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Formic and acetic acids measurements made during the year 1986 are reported for eight sites in the Los Angeles basin and one remote offshore site. Formic and acetic acids concentrations measured in marine air upwind of the Los Angeles area over an 8-month period average 1.4 and 0.6 ppb, respectively, while concentrations within the Los Angeles urban area average 2.7−5.8 ppb formic acid and 2.9−4.2 ppb acetic acid. Average formic and acetic acids concentrations exceed average HNO3 and HCl concentra tions, making them the most abundant gas phase acids in the southern California atmosphere throughout the year. Formic and acetic 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 as motor vehicle traffic. Downwind of Los Angeles, formic and acetic acids concentrations peak during the summer photochemi cal smog season, and concentration changes track both changes in atmospheric oxidant concentrations 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 the relative importance of these pathways varying spatially over the area surveyed.