Airborne measurement of OH reactivity during INTEX-B

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 out-flow 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-△ln(signal)/△ 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 reactiv-ity 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.