Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide

Abstract

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 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 cloudcover 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(±0.2)×10 6 OH molecules cm −3 from the DMS budget, which represents a monthly effective concentration and is well within the range of previous estimates. Furthermore, when linearly proportioned according to the intensity of solar flux, the resultant diel OH profile, together with DMS surface and entrainment fluxes, enables us to accurately replicate the observed diurnal cycle in DMS (correlation coefficient over 0.9). The nitrate radical (NO 3 ) is found to have little contribution to DMS oxidation during VOCALS-REx. An upper limit estimate of 1 pptv of bromine oxide radical (BrO) would account for 30% of DMS oxidation and lower the OH concentration to 1.0×10 6 OH molecules cm −3 . Our effective OH estimate includes the oxidation of DMS by such radicals.

Measurements of OVOC fluxes by eddy covariance using a proton-transfer-reaction mass spectrometer – method development at a coastal site

“…Atmospheric dimethyl sulfide (DMS) mixing ratio was also detected by the PTR-MS, which confirmed the marine influence. Consistent with previous diel cycle observations (e.g., Bandy et al, 1996;Yang et al, 2009), DMS mixing ratio decreased in the afternoon, when photochemical destruction outpaced sea-toair emission, until a few hours before sunset. Contrastingly, acetaldehyde mixing ratio generally increased with solar irradiance.…”

Ship-based detection of glyoxal over the remote tropical Pacific Ocean

“…It is determined primarily by the abundance of solar radiation around the Equator and, to a lesser extent, OH-initiated CHOCHO loss. The 24 h average OH concentration during VOCALS-REx was estimated to be 1.4 × 10 6 molec/cm 3 (Yang et al, 2009), corresponding to a peak daytime OH of about 5 × 10 6 molec/cm 3 . This is in reasonable agreement with observations over the tropical Atlantic Ocean (on average several 10 6 molec/cm 3 , though midday peaks reached almost 10 7 molec/cm 3 ; Whalley et al, 2010).…”

Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem

“…(4) of Yang et al (2009) with 5-min model outputs of tracer concentrations. It is worth noting that the estimated entrainment rate is larger than the nighttime entrainment rate (0.4 cm s −1 ) estimated by Yang et al (2009) using budget analysis of measured DMS during the VOCALS-REx. This is very likely due to vertical resolution near cloud top and in the inversion layer.…”