OZONE OBSERVATIONS AND A MODEL OF MARINE BOUNDARY-LAYER PHOTOCHEMISTRY DURING SAGA-3

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 betwee n Hilo, Hawaii, and Pago-Pago, American Samoa, in February and March 1 990, shipboard measurements were made Of O3, CO, CH4, nonmethane hydro carbons (NMHC), NO, dimethyl sulfide (DMS), H2S, H2O2, organic peroxid es, and total column O3. Postcruise analysis was performed for alkyl n itrates and a second set of nonmethane hydrocarbons. A latitudinal gra dient 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) a nd less than 12 ppbv south of the ITCZ but never less-than-or-equal-to 3 ppbv as observed on some previous equatorial Pacific cruises (Piotr owicz et al., 1986; Johnson et al., 1990). Total column O3 (230-250 Do bson units (DU)) measured from the Akademik Korolev was within 8% of t he corresponding total ozone mapping spectrometer (TOMS) satellite obs ervations and confirmed the equatorial Pacific as a low O3 region. In terms of number of constituents measured, SAGA 3 may be the most photo chemically complete at-sea experiment to date. A one-dimensional photo chemical model gives a self-consistent picture of O3-NO-CO-hydrocarbon interactions taking place during SAGA 3. At typical equatorial condit ions, mean O3 is 10 ppbv with a 10-15% diurnal variation and maximum n ear sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain mode l-calculated OH to 9 x 10(5) cm-3 for 10 ppbv O3 at the equator. For D MS (300-400 parts per trillion by volume (pptv)) this OH abundance req uires a sea-to-air flux of 6-8 x 10(9) cm-2 s-1, which is within the u ncertainty range of the flux deduced from SAGA 3 measurements of DMS i n seawater (Bates et al., this issue). The concentrations of alkyl nit rates on SAGA 3 (5-15 pptv total alkyl nitrates) were up to 6 times hi gher than expected from currently accepted kinetics, suggesting a larg ely 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.