If the base case is taken as the current year, the base-case air quality impacts (pollutant concentrations) are established by one of the following methods: (a) measuring the pollutant concentrations directly using air quality monitoring equipment, or (b) carrying out steps similar to steps 1.1 to 1.6 using current-year data on emission rates, traffic operations, and so on. If the base case is for some future year projected from a current do-nothing situation, then estimates of the base case air quality can be predicted by carrying out steps similar to steps 1.1 to 1.6 using data projected for emission rates, traffic operations, and so on, at the future year of interest.