The sulfur dioxide budget in the tropical South Pacific marine boundary layer (12S, 135W)

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D3, PAGES 6911-6918, MARCH 20, 1996 Atmospheric sulfur cycling in the tropical Pacific marine boundary layer (12oS, 135oW): A comparison of field data and model results 2. Sulfur dioxide S. A. Yvon 1 and E. S. Saltzman Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 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 12oS, 135oW for a 6-day period from March 3 through March 9, 1992. The mean SO2 and DMS mole fractions were 71 _+ 56 pmol mol 4 (lc0 and 453 _+ 93 pmol mo1-1 (lc 0 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 SO• in this region. Estimates of vertical entrainment velocities based on the tropospheric ozone budget suggest that vertical entrainment is a minor source of SO:. The relative rates of various loss mechanisms for SO: are dry deposition to the sea surface (58%), in-cloud oxidation (9%), OH oxidation (5%), and uptake by sea-salt aerosols (28%). Introduction et al., 1991; Bandy et al., 1992; Huebert et al., 1993; Thornton et al., 1995]. With the exception of those ofFerek et al. [1991], Dimethylsulfide (DMS) is ubiquitous in the marine these measurements have shown surprisingly low SO2 mole atmosphere. It is produced in the surface ocean by certain fractions given the amounts of DMS present. The measurements species of phytoplankton and is emitted to the atmosphere reported by Ferek et al. [1991 ] were made from an aircraft in the through air/sea exchange processes. In the marine boundary same region and during the same time period as the shipboard layer, DMS is oxidized by OH radicals to produce sulfuric acid, measurements reported by l/yon et al. [1991] and Bandy et al. methanesulfonic acid (MSA), and possibly dimethylsulfoxide [1992] as part of the Pacific Stratus/Sulfur Investigation (PSI 3). (DMSO) and dimethylsulfone (DMSO2) [I/in et al., 1990a, b, The above mentioned studies focused on the spatial rather than and references therein]. The relative yields of these products in temporal distributions of these species. Of the various methods the remote marine boundary layer is not well established owing employed in the studies mentioned above, only the methods used to the difficulties in creating a laboratory environment that is by l/yon et al. [1991] and Bandy et al. [1992] utilize sampling suitably similar to that found over the remote oceans. times that are short enough to allow the results to be examined Sulfur dioxide (SO2) is an intermediate in the production of for diel cycles and short timescale trends. sulfuric acid from DMS oxidation in the marine boundary layer. Simultaneous measurements of atmospheric DMS and SO2 It is this SO2 and its oxidation product sulfuric acid that are were made during a recent cruise aboard the R/V John V. thought to be the major sources of non-sea-salt sulfate (nss Vickers in the equatorial Pacific Ocean. This field campaign sulfate) in aerosols in the remote marine boundary layer. was part of the International Global Atmospheric Oxidation of DMS is the largest biogenic source of SO2 in the Chemistry/Marine Aerosols and Gases Experiment remote marine boundary layer. Hydrogen sulfide (H2S), another (IGAC/MAGE). A station at 12oS, 135oW was occupied for 6 biogenic reduced sulfur gas, also oxidizes to form SO2. days (Julian days 64.0-70.5, GMT) in March 1992 in order to However, H2S is present only at very low concentrations over the study the diel cycles of these and other trace atmospheric and remote open ocean and is not considered to be a significant surface seawater species. The results of these atmospheric DMS source of SO2 [Saltzman and Cooper, 1988; l/yon et al., 1993]. and SO2 measurements are investigated by using a gas phase Recently, simultaneous measurements of atmospheric DMS photochemical box model [l/yon and $altzman, 1993]. and SO2 have been made in the marine boundary layer [Berresheim 1987; Andreae et al., 1988; Ferek et al., 1991; l/yon Experimental Methods 1Now at National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado. Copyright 1996 by the American Geophysical Union. Paper number 95JD03355. 0148-0227/96/95 JD-03355505.00 The sample inlets were mounted approximately 15 m above sea level on the forward facing side of a laboratory van which was bolted to the deck of the flying bridge. The van was located forward of the ship's stacks. The ship was positioned with its bow facing into the wind while we were at the station to prevent contamination of the air samples by stack gases. Samples were