Acid rain is the phenomenon in which oxidized atmospheric pollutants are scavenged by raindrops. By short time observations of ion concentration changes in rainwater, various information about ions in the atmosphere is obtained. In this study, short time and continuous observations of acid rain were performed with an automatic rain analyzer which was originally remodeled. A change in ion concentration in the rainwater as well as a change in the air pollution gas concentration and weather, were observed at a same time in order to understand the ion scavenging mechanism. During the early stage of precipitation, highly concentrated ions in rainwater were observed due to acidic air contaminants formed in the atmosphere, and ion concentrations fell as a result of the ion scavenging by the rain with time. A decreasing ratio of ion concentration was found to be different based on ionic species, and the deposition velocity of sulfate, which was suspended as a small particle in the air, was quantitatively obtained and was clearly small when compared with nitrate. The deposition velocity of nitrate was considered to be difficult to measure because of its local distribution. However, it could be estimated from the nitrate concentration change in rainwater under calm weather conditions when no of change in the wind direction or the transportation of pollution occurred. The deposition velocity constant could be expressed as a linear function of the rainfall intensity in this study, and evaluation of the ion deposition flux during gentle precipitation or acidic mist, which were not able to be previously expressed by a power function, became possible. A change in ion concentration and pH of the sequel could be predicted by knowing the initial concentration and rainfall intensity changes, and these results were in good agreement with the observations.

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