Interpretation of the concentrations of aldehydes in rainwater over a wide area and local areas of Japan by some dominant factors

Rainwater pollution with formaldehyde, acetaldehyde and other aliphatic aldehydes was investigated from May 2008 to June 2011 at 19 locations over a wide area in Japan to survey the pollution level. The pollution level was remarkably controlled by the amount of precipitation. The concentrations of total aliphatic aldehydes and formaldehyde sometimes exceeded 0.6 mg L−1 and 0.2 mg L−1 when the rainfall was small, and decreased rapidly with increasing the amount of rainfall. And in larger rainfall, the concentrations were converged to ca. 0.09 mg L−1 and 0.06 mg L−1 respectively. It was found that the concentration of the pollutants collected in a bottle for a single rainfall event is expressed by an equation based on a dilution model where heavy deposition occurred only at early rainfall followed by continuous dilution of the water with less polluted rainwater. These results were interpreted that the aldehydes in air were washed out very quickly due to their high solubility in water. As progressing of rainfall, the pollution level attained to an equilibrium state between influx and outflux of the pollutants. Precise investigation through a sequential sampling of rainwater by 1 mm during a rainfall event was also performed at several sites. It was found that a break of rainfall is another dominant factor to control the pollution level. A break of rainfall quickly enhanced the pollution level of the resumed rain. Regional difference on rainwater pollution level was observed only at the early stage throughout a rainfall event. The air-pollution levels were high just before beginning rainfall, and they decreased quickly with increasing the amount of rainfall, but never converged to zero. These results suggest that continuous influx of the pollutants occurs in a wide area in Japan besides regional influx.