1Department of Meteorology, Pennsylvania State University, University Park, PA, USA
2Science Directorate, NASA Langley Research Center, Hampton, VA, USA
3Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
4School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
5Department of Chemistry and Department of Earth and Planetary Science, University of California Berkeley, Berkeley, CA, USA
6Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA
7NASA Ames Research Center, Moffett Field, CA, USA
8Department of Chemistry, University of California, Irvine, CA, USA
9Science Directorate, NASA Langley Research Center, Hampton, VA, USA
10Atmospheric 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
Received: 01 Jul 2008 – Discussion started: 24 Jul 2008
Abstract. 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±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. 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.
Published: 12 Jan 2009
Mao, J., Ren, X., Brune, W. H., Olson, J. R., Crawford, J. H., Fried, A., Huey, L. G., Cohen, R. C., Heikes, B., Singh, H. B., Blake, D. R., Sachse, G. W., Diskin, G. S., Hall, S. R., and Shetter, R. E.: Airborne measurement of OH reactivity during INTEX-B, Atmos. Chem. Phys., 9, 163-173, doi:10.5194/acp-9-163-2009, 2009.