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.

摘要

Vertical fluxes of dimethyl sulfide (DMS) and sulfur dioxide (SO ₂) 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 measurements. The average DMS flux near the surface was (1.7 ± 0.2) × 1013 molecules m−2 s−1. A flux gradient for DMS generated a DMS surface flux of (2.3 ± 0.7) × 10 13 molecules in m−2 s−1 and a DMS entrainment flux of (7.5 ± 4.4) × 1012 molecules m−2 s−1. High resolution altitude profiles of DMS allowed for the determination of the DMS entrainment velocity (1.9 ± 1.1 cm/s). A SO₂ flux of (4.3 ± 1.9) × 1012 molecules m−2 s−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.