO-O3化学发光（CL）测定环境空气中的标准污染物二氧化氮（NO2）。识别并通过实验研究了四种产生干扰的机理：亚硝酸的直接光解，其速率恒定为NO2的6％，PAN和N2O5的热分解，涉及硝酸的表面促进化学，以及涉及NO的二次化学。连接转换器和CL分析仪的油管。预计这些干扰会在所有NO2 P-CL系统中发生，但可以通过适当的热管理和高流速运行来避免。