Estimation of bubble-mediated air–sea gas exchange from concurrent DMS and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; transfer velocities at intermediate–high wind speeds

Thomas G Bell,Scott D Miller,Brian Scanlon,Adrian H Callaghan,Brian Ward,Eric S Saltzman,Warren J De Bruyn,Sebastian Landwehr,Mingxi Yang

doi:10.5194/acp-17-9019-2017

Abstract

Abstract. Simultaneous air–sea fluxes and concentration differences of dimethylsulfide (DMS) and carbon dioxide (CO2) were measured during a summertime North Atlantic cruise in 2011. This data set reveals significant differences between the gas transfer velocities of these two gases (Δkw) over a range of wind speeds up to 21 m s−1. These differences occur at and above the approximate wind speed threshold when waves begin breaking. Whitecap fraction (a proxy for bubbles) was also measured and has a positive relationship with Δkw, consistent with enhanced bubble-mediated transfer of the less soluble CO2 relative to that of the more soluble DMS. However, the correlation of Δkw with whitecap fraction is no stronger than with wind speed. Models used to estimate bubble-mediated transfer from in situ whitecap fraction underpredict the observations, particularly at intermediate wind speeds. Examining the differences between gas transfer velocities of gases with different solubilities is a useful way to detect the impact of bubble-mediated exchange. More simultaneous gas transfer measurements of different solubility gases across a wide range of oceanic conditions are needed to understand the factors controlling the magnitude and scaling of bubble-mediated gas exchange.

Highlights

Air–sea exchange is a significant process for many compounds that have biogeochemical and climatic importance
This study shows that kw is near zero (< 4.5 cm h−1) at low wind speeds (U10 ≤ 4.5 m s−1), which is consistent with the wind speed at which whitecap fraction becomes significant (WT > 10−5, Fig. 2a)
The Knorr_11 concurrent measurements of DMS and CO2 gas transfer velocities show significant differences in gas transfer between the two gases at intermediate–high wind speeds. These data indicate that (i) bubble-mediated gas transfer becomes significant for CO2 at or above the threshold for wave-breaking and (ii) the wind speed dependence is www.atmos-chem-phys.net/17/9019/2017/

Summary

Introduction

Air–sea exchange is a significant process for many compounds that have biogeochemical and climatic importance. The biogenic gas dimethylsulfide (DMS) is a major contributor to the mass of marine atmospheric aerosol (Virkkula et al, 2006). Volatile organic compounds (VOCs) such as isoprene, acetone and acetaldehyde alter the oxidising capacity of the troposphere (Carpenter et al, 2012). The solubility differences between these VOCs mean that their exchange is controlled to differing degrees by processes on the water and air side of the air–sea interface (Yang et al, 2014). Many of the factors influencing air–sea gas exchange will be altered by future changes in climate, ocean circulation and biology. Earth system models and air quality models require more accurate understanding of the processes that influence air–sea gas transfer

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