Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 1 2009 10.5194/acp-9-163-2009 http://www.atmos-chem-phys.net/9/163/2009/ http://www.atmos-chem-phys.net/9/163/2009/acp-9-163-2009.html http://www.atmos-chem-phys.net/9/163/2009/acp-9-163-2009.pdf 163 173 2009-01-12 Airborne measurement of OH reactivity during INTEX-B J. Mao mao@fas.harvard.edu X. Ren W. H. Brune J. R. Olson J. H. Crawford A. Fried L. G. Huey R. C. Cohen B. Heikes H. B. Singh D. R. Blake G. W. Sachse G. S. Diskin S. R. Hall R. E. Shetter Department of Meteorology, Pennsylvania State University, University Park, PA, USA Science Directorate, NASA Langley Research Center, Hampton, VA, USA Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA Department of Chemistry and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA NASA Ames Research Center, Moffett Field, CA, USA Department of Chemistry, University of California, Irvine, CA, USA Science Directorate, NASA Langley Research Center, Hampton, VA, USA Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA now at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA now at: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during the second phase of Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign, which was focused on the Asian pollution outflow over Pacific Ocean and was based in Hawaii and Alaska. The OH reactivity was measured by adding OH, generated by photolyzing water vapor with 185 nm UV light in a moveable wand, to the flow of ambient air in a flow tube and measuring the OH signal with laser induced fluorescence. As the wand was pulled back away from the OH detector, the OH signal decay was recorded; the slope of −Δln(signal)/Δ time was the OH reactivity. The overall absolute uncertainty at the 2σ confidence levels is about 1 s^−1 at low altitudes (for decay about 6 s^−1), and 0.7 s^−1 at high altitudes (for decay about 2 s^−1). From the median vertical profile obtained in the second phase of INTEX-B, the measured OH reactivity (4.0±1.0 s^−1) is higher than the OH reactivity calculated from assuming that OH was in steady state (3.3&plusmn0.8 s^−1), and even higher than the OH reactivity that was calculated from the total measurements of all OH reactants (1.6±0.4 s^−1). Model calculations show that the missing OH reactivity is consistent with the over-predicted OH and under-predicted HCHO in the boundary layer and lower troposphere. The over-predicted OH and under-predicted HCHO suggest that the missing OH sinks are most likely related to some highly reactive VOCs that have HCHO as an oxidation product. Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., Troe, J., and IUPAC Subcommittee: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II - gas phase reactions of organic species, Atmos. Chem. Phys., 6, 3625–4055, 2006. 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