Concentrations, Deposition Rates and Sources of Organic Acids in Precipitation Collected in Higashi-Hiroshima, Japan

Takayuki MIYAKE, Kazuhiko TAKEDA, Kitao FUJIWARA and Hiroshi SAKUGAWA*

Graduate school of Biosphere Sciences, Hiroshima University; 1-7-1 Kagamiyama,
Higashi-Hiroshima-shi 739-8521 Japan
Faculty of Integrated Arts and Sciences, Hiroshima University; 1-7-1 Kagamiyama,
Higashi-Hiroshima-shi 739-8521 Japan

Precipitation samples (n = 188) were collected in Higashi-Hiroshima, Japan during September 1995--March 1999, and the concentration of organic acids such as formic, acetic and oxalic acids was measured by ion chromatography. Volume weighted average concentrations of organic acids in precipitation samples were 3.4μM for formic acid, 2.3μM for acetic acid and 0.24μM for oxalic acid, respectively. Volume weighted average annual depositions of these acids for three years (1996--1998) were 4.8 mmol m-2 year-1 for formic acid, 3.3 mmol m-2 year-1 for acetic acid and 0.32<+4>mmol m-2 year-1 for oxalic acid, respectively. Seasonal variations of volume weighted average concentrations and deposition rates were showed to be high in spring and summer seasons, and low in autumn and winter seasons, probably due to the variation of photochemical activities corresponding to solar intensity. Organic acids contributed 7.9% (in the range of 0.79--21.3%) of total acidic species (non-seasalt sulfuric acid+nitric acid+non-seasalt hydrochloric acid+organic acids) in volume weighted average and the percentages exhibited the same seasonal variations as the concentrations and deposition rates. The result of linear regression and principal component analyses indicate that organic acids well correlated with man-made acidic species such as nitrate and non-seasalt sulfate. Thus, organic acids in precipitation collected in Higashi-Hiroshima may originate mostly from anthropogenic sources, via the direct emission and/or formed through photochemical reactions of precursors like hydrocarbons and aldehydes. The calculated equilibrium concentrations of gas-phase formic and acetic acids are comparatively in good agreement with reported gas-phase concentrations measured in Higashi-Hiroshima.

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