NO₂, is the lesser of the two emitted NO_x gases from high temperature combustion in air; NO the other. NO_x production occurs in all internal combustion engines that use atmospheric oxygen from the air as oxidant, a process that brings atmospheric N₂ along with it. NO₂ is a very important species in the atmosphere. Since it absorbs in the visible wavelength region--creating the Brown Cloud over Denver, LA, Mexico City, Beijing, etc. --and can be photolyzed and yield oxygen atoms that can react with molecular oxygen to create ozone, NO₂ and the NO/NO₂ ratio are important in tropospheric chemistry. See nitrogen monoxide.