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24 CHAPTER 7 THE VEHICLE EMISSION MODULE This chapter describes the method for estimating vehicle v(i, j) = VHT at speed i and at acceleration j. emissions based on VHT by speed and acceleration category. CMEM calculates emission rates for feasible values of USER'S GUIDE vehicle speeds and accelerations based on vehicle weight and engine power output. The development of speed-acceleration 7.1 METHODOLOGY DEVELOPMENT vehicle activity in the traffic module must be constrained to The underlying concept for traditional on-road emission these feasible values. Otherwise, emissions will be under- inventory development using composite emission factors estimated, as vehicles will be assumed to travel at higher- expressed in grams per mile can be thought of as "traffic on than-achievable speeds (and for shorter time periods) than roads." That is, the fundamental processes affecting emis- would actually be the case. sions can be decomposed to roadway segments and charac- Because of a lack of the necessary data, the emission terized by the nature of traffic occurring on them. estimates do not take into account the following emission Currently, no single model addresses the range of specific effects that might be potentially impacted by traffic-flow emission processes in sufficient detail to capture the effects improvements: of traffic-flow improvement projects. CMEM provides the most detailed and best tested estimates of hot-stabilized vehi- Starts and stops (e.g., cold starts and hot soaks), cle exhaust emissions at different speeds and accelerations. Heavy-duty vehicle emissions, and Similarly, EMFAC2000 provides the most detailed estimates PM emissions. of process-specific evaporative emissions and excess start emissions. The methodology described here relies on emis- sion rate estimates from these two models. (As described pre- viously, no currently available models address either heavy- 7.2 METHODOLOGY APPLICATION duty vehicle emissions or PM emissions at the same level of detail as CMEM.) Three emission rate tables (hydrocarbons [HC], CO, and The rates depend on ambient temperature, which fluctuates NOX; see Tables 11 through 13, respectively) are used to by time of day and season of the year. A typical afternoon convert estimates of vehicle activity by speed and acceler- peak-hour temperature for a summer day is selected for the ation into estimates of emissions. One simply looks up the total hydrocarbons (THC) and nitric oxides (NOX) emission appropriate rate for the speed and acceleration category rates. A typical afternoon peak hour for an average winter and multiplies that rate by the VHT in the speed and accel- day is selected for CO analyses. eration category to obtain the vehicle emissions for that The exhaust emissions for THC, NOX, and CO are esti- pollutant. mated using the following equation: ER = q R (i, j ) v(i, j ) Equation 14 7.3 NONTECHNOLOGY UPDATES ij TO VEHICLE EMISSION MODULE Where: Emission rate models are frequently being updated. To the ER = emissions for pollutant R in terms of grams, extent that new CMEM rates become available, the analyst qR(i, j) = CMEM emission rate for pollutant R in terms of will need to exercise CMEM to develop new tables of aver- grams per hour for movement at speed i and accel- age rates for each acceleration and speed category in Tables eration j, and 11 through 13.