Trace gas air-sea exchange using eddy correlation
ACRONYM
TRASE-EC
Title
Trace gas air-sea exchange using eddy correlation
General information
Oceanic biological, chemical, and physical processes can significantly impact atmospheric trace gas chemistry through air-sea exchange, with important consequences for climate and life on earth. Therefore, many studies have investigated surface ocean processes as well as the physical constraints on air-sea flux. One major goal of these studies is to understand how to predict ocean fluxes of trace gases. The flux (F) is typically prescribed by the bulk equation, F = k ΔC, where k is the gas transfer coefficient (physical constraint on flux) and ΔC is the gas concentration disequilibrium between the bulk atmosphere and bulk ocean phases (chemical constraint on flux). There has long been interest in having simple parameterizations to compute fluxes, although this goal has not yet been fully realized.
Direct air-sea flux measurements can be a useful tool for studying both the biogeochemical cycling of trace gases and the physical constraints on air-sea gas flux. In one example, Marandino et al. (2005) illustrated that the directly measured open ocean fluxes of acetone did not agree with those calculated by methods commonly used in the research community. Additionally, commonly used wind speed based parameterizations of k are significantly different, up to ~5 times at wind speeds of 9 m s-1. Direct flux measurements can be used to understand these discrepancies and test the efficacy of using wind speedbased parameterizations of k. The most direct flux measurement technique is eddycorrelation, which involves simultaneously measuring the fluctuations in vertical wind speed and the fluctuations in trace gas concentration in order to compute their covariance (i.e. flux). The measurement requirements for open ocean eddy correlation are: 1)Time response to capture fluctuations due to atmospheric turbulent motions (10-4 -1 Hz), 2)Sensitivity to discern fluctuations above background concentrations (~1 ppt level), 3)High-resolution motion-corrected vertical wind speed measurements synchronized to chemistry measurements.
Despite the amount of interest in this topic, direct air-sea fluxes of a variety of trace gases are not yet measured. This is largely due to the difficulty in performing these measurements, especially on a moving platform such as a research ship. In recent years, atmospheric pressure chemical ionization mass spectrometry (APCI-MS) has been developed to perform high-quality atmospheric eddy-correlation measurements. It has demonstrated the time response and sensitivity required to measure the turbulent fluctuations of DMS, acetone, and methanol in the atmosphere. In addition, the measurements from APCI-MS can be easily synchronized to wind and motion measurements and the instrument itself is not affected by ship motion.
This proposed research seeks to establish an APCI-MS eddy correlation flux measurement system at IFM-GEOMAR to directly determine the direction and magnitude of a variety of trace gas fluxes. This will be the first (non CO2) trace gas eddy correlation technique developed in Germany. In addition, the APCI-MS will be coupled to an equilibrator in order to measure surface ocean concentrations of the same trace gases. In this way, the gas transfer coefficient can be directly determined (k = F/ΔC). The system will be initially developed to measure DMS and low molecular weight alcohols, aldehydes, and ketones. The system will be further developed to measure organohalogen fluxes in collaboration with other researchers at IFM-GEOMAR (e.g. Dr. Birgit Quack). By measuring a range of gases, several constraints on gas exchange can be examined in tandem. The systems will be deployed on research ships to perform flux and concentration measurements.
Start
January, 2012
End
December, 2016
Funding (total)
-
Funding (GEOMAR)
625000
Funding body / Programme
-
Helmholtz-Gemeinschaft
/ Impuls- und Vernetzungsfond / Helmholtz Young Investigator Group
Coordination
null
Contact
