Air-sea exchange of acetone, acetaldehyde, DMS and isoprene at a UK coastal site

Abstract

Abstract. Volatile organic compounds (VOCs) are ubiquitous in the atmosphere and are important for atmospheric chemistry. Large uncertainties remain in the role of the ocean in the atmospheric VOC budget because of poorly constrained marine sources and sinks. There are very few direct measurements of air–sea VOC fluxes near the coast, where natural marine emissions could influence coastal air quality (i.e. ozone, aerosols) and terrestrial gaseous emissions could be taken up by the coastal seas. To address this, we present air–sea flux measurements of acetone, acetaldehyde and dimethylsulfide (DMS) at the coastal Penlee Point Atmospheric Observatory (PPAO) in the South-West UK during the spring (Apr–May 2018). Fluxes of these gases are quantified simultaneously by eddy covariance (EC) using a proton transfer reaction quadrupole mass spectrometer. Comparisons are made between two wind sectors representative of different air–water exchange regimes: the open water sector facing the North Atlantic Ocean and the fetch-limited Plymouth Sound fed by two estuaries. Mean EC (± 1 standard error) fluxes of acetone, acetaldehyde and DMS from the open-water wind sector were −8.0 ± 0.8, −1.6 ± 1.4 and 4.7 ± 0.6 μmol m−2 d−1 respectively (− sign indicates net air-to-sea deposition). These measurements are generally comparable (same order of magnitude) to previous measurements in the Eastern North Atlantic Ocean at the same latitude. In comparison, the terrestrially influenced Plymouth Sound wind sector showed respective fluxes of −12.9 ± 1.4, −4.5 ± 1.7 and 1.8 ± 0.8 μmol m−2 d−1. The greater deposition fluxes of acetone and acetaldehyde within the Plymouth Sound were likely to a large degree driven by higher atmospheric concentrations from the terrestrial wind sector. The reduced DMS emission from the Plymouth Sound was caused by a combination of lower wind speed and likely lower dissolved concentrations as a result of the freshwater estuarine influence (i.e. dilution). In addition, we measured the near surface seawater concentrations of acetone, acetaldehyde, DMS and isoprene from a marine station 6 km offshore. Comparisons are made between EC fluxes from the open water and bulk air–sea VOC fluxes calculated using air/water concentrations with a two-layer (TL) model of gas transfer. The calculated TL fluxes are largely consistent with the EC measurements with respect to the directions and magnitudes of fluxes. Accordingly, the computed transfer velocities of DMS and acetone from the EC fluxes and air/water concentrations are largely consistent with previous transfer velocity estimates from the open ocean.