Search for: All records

Abstract. A fast-response (10 Hz) chemiluminescence detector forozone (O3) was used to determine O3 fluxes using the eddy covariancetechnique at the Penlee Point Atmospheric Observatory (PPAO) on the southcoast of the UK during April and May 2018. The median O3 flux was −0.132 mg m−2 h−1 (0.018 ppbv m s−1),corresponding to a deposition velocity of 0.037 cm s−1(interquartile range 0.017–0.065 cm s−1) – similar to thehigher values previously reported for open-ocean flux measurements but notas high as some other coastal results. We demonstrate that a typical singleflux observation was above the 2σ limit of detection but hadconsiderable uncertainty. The median 2σ uncertainty of depositionvelocity was 0.031 cm s−1 for each 20 min period, whichreduces with the square root of the sample size. Eddy covariance footprintanalysis of the site indicates that the flux footprint was predominantlyover water (> 96 %), varying with atmospheric stability and, toa lesser extent, with the tide. At very low wind speeds when the atmospherewas typically unstable, the observed ozone deposition velocity was elevated,most likely because the footprint contracted to include a greater landcontribution in these conditions. At moderate to high wind speeds whenatmospheric stability was near-neutral, the ozone deposition velocityincreased with wind speed and showed a linear dependence with frictionvelocity. This observed dependence on friction velocity (and therefore alsowind speed) is consistent with the predictions from the one-layer model ofFairall et al. (2007), which parameterisesthe oceanic deposition of ozone from the fundamental conservation equation,accounting for both ocean turbulence and near-surface chemical destruction,while assuming that chemical O3 destruction by iodide is distributed overdepth. In contrast to our observations, the deposition velocity predicted bythe recently developed two-layer model of Luhar et al. (2018) (whichconsiders iodide reactivity in both layers but with molecular diffusivitydominating over turbulent diffusivity in the first layer) shows no majordependence of deposition velocity on wind speed and underestimates themeasured deposition velocities. These results call for further investigationinto the mechanisms and control of oceanic O3 deposition.