Atmos. Chem. Phys. Discuss., 13, 5233-5278, 2013
www.atmos-chem-phys-discuss.net/13/5233/2013/
doi:10.5194/acpd-13-5233-2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
OH reactivity in a South East Asian Tropical rainforest during the Oxidant and Particle Photochemical Processes (OP3) project
P. M. Edwards1,*, M. J. Evans2,**,***, K. L. Furneaux1,†, J. Hopkins4,5, T. Ingham1,3, C. Jones4,****, J. D. Lee4,5, A. C. Lewis4,5, S. J. Moller4,5, D. Stone1, L. K. Whalley1,3, and D. E. Heard1,3
1School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
2School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
3National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
4Department of Chemistry, University of York, Heslington, YO10 5DD, UK
5National Centre for Atmospheric Science, University of York, Heslington, YO10 5DD, UK
*now at: Earth System Research Laboratory, Chemical Sciences Division, NOAA, 325 Broadway, Boulder, Colorado 80305, USA
**now at: Department of Chemistry, University of York, Heslington, YO10 5DD, UK
***now at: National Centre for Atmospheric Science, University of York, Heslington, YO10 5DD, UK
****now at: Graduate School of Global Environmental Studies, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Sadly passed away on 28 July 2009

Abstract. OH reactivity, the reciprocal of its lifetime from reaction with its sinks, was measured for 12 days in April 2008 within a tropical rainforest on Borneo as part of the OP3 project. The maximum observed value was 83.8 ± 26.0 s−1 with the campaign averaged noon-time maximum being 29.1 ± 8.5 s−1. The maximum OH reactivity calculated using the campaign averaged noon-time concentrations of observed sinks was ~18 s−1, significantly less than the observations, consistent with other studies in similar environments. OH reactivity was dominated by reaction with isoprene. Numerical simulations of isoprene oxidation using the Master Chemical Mechanism (v3.2) in a highly simplified physical and chemical environment show that the steady state OH reactivity is a linear function of the OH reactivity due to isoprene alone, with a maximum multiplier being equal to the number of isoprene OH attackable bonds (10). Thus the emission of isoprene constitutes a significantly larger emission of reactivity than is offered by the primary reaction with isoprene alone, with significant scope for the secondary oxidation products of isoprene to constitute the missing reactivity. A physically and chemically more sophisticated simulation (including physical loss, photolysis, and other oxidants) showed that the calculated OH reactivity is reduced by the removal of the OH attackable bonds by other oxidants and photolysis, and by physical loss (mixing and deposition). The calculated OH reactivity is increased by peroxide cycling, and by the OH concentration itself. Notable in these calculations is that the lifetime of OH reactivity is significantly longer than the lifetime of isoprene and critically depends on the chemical and physical lifetime of intermediate species. When constrained to the observed campaign averaged diurnal concentrations of primary volatile organic compounds (VOCs), O3, nitrogen oxides (NOx) and other parameters, the model underestimated the observed mean OH reactivity by 30%. However, it was found that: (1) the short lifetimes of isoprene and OH lead to a large variability in their concentrations and so significant variation in the calculated OH reactivity, (2) uncertainties in the OH chemistry in these high isoprene environments can lead to an underestimate of the OH reactivity, and (3) the physical loss of species that react with OH plays a significant role in the calculated OH reactivity, (4) a missing primary source of reactive carbon would have to be emitted at a rate equivalent to 50% that of isoprene to account for the missing OH sink. A clear argument for a significant missing flux of primary emitted VOC compounds to account for the unmeasured reactivity is not found and the development of techniques for the measurement of secondary multifunctional carbon compounds is needed to close the OH reactivity budget.

Citation: Edwards, P. M., Evans, M. J., Furneaux, K. L., Hopkins, J., Ingham, T., Jones, C., Lee, J. D., Lewis, A. C., Moller, S. J., Stone, D., Whalley, L. K., and Heard, D. E.: OH reactivity in a South East Asian Tropical rainforest during the Oxidant and Particle Photochemical Processes (OP3) project, Atmos. Chem. Phys. Discuss., 13, 5233-5278, doi:10.5194/acpd-13-5233-2013, 2013.
 
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