[MW:g/mol]*(2/3)*pow([u:m/s], 0.78)*pow(18/[MW:g/mol], 1/3)*[AP:cm2]*[Pv:atm]*[RMP_TCF]/(82.05*298)

• U.S. EPA, "Risk Management Program Guidance for Offsite Consequence Analysis", 1999
• U.S. EPA; FEMA; U.S. DoT, "Technical Guidance for Hazards Analysis: Emergency Planning for Extremely Hazardous Substances", 1987

See U.S. EPA (1999), Equation D-1 (section D.2.1, page D-1). Since temperature dependent vapor pressure is not supported, temperature correction factor (TCF) given in Equation D-5 (section D.2.2, page D-3) is incorporated into the equation.
For the original equation, see Section G.2 (Estimation of airborne quantity released for liquid) in "Technical Guidance for Hazards Analysis" (1987).
Mass transfer coefficient of 0.67 (2/3) is based on MacKay and Matsugu (1973).