# What is the chemical composition of acid rain?

Feb 27, 2016

Sulfuric acid (${H}_{2} S {O}_{4}$), nitric acid ($H N {O}_{3}$), and carbonic acid (${H}_{2} C {O}_{3}$) are the major components of acid rain.

#### Explanation:

These chemicals are released into the atmosphere naturally however prior to industrialization and the advent of factories and reliance on hydrocarbons (coal, gasoline, crude oil, et cetera) acid rain was a rare event. In recent decades acid rain has become an increasingly common event.

Wikipedia has a good general summary of the movement of these chemicals:

Courtesy of: NHSavage, Obtained from: https://commons.wikimedia.org/w/index.php?curid=7038792 (Public Domain)

Sulfuric Acid (${H}_{2} S {O}_{4}$)

Sulfuric acid forms from sulfur dioxide ($S {O}_{2}$). This occurs naturally though volcanic eruptions (or the release of volcanic gases). Humans contribute to the release of $S {O}_{2}$ through combustion of fossil-fuels. The initial equation looks like this:

$S {O}_{2} + O H \cdot \to H O S {O}_{2} \cdot$

$H O S {O}_{2}$ is an intermediary so it's immediately broken down through exposure to oxygen forming a hydroperoxyl molecule/ radical ($H {O}_{2} \cdot$) and sulfur trioxide molecule ($S {O}_{3}$):

$H O S {O}_{2} \cdot + {O}_{2} \to H {O}_{2} \cdot + S {O}_{3}$

$S {O}_{3}$ is another intermediary and is immediately broken down into a sulfuric acid molecule (${H}_{2} S {O}_{4}$) when water (${H}_{2} O$) is present:

$S {O}_{3} \left(g\right) + {H}_{2} O \left(l\right) \to {H}_{2} S {O}_{4} \left(a q\right)$

Voila! Sulfuric acid!

Nitric Acid ($H N {O}_{3}$)

Nitric acid forms through the mixing of nitrogen dioxide ($N {O}_{2}$) and a hydroxide ion ($O H \cdot$) in the atmosphere. Naturally $N {O}_{2}$ comes from electrical discharges such a lightening strikes. Humans increase the $N {O}_{2}$ levels through burning of gasoline and oil (typically from a vehicle). The chemical equation looks like this:

$N {O}_{2} + H O \cdot \to H N {O}_{3}$

Carbonic Acid

Carbonic acid forms from the mixing of carbon dioxide ($C {O}_{2}$) and water (${H}_{2} O$). Carbonic acid in the atmosphere is usually attributed to natural sources such as the decomposition of dead plants and animals and not to industrial processes or human consumption of hydrocarbons. The chemical equation looks like this:

${H}_{2} O + C {O}_{2} \to {H}_{2} C {O}_{3}$