Atmos. Chem. Phys. Discuss., 11, 28851-28894, 2011
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Comparisons of observed and modeled OH and HO2 concentrations during the ambient measurement period of the HOxComp field campaign
Y. Kanaya1, A. Hofzumahaus2, H.-P. Dorn2, T. Brauers2, H. Fuchs2, F. Holland2, F. Rohrer2, B. Bohn2, R. Tillmann2, R. Wegener2, A. Wahner2, Y. Kajii3, K. Miyamoto3, S. Nishida3,*, K. Watanabe3, A. Yoshino3,**, D. Kubistin4, M. Martinez4, M. Rudolf4, H. Harder4, H. Berresheim5,***, T. Elste5, C. Plass-Dülmer5, G. Stange5, J. Kleffmann6, Y. Elshorbany6,****, and U. Schurath7
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: National University of Ireland Galway, Department of Physics, Galway, Ireland
****now at: Max Planck Institute for Chemistry, Atmospheric Chemistry Department, 55020 Mainz, Germany

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. OH and HO2 levels, observed by four and three instruments, respectively, 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. Adding isomerization of isoprene peroxy radicals to the model increased OH and HO2 by 43% and 48% on average. Although these are still only 15% and 21% higher than the observations made by one of the instruments, larger overestimations (>60%) occurred with respect to the observations made by the other three instruments, suggesting that the rates of the isomerization were not readily supported by the ensemble of radical observations. These model runs tend to underestimate observed OH reactivity which may be explained by unmeasured hydrocarbon species. By selecting hydrocarbon types to be added to the model in amounts that accounted for the missing fractions of observed OH reactivity, the gaps between HOx observations and model results with and without isomerization could be individually diminished to within uncertainty levels. In this case, however, the HO2/OH ratio rose on addition of hydrocarbons and diverged from observations. In the case where we used modeled HO2(*), 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, the model's overestimation for HO2 became evident (by factors of more than 1.8). These results strongly indicated that more loss processes for peroxy radicals were necessary to explain the 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 (and HO2(*)) with respect to observations made by the three instruments. We found that this tendency is associated with high NOx concentrations, suggesting that some HO2 production or HO2 regeneration processes under high NOx conditions were being overlooked; this might require revision of ozone production regimes.

Citation: 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. Discuss., 11, 28851-28894, doi:10.5194/acpd-11-28851-2011, 2011.
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