Comparisons of observed and modeled OH and HO<sub>2</sub> concentrations during the ambient measurement period of the HO<sub>x</sub>Comp field campaign

Abstract

<jats:p>Abstract. A photochemical box model constrained by ancillary observations was used to simulate OH and HO2 concentrations for three days of ambient observations during the HOxComp field campaign held in Jülich, Germany in July 2005. Daytime OH levels observed by four instruments were fairly well reproduced to within 33% by a base model run (Regional Atmospheric Chemistry Mechanism with updated isoprene chemistry adapted from Master Chemical Mechanism ver. 3.1) with high R2 values (0.72–0.97) over a range of isoprene (0.3–2 ppb) and NO (0.1–10 ppb) mixing ratios. Daytime HO2(*) levels, reconstructed from the base model results taking into account the sensitivity toward speciated RO2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO2 measurement mode, were 93% higher than the observations made by the single instrument. This also indicates an overprediction of the HO2 to OH recycling. Together with the good model-measurement agreement for OH, it implies a missing OH source in the model. Modeled OH and HO2(*) could only be matched to the observations by addition of a strong unknown loss process for HO2(*) that recycles OH at a high yield. Adding to the base model, instead, the recently proposed isomerization mechanism of isoprene peroxy radicals (Peeters and Müller, 2010) increased OH and HO2(*) by 28% and 13% on average. Although these were still only 4% higher than the OH observations made by one of the instruments, larger overestimations (42–70%) occurred with respect to the OH observations made by the other three instruments. The overestimation in OH could be diminished only when reactive alkanes (HC8) were solely introduced to the model to explain the missing fraction of observed OH reactivity. Moreover, the overprediction of HO2(*) became even larger than in the base case. These analyses imply that the rates of the isomerization are not readily supported by the ensemble of radical observations. One of the measurement days was characterized by low isoprene concentrations (∼0.5 ppb) and OH reactivity that was well explained by the observed species, especially before noon. For this selected period, as opposed to the general behavior, the model tended to underestimate HO2(*). We found that this tendency is associated with high NOx concentrations, suggesting that some HO2 production or regeneration processes under high NOx conditions were being overlooked; this might require revision of ozone production regimes.</jats:p>

Insights into hydroxyl measurements and atmospheric oxidation in a California forest

“…Using the fast isomerization rates calculated by Peeters et al (2009) and Peeters and Müller (2010), the box model calculated HO 2 mixing ratios which are up to 3 times greater than the HO 2 measurements (Fig. S3 in the Supplement), consistent with recent model-to-observation comparisons in other forest studies (Kanaya et al, 2012;Lu et al, 2011;Whalley et al, 2011). However, when the isomerization rate is reduced for the unimolecular isomerization channel in accordance with recent laboratory studies , the HO 2 from the Peeters mechanism comes into better agreement with the HO 2 measurements (Fig.…”

Missing OH source in a suburban environment near Beijing: observed and modelled OH and HO&lt;sub&gt;2&lt;/sub&gt; concentrations in summer 2006

“…Studies of ambient HO x during the field campaign HOx-Comp2005 in Jülich have shown relatively good agreement (within 30 %) between modelled and measured OH Kanaya et al, 2012). Interestingly, agreement was found both for high and low NO concentrations, contrasting the PRD and Yufa results where significant OH discrepancies were found at similar isoprene concentrations at low NO.…”

“…It has been speculated that the missing recycling is related to isoprene (e.g. Lelieveld et al, 2008;Ren et al, 2008;Paulot et al, 2009;Peeters et al, 2009;Peeters and Müller, 2010;Whalley et al, 2011;Lu et al, 2012), but a consistent chemical mechanism explaining the measured concentrations of both OH and HO 2 has not been identified so far (Crounse et al, 2011;Whalley et al, 2011;Lu et al, 2012;Kanaya et al, 2012).…”

Measurement of interferences associated with the detection of the hydroperoxy radical in the atmosphere using laser-induced fluorescence

“…96 M. M. Lew et al: Hydroperoxy radical in the atmosphere using laser-induced fluorescence 2009b; Michoud et al, 2012;Lu et al, 2013;Ren et al, 2013;Griffith et al, 2016), while other times the measurements were found to be both lower (George et al, 1999;Konrad et al, 2003) and higher than model predictions Ren et al, 2003;Emmerson et al, 2005;Kanaya et al, 2007a;Chen et al, 2010;Sheehy et al, 2010;Czader et al, 2013;Griffith et al, 2016). In forested environments, measured HO 2 concentrations were sometimes found to agree with model predictions (Tan et al, 2001;Ren et al, 2005Ren et al, , 2006 but were often found to be either lower (Carslaw et al, 2001;Kanaya et al, 2007bKanaya et al, , 2012Whalley et al, 2011;Mao et al, 2012;Griffith et al, 2013) or higher than model predictions (Carslaw et al, 2001;Kubistin et al, 2010;Kim et al, 2013;Hens et al, 2014).…”