Acid deposition occurs when sulfur dioxide and nitrogen oxides emitted from the burning of fossil fuels react with water, oxygen, and other chemicals in the atmosphere to form acids. These acids can fall to Earth as rain, snow, fog or dry particles. Natural processes also produce acid deposition, but human activities such as burning coal and oil have increased acid deposition levels. Acid rain has a pH lower than 5.6 and can damage sensitive forests and lakes. The main pollutants that cause acid rain are sulfur dioxide and nitrogen oxides.

1. Acid deposition

3. is a weak acid and dissociates
partially according to the equation :
H2CO3 (aq) ⇆ H+
(aq) + HCO3
-
(aq)

4. • Because of these reaction, the pH of rain
water is about 5.6.
• This is a natural phenomenon- rain with a pH
between 5.6 and 7 is not considered to be
‘acid rain’.

5. • Acid rain is considered to be rain with a pH
lower than 5.6.
• The average pH of rain in some areas can be
as low as 4.

6. • Acidic pollutants include the oxides of sulphur
and nitrogen.
• Sulphur dioxide (sulfur(IV) oxide) can be
formed by various natural and anthropogenic
process – such as the burning of sulfur-
containing fuels :

7. • The process by which SO2 is converted into
SO3 and H2SO4 in the atmosphere is complex
and does not involve simple oxidation by
atmospheric oxygen – interactions with
hydroxyl radicals, ozone or hydrogen peroxide
are involved.

8. • However, the reactions can be summarised :
2SO2(g) + O2(g)  2SO3 (g)
2SO3(g) + H2O (l)  H2SO4 (aq)

9. • SO2 can also dissolve in water to produce
sulfuric acid (IV) (sulfurous acid) :
SO2 (g) + H2O (l)  H2SO3 (aq)

10. • The main anthropogenic sources of nitrogen
oxides are the internal combustion engine,
coal, gas, oil-fuelled power stations and heavy
industry power generation.

11. • The combustion temperatures of fuels in
these process are very high and oxidation of
atmospheric nitrogen occurs forming NO
(nitrogen monoxide, nitric oxide or nitrogen(II)
oxide) :
N2(g) + O2 (g)  2NO (g)

12. • NO can be oxidised in the atmosphere to NO2.
• Again, the exact nature of the process is
complex but the reaction can be summarised
as :
2NO (g) + O2 (g)  2NO2 (g)

13. • The NO2 can then react with a hydroxyl free
radical (HO.) to form nitric (V) acid :
NO2 (g) + HO. (g)  HNO3 (g)

14. • Other reactions can occur and the formation
of nitric acid can also be shown as :
4NO2 (g) + O2 (g) + 2H2O (l)  4HNO3 (aq)
Or 2NO2 (g) + H2O (l)  HNO2 (aq) + HNO3(aq)