[These remaining trace gases exist in such small quantities that they are measured in terms of a mixing ratio.....parts per million by volume (ppmv), parts per billion by volume (ppbv) or parts per trillion by volume (pptv).]Atmospheric temperature and chemistry are believed to be controlled by the trace gases. [There is increasing evidence that the percentages of environmentally significant trace gases are changing because of both natural and human factors.]
Purely man-made gases include chlorofluorocarbons CFC-11 and CFC-12 and halons. Carbon dioxide, nitrous oxide, and methane (CH4) are produced by the burning of fossil fuels, and some natural sources.