Ve (lb/gal) = MW(lb/mol) lbmol/379.6 ft3 0.1337 ft3/gal TVP/14.7 psi 520R/T (1)
T = gas and liquid temperature (R)
TVP = true vapor pressure (psi) at the equilibrium temperature
Table 4-28 shows the vapor space concentrations from various liquid fuels. Since the ARB inventory is based on test data that represents a range of gas temperatures and actual saturation conditions, a representative condition was modeled that reflects the inventory value. An effective fuel temperature was estimated for the ARB inventory values. The values in the top portion of Table 4-28 illustrate what temperature conditions would yield vapor concentrations that are consistent with the ARB inventory values (9.5, 1.0. and 10.0 lb/1000 gal for uncontrolled NMOG vapor mass). The 10 lb/1000 gal emission factor can then be calculated from a temperature of 80F, TVP of 6.2 psi and vapor molecular weight of 70. A higher molecular weight corresponds to lower RVP gasolines since they contain less butane (MW=58). One could ignore the inventory data and simply project vapor emissions based on an assessment of vapor temperatures. This approach could be used to parametrically evaluate the effect of temperature on fuel-cycle emissions, but was not performed in this study. All of the emphasis was placed on comparison of emissions in the context of the State inventory values.
Vapor concentration (uncontrolled NMOG vapor mass) for this study was determined from equilibrium vapor densities that correspond to 70F for underground tank vapors, and 80F for vehicle fuel tank vapors. Actual vehicle vapor temperatures can be much higher.
The same temperature conditions can then be applied to a range of liquid fuels to generate vapor space concentrations or uncontrolled emission estimates that are with California inventories. This effectively results in an equivalent equilibrium temperature that reflects the actual range of fuel temperatures and saturation conditions that correspond to test data. The underlying assumption with this approach is that the inventory data is based on a broad range of conditions and reflects the suitable conditions. Also shown in Table 4-28 are the vapor densities that would correspond to underground storage at 60F and vehicle fuel storage at 100F. These values were not used further in this study.
The inventory values reflect an equivalent equilibrium temperature of about 75F for storage tank vapors. This value appears higher than what might be expected for soil temperatures. The effective temperature for vehicle tank vapors is 80F that appears reasonable and agrees with fuel vapor and liquid measurements. Since this study is aimed at evaluating summer conditions these temperatures were used for estimating uncontrolled emissions.