Inventory emission factors
ARB determines emission factors from gasoline dispensing facilities (Asregadoo 1992). The emission sources are broken into the categories shown in Table 4-26. Each category has a an emission factor for uncontrolled and controlled fueling operations. Spills from tank truck unloading are not explicitly accounted for; however, they may be implicitly included in an adjustment factor. Emissions from working losses and tank breathing are based on a combination of ideal gas law calculations in AP-42 and ARB's certification testing results. The current document shows 0.7 lb/1000 gal for fuel spillage. All of the spilled liquid will evaporate from the concrete surface at service stations. Uncontrolled emissions are determined from the gas concentration in the tank ullage space.
Table 4-26: ARB Emission Control Factors for Gasoline Dispensing Facilities
Control Factor (%)
Defect Rate (%)
Under ground tank working loss
Under ground tank breathing loss
Vehicle refueling vapor displacement
Vehicle refueling spillage
Since the values in this study will be compared to emission inventories it is important to understand how inventories are developed and how those values compare to this study. Emission inventories are based on the mix of controlled and uncontrolled sources and estimate of the number of operations with defective emission controls. Stage I and Stage II vapor recovery are considered to be 95 percent effective; so controlled emissions are 5 percent of the uncontrolled value. Breathing losses from underground tanks are estimated at 10 percent of the uncontrolled value if both Stage I and Stage II controls are in effect. No controls are assumed for spillage. The mix between controlled and uncontrolled service stations varies across the State. Some smaller operations have been exempt from Stage II vapor recovery requirements. Stage I vapor recovery is always used. In addition to reducing emissions, it returns vapor product to the refinery and provides for safer operations.
Statewide inventories are comprised of emission factors that are adjusted for malfunctions or failures in the vapor recovery system or defect rate. For example, emissions from vehicle refueling vapor displacement or working losses with 95 percent control and a 4 percent defect rate would be the following:
Emissions (lb/1000 gal) = 10 0.96 0.05 + 10 0.04 = 0.88
The defect rate has a profound effect on the overall emission estimate and was incorporated into the vapor emissions in this study. Emission inventories include