Radical chemistry at a rural site (Wangdu) in the North China Plain: Observation and model calculations of OH, HO<sub>2</sub> and RO<sub>2</sub> radicals

Abstract

A comprehensive field campaign was carried out in summer 2014 in Wangdu located in the North China Plain. A month of continuous OH, HO 2 and RO 2 measurements were achieved.Observations of radicals by laser induced fluorescence (LIF) technique gave daily maximum concentrations between (5-15)×10 6 cm −3 , (3-14)×10 8 cm −3 and (3-15)×10 8 cm −3 for OH, HO 2 and RO 2 , respectively. Measured OH reactivities (inverse OH lifetimes) were 10 to 20 s −1 during day-5 time. A chemical box model constrained by trace-gas observations and based on a state-of-the-art chemical mechanism is used to interpret the observed radical concentrations. In general, the model can reasonably well reproduce measured radical concentrations during daytime. Like in previous field campaigns in China, modelled and measured OH concentrations agree for NO mixing ratios higher than 1 ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios 10 of less than 300 pptv (the model-measurement ratio is between 1.4 to 2 in this case). If additional OH recycling equivalent to 100 pptv NO is assumed, the model is also capable of reproducing the observed OH concentrations for conditions of high VOC and low NO x concentrations with good agreement in HO 2 and RO 2 . Observed RO 2 concentrations are underestimated in the morning hours by a factor of 3 to 5. This indicates that an additional chemical source of RO 2 is missing in 15 the model. The OH reactivity is also underpredicted in the early morning. Increasing VOC concentrations to match measured OH reactivity helps to reduce the discrepancy between modelled and measured RO 2 . The underprediction of RO 2 coincides with high NO concentrations and therefore 1 Atmos. Chem. Phys. Discuss., leads to a significant underestimation of the local ozone production rates determined from the peroxy radical (HO 2 and RO 2 ) reactions with NO. The underestimation corresponds to a daily integral 20 ozone production of about 20 ppbv per day.2 Atmos. Chem. Phys. Discuss.,

Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)

“…In contrast to some of the earlier HO2 model-measurement comparisons which diverged at NO concentrations > 1ppbv with models increasingly under-predicting the levels of HO2 observed, the predicted levels of HO2 were in good agreement with HO2 observations made in Wangdu over the whole range (0.1 -4 ppbv) of NO encountered (Tan et al, 2017). However, the authors did report an increasing model under-prediction of the observed 10 RO2 with increasing NO (Tan et al, 2017).…”

“…The same assumption was drawn in the recent ROx study in Wangdu, China (Tan et al, 2017). This assumption means that the 25 concentration of RO2 observed may be a lower estimate.…”

“…OH and HO2 only. In the Wangdu study, however, Tan et al, (2017) using observed RO2 demonstrated that models may underpredict ozone production at high NO due to an underestimation of the RO2 radical concentration.…”

“…At urban sites (including London), significant 30 concentrations of HONO (often several hundred pptv) have been reported to persist throughout the day (Lee et al, 2016), and, as such, HONO should be considered an important OH source throughout sunlit hours, not just at sunrise, in these environments. Dusanter et al (2009) Tan et al (2017) found that HONO photolysis was the most important primary radical source in Wangdu in the North China Plain.…”

“…2016) and radical measurements from the MEGAPOLI project which took place at a suburban site close to Paris employed the chemical ionisation mass spectrometry (CIMS) technique to make observations of OH and the sum of HO2 and RO2 species rather than FAGE (Michoud et al, 2012). HO2 * (= [HO2] +∑iαi [RO2i], and αi is the mean fractional contribution of the RO2 species that interfere (RO2i)) model-measurement comparisons are now often reported (rather than HO2) to take into account contributions from RO2 species (Lu et al, 2013;Griffith et al, 2016) and very recently Tan et al (2017) presented interference-5 free HO2 observations alongside RO2 observations which were made using the FAGE technique coupled to a flow reactor (Fuchs et al, 2008) at a rural site in Wangdu, China. In contrast to some of the earlier HO2 model-measurement comparisons which diverged at NO concentrations > 1ppbv with models increasingly under-predicting the levels of HO2 observed, the predicted levels of HO2 were in good agreement with HO2 observations made in Wangdu over the whole range (0.1 -4 ppbv) of NO encountered (Tan et al, 2017).…”