Measured OH Reactivity in ATom

The frequency of hydroxyl (OH) reactions with all atmospheric constituents is called the OH reactivity. It determines the rate at which a substantial portion of atmospheric chemistry can proceed. Over oceans, modeled OH reactivity is no more than a few s^-1 in the lowest two kilometers and is less than 0.5 s^-1 in the middle and upper troposphere. Despite these low values compared to typical values of 5 s^-1 to 30 s^-1 over land, the OH reactivity over oceans plays an important role in setting the global atmospheric oxidation rate. However, such low values present a challenge for the instruments that directly measure the OH reactivity. We will discuss the OH reactivity measurement, the measured OH reactivity, and its comparison with OH reactivity calculated from the other measurements on the DC-8 and from the model for ATom, phases one and two. At the current level of analysis, these ATom OH reactivity measurements show higher-than-expected OH reactivity of a few s^-1 in the lowest 2 kilometers, as was observed by Mao et al. (ACP, 9, 163, 2009) during the INTEX-B NASA DC-8 campaign over the northern central Pacific. In this presentation, we will look at the global distribution of OH reactivity, particularly in the lowest two kilometers, and will focus on the causes of any differences between measured and modeled OH reactivity.