Nighttime HOx chemistry was investigated in two ground-based field campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by comparison of measured and modeled concentration data of OH and HO2. The measurement sites were located in a rural environment in the Pearl River Delta (PRD) under urban influence and in a suburban area close to Beijing, respectively. In both locations, significant nighttime concentrations of radicals were observed under conditions with high total OH reactivities of about 40–50 s?1 in PRD and 25 s?1 near Beijing. For OH, the nocturnal concentrations were within the range of (0.5–3) × 106 cm?3, implying a significant nighttime oxidation rate of pollutants on the order of several ppb per hour. The measured nighttime concentration of HO2 was about (0.2–5) × 108 cm?3, containing a significant, model-estimated contribution from RO2 as an interference. A chemical box model based on an established chemical mechanism is capable of reproducing the measured nighttime values of the measured peroxy radicals and $k_{ext{OH}}$, but underestimates in both field campaigns the observed OH by about 1 order of magnitude. Sensitivity studies with the box model demonstrate that the OH discrepancy between measured and modeled nighttime OH can be resolved, if an additional ROx production process (about 1 ppb h?1) and additional recycling (RO2 → HO2 → OH) with an efficiency equivalent to 1 ppb NO is assumed. The additional recycling mechanism was also needed to reproduce the OH observations at the same locations during daytime for conditions with NO mixing ratios below 1 ppb. This could be an indication that the same missing process operates at day and night. In principle, the required primary ROx source can be explained by ozonolysis of terpenoids, which react faster with ozone than with OH in the nighttime atmosphere. However, the amount of these highly reactive biogenic volatile organic compounds (VOCs) would require a strong local source, for which there is no direct evidence. A more likely explanation for an additional ROx source is the vertical downward transport of radical reservoir species in the stable nocturnal boundary layer. Using a simplified one-dimensional two-box model, it can be shown that ground-based NO emissions could generate a large vertical gradient causing a downward flux of peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN). The downward transport and the following thermal decomposition of these compounds can produce up to 0.3 ppb h?1 radicals in the atmospheric layer near the ground. Although this rate is not sufficient to explain the complete OH discrepancy, it indicates the potentially important role of vertical transport in the lower nighttime atmosphere.
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机译：2006年夏季在2006年夏季在2006年夏季调查了夜间Hox化学，通过测量和建模的OH和HO2的模拟浓度数据。测量部位位于城市影响下的珠江三角洲（PRD）的农村环境中，分别靠近北京的郊区。在两个地方，在北京附近的高度为40-50秒的ob orectivity的条件下，在高分子和25 s？1的条件下观察到显着的夜间浓度。对于OH，夜间浓度在（0.5-3）×106cm≤3的范围内，暗示每小时几种PPB的污染物的显着夜间氧化速率。 HO2的测量夜间浓度约为（0.2-5）×108cm≤3，含有从RO2作为干扰的显着模型估计的贡献。基于建立的化学机制的化学盒模型能够再现测量的过氧自由基的测量夜间值和$ k _ { text {oh}} $，但在这两个场上的竞争中低估了观察到的oh大约1级。具有盒式模型的敏感性研究表明，如果额外的ROX生产过程（约1ppb h？1）和额外的回收（RO2→HO2→OH），则可以解决测量和建模夜间oh之间的oh差异，并且具有相当于的效率假设1 ppb否。还需要额外的再循环机制来在白天在白天在同一位置进行oh观察，在没有混合比率低于1 ppb的情况下的条件。这可能是一个指示同一缺失过程在白天和夜间运行。原则上，所需的初级ROX源可以通过柚皮臭氧溶解，其在夜间气氛中与臭氧更快地反应。然而，这些高反应性生物挥发性有机化合物（VOC）的量将需要强大的局部来源，因此没有直接证据。对额外的ROX源的更可能的解释是稳定夜间边界层中的自由基储存器种类的垂直向下传输。使用简化的一维双箱模型，可以显示出基于地面的不排放可以产生大的垂直梯度，导致过氧醋酸乙腈（PAN）和过氧键丙烯酰硝酸酯（MPAN）的向下通量。向下的运输和以下热分解这些化合物可以在地面附近的大气层中产生高达0.3ppb的H 2次自由基。虽然这种速度不足以解释完全哦差异，但它表明垂直运输在较低的夜间气氛中的潜在重要作用。
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