Abstract. The atmospheric mixing ratio of formaldehyde (HCHO) was measured continuously in clean marine air at the Australian Baseline Station at Cape Grim between mid November and mid December 1993. A diurnal cycle in mixing ratio was observed, consistent in amplitude with the expected photochemical source of this species. However the absolute values of the HCHO mixing ratio were higher than expected if the major source of HCHO under clean, low-NOx conditions is photolysis of methyl hydroperoxide (CH3OOH) derived from oxidation of methane by OH radicals. Possible explanations for elevated HCHO levels are considered. One sufficient to explain the observed HCHO levels is that reaction between hydroperoxy (HO2) and methylhydroperoxy (CH302) radicals may not proceed with 100% efficiency to form CH3OOH, but may have an additional branch yielding HCHO in clean marine conditions. There is some evidence from laboratory studies consistent with this proposal.

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Abstract. Seasonal emissions of isoprene and other light hydrocarbons were measured from both an elevated dry site (hummock) and low wet site (flark) on a Sphagnum fen in Sweden, and diurnal emissions were measured from a wet site on a Sphagnum fen in Finland. Both fens are in the southern boreal zone. Samples were taken from a Teflon chamber with stainless steel canisters, and analysis was done by gas chromatography with flame ionization detection. Isoprene clearly dominated the flux of light hydrocarbons, while ethene, propene, and 2-butene, as well as traces of n-butane and propane, were also observed. The isoprene emission season extended from the spring thaw to the occurrence of ground frost in the fall. The highest fluxes of isoprene were observed from wetter sites 25,513

Atmospheric peroxides and formaldehyde were measured during the Polar Sunrise Experiment 1992 in Alert, Canada (82.5°N, 62.3°W). Two measurement periods, in the dark winter and in the sunlit spring, were chosen to investigate the effects of photochemistry. Continuous measurements were performed using diffusion scrubbers coupled to aqueous phase fluorometry. The concentration of total peroxides varied from below the detection limit (∼10 parts per trillion by volume (pptv)) to 40 pptv in the dark and 100 to 400 pptv in the sunlit period with large variations in the ratio between H2O2 and organic peroxides. The CH2O concentrations measured in the dark were between 100 and 700 pptv and showed good correlation with a number of atmospheric constituents such as CH4, CO2, and Rn but anticorrelation to O3. A fraction of the observed CH2O concentrations is believed to be formed by nonphotochemical O3‐alkene chemistry. In the presence of sunlight the CH2O concentrations ranged between 30 to 600 pptv without correlation to CH4, CO2, or Rn. The maximum CH2O concentrations were associated with air, depleted in O3, coming from the Arctic Ocean. During the O3 depletions, decreased peroxide concentrations were observed. The origin of the air mass was a very important factor during both periods in explaining the observed variabilities in CH2O and peroxide concentrations.