photochemical smog

2. • When fossil fuels are burned, a
variety of pollutants are emitted into
the earth's troposphere
• Two of the pollutants that are
emitted are hydrocarbons (unburned
fuel) and nitric oxide (NO)
• When the gas is introduced to
sunlight the nitric oxide (NO) is
converted to nitrogen dioxide (NO2).
• NO2 is a brown gas and at
sufficiently high levels can
contribute to urban haze.
• NO2 can absorb sunlight and break
apart to produce oxygen atoms that
combine with the O2 in the air to
produce ozone (O3)

3. • Ozone is a powerful
oxidizing agent, and a
toxic gas
• it is believed that the
natural level of ozone
in the clean
troposphere is 10 to 15
parts-per-billion.
• scientists have found
that ozone levels in
"clean air" are now
approximately 30 parts-
per-billion.

4. • The presence of the ozone
layer in the stratosphere is
vital to life at the surface since
the ozone layer absorbs UV
light that would otherwise
reach the surface and cause
damage to both animal and
plant life.
• the ozone layer is becoming
chemically perturbed due to
the presence of
chlorofluorocarbons (CFCs)
in the stratosphere
• CFCs are used in air
conditioners and as cleaning
and blowing agents in the
chemical industry

5. • Located over
Antarctica
• it appears every year
in October
• The amount of ozone
in the stratosphere
over Antarctica has
decreased to half the
natural level

6. • Among the chemicals that may pose a human
health risk are pesticides, PCBs, polycyclic
aromatic hydrocarbons (PAHs), dioxins, and
volatile organic compounds (benzene, carbon
tetrachloride).
• Many of the more environmentally persistent
compounds ( PCBs) have been measured in
Arctic wildlife and, for example, in tissues of
the local Innuit population.