Q623. The rule stipulates what standard temperature and pressure are, but how, for an annual average, is actual temperature and pressure defined?

A623. For most emission calculations, actual temperature and pressure refers to the actual temperature and pressure of the stream at the time of the measurement or release. As noted in 40 CFR 98.233(t) “Calculate volumetric emissions at standard conditions as specified in paragraphs (t)(1) or (2) of this section, with actual pressure and temperature determined by engineering estimates based on best available data unless otherwise specified.” There may be times where an “annual average” actual temperature and pressure may be used in the calculation. For blowdowns, for example, if the actual temperature and pressure of the stream is not measured just prior to the blowdown, it is reasonable to use the typical or annual average operating conditions of the system as the actual temperature and pressure. If the system subject to calculations operates at ambient conditions, the ambient conditions at the time of the measurement or release may be used as the actual temperature and pressure. If the release occurs continuously throughout the year, it is reasonable to use the annual average temperature and pressure as the actual temperature and pressure.