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 (April–May 2018). Fluxes of these gases were measured 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 terrestrially influenced 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 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 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 and water concentrations with a two-layer (TL) model of gas transfer. The calculated TL fluxes agree with the EC measurements with respect to the directions and magnitudes of fluxes, implying that any recently proposed surface emissions of acetone and acetaldehyde would be within the propagated uncertainty of 2.6 µmol m−2 d−1. The computed transfer velocities of DMS, acetone and acetaldehyde from the EC fluxes and air and water concentrations are largely consistent with previous transfer velocity estimates from the open ocean. This suggests that wind, rather than bottom-driven turbulence and current velocity, is the main driver for gas exchange within the open-water sector at PPAO (depth of ∼ 20 m).

Data supporting the North Atlantic Climate System: Integrated Studies (ACSIS) programme, including atmospheric composition, oceanographic and sea ice observations (2016–2022) and output from ocean, atmosphere, land and sea-ice models (1950–2050)

“…In close proximity to the Western Channel Observatory marine sampling stations, high frequency observations at PPAO enable both long-term and process-based studies of atmosphere-ocean interactions. Current/recent work has assessed trace gas burdens and air-sea fluxes including greenhouse gases (Yang et al 2016b(Yang et al , 2016c(Yang et al , 2019a, volatile organic carbon (Phillips et al, 2021), sulfur- (Yang et al, 2016c), halogen- (Sommariva et al, 2018), and nitrogen-containing gases (ongoing). Further works include aerosol composition and fluxes, with particular foci on ship emissions (ongoing as a part of the ACRUISE project), sea spray production (Yang et al, 2019b), macro/micro nutrient deposition (White et al, 2021), and reaction between atmospheric ozone and the sea surface microlayer (Loades et al, 2020) Continuous observations most relevant to ACSIS include ground-based ozone and methane from PPAO as well as column aerosols from the rooftop of PML (10 km north/northeast of PPAO).…”

Production of oxygenated volatile organic compounds from the ozonolysis of coastal seawater

“…5). It is worth noting that the two largest contributors to 𝜑 𝑉𝑂𝐶,𝑓𝑖𝑒𝑙𝑑 , C5H9 + and C6H9 + , are two RT-Vocus ions where marine BVOC can be detected, namely isoprene at C5H9 + and monoterpenes at C6H9 + (Kim et al, 2010;Phillips et al, 2021). Without field GC measurements, it is possible that these numbers could be inflated by incorrectly assigning some BVOC to an abiotic source since the ocean surface is continually being replenished and influenced by ocean biological dynamics.…”

Concentrations of dissolved dimethyl sulfide (DMS), methanethiol and other trace gases in context of microbial communities from the temperate Atlantic to the Arctic Ocean

“…A positive net flux of acetone is generally observed in biologically productive areas such as tropical upwelling zones, whereas high-latitude and oligotrophic waters represent sinks (Lawson et al, 2020). As OVOCs mainly originate from terrestrial sources, their air-sea fluxes can also be a net deposition, when their marine atmospheric concentrations are directly influenced by air masses originating from continents (Phillips et al, 2021). Furthermore, OVOCs can show seasonal variation (Davie- Martin et al, 2020).…”