Precipitation Chemistry Trends In The Northeastern United States

Historical trends of the concentrations of the various trace substances in precipitation can serve as an indication of the changes in the levels of these substances in the air. Three topics related to the precipitation chemistry data base for the United States are examined. First, since the pH of precipitation is of major interest, techniques to calculate this quantity are considered. Next, the changes in the precipitation chemistry for a central Illinois site are discussed. This site is at the western edge of present high atmospheric sulfate levels, and thus it is interesting to examine changes in the precipitation pH at this site and the reasons for them. Changes in the sulfate and nitrate at this central Illinois site and other sites further East are also discussed, and these changes are compared to the time trends in the anthropogenic source emissions. Results suggest that at present the best approach to calculating the pH of a precipitation sample is to assume that a charge balance exists and then to apply an empirical correction. With this approach the 1977 central Illinois samples are shown to be much more acid than those from 1954. The primary reason for this pH decrease is the decreased concentration of calcium and magnesium in the samples. For the mid-1970's compared to the mid-1950's the source emissions estimates for the United States were up by a factor of 1.3 for sulfur and 2.1 for nitrogen. During this same time interval the sulfate concentrations for the precipitation chemistry sites in the northeastern United States remained about the same and the nitrate concentrations increased by factors varying from 1.3 to 1.8. Since the emissions estimates increased more than the precipitation concentrations, it would seem that a smaller fraction of the pollutants were being removed over the northeast by wet processes in the mid-1970's than in the mid-1950's.

This is the twelfth in a series of conferences on environmental toxicity sponsored by the Department of Radiation Biology and Bio physics. The topics selected are intended to be of timely interest with a focus somewhat different from those with with similar titles. I would like to share with you some of the thought which went into the development of this year's program. Everyone here is aware of the great amount of publicity being given to some phase of this year's topic of "Polluted Rain", but I wonder how many of the younger generation realize how the entire world got into this kind of predicament. I can remember as a young ster that precipitation from the sky in the form of rain or snow was considered one of the ultimates in purity. That was in the era when a small number of automobiles were in use and the airplane was more of a curiosity in the developmental stage than anything of commer cial significance. Many homes used iceboxes for refrigeration, and summer cooling was accomplished by electric fans by the more afflu ent families.