Nighttime HO chemistry was investigated in two ground-based fieldcampaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China bycomparison of measured and modeled concentration data of OH and HO. Themeasurement sites were located in a rural environment in the Pearl RiverDelta (PRD) under urban influence and in a suburban area close to Beijing,respectively. In both locations, significant nighttime concentrations ofradicals were observed under conditions with high total OH reactivities ofabout 40–50 s in PRD and 25 s near Beijing. For OH, thenocturnal concentrations were within the range of (0.5–3) × 10 cm, implying a significant nighttimeoxidation rate of pollutants on the order of several ppb per hour. Themeasured nighttime concentration of HO was about(0.2–5) × 10 cm, containing a significant,model-estimated contribution from RO as an interference. A chemical boxmodel based on an established chemical mechanism is capable of reproducingthe measured nighttime values of the measured peroxy radicals and$k_{ext{OH}}$, but underestimates in both field campaigns the observed OHby about 1 order of magnitude. Sensitivity studies with the box modeldemonstrate that the OH discrepancy between measured and modeled nighttime OHcan be resolved, if an additional RO production process (about1 ppb h) and additional recycling (RO → HO → OH) with an efficiencyequivalent to 1 ppb NO is assumed. The additional recycling mechanismwas also needed to reproduce the OH observations at the same locations duringdaytime for conditions with NO mixing ratios below 1 ppb. This couldbe an indication that the same missing process operates at day and night. Inprinciple, the required primary RO source can be explained byozonolysis of terpenoids, which react faster with ozone than with OH in thenighttime atmosphere. However, the amount of these highly reactive biogenicvolatile organic compounds (VOCs) would require a strong local source, forwhich there is no direct evidence. A more likely explanation for anadditional RO source is the vertical downward transport ofradical reservoir species in the stable nocturnal boundary layer. Using asimplified one-dimensional two-box model, it can be shown that ground-basedNO emissions could generate a large vertical gradient causing a downward fluxof peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN).The downward transport and the following thermal decomposition of thesecompounds can produce up to 0.3 ppb h radicals in theatmospheric layer near the ground. Although this rate is not sufficient toexplain the complete OH discrepancy, it indicates the potentially importantrole of vertical transport in the lower nighttime atmosphere.
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