Measurement of Air-Sea Methane Fluxes in the Baltic Sea Using the Eddy Covariance Method

Lucía Gutiérrez-Loza,Annette Kock,Marcus B Wallin,Erik Nilsson,Hermann W Bange,Erik Sahlée,Anna Rutgersson

doi:10.3389/feart.2019.00093

Lucía Gutiérrez-Loza, Annette Kock + Show 5 more

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https://doi.org/10.3389/feart.2019.00093

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Abstract

Methane (CH4) is the second-most important greenhouse gas in the atmosphere having a signiﬁcant effect on global climate. The ocean—particularly the coastal regions—have been recognized to be a net source of CH4, however, the constraints on temporal and spatial resolution of CH4 measurements have been the limiting factor to estimate the total oceanic contributions. In this study, the viability of micrometeorological methods for the analysis of CH4 ﬂuxes in the marine environment was evaluated. We present one year of semi-continuous eddy covariance measurements of CH4 atmospheric dry mole fractions and air–sea CH4 ﬂux densities at the Ostergarnsholm station at the east coast of the Gotland Island in the central Baltic Sea. The mean annual CH4 ﬂux density was positive, indicating that the region off Gotland are a net source of CH4 to the atmosphere with monthly mean ﬂux densities ranging between -0.1 and 36 nmol m−2s−1. Both the air–water concentration gradient and the wind speed were found to be crucial parameters controlling the ﬂux. The results were in good agreement with other measurements in the Baltic Sea reported in the MEMENTO database. Our results suggest that the eddy covariance technique is a useful tool for studying CH4 ﬂuxes and improving the understanding of air-sea gas exchange processes with high-temporal resolution. Potentially, the high resolution of micrometeorological data can increase the understanding of the temporal variability and forcing processes of CH4 ﬂux.

Highlights

Methane (CH4) is an atmospheric trace gas considered to be the second-most important greenhouse gas after carbon dioxide (CO2)
During the period of this study, the wind speed did not show a clear seasonal pattern, higher wind-speed events were observed during autumn and winter causing relatively higher monthly means between September and March than those observed during summer (4.2 to 6.1 m s−1)
Air-sea FCH4 calculated in this study using the eddy covariance (EC) method are, to our best knowledge, the first continuous measurements of FCH4 in the Baltic Sea

Summary

Introduction

Methane (CH4) is an atmospheric trace gas considered to be the second-most important greenhouse gas after carbon dioxide (CO2). The estimated warming potential per molecule of CH4 is 28 times greater than CO2 over a 100-years horizon, and 72 times greater over a 20-years horizon (IPCC, 2013). The global average atmospheric concentration of CH4 has more than doubled since the pre-industrial era, reaching values of over 1,800 ppb (WDCGG, 2015). CH4 is emitted to the atmosphere by natural and anthropogenic sources, the rapid increase in the atmospheric CH4 concentrations has been attributed to anthropogenic activities. The ocean is a net source of CH4 to the atmosphere. Considering biogenic, geological and hydrate sources, the global oceanic emissions have been estimated at 14 Tg yr−1 (range 5-25) value that represent about 1–3% of the total global sources of atmospheric CH4 (Saunois et al, 2016)

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