In 1968, an estimated 9,600,000 tons of nitrogen oxides were emitted from stationary sources. The manufacture of nitric acid generated emissions of approximately 150,000 tons of NOx .6% of the total NOx emitted from stationary sources. Though emissions from nitric acid plants do not account for a major proportion of NOx on a nationwide basis, large individual plants can be large point sources of this pollutant. Approximately 50% of this emission will usually be in the form of nitrogen dioxide, a very visible reddish-brown colored gas. Most new plants utilize catalytic combustion devices to decrease operating costs and to reduce NOx emissions.