In adapting MOBILE6 for use in SMOKE, MOBILE6 has been changed somewhat in order to output emission factors at the level of aggregation required by SMOKE, generate multiple sets of emission factors in the same run, and create a better interface with Emisfac. The mapping of SMOKE road classes to MOBILE6 facilities are given in Table 2.3, “Road class and corresponding MOBILE6 road type”. The mapping of SMOKE vehicle types to MOBILE6 vehicle types are provided in the M6MAP file.

The annual VMT and average speed values for rural and urban interstates and urban freeways typically account for travel on freeway ramps. Since MOBILE6 calculates freeway and freeway ramp emission factors separately, SMOKE must create a composite emission factor that is used for rural and urban interstates and urban freeways. The approach SMOKE uses is intended to duplicate the way MOBILE6 is used outside of SMOKE to compute the NEI inventories.

Since the speeds provided in the on-road mobile inventory are an average of speeds on freeways and ramps, SMOKE first calculates the nonramp freeway speed using the following formula:

S_f = (1 - V_r)/[(1/S_i) - (V_r/S_r)]

where

Based on the MOBILE6 documentation, SMOKE assumes a ramp VMT fraction (V_r) of 0.08 and a constant ramp speed (S_r) of 34.6 mph. The calculated nonramp freeway speed (S_f) is passed to MOBILE6 and used to calculate the freeway emission factors. MOBILE6 uses a fixed ramp speed of 34.6 mph to calculate the freeway ramp emission factors. The environment variables ADJUST_INV_SPEED and ADJUST_HR_SPEED are used to control whether or not the inventory and/or hourly speeds are adjusted for freeway ramps.

SMOKE creates a composite emission factor from the freeway and ramp emission factors as follows:

C_ef = V_r R_ef + (1 - V_r) F_ef

where

Again, SMOKE assumes a ramp VMT fraction (V_r) of 0.08.