understanding acid rain

When Gene Likens discovered that rainwater collected in the Hubbard Brook Experimental Forest in 1963 had a pH of 3.7, this began a long series of efforts to understand why this rain was so acidic, how this acidity compared to other places on earth, and what the source of this acidity was. It took more than a decade for researchers to establish that this rainwater acidity was not natural (rain in the most remote regions of the Earth typically is 100x less acidic than samples collected from Hubbard Brook in the 1960s) and that the source of this acidity in the rain falling in central New Hampshire was from the smokestacks of coal fired power plants in the Ohio River valley. (read more about this discovery here)

FIGURE 1 – Hydrogen ion concentrations as pH from measurements made by the National Atmospheric Deposition Program in a) 1985 and b) 2015. Note that each dot represents a precipitation monitoring station. Maps for all years available at http://nadp.isws.illinois.edu/maplib

Have we fixed the problem of acid rain?

The answer is not everywhere and not quite.

Not Everywhere:

While environmental regulations to reduce the production of volatiles from fossil fuel combustion have significantly reduced the problem of acid rain in North American and eastern Europe, the problem of acid rain is on the rise in other parts of the world (Klimont et al. 2013) where fossil fuel combustion is increasing without corresponding regulatory protections for air quality (Figure 2).

FIGURE 2. A map of the change in global sulfur dioxide (SO2) emissions between 2005 and 2010. Note the decline throughout North American and Europe and the widespread increase in southeast Asia (this figure is from Klimont et al. 2013 and is available at this link.

Not Quite:

Even in the places, like Hubbard Brook, where the acidity of rain has returned to a more natural range (pH ~ 5, Figure 1B), the forests and the streams have been altered by decades of acid rain. As sulfuric acid rain fell onto soils, it acidified the soil and leached important plant nutrients such as calcium and magnesium and toxic metals (such as Al) into receiving streams. Over time, these important nutrients and trace elements have been depleted from the soil, limiting forest growth and contributing to toxicity issues in affected rivers and lakes. It will take many decades for rock weathering to replenish the supply of these critical elements for forest growth and aquatic food webs. You can see what happens when these acid rain impacted forests were experimentally fertilized with calcium by going to the Calcium Experiment data story on our webpage. There is encouraging news from a recent study by Lawrence et al.

What to look for in this data story

You can see the change over time in the pH and sulfur content of rain and the corresponding changes in the chemistry of receiving streams. As soil nutrients were depleted, fewer and fewer soil minerals were available to be leached into streams and thus streamwater grew increasingly dilute over time.

Was there an effect of the Clean Air Act of 1970 or the Clean Air Act Amendments of 1990 on the amount of H+ or SO4 (the constituents of sulfuric acid) in precipitation?

What is more acidic, rain or stream water? How have both changed over time?

How much calcium and magnesium enters these forests in rain? How much is leaving in streams? How has this changed over time?


Free internet resources

From US EPA, What is Acid Rain?


Holmes, R. T., & Likens, G. E. (2016). Hubbard Brook: The story of a forest ecosystem. New Haven: Yale University Press.

Key Papers

Likens, G.E., R.F. Wright, J.N. Galloway, and T.J. Butler. Acid Rain Scientific American 241: 43-51

Likens, G.E., C.T. Driscoll and D.C. Buso. 1996. Long-term effects of Acid Rain: Response and Recovery of a Forest Ecosystem. Science 272 244-246

How does pH change when acid rain is mitigated?

How does acid rain affect solute concentrations?

How have policies altered the effects of acid rain? (As seen in ws6)

Acid Rain Key Events Timeline