Numerical tools to estimate the flux of a gas across the air–water interface and assess the heterogeneity of its forcing functions

R Santos,F Martins,J Silva,V M N C S Vieira

doi:10.5194/os-9-355-2013

Abstract

Abstract. A numerical tool was developed for the estimation of gas fluxes across the air–water interface. The primary objective is to use it to estimate CO2 fluxes. Nevertheless application to other gases is easily accomplished by changing the values of the parameters related to the physical properties of the gases. A user-friendly software was developed allowing to build upon a standard kernel a custom-made gas flux model with the preferred parameterizations. These include single or double layer models; several numerical schemes for the effects of wind in the air-side and water-side transfer velocities; the effects of atmospheric stability, surface roughness and turbulence from current drag with the bottom; and the effects on solubility of water temperature, salinity, air temperature and pressure. An analysis was also developed which decomposes the difference between the fluxes in a reference situation and in alternative situations into its several forcing functions. This analysis relies on the Taylor expansion of the gas flux model, requiring the numerical estimation of partial derivatives by a multivariate version of the collocation polynomial. Both the flux model and the difference decomposition analysis were tested with data taken from surveys done in the lagoon system of Ria Formosa, south Portugal, in which the CO2 fluxes were estimated using the infrared gas analyzer (IRGA) and floating chamber method, whereas the CO2 concentrations were estimated using the IRGA and degasification chamber. Observations and estimations show a remarkable fit.

Highlights

The appropriate algorithms for the estimation of gas fluxes across the air–water interface have been the subject of great concern by the scientific community
Wide spatial and temporal variabilities of gas concentrations in the water, in the overlying air and their fluxes across the air–water interface are widely documented for the open oceans, the coastal oceans and riverine systems
These gas fluxes have a multitude of potential forcing functions. Their integration and the establishment of their relative importance has been underachieved. This is evident from how atmospheric stability, sea-surface roughness and current drag with the bottom have often been devalued in studies about riverine systems and coastal waters

Summary

Introduction

The appropriate algorithms for the estimation of gas fluxes across the air–water interface have been the subject of great concern by the scientific community. The marine and aquatic environments may work as either net sinks or net sources of CO2 for the atmosphere This shows a great spatial and temporal variability (Smith and Hollibaugh, 1993; Duarte and Prairie, 2005; Borges, 2005; Borges et al, 2005). The flux of CO2 across the air–water interface is fundamental to estimate the carbon budget of marine and aquatic ecosystems and classify them as either autotrophic, upon net CO2 consumption by primary producers, or heterotrophic, upon net CO2 production by bacterial degradation of organic carbon. Coastal oceans and riverine systems are believed to be globally heterotrophic, remineralizing organic carbon imported from terrestrial ecosystems (Smith and Hollibaugh, 1993; Cole and Caraco, 2001; Duarte and Prairie, 2005; Borges et al, 2005).

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