Concentrations of formic and acetic acids in the marine atomsphere were measured at the route of an oil tanker over the East China Sea, South China Sea, Bay of Bengal, Arabian Sea, and Persian Gulf during July-September, 1991 in order to investigate their behavior over the ocean. Formic and acetic acids were measured by an automatic measurement system which consisted of a mist chamber, an ion chromatograph, a timer and so on. As a result, average concentrations in the marine atmosphere were 1.18ppbv (n = 1068) for formic acid, 0.60ppbv (n = 1061) for acetic acid. Concentration of formic acid was higher than that of acetic acid in all sea areas. Concentrations of formic and acetic acids increased in the daytime and decreased in the nighttime. Therefore, it is considered that these acids are produced by photochemical reaction. A good correlation between concentrations of formic and acetic acids was observed in all sea areas. mAs for the source of these acids in the marine atmosphere, it was found that the effect of the transport from free troposphere or land is larger than local photochemical reactions. On the other hand, the sink of these acids was mainly dry deposition except for wet deposition, and a gas-phase destruction was a minor sink. Taking into acount air-sea gas exchange for formic and acetic acids, the net flux of these gases was from the atmosphere into the ocean, while those of carbon dioxide, carbon monoxide and methane were from the ocean to the atmosphere. Therefore, it is considered that formic and acetic acids have an important role in the global carbon cycle, that is, these acids are impotant sources of carbon into the ocean.
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