The effects of experimental uncertainty in parameterizing air-sea gas exchange using tracer experiment data

W E Asher

doi:10.5194/acp-9-131-2009

Abstract

Abstract. It is not practical to measure air-sea gas fluxes in the open ocean for all conditions and areas of interest. Therefore, in many cases fluxes are estimated from measurements of air-phase and water-phase gas concentrations, a measured environmental forcing function such as wind speed, and a parameterization of the air-sea transfer velocity in terms of the environmental forcing function. One problem with this approach is that when direct measurements of the transfer velocity are plotted versus the most commonly used forcing function, wind speed, there is considerable scatter, leading to a relatively large uncertainty in the flux. Because it is known that multiple processes can affect gas transfer, it is commonly assumed that this scatter is caused by single-forcing function parameterizations being incomplete in a physical sense. However, scatter in the experimental data can also result from experimental uncertainty (i.e., measurement error). Here, results from field and laboratory results are used to estimate how experimental uncertainty contributes to the observed scatter in the measured fluxes and transfer velocities as a function of environmental forcing. The results show that experimental uncertainty could explain half of the observed scatter in field and laboratory measurements of air-sea gas transfer velocity.

Highlights

Advances in techniques for measuring air-sea fluxes have resulted in several new oceanic data sets of oceanic gas fluxes (Edson et al, 2004; Ho et al, 2006; Jacobs et al, 1999; McGillis et al, 2001a, b; Nightingale et al, 2000a, b; Wanninkhof et al, 1993, 1997, 2004)
The air-sea flux of a sparingly soluble nonreactive gas at low to moderate wind speeds can be written as the product of a kinetic term, the air-sea gas transfer velocity kL, and a thermodynamic driving force defined in terms of the disequilibrium in chemical potential of the gas between the ocean and the atmosphere
A similar relation exists for the analysis of purposeful dual-tracer method (PDTM) data collected during oceanic air-sea gas exchange measurements

Summary

Introduction

Advances in techniques for measuring air-sea fluxes have resulted in several new oceanic data sets of oceanic gas fluxes (Edson et al, 2004; Ho et al, 2006; Jacobs et al, 1999; McGillis et al, 2001a, b; Nightingale et al, 2000a, b; Wanninkhof et al, 1993, 1997, 2004). It is assumed that much of the scatter in the data represents variability in the transfer velocity imposed by variability in the environmental conditions This could occur if, for example, the levels of aqueous-phase turbulence generated at a particular value of U depend on factors other than the U itself. The purpose of this paper is to explore whether it is possible to explain the scatter in various measurements of the kL in terms of the uncertainty in measuring the underlying parameters This will be done using the dual-tracer data shown in Fig. 1 and using gas transfer data collected in a wind-wave tunnel

Theory

Wind-wave tunnel measurements

Oceanic purposeful dual-tracer measurements

Findings

Conclusions