(a) Correct the emissions in a gaseous sample for background using the following equation:

Where:
*x*[emission]dexh = measured emission concentration in dilute exhaust (after dry-to-wet correction, if applicable).
*x*[emission]bkgnd = measured emission concentration in the dilution air (after dry-to-wet correction, if applicable).
*DF* = dilution factor, as determined in paragraph (b) of this section.
Where:
*x*CO2 = amount of CO2 measured in the sample over the test interval.
*x*NMHC = amount of C1-equivalent NMHC measured in the sample over the test interval.
*x*CH4 = amount of CH4 measured in the sample over the test interval.
*x*CO = amount of CO measured in the sample over the test interval.
a = atomic hydrogen-to-carbon ratio of the test fuel. You may measure a or use default values from Table 1 of 40 CFR 1065.655.
b = atomic oxygen-to-carbon ratio of the test fuel. You may measure b or use default values from Table 1 of 40 CFR 1065.655.
(c) Determine the dilution factor, *DF*, over the test interval for partial-flow dilution sample systems using the following equation:

Where:
*V*dexhstd = total dilute exhaust volume sampled over the test interval, corrected to standard reference conditions.

*V*exhstd = total exhaust volume sampled from the vehicle, corrected to standard reference conditions.

(d) Determine the time-weighted dilution factor, *DF*w, over the duty cycle using the following equation:

Where:
*N* = number of test intervals
*i* = test interval number
*t* = duration of the test interval
*DF* = dilution factor over the test interval
Example:
*N* = 3
*DF*1 = 14.40
*t*1 = 505 s
*DF*2 = 24.48
*t*2 = 867 s
*DF*3 = 17.28
*t*3 = 505 s
[79 FR 23823, Apr. 28, 2014, as amended at 86 FR 34583, June 29, 2021]