Dimethylsulfide gas transfer coefficients from algal blooms in the Southern Ocean

T G Bell,C S Law,E S Saltzman,M J Smith,W De Bruyn,S D Miller,C A Marandino

doi:10.5194/acp-15-1783-2015

Abstract

Abstract. Air–sea dimethylsulfide (DMS) fluxes and bulk air–sea gradients were measured over the Southern Ocean in February–March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (> 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer–wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

Highlights

Gas exchange across the ocean–atmosphere interface influences the atmospheric abundance of many compounds of importance to climate and air quality
This paper presents data collected in the Southern Ocean summer (February–March 2012) as part of the New Zealand Surface Ocean Aerosol Production (SOAP) cruise (Fig. 1)
The SOAP k660 bin average values are in good agreement with previous gas transfer studies using eddy covariance of DMS (Yang et al, 2011; Bell et al, 2013; Marandino et al, 2007)

Summary

Introduction

Gas exchange across the ocean–atmosphere interface influences the atmospheric abundance of many compounds of importance to climate and air quality. Such compounds include greenhouse gases, aerosol precursors, stratospheric ozonedepleting substances, and a wide range of photochemically reactive volatile organic carbon compounds that influence tropospheric ozone. Estimating the air–sea fluxes of all of these compounds requires knowledge of their distributions in near-surface air and seawater and an understanding of the transport processes controlling gas exchange across the air– sea interface. The transport processes are not well understood, in large part because of the paucity of direct air–sea gas flux observations. The parameterization of gas exchange is a significant source of uncertainty in ocean–atmosphere exchange in global models, at high wind speeds (Elliott, 2009).

Methods

Results

Conclusion