Once a vehicle’s engine/catalyst reaches operating temperature, the ambient temperature has little effect on the emissions performance of the vehicle. These “running” emission rates in MOVES have no temperature adjustment. However, during summer months at warm temperatures, MOVES assumes extensive use of air conditioning (which is a function of both temperature and humidity), which increases emissions.
MOVES calculates the emissions impact of engine starts separately from running emissions. During initial cold engine operation after an engine start, the ambient temperature has a significant effect on emissions, which MOVES determines using ambient temperature. This means, when you combine both the engine start with the running emissions, CO emissions during warm summer months will generally be significantly less than colder winter months.