Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 4 2009 10.5194/acpd-9-16267-2009 http://www.atmos-chem-phys-discuss.net/9/16267/2009/ http://www.atmos-chem-phys-discuss.net/9/16267/2009/acpd-9-16267-2009.html http://www.atmos-chem-phys-discuss.net/9/16267/2009/acpd-9-16267-2009.pdf 16267 16294 2009-07-30 Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide M. Yang B. W. Blomquist B. J. Huebert huebert@hawaii.edu Department of Oceanography, University of Hawaii, Honolulu, HI, USA 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 boundary 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 troposphere (FT) greatly simplify the dynamics in this region and make our budget estimate possible. From the observed diurnal cycle in DMS concentration, the nighttime entrainment velocity at the inversion is estimated to be 4 mm s^−1. We calculate 1.4×10⁶ OH molecules cm^−3 from the DMS budget, which represents a ~monthly effective OH concentration and is well within the range of previous estimates. 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