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).
Highlights
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere and play an important role in atmospheric chemistry and carbon cycling in the biosphere (Heald et al, 2008)
These VOCs have been detected in the surface ocean at concentrations of up to tens of nanomolar (Zhou and Mopper, 1997; Williams et al, 2004), with known oceanic sources including the photochemical degradation of dissolved organic matter (DOM) in bulk seawater (Kieber et al, 1990; Zhou and Mopper, 1997; Zhu and Kieber, 2018) and possibly autotrophic/heterotrophic biological processes (Halsey et al, 2017; Schlundt et al, 2017)
The acetaldehyde lifetime is in good agreement with literature values (0.1–0.8 d; Millet et al, 2010; Wang et al, 2019) because photochemistry is the main loss mechanism for marine boundary layer (MBL) acetaldehyde rather than deposition, and the April surface 50◦ N zonal average OH concentration is similar to the tropospheric mean (1.1 × 106 molec. cm−3; Li et al, 2018)
Summary
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere and play an important role in atmospheric chemistry and carbon cycling in the biosphere (Heald et al, 2008). Acetone and acetaldehyde are carbonyl-based VOCs that have been shown to make up to ∼ 57 % of the carbon mass of all non-methane organic carbon compounds in remote marine air over the North Atlantic Ocean (Lewis et al, 2005) These VOCs have been detected in the surface ocean at concentrations of up to tens of nanomolar (nM) (Zhou and Mopper, 1997; Williams et al, 2004), with known oceanic sources including the photochemical degradation of dissolved organic matter (DOM) in bulk seawater (Kieber et al, 1990; Zhou and Mopper, 1997; Zhu and Kieber, 2018) and possibly autotrophic/heterotrophic biological processes (Halsey et al, 2017; Schlundt et al, 2017). From the EC fluxes and air and sea concentrations, we derive the gas transfer velocities of DMS, acetone and acetaldehyde and compare them against previous observations from the open ocean.
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