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How Much CO2 Is in Rain Water?

Perhaps it would be too simple to assume rainwater consists of only molecules containing two hydrogen atoms and one oxygen atom. H2O, however, reacts with gases in the atmosphere to form rain that is more acidic than pure water. While pure water has a pH of 7.0, which is neutral, rainwater has a pH of about 5.65, which is considered acidic. The acidity of water comes from gases like carbon dioxide.
  1. CO2 Concentrations

    • Carbon dioxide is the major contributor of natural acidity in rainwater, as it is present in a much higher concentration than gases like nitrous oxide (NO2) and sulfur dioxide (SO2). The concentration of CO2 in rainwater is about 355 parts per million, according to Rachel Casiday and Regina Frey of the Washington University Department of Chemistry. While 0.0355 percent seems like a small amount, it is enough CO2 to significantly lower the pH of water.

    Where CO2 Enters

    • CO2 combines with rainwater in the atmosphere, and it often reacts with the water to form carbonic acid (H2CO3). Carbonic acid can dissociate to produce the hydrogen ion (H+) and the hydrogen carbonate ion (HCO3-). The presence of the hydrogen ion classifies the molecule as an acid, which in turn lowers the pH of rainwater.

    Polluted Rainwater

    • While CO2 concentration is at an average of 355 parts per million in natural rainwater, human activity can increase levels of CO2, thus making the pH of polluted rainwater more acidic. According to Casiday and Frey, the pH of some rainwater in the United States could be 3.0 or lower. Nitrous oxide and sulfur dioxide, while present in lower concentrations than CO2, form stronger acids, which contribute greatly to the acidity of rainwater.

    Acid Rain

    • The carbon dioxide found in rainwater is in equilibrium with the atmosphere, which gives water a pH of around 5.6. According to the Environmental Protection Agency, however, typical acid rain has a pH of around 4.0. While the acidification from CO2 in rainwater is considered normal, the additional increase of acidity from SO2 and NO2 is of growing concern for the health of the environment. Acid rain contributes to the acidification of soil, which damages plants and forests. Acid rain also affects aquatic ecosystems and contributes to decay of buildings and landmarks.


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