Methane concentrations, emissions, and flow rates (metered sources) are commonly reported using a wide range of units. This appendix focuses primarily on units used in this report.
Volumetric mixing ratio (v/v or l3/l3): Parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt); 1 ppm = 1,000 ppb; 1 ppm = 1,000,000 ppt.
Mass per unit volume (m/v or m/l3) at standardized temperature and pressure: milligrams or grams per cubic meter (mg m–3, g m–3) at standardized temperature and pressure conditions (commonly 0°C and 101.325 kilopascals)
Note: Conversions from volumetric to mass-based units require use of the ideal gas law equation: PV = NRT, where
P = pressure in atm,
T = temperature in Kelvin [=deg C + 273],
R = molar gas constant, where R = 0.082058 L atm mol–1 K–1
1 mole methane = 16.04 g.
Mass per time (m/t): megagrams, or terragrams per year (Mg yr–1, Tg yr–1)
Mass per unit area per time (m/(l2 t): grams per square meter per day (g m–2 d–1). Used for area sources (i.e., manured soils; landfills).
Mass CO2 equivalents per time: Requires multiplication of mass methane by global warming potential (GWP) for methane. GWP varies through time (=28 per IPCC 5th Assessment Report).
METHANE FLOW RATES (METERED SYSTEMS)
Standard cubic feet per minute: Cubic feet per minute at 60°F and 14.7 pounds per square inch (psi) pressure.
Standard cubic meters per minute: Cubic meters per minute at 15°C and 100 kilo-pascal pressure.
Normal cubic meters per minute: Cubic meters per minute at 0°C and 101.325 kilo-pascal pressure.
Multiplication factors for common metric units