Mobile source emission inventories--monthly or annual average inputs to MOBILE6?

In many urban areas, on-road vehicles are the biggest contributing source category of volatile organic compounds (VOCs) and nitrogen oxides (NOx). Based on a recently completed emission inventory study for three counties in central Florida, the major source by far of anthropogenic VOCs and NOx was on-road mobile sources, even though other sources (such as construction equipment, lawn and garden equipment, and various point sources) were also significant. Although there is specific guidance for conducting an ozone-season inventory for mobile sources, there is a lack of detailed guidance as to how to employ the U.S. Environmental Protection Agency's (EPA) latest mobile source emission factor program, MOBILE6, for an annual inventory. Several of the MOBILE6 inputs that significantly influence emission factors (e.g., temperature) can vary widely throughout the year, and the annual average value may not be appropriate. Rather, it may be better to utilize monthly values of these parameters. This paper investigated the sensitivity of the annual emission inventory results to using annual or monthly values of temperature, Reid Vapor Pressure of gasoline, and humidity. The results show that, for a three-county area in central Florida representing metropolitan Orlando, the annual emission inventory based on the sum of individual monthly averages is not significantly different from that calculated using one set of annual average inputs to MOBILE6.