SEASONAL AND SPATIAL CHARACTERISTICS OF FORMIC AND ACETIC-ACIDS CONCENTRATIONS IN THE SOUTHERN CALIFORNIA ATMOSPHERE

Formic and acetic acids measurements made during the year 1986 are rep orted for eight sites in the Los Angeles basin and one remote offshore site. Formic and acetic acids concentrations measured in marine air u pwind of the Las Angeles area over an 8-month period average 1.4 and 0 .6 ppb, respectively, while concentrations within the Los Angeles urba n area average 2.7-5.8 ppb formic acid and 2.9-4.2 ppb acetic acid. Av erage formic and acetic acids concentrations exceed average HNO3 and H Cl concentrations, making them the most abundant gas phase acids in th e southern California atmosphere throughout the year. Formic and aceti c acids concentrations near the coast change in proportion to changes in atmospheric dilution potential, as would be expected if formic and acetic acids were emitted directly from widespread area sources such a s motor vehicle traffic. Downwind of Los Angeles, formic and acetic ac ids concentrations peak during the summer photochemical smog season, a nd concentration changes track both changes in atmospheric oxidant con centrations and markers for heterogeneous conversion within clouds or fog. Formic and acetic acids concentrations thus appear to arise both from direct emissions and from atmospheric chemical production, with t he relative importance of these pathways varying spatially over the ar ea surveyed.