Ozone observations and a model of marine boundary layer photochemistry during SAGA 3

Abstract

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 × 109 cm⁻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.