48 Chapter Three
Fig. 3.5The London smog of 1952. Photograph courtesy of Popperfoto Northampton, UK.Victorian England realized that the fogs were affecting health, but they, along
with others, were not able to legislate smoke out of existence. Even where there
was a will, and indeed there were enthusiasts in both Europe and North America
who strove for change, the technology was far too naïve to achieve really notice-
able improvements. The improvements that did come about were often due to
changes in fuel, in location of industry or in climate.3.6.2 Los Angeles smog—secondary pollution
The air pollutants that we have been discussing so far have come from station-
ary sources. Traditionally, industrial and domestic activities in large cities burnt
coal. The transition to petroleum-derived fuels this century has seen the emer-
gence of an entirely new kind of air pollution. This newer form of pollution is
the result of the greater volatility of liquid fuels. The motor vehicle is such an
important consumer of liquid fuels that it has become a major source of con-
temporary air pollution. However, the pollutants really responsible for causing
the problems are not themselves emitted by motor vehicles. Rather, they form in
the atmosphere. These secondary pollutants are formed from the reactions of
primary pollutants, such as NO and unburnt fuel, which come directly from the
automobiles. Chemical reactions that produce the secondary pollutants proceed
most effectively in sunlight, so the resulting air pollution is called photochemi-
cal smog.