Long-term trend of chemical constituents in precipitation in Tokyo metropolitan area, Japan, from 1990 to 2002.

In order to understand the actual status and mechanism of acid rain, it is important to know the pH of precipitation and its chemical constituents on a continuous and regular basis over a wide area. This study examines acid rain over a wide area using an observational network in the Tokyo metropolitan area of Japan, and analyzes the major chemical constituents of every precipitation sample. Precipitation was collected continuously for a period of 12 years from June 1990 to May 2002 at several sampling sites in the Tokyo metropolitan area, and its pH and chemical constituent concentrations were measured. The average pH ranged from 4.23 to 4.62, clearly indicating acidification of precipitation over the entire Tokyo metropolitan area. A time-trend model was applied to describe temporal variations of chemical constituent concentrations, including annual change rate, seasonal variation, and precipitation effects. Seasonal and annual trends for the past 12 years were examined with the model, using the least squares method. Nonsea salt (nss)-Ca2+ shows a maximum value in early spring, a seasonality probably caused by calcium-rich particles in airborne yellow dust from Asia. Slightly decreasing annual trends of nss-SO4(2-) may correspond to the recent decreasing trend of atmospheric SO2 gas concentrations in the Tokyo metropolitan area. The annual trends of NO3-, NH4+, and nss-Ca2+ show a large site-to-site difference. The increasing NO3-, NH4+, and nss-Ca2+ concentrations at inland suburban sites may be caused by increases in their local sources such as vehicle traffic and municipal waste incineration. The annual change rate of H+ is slightly negative or almost zero at every site, so the acidification of precipitation has not become worse since 1990 over the Tokyo metropolitan area.