Determination of vertical fluxes of sulfur dioxide and dimethyl sulfide in the remote marine atmosphere by eddy correlation and an airborne isotopic dilution atmospheric pressure ionization mass spectrometer

Abstract

Vertical fluxes of dimethyl sulfide (DMS) and sulfur dioxide (SO2) were determined by eddy correlation and an isotopic dilution atmospheric pressure ionization mass spectrometer (APIMS) on an aircraft platform. The sampling frequency of the isotopic dilution APIMS ranged from 1 Hz to 25 Hz for real-time measurements. Measurements were made near the surface in the marine boundary layer to over 6 km in the free troposphere. The APIMS demonstrated an average sensitivity of 80 cps/pptv for DMS and SO2 with a signal-to-noise (S/N) ratio > 5. A lower limit of detection of 0.1 pptv in a one second integration period was also determined for DMS and SO2 in airborne atmospheric measurements. Use of the isotopic dilution technique provided an internal calibration of every ambient sample along with manifold conditioning for rapid and efficient transport of the ambient species through the manifold. As a result the eddy correlation flux determinations were accurate, precise, and reproducible. Laboratory results suggest the addition of ozone in excess of 45 ppbv to the APIMS sampling manifold to ensure unaltered formation of SO5- ion (the ion detected for SO2). Use of a Nafion dryer was warranted for sensitive APIMS detection of SO2 in humid air. A Nafion dryer along with an air temperature of 400°C was required for sensitive APIMS detection of DMS in humid air. Results from an initial airborne test deployment in continental air include a successful SO2 intercomparison between the APIMS and a GC/MS/ILS (isotopically labeled standard-gas chromatography/mass spectrometer). Fast SO2 measurements in thin moist layers and pollution plumes demonstrated the utility of the fast isotopic dilution APIMS technique. Results from a second airborne test deployment in the remote marine boundary layer include a successful DMS intercomparison and eddy correlation fluxes of DMS and SO2 from isotopic dilution APIMS. The average DMS flux near the surface was (1.7±0.2)X1013 molecules m-2 s-1. A flux gradient for DMS generated a DMS surface flux of (2.3±0.7)X1013 molecules m-2 s-1 and a DMS entrainment flux of (7.5±4.4)X1012 molecules m-2s-1. High resolution altitude profiles of DMS allowed for the determination of the DMS entrainment velocity (1.91±1.1 cm/s). A SO2 flux of (4.3±1.9)X1012 molecules m-2s-1 and a deposition velocity of 2.4±1.1 mm/s also was determined. All values reported are in good agreement with the literature.%%%%Ph.D., Chemistry – Drexel University, 2001