Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 4 2008 10.5194/acpd-8-14217-2008 http://www.atmos-chem-phys-discuss.net/8/14217/2008/ http://www.atmos-chem-phys-discuss.net/8/14217/2008/acpd-8-14217-2008.html http://www.atmos-chem-phys-discuss.net/8/14217/2008/acpd-8-14217-2008.pdf 14217 14246 2008-07-24 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 Dept. of Meteorology, Pennsylvania State Univ., University Park, PA, USA Science Directorate, NASA Langley Research Center, Hampton, VA, USA Earth Observing Laboratory, National Center for Atmos. Research, Boulder, CO, USA School of Earth and Atmos. Sciences, Georgia Inst. of Technology, Atlanta, GA, USA Dept. of Chemistry and Dept. of Earth and Planet. Sci., Univ. of California Berkeley, CA, USA Graduate School of Oceanography, Univ. of Rhode Island, Narragansett, RI, USA NASA Ames Research Center, Moffett Field, CA, USA Dept. of Chemistry, Univ. of California, Irvine, CA, USA Science Directorate, NASA Langley Research Center, Hampton, VA, USA Atmos. Chemistry Division, National Center for Atmos. Research, Boulder, CO, USA now at: School of Eng. and Applied Sciences, Harvard Univ., Cambridge, MA, USA now at: Rosenstiel School of Marine and Atmos. Science, Univ. of Miami, Miami, FL, USA The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate the atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign. 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)/Δtime was the OH reactivity. 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±0.8 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. 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