Abstract
Abstract. A fast-response (10 Hz) chemiluminescence detector for ozone (O3) was used to determine O3 fluxes using the eddy covariance technique at the Penlee Point Atmospheric Observatory (PPAO) on the south coast 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 the higher values previously reported for open-ocean flux measurements but not as high as some other coastal results. We demonstrate that a typical single flux observation was above the 2σ limit of detection but had considerable uncertainty. The median 2σ uncertainty of deposition velocity was 0.031 cm s−1 for each 20 min period, which reduces with the square root of the sample size. Eddy covariance footprint analysis of the site indicates that the flux footprint was predominantly over water (> 96 %), varying with atmospheric stability and, to a lesser extent, with the tide. At very low wind speeds when the atmosphere was typically unstable, the observed ozone deposition velocity was elevated, most likely because the footprint contracted to include a greater land contribution in these conditions. At moderate to high wind speeds when atmospheric stability was near-neutral, the ozone deposition velocity increased with wind speed and showed a linear dependence with friction velocity. This observed dependence on friction velocity (and therefore also wind speed) is consistent with the predictions from the one-layer model of Fairall et al. (2007), which parameterises the 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 over depth. In contrast to our observations, the deposition velocity predicted by the recently developed two-layer model of Luhar et al. (2018) (which considers iodide reactivity in both layers but with molecular diffusivity dominating over turbulent diffusivity in the first layer) shows no major dependence of deposition velocity on wind speed and underestimates the measured deposition velocities. These results call for further investigation into the mechanisms and control of oceanic O3 deposition.
Highlights
Tropospheric ozone is important due to its considerable effects on human health (Medina-Ramón et al, 2006), agricultural yields (Heck et al, 1982), and global warming (Stevenson et al, 2013)
The resulting distribution of vd values was compared to that obtained with the lag time set to 180 s and was significantly different from the results of the disjoined data (Kolmogorov–Smirnov test, p < 0.001; Fig. S3), rejecting the null hypothesis that the two sets of values could be taken by chance from the same distribution
This confirms that the experimental set-up used here has a sufficiently low limit of detection to discern the flux from noise over the whole duration of the measurements
Summary
Tropospheric ozone is important due to its considerable effects on human health (Medina-Ramón et al, 2006), agricultural yields (Heck et al, 1982), and global warming (Stevenson et al, 2013). Dry deposition is a major sink of tropospheric ozone, comprising as much as 25 % of total loss from the troposphere (Ganzeveld et al, 2009; Lelieveld and Dentener, 2000; Pound et al, 2020). Deposition to the sea surface is the greatest source of uncertainty in global estimates. Loades et al.: Ozone deposition to a coastal sea of total ozone dry deposition (Hardacre et al, 2015) due to deposition occurring at a slow and highly uncertain rate but over a vast area
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