1Research Institute for Global Change (formerly FRCGC), Japan Agency for Marine-Earth Science and Technology, Yokohama 236-0001, Japan
2Forschungszentrum Jülich, IEK-8: Troposphäre, 52425 Jülich, Germany
3Tokyo Metropolitan University, Department of Applied Chemistry, Tokyo 192-0397, Japan
4Max Planck Institute for Chemistry, Atmospheric Chemistry Department, 55020 Mainz, Germany
5Deutscher Wetterdienst, Meteorologisches Observatorium, 82383 Hohenpeissenberg, Germany
6Bergische Universität Wuppertal, 42097 Wuppertal, Germany
7Karlsruhe Institute of Technology (KIT), IMK-AAF, 76021 Karlsruhe, Germany
*now at: Gifu University, Gifu 501-1193, Japan
**now at: Tokyo University of Agriculture and Technology, Tokyo 183-8538, Japan
***now at: University of Wollongong, School of Chemistry, Wollonong, NSW, Australia
****now at: National University of Ireland Galway, Department of Physics, Galway, Ireland
*****now at: Max Planck Institute for Chemistry, Atmospheric Chemistry Department, 55020 Mainz, Germany
Received: 21 Sep 2011 – Discussion started: 26 Oct 2011
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.
Revised: 11 Feb 2012 – Accepted: 13 Feb 2012 – Published: 07 Mar 2012
Kanaya, Y., Hofzumahaus, A., Dorn, H.-P., Brauers, T., Fuchs, H., Holland, F., Rohrer, F., Bohn, B., Tillmann, R., Wegener, R., Wahner, A., Kajii, Y., Miyamoto, K., Nishida, S., Watanabe, K., Yoshino, A., Kubistin, D., Martinez, M., Rudolf, M., Harder, H., Berresheim, H., Elste, T., Plass-Dülmer, C., Stange, G., Kleffmann, J., Elshorbany, Y., and Schurath, U.: Comparisons of observed and modeled OH and HO2 concentrations during the ambient measurement period of the HOxComp field campaign, Atmos. Chem. Phys., 12, 2567-2585, doi:10.5194/acp-12-2567-2012, 2012.