# 1066.615 NO intake-air humidity correction.
You may correct NO<sub>X</sub> emissions for intake-air humidity as described in this section if the standard-setting part allows it. See § 1066.605(c) for the proper sequence for applying the NO<sub>X</sub> intake-air humidity correction.
(a) For vehicles at or below 14,000 pounds GVWR, apply a correction for vehicles with reciprocating engines operating over specific test cycles as follows:
(1) Calculate a humidity correction using a time-weighted mean value for ambient humidity over the test interval. Calculate absolute ambient humidity, *H*, using the following equation:
Where:
*M*<sub>H2O</sub> = molar mass of H<sub>2</sub>O.
*p*<sub>d</sub> = saturated vapor pressure at the ambient dry bulb temperature.
*RH* = relative humidity of ambient air
*M*<sub>air</sub> = molar mass of air.
*p*<sub>atmos</sub> = atmospheric pressure.
*M*<sub>H2O</sub> = 18.01528 g/mol
*p*<sub>d</sub> = 2.93 kPa
*RH* = 37.5% = 0.375
*M*<sub>air</sub> = 28.96559 g/mol
*p*<sub>atmos</sub> = 96.71 kPa
(2) Use the following equation to correct measured concentrations to a reference condition of 10.71 grams H<sub>2</sub>O vapor per kilogram of dry air for the FTP, US06, LA-92, SC03, and HFET test cycles:
Where:
χ<sub>NOx</sub> = measured NO<sub>X</sub> emission concentration in the sample, after dry-to-wet and background corrections.
*H*<sub>s</sub> = humidity scale. Set = 1 for FTP, US06, LA-92, and HFET test cycles. Set = 0.8825 for the SC03 test cycle.
*H* = ambient humidity, as determined in paragraph (a)(1) of this section.
*H* = 7.14741 g H<sub>2</sub>O vapor/kg dry air time weighted over the FTP test cycle
χ<sub>NOx</sub> = 1.21 ppm
(b) For vehicles above 14,000 pounds GVWR, apply correction factors as described in 40 CFR 1065.670.
[80 FR 9121, Feb. 19, 2015, as amended at 81 FR 74207, Oct. 25, 2016]