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Photochemical Smog
Stack gases emitted from industrial processes and automobile exhausts are toxic to plants. The term' smog' is used for mixture of smoke and moisture in the air. The smog may be reducing or oxidising-conversion of SO₂ to its acid aerosol is reducing while automobile exhaust gases, by the action of sunlight, form phytotoxic oxidants which are toxic to plants and this is oxidising , smog'. Ozone and peroxyacetyl nitrate (PAN) are the two most important photochemical phyto-oxidants. Nitrogen dioxide and hydrocarbons of the automobile exhausts are energised by the ultraviolet light to react with oxygen resulting in production of ozone at a rate faster than its rate of decomposition to oxygen.