Introduction Page 13
Instead of measuring pH values close to 5.6, expected in natural precipitation, scientists were frequently recording values between 4.0 and 4.2. In fact, they once measured a rain sample with a pH of 3.0 - the same pH as orange juice!
The precipitation they were collecting was slightly acidic.
The scientists who discovered acid precipitation (sometimes referred to as acid rain) were not initially interested in precipitation chemistry. Rather, they were conducting research on biogeochemical cycles of the northern hardwood forest. Nevertheless, as is often the case with long-term research, these interesting results were noticed - and thus began a study that has resulted in the longest dataset on acid deposition in the entire country.
The pH scale
The measure of the acidity of a liquid sample is referred to as "pH." Acidic substances (like lemon juice) have low pH values, while basic substances (like ammonia) have high pH values. The pH scale runs from 0 (acid) to 14 (basic), with 7 being neutral.
Natural rainfall that has not been affected by humans is slightly acidic, and would have a pH of ~ 5.6 in the HBEF. "Acid precipitation" refers to precipitation that has a pH lower than 5.6 because of human influences such as the burning of fossil fuels (coal, oil, and gasoline).
Measuring acid precipitation at the HBEF
Since 1965 scientists and their assistants have collected precipitation samples throughout the entire HBEF. Once a week technicians walk, hike, drive, 4-wheel, or snowmobile out into the woods and carefully collect samples of precipitation that has fallen on the HBEF during the previous 7 days. Then the samples are brought back to the lab where pH values are measured and then entered into the long-term database. Extra samples are also archived for future study.
The utmost care is taken when collecting and analyzing precipitation samples. First, rain collectors similar to the one on the left were placed throughout the HBEF. Scientists are careful to maintain the quality of these collectors by keeping them clean and in good repair.
The collectors are designed to allow rain to enter the funnel and flow through it into a clean bottle located in the metal column below. Each week a technician removes a bottle containing the sample and places a new, clean bottle at the bottom of the funnel to collect the next week's rain. Each bottle has an overflow valve and is designed to prevent evaporation (and loss of sample).
Before being taken into the field each bottle is very carefully washed, using a mild acid solution to remove contaminants, and then thoroughly rinsed. Bottles are wrapped in clean plastic bags for transport to the precipitation collectors.
Of course, as you might imagine, there can be problems: leaves and other contaminants fall into the funnels, and it's often raining on sampling days. Can you think of other problems with sampling precipitation? How would you solve some of these problems?
To learn more about acid precipitation at the HBEF, click here.