Measurements of air–sea gas transfer velocities in the Baltic Sea

Leila Nagel,Kerstin E Krall,Bernd Jähne

doi:10.5194/os-15-235-2019

Abstract

Abstract. Heat transfer velocities measured during three different campaigns in the Baltic Sea using the active controlled flux technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented. Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows us to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter. The measured data and other field data clearly show that some gas transfer velocities are much lower than those based on the empirical wind speed parameterizations. This indicates that the dependencies of the transfer velocity on the fetch, i. e., the history of the wind and the age of the wind-wave field, and the effects of surface-active material need to be taken into account.

Highlights

The transfer of a trace gas across the air–sea interface is commonly characterized by the gas transfer velocity k, which links the gas flux j with the concentration difference across the interface, c: j = k c. (1)Traditionally, k is parameterized with the wind speed measured at a height of 10 m, u10, since wind speed is the most readily available parameter
Heat transfer velocities measured during three different campaigns in the Baltic Sea using the active controlled flux technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented
Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows us to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter

Summary

Introduction

Wanninkhof et al (2009) gives an overview of the most commonly used techniques to measure the gas transfer velocity. The dual-tracer technique, especially with the tracer pair 3He−SF6, as well as eddy covariance measurements of the gases CO2 and dimethylsulfide (DMS), has become state of the art for measuring the gas transfer velocity in situ. As the best fit to all available 3He−SF6 dual-tracer data points, where k600 denotes the transfer velocity scaled to a CO2-equivalent transfer velocity at 20 ◦C. Mass balance techniques such as the dual-tracer method have a large time constant of up to weeks and large spatial scales of a few tens of kilometers, smoothing away varying micrometeorological and surface conditions (e.g., the degree of surface contamination by surface-active material)

Methods

Results

Conclusion