# 40 CFR § 1066.615 - NOX intake-air humidity correction.

§ 1066.615 NOX intake-air humidity correction.

You may correct NOX 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 NOX 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:

$\begin{array}{c}H=\frac{1000·{M}_{H2O}·{p}_{d}·RH}{{M}_{\mathrm{air}}·\left({p}_{\mathrm{atmos}}-{p}_{d}·RH\right)}\\ \text{Eq. 1066.615-1}\end{array}$

Where:
MH2O = molar mass of H2O.
pd = saturated vapor pressure at the ambient dry bulb temperature.
RH = relative humidity of ambient air
Mair = molar mass of air.
patmos = atmospheric pressure.
Example:
MH2O = 18.01528 g/mol
pd = 2.93 kPa
RH = 37.5% = 0.375
Mair = 28.96559 g/mol
patmos = 96.71 kPa

$H=\frac{1000·18.01528·2.93·0.375}{28.96559·\left(96.71-2.93·0.375\right)}=7.14741\phantom{\rule{0ex}{0ex}}g\phantom{\rule{0ex}{0ex}}{H}_{2}O\phantom{\rule{0ex}{0ex}}\mathrm{vapor}/\mathrm{kg}\phantom{\rule{0ex}{0ex}}\mathrm{dry}\phantom{\rule{0ex}{0ex}}\mathrm{air}$

(2) Use the following equation to correct measured concentrations to a reference condition of 10.71 grams H2O vapor per kilogram of dry air for the FTP, US06, LA-92, SC03, and HFET test cycles:

${x}_{\mathrm{NOxcor}}={x}_{\mathrm{NOx}}·\frac{{H}_{s}}{1-0.0329·\left(H-10.71\right)}\phantom{\rule{0ex}{0ex}}\text{Eq. 1066.615-2}$

Where:
χNOx = measured NOX emission concentration in the sample, after dry-to-wet and background corrections.
Hs = 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.
Example:
H = 7.14741 g H2O vapor/kg dry air time weighted over the FTP test cycle
χNOx = 1.21 ppm

${x}_{\mathrm{NOxcor}}=1.21·\frac{1}{1-0.0329·\left(7.14741-10.71\right)}=1.08305\phantom{\rule{0ex}{0ex}}\mathrm{ppm}$

(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]