In the 1950s through studies in Los Angeles, it became clear that emissions from
automobiles were a major contributor to urban air pollution. This smog, formed
in the atmosphere as a result of complex photochemistry between hydrocarbonsoften
called volatile organic compounds (HC or VOC), and oxides of nitrogen
(NOx)-on warm spring, summer, and fall days, results in high ambient levels of
ozone and other oxidants. In addition, automobiles are the dominant source of
carbon monoxide (CO) and of lead. It is not just cars: Light trucks, heavy trucks,
and off-road vehicles also contribute significantly. So do stationary combustion
systems. Even natural (i.e., biogenic) hydrocarbon emissions are important.
Starting in the late 1960s, vehicle emissions in the developed world have
been regulated with increasing strictness. More recently, the fuels that the sparkignition
and diesel engines in these vehicles use (i.e., gasoline/petrol and diesel)
have been or are about to be subject to more stringent constraints with the intent
of further reducing emissions. This introduction traces the history of our efforts to
understand this important environmental issue and to find effective solutions. We
have made steady progress on improving urban air quality, yet the full resolution
of the problem still eludes us. Looking at this problem of motor vehicles and air
pollution from a broader perspective, there are several important questions.