Chemistry, Third edition

ATMOSPHERIC POLLUTION

Acid deposition

This is a term used to describe all precipitation (rain, snow or fog) which is made

acidic by acids stronger than aqueous CO 2. The gases SO 2 and NO 2 , mainly emitted

from fossil fuel combustion, are major contributors towards acid rain since these

gases are oxidized to acids by the oxygen in the atmosphere:

SO 2 (g) +^1 ⁄ 2 O 2 (g) + H 2 O(l) H 2 SO 4 (aq)

and

2NO 2 (g) +^1 ⁄ 2 O 2 (g) + H 2 O(l) 2HNO 3 (aq)

Acid rain is toxic to vegetation and aquatic life, damages buildings and statues,

and dissolves heavy metalsfrom soils, rocks and sediments. Power stations are a

major source of sulfur dioxide emissions, whilst nitrogen oxides originate from the

exhausts of motor vehicles. Catalytic converterssignificantly reduce nitrogen oxide

emissions from cars by reducing them to nitrogen (see Exercise 22B).

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BOX 22.1

The filtering of UV light by ozone

Ultraviolet radiation (roughly any radiation in the wavelength range 50–400 nm) can be

classified into one of three regions, depending upon its wavelength:

Wavelength/nm Sub-classification

 290 UV-C

290–320 UV-B

320–400 UV-A

The ultraviolet light wavelengths absorbed by ozone molecules are shown in Fig. 22.2.

Ozone O 3 has a different molecular structure, and therefore a different set of energy levels

from diatomic oxygen, O 2. The absorption spectrum of O 2

is therefore not the same. O 2 absorbs ultraviolet

radiation of much shorter wavelengths. Diatomic oxygen

and other components of the atmosphere filter out

harmful radiation from the sun, in the ultraviolet region,

up to 220 nm. This radiation would damage our eyes and

skin if it reached the surface of the Earth.

Ozone, helped to a certain extent by diatomic oxygen,

filters out all of the sun’s radiation in the UV-C range, but

can only absorb someof the UV-B light in the 290–320

range (look at Figure 22.2). The unabsorbed radiation

reaches the surface of the Earth. No component of the

atmosphere absorbs UV-A radiation to any great extent,

so most of this radiation reaches the Earth’s surface.

However, UV-A radiation is the least harmful of light

within the ultraviolet range. A reduction in ozone in the atmosphere would lead to more UV-B

radiation reaching the surface of the Earth and increased sunburn of human skin (this can lead

to skin cancer because UV-B is absorbed by DNA). Increased exposure to UV-B may also lead

to more eye cataracts and reduce the effectiveness of our immune systems.

Fig. 22.2The absorption

spectrum of ozone.