FORMALDEHYDE PRODUCTION IN CLEAN MARINE AIR

The atmospheric mixing ratio of formaldehyde (HCHO) was measured conti nuously in clean marine air at the Australian Baseline Station at Cape Grim between mid November and mid December 1993. A diurnal cycle in m ixing ratio was observed, consistent in amplitude with the expected ph otochemical source of this species. However the absolute values of the HCHO mixing ratio were higher than expected if the major source of HC HO under clean, low-NOx conditions is photolysis of methyl hydroperoxi de (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 hydropero xy (HO2) and methylhydroperoxy (CH3O2) radicals may not proceed with 1 00% efficiency to form CH3OOH, but may have an additional branch yield ing HCHO in clean marine conditions. There is some evidence from labor atory studies consistent with this proposal.