Speciation of OH reactivity above the canopy of an isoprene-dominated forest

Abstract

Measurements of OH reactivity, the inverse lifetime of the OH radical, can provide a top-down estimate of the total amount of reactive carbon in an air mass. Using a comprehensive measurement suite, we examine the measured and modeled OH reactivity above an isoprenedominated forest in the southeast United States during the 2013 Southern Oxidant and Aerosol Study (SOAS) field campaign. Measured and modeled species account for the vast majority of average daytime reactivity (80-95 %) and a smaller portion of nighttime and early morning reactivity (68-80 %). The largest contribution to total reactivity consistently comes from primary biogenic emissions, with isoprene contributing ∼60% in the afternoon, and ∼30-40% at night and monoterpenes contributing ∼15-25% at night. By comparing total reactivity to the reactivity stemming from isoprene alone, we find that ∼20% of the discrepancy is temporally related to isoprene reactivity, and an additional constant ∼1 s-1 offset accounts for the remaining portion. The model typically overestimates measured OVOC concentrations, indicating that unmeasured oxidation products are unlikely to influence measured OH reactivity. Instead, we suggest that unmeasured primary emissions may influence the OH reactivity at this site.