atmosphere. Weather plays an important role in the
formation of secondarily formed air pollutants, as
discussed later in the Ozone and Particle Pollution
sections.
EPA tracks direct emissions of air pollutants and
emissions that contribute to the formation of key
pollutants, also known as precursor emissions.
Emissions data are compiled from many different
organizations, including industry and state, tribal, and
local agencies. Some emissions data are based on actual
measurements while others are estimates.
Generally, emissions come from large stationary fuel
combustion sources (such as electric utilities and
industrial boilers), industrial and other processes
(such as metal smelters, petroleum refineries, cement
kilns, manufacturing facilities, and solvent utilization),
and mobile sources including highway vehicles and
non-road sources (such as recreational and construction
equipment, marine vessels, aircraft, and locomotives).
Sources emit different combinations of pollutants.
For example, electric utilities release SO2
, NOx
, and
particles.
Figure 2 shows the distribution of national total
emissions estimates by source category for specific
pollutants in 2008. Electric utilities contribute about
70 percent of national SO2
emissions. Agricultural
operations (other processes) contribute over 80 percent
of national NH3
emissions. Almost 50 percent of the