FY 1993

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

Thompson, A.M., J.E. Johnson, A.L. Torres, T.S. Bates, K.C. Kelly, E. Atlas, J.P. Greenberg, N.M. Donahue, S.A. Yvon, E.S. Saltzman, B.G. Heikes, B.W. Mosher, A.A. Shashkov, and V.I. Yegorov

J. Geophys. Res., 98(D9), 16,955–16,968 (1993)

A major purpose of the third joint Soviet-American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O 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 O, CO, CH, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), HS, HO, organic peroxides, and total column O. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O was observed on SAGA 3, with O 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 O (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 O 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 O-NO-CO-hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O is 10 ppbv with a 10-15% diurnal variation and maximum near sunrise. Measurements of O, CO, CH, NMHC, and HO constrain model-calculated OH to 9 × 10 cm for 10 ppbv O 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 × 10 cm s, 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.