How does acid rain form?

Acid rain forms when water molecules in the atmosphere react with sulfur dioxide or nitrogen oxides to form an acidic compound. The resulting compound has a lower pH.

The chemical reactions that occur when water molecules mix with sulfur dioxide and nitrogen dioxide are below:

Nitrogen oxides can be produced naturally by lightening strikes and sulfur dioxides is formed when volcanoes explode, but acid rain can also be caused by emissions due to humans.

SULFUR OXIDES

1) Sulfur dioxide #("SO"_2)# is produced industrially from the combustion of sulfur-containing fossil fuels and smelting of sulfide ores

#"S"(s) + "O"_2(g) -> "SO"_2(g)#

2) Sulfur dioxide #("SO"_2)# is then oxidized by sunlight to form sulfur trioxide #("SO"_3)#

#2"SO"_2(g) + "O"_2(g) -> 2"SO"_3(g)#

3) The oxides react with water to form acids

#"SO"_2(g) + "H"_2"O"(l) -> "H"_2"SO"_3(aq)#

#"SO"_3(g) + "H"_2"O" (l) -> "H"_2"SO"_4(aq)#

NITROGEN OXIDES

1) Nitrogen monoxide #("NO")# is produced from internal combustion of engines

#"N"_2(g) + "O"_2(g) -> 2"NO"(g)#

2) Nitrogen monoxide #("NO")# oxidizes to form nitrogen dioxide

#2"NO"(g) + "O"_2(g) -> 2"NO"_2(g)#

3) Nitrogen dioxide reacts with water to form nitric acid #("HNO"_3)# and nitrous acid #("HNO"_2)#

#2"NO"_2(g) + "H"_2"O"(l) -> "HNO"_3(aq) + "HNO"_2(aq)#

or it reacts with oxygen and water and becomes nitric acid

#4"NO"2_(g) + "O"_2(g) + "H"_2"O"(l) -> 4"HNO"_3(aq)#

The above was taken from Oxford IB Chemistry Study Guide by Geoffrey Neuss textbook (pg 62)