5.5.1 Acid Rain

Acid rain is a serious environmental problem around the world, particularly affecting Asia, Europe, and large parts of the U.S. and Canada. The acidic pollutants such as SO₂ and NO_x are emitted into the environment by combusting fossil fuels.

Most of the sulfur in any fuel combines with oxygen and forms SO₂ in the combustion chamber. This SO_2, when emitted into the atmosphere, slowly oxidizes to SO₃. SO₃ is readily soluble in water in the clouds and forms H₂SO₄ (sulfuric acid).

$${\text{S + O}}_{\text{2}}{\text{ → SO}}_{\text{2}}{\text{ + 1/2 O}}_{\text{2}}{\text{ (in the atmosphere) → SO}}_{\text{3}}{\text{ +\hspace{0.17em}H}}_{\text{2}}{\text{O → H}}_{\text{2}}{\text{SO}}_{\text{4}}\text{ (sulfuric acid)}$$

Most of the NO_x that is emitted is in the form of NO. This NO is oxidized in the atmosphere to NO₂. NO₂ is soluble in water and forms HNO₃ (nitric acid).

$${\text{NO + 1/2 O}}_{\text{2}}{\text{ (in the atmosphere) → NO}}_{\text{2}}{\text{ +\hspace{0.17em}H}}_{\text{2}}{\text{O → HNO}}_{\text{3}}\text{ (nitric acid) }$$

Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3. By the 2020's most locations of the US have precipitation with pH of 5.0-5.5. 

Below is a video demonstration that replicates the effect of acid rain on plant life. In this video, beans are placed in: a) water, b) slightly acidic water and c) acidic water, and their growth is observed over a period of three days. Please watch the following 5:35 video:

Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. However, the pollutants that cause acid rain also damage human health.

• Effects of Sulfur Dioxide (SO₂): These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled deep into people's lungs. Fine particles can also penetrate indoors. Many scientific studies have identified a relationship between elevated levels of fine particles and increased illness and premature death from heart and lung disorders, such as asthma and bronchitis.
• Effects of Nitrogen Oxide (NO_x): Decrease in nitrogen oxide emissions are also expected to have a beneficial impact on human health by reducing the nitrogen oxides available to react with volatile organic compounds and form ozone. Ozone impacts on human health include a number of morbidity and mortality risks associated with lung inflammation, including asthma and emphysema.

Text description of the Annual Wet Sulfate Deposition image.

The image is a comparison of two maps of the United States showing annual wet sulfate (SO₄²⁻) deposition over two different time periods, 1989-1991 and 2020-2022. Each map displays the continental U.S. with color gradients representing the levels of sulfate deposition.

On the left, the map from 1989-1991 shows high levels of sulfate deposition concentrated mostly in the eastern and central regions, especially pronounced in the Ohio Valley and surrounding areas, depicted in dark red and orange, indicating higher levels. The western regions are shown in lighter greens and yellows, indicating lower levels of deposition.

On the right, the map from 2020-2022 has a more uniform teal color across the entire country, suggesting significantly reduced sulfate deposition levels compared to the earlier period. This illustrates a substantial improvement in air quality over time.

Credit: Acid Rain Program Results. NADP, PRISM, USEPA. Accessed May 29, 2026.

Text description of the Annual Sulfur Dioxide Emissions image.

The image is a bar and line graph titled "Annual Sulfur Dioxide Emissions, 1990–2020." It displays data on sulfur dioxide emissions and gross electricity generation in the United States over the period of 1990 to 2020. The vertical axis on the left measures sulfur dioxide emissions in million short tons, ranging from 0 to 17.5, while the right vertical axis measures gross generation in billion megawatt-hours (MWh), ranging from 0 to 3.5. The bars in blue represent the sulfur dioxide emissions for each year, starting at 15.73 million short tons in 1990 and decreasing to 0.79 million short tons in 2020. The green line indicates gross generation, which shows a generally increasing trend from 1990 until around 2008, then slightly decreasing towards 2020. The graph includes a note that data for sulfur dioxide emissions from 1991 to 1994 are not available.

Data Table for the Annual Sulfur Dioxide Emissions, 1990-2020

Year	Sulfur Dioxide (million short tons)	Gross Generation (Billion MWh)
1990	15.73
1991
1992
1993
1994
1995	11.83
1996	12.51
1997	12.94	2.25
1998	13.09	2.34
1999	12.45	2.39
2000	11.20	2.49
2001	10.64	2.46
2002	10.20	2.48
2003	10.59	2.53
2004	10.26	2.58
2005	10.22	2.74
2006	9.39	2.72
2007	8.93	2.83
2008	7.62	2.78
2009	5.82	2.65
2010	5.17	2.80
2011	4.55	2.73
2012	3.32	2.71
2013	3.24	2.69
2014	3.16	2.70
2015	2.22	2.66
2016	1.49	2.59
2017	1.34	2.49
2018	1.26	2.61
2019	0.97	2.53
2020	0.79	2.38

Credit: Whatever Happened to Acid Rain?. American Council on Science and Health. Accessed May 29, 2026.