A new method for continuous measurement of methane and carbon dioxide in surface waters using off-axis integrated cavity output spectroscopy (ICOS): An example from the Baltic Sea

摘要

A new system is presented that allows the continuous measurement of methane and carbon dioxide concentrations in surface waters autonomously using ships of opportunity. The analytical setup consists of a methane carbon dioxide-Analyzer (MCA, Los Gatos Research) joined to an established equilibrator setup. The analyzer uses off-axis integrated cavity output spectroscopy (ICOS) and combines a highly specific infrared band laser with a set of strongly reflective mirrors to obtain an effective laser path length of several kilometers. This allows detecting methane and carbon dioxide in the equilibrated gas phase with high recision (less than 0.1%) and frequency.udThe system was installed on the cargo ship Finnmaid (Finnlines) in November 2009, which commutes regularly in the Baltic Sea between Travemünde (Germany), Gdynia (Poland), and Helsinki (Finland). Methane concentrations of the equilibrated gas phase measured by gas chromatography and by the MCA during lab tests are in excellent agreement. The comparison of carbon dioxide data measured by the MCA system to CO2 values gathered from the same type of equilibrator in combination with a LICOR CO2 detector (Schneider et al. 1992) during onboard operation show concordant results. The time constant for the system in freshwater at room temperature was determined to be 676 s for CH4 and 226 s for CO2. Additional performance tests are presented. First field results show large regional differences with remarkable features, especially in shallow regions, demonstrating the need for the high spatiotemporal data coverage provided by the instrument.