Abstract
The atmospheric chemistry of sulfur dioxide over the tropical South Pacific Ocean is investigated by using results from field measurements and numerical models. Simultaneous real time measurements of sulfur dioxide and its biogenic precursor dimethylsulfide were made at 12°S, 135°W for a 6‐day period from March 3 through March 9, 1992. The mean SO2 and DMS mole fractions were 71 ± 56 pmol mol−1 (1σ) and 453 ± 93 pmol mol−1 (1σ) respectively. These concentrations are compared to those predicted by a time‐dependent photochemical box model of the marine boundary layer. Model estimates of the yield of SO2 from DMS oxidation range from 27% to 54%. Even with low yields, DMS is the dominant source of SO2 in this region. Estimates of vertical entrainment velocities based on the tropospheric ozone budget suggest that vertical entrainment is a minor source of SO2. The relative rates of various loss mechanisms for SO2 are dry deposition to the sea surface (58%), in‐cloud oxidation (9%), OH oxidation (5%), and uptake by sea‐salt aerosols (28%).
Highlights
Introduction et al, 1991;Bandyet al., 1992;Huebertet al., 1993;Thornton etal., 1995].WiththeexceptioonfthoseofFereketal. [1991], Dimethylsulfide (DMS) is ubiquitous in the marine thesemeasurementhsave shownsurprisingllyow SO2mole atmosphere.It is producedin the surfaceoceanby certain fractionsgiventheamountosfDMS present.Themeasurements speciesof phytoplanktonand is emittedto the atmosphere reportedbyFereketal. [1991] weremadefroman aircraftin the throughair/seaexchangeprocesses.In the marineboundary sameregionandduringthe sametimeperiodasthe shipboard layer,DMS is oxidizedby OH radicalsto producesulfuricacid, measurementrseportedby l/yonet al. [1991] andBandyet al
Andreaeet al. [1988] measuredDMS and SO2from an aircrai concentrationasndlaboratoryrate constantsF. or the datafrom over the Northeast Pacific Ocean. Their results suggesta DMS/SO2ratio of 6. It hasbeensuggestedthat thesehigh ratios may be due to a low yield of SO2/om the oxidationof DMS [Bandy et al, 1992]
A 50% increase in the rate of homogeneouosxidationof SO2wouldresultin a predictedyield of SO2 of 28% fromthe daytimeoxidationof DMS, andthe contributionof homogeneouosxidationto the totallossof SO2 would become 7%
Summary
Introduction et al, 1991;Bandyet al., 1992;Huebertet al., 1993;Thornton etal., 1995].WiththeexceptioonfthoseofFereketal. [1991], Dimethylsulfide (DMS) is ubiquitous in the marine thesemeasurementhsave shownsurprisingllyow SO2mole atmosphere.It is producedin the surfaceoceanby certain fractionsgiventheamountosfDMS present.Themeasurements speciesof phytoplanktonand is emittedto the atmosphere reportedbyFereketal. [1991] weremadefroman aircraftin the throughair/seaexchangeprocesses.In the marineboundary sameregionandduringthe sametimeperiodasthe shipboard layer,DMS is oxidizedby OH radicalsto producesulfuricacid, measurementrseportedby l/yonet al. [1991] andBandyet al. Oxidationof DMS is the largestbiogenicsourceof SO2in the Chemistry/Marine Aerosols and Gases Experiment remotemarineboundarylayer. Calculate the DMS oxidation rate based on model OH It hasbeensuggestedthat thesehigh ratios may be due to a low yield of SO2/om the oxidationof DMS [Bandy et al, 1992].
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