Currently, most governments, including the US government, estimate carbon emissions with a "bottom-up" approach, using emission factors which give the rate of carbon emissions per unit of a certain activity, and data on how much of that activity has taken place. For example an emission factor can be determined for the amount of carbon dioxide emitted per gallon of gasoline burned, and this can be combined with data on gasoline sales to get an estimate of carbon emissions from light duty vehicles. Other examples include determining the number of cows in various locations, or the mass of coal burned at power plants, and combining these data with the appropriate emissions factors to estimate methane or carbon dioxide emissions. Sometimes "top-down" methods are used to monitor carbon emissions. These involve measuring the concentration of a greenhouse gas in the atmosphere and using these measurements to determine the distribution of emissions which caused the resulting concentrations.