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%).

Simultaneous measurements are reported of the nitrate radical (NO₃), nitrogen dioxide (NO₂), ozone (O₃), and dimethylsulfide (DMS) in the nighttime marine boundary layer over Biscayne Bay in South Florida. These field observations are analyzed and used to initialize a boundary layer box model which examines the relative importance of the various sinks for NO_x in the marine boundary layer. The results show that the observed lifetime of NO₃ (≤6 min.) is probably controlled both by the loss of nitrogen pentoxide (N₂O₅) to reaction with water vapor and aerosols and by the reaction between NO₃ and DMS. The model is then extended to investigate the loss of nitrogen oxides from an air parcel that remains in the boundary layer with a constant sea-to-air DMS flux for several days. The principal conclusions are (1) that DMS is a much more important sink for NO₃ at lower NO₂ levels and (2) that the reaction between NO₃ and DMS is an important sink for DMS in the marine boundary layer and could exceed that of the daytime removal by OH.

Shipboard measurements of atmospheric and seawater DMS were made at 12°S, 135°W for 6 days during March 1992. The mean seawater DMS concentration during this period was 4.1 ± 0.45 nM (1σ, n = 260) and the mean atmospheric DMS mole fraction was 453 ± 93 pmol mol⁻¹ (1σ, n = 843). Consistent atmospheric diel cycles were observed, with a nighttime maximum and daytime minimum and an amplitude of approximately 85 pmol mol⁻¹. Photochemical box model calculations were made to test the sensitivity of atmospheric DMS concentrations to the following parameters: 1) sea-to-air flux, 2) boundary layer height, 3) oxidation rate, and 4) vertical entrainment velocities. The observed relationship between the mean oceanic and atmospheric DMS levels require the use of an air-sea exchange coefficient which is at the upper limit end of the range of commonly used parameterizations. The amplitude of the diel cycle in atmospheric DMS is significantly larger than that predicted by a photochemical model. This suggests that the sea-to-air DMS flux is higher than was previously thought, and the rate of daytime oxidation of DMS is substantially underestimated by current photochemical models of DMS oxidation.

A major purpose of the third joint Soviet-American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O₃ and photochemical cycles in detail. On leg 1, which took place between Hilo, Hawaii, and Pago-Pago, American Samoa, in February and March 1990, shipboard measurements were made of O₃, CO, CH₄, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), H₂S, H₂O₂, organic peroxides, and total column O₃. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O₃ was observed on SAGA 3, with O₃ north of the intertropical convergence zone (ITCZ) at 15–20 parts per billion by volume (ppbv) and less than 12 ppbv south of the ITCZ but never ≤3 ppbv as observed on some previous equatorial Pacific cruises (Piotrowicz et al., 1986; Johnson et al., 1990). Total column O₃ (230–250 Dobson units (DU)) measured from the Akademik Korolev was within 8% of the corresponding total ozone mapping spectrometer (TOMS) satellite observations and confirmed the equatorial Pacific as a low O₃ region. In terms of number of constituents measured, SAGA 3 may be the most photochemically complete at-sea experiment to date. A one-dimensional photochemical model gives a self-consistent picture of O₃-NO-CO-hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O₃ is 10 ppbv with a 10–15% diurnal variation and maximum near sunrise. Measurements of O₃, CO, CH₄, NMHC, and H₂O constrain model-calculated OH to 9 × 10⁵ cm⁻³ for 10 ppbv O₃ at the equator. For DMS (300–400 parts per trillion by volume (pptv)) this OH abundance requires a sea-to-air flux of 6–8 × 10⁹cm⁻² s⁻¹, which is within the uncertainty range of the flux deduced from SAGA 3 measurements of DMS in seawater (Bates et al., this issue). The concentrations of alkyl nitrates on SAGA 3 (5–15 pptv total alkyl nitrates) were up to 6 times higher than expected from currently accepted kinetics, suggesting a largely continental source for these species. However, maxima in isopropyl nitrate and bromoform near the equator (Atlas et al., this issue) as well as for nitric oxide (Torres and Thompson, this issue) may signify photochemical and biological sources of these species.