Determining the sea-air flux of dimethylsulfide by eddy correlation using mass spectrometry

Abstract

Abstract. Mass spectrometric measurement of DMS by atmospheric pressure ionization with an isotopically labeled standard (APIMS-ILS) is a sensitive method with sufficient bandpass for direct flux measurements by eddy correlation. Use of an isotopically labeled internal standard greatly reduces instrumental drift, improving accuracy and precision. APIMS-ILS has been used in several recent campaigns to study ocean-atmosphere gas transfer and the chemical budget of DMS in the marine boundary layer. This paper provides a comprehensive description of the method and errors associated with DMS flux measurement from ship platforms. The APIMS-ILS instrument used by most groups today has a sensitivity of 100-200 counts s −1 pptv −1 , which is shown to be more than sufficient for flux measurement by eddy covariance. Mass spectral backgrounds (blanks) are determined by stripping DMS from ambient air with gold. The instrument is found to exhibit some high frequency signal loss, with a half-power frequency of ≈1 Hz, but a correction based on an empirically determined instrument response function is presented. Standard micrometeorological assumptions of steady state and horizontal uniformity are found to be appropriate for DMS flux measurement, but rapid changes in mean DMS mixing ratio may serve as a warning that measured flux does not represent the true surface flux. In addition, bias in surface flux estimates arising from the flux divergence is not generally significant in the surface layer, but under conditions of lowered inversion and high flux may become so. The effects of error in motion corrections and of vertical motion within the surface layer concentration gradient are discussed and the estimated maximum error from these effects is ≤18%.

Using eddy covariance to measure the dependence of air–sea CO<sub>2</sub> exchange rate on friction velocity

“…The flux loss was estimated as the ratio of the cumulative sums counting from low to high frequencies (ogives) at n = 0.3 Hz (Marandino et al, 2007;Blomquist et al, 2010). Highfrequency fluctuations of CO 2 are attenuated by the measurement system due to passage of air through the intake tubing, drier, and closed-path detector.…”

“…The last assumption was tested by using slightly higher and lower frequencies for the loss estimation. The CO 2 flux data were corrected by applying a gain (g CO 2 ) computed as the ratio of the cumulative sums (from low to high frequencies) of sensible heat and CO 2 cospectra at n = 0.3 Hz (Marandino et al, 2007;Blomquist et al, 2010). In Fig.…”

Direct measurement of the oceanic carbon monoxide flux by eddy correlation

“…A short pulse of nitrogen injected at the inlet tip each hour facilitated precise synchronization of wind and CO measurements. Procedures for correcting wind data for ship motion interference have been described previously (Edson et al, 1998;Blomquist et al, 2010).…”

Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes

“…Values of σ 2 C a were calculated as the second point of the autocovariance of the concentration signal (Yang et al, 2016). Blomquist et al (2010) estimated τ wc using the peak frequency of the variance cospectrum. Peak frequencies here were modelled via the equation given in Horst (1997).…”

“…Flux detection limits (δF ) were calculated using the method described in Blomquist et al (2010) and Yang et al (2016) where the detection limit is calculated using the instrument noise and the variability of the concentration signal:…”