OZONE OBSERVATIONS AND A MODEL OF MARINE BOUNDARY-LAYER PHOTOCHEMISTRY DURING SAGA-3
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abstract
A major purpose of the third joint SovietAmerican 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 PagoPago, 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 1520 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 (230250 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 atsea experiment to date. A onedimensional photochemical model gives a selfconsistent picture of O3NOCOhydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O3 is 10 ppbv with a 1015% diurnal variation and maximum near sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain modelcalculated OH to 9 105 cm3 for 10 ppbv O3 at the equator. For DMS (300400 parts per trillion by volume (pptv)) this OH abundance requires a seatoair flux of 68 109 cm2 s1, 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 (515 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.
