Constraining the concentration of the hydroxyl radical

Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract The hydroxyl radical (OH) is an important oxidant in the troposphere due to its high reactivity and relative abundance. Measuring the concentration of OH in situ, however , is technically challenging. Here we present a robust yet simple method of estimating an OH-equivalent oxidant concentration (" effective OH ") in the marine bound-5 ary layer (MBL) from the mass balance of dimethylsulfide (DMS). We use shipboard eddy covariance measurements of the sea-to-air DMS flux from the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. The persistent stratocumulus cloud-cover off the west coast of South America and the associated strong inversion between MBL and the free tro-10 posphere (FT) greatly simplify the dynamics in this region and make our budget estimate possible. From the observed diurnal cycle in DMS concentration, the night-time entrainment velocity at the inversion is estimated to be 4 mm s −1. We calculate 1.4×10 6 OH molecules cm −3 from the DMS budget, which represents a ∼monthly effective OH concentration and is well within the range of previous estimates. Furthermore, 15 when fitted to the measured intensity of solar flux, the resultant diel variation in OH concentration, together with the DMS surface and entrainment fluxes, enables us to accurately replicate the observed diurnal cycle in DMS (correlation coefficient exceeding 0.9). The nitrate radical is found to have little contribution to DMS oxidation during VOCALS-REx.