# Reference method for measuring emissions of nitrogen oxides from stationary sources: chapter 10

## 10 Calculations

### 10.1 Gas Sample Volume Correction

Correct the volume of the dry gas sample (after removal of absorbed constituents) to 25°C and 760 mm Hg.

10.1.1 Calculate the volume of gas sample as follows:

(1)

Where:

• Vc - corrected volume of sample gas, ml
• Vf - volume of sample collection flask, ml
• Va - volume of absorbing solution used, 25 ml
• Pi - initial absolute pressure in flask prior to sampling, mm Hg
• Pf - final absolute pressure in flask after absorption period, mm Hg
• Ti- initial absolute temperature of flask prior to sampling, degrees K (Celsius + 273)
• Tf - final absolute temperature of flask after absorption period, degrees K (Celsius + 273)
• Tref - reference temperature, 298°K
• Pref - reference pressure, 760 mm Hg
Description of Equation (1)

Vc, the corrected volume of sample gas, Vc, in mililitres, equals Vf the difference between the volume of sample collection flask, in millilitres, minus Va the volume of absorbing solution used, 25 millilitres, times Pf, the difference between the initial absolute pressure in flask prior to sampling, in mm Hg, divided by Tf , the final absolute temperature of flask after absorption period, in degrees K, minus Pi, the initial absolute pressure in flask prior to sampling, in mm Hg, divided by Ti, initial absolute temperature of flask prior to sampling, in degrees Kelvin. This is then multiplied by 0.392 which is calculated from Tref, the reference temperature, at 298°K, divided by Pref, the reference pressure, 760 mm Hg.

10.2 Calculate the concentration of nitrogen oxides (as NO2) as follows:

(2)

Where:

• CNO2 - concentration of NO2 in ppm on a dry basis at 25°C and 760 mm Hg
• VC - corrected volume of gas sample, ml
• W - weight of NO2 found in gas sample, µg
• 24.45 x 10³ - standard molar gas volume, ml at 25°C and 760 mm Hg
• 46.01 - molecular weight of NO2
Description of Equation (2)

Concentration of NO2 equals 24.45 × 10³ divided by 46.01 multiplied by the weight of NO2 in the gas sample, in micrograms, divided by the corrected volume of gas sample, in millilitres.

Note 8 - Parts per million by volume of NO2 may be converted to milligrams per cubic metre (mg/m³) or to pounds per cubic foot (lb/ft³) by multiplying by 1.88 and 1.17 x 10-7, respectively. Figure 3 is a useful table for calculation purposes.

10.3 Calculate the mass emission of nitrogen oxides as follows:

(3)  ERNO2 = QS x CNO2 x 1.88 x 10-6

Where:

• ERNO2 - emission rate of NOx (as NO2) on a dry basis at 25°C and 760 mm Hg, kg/h
• CNO2 - concentration of NO2 in ppm on a dry basis at 25°C and 760 mm Hg
• QS - volumetric stack gas flow rate on a dry basis at 25°C and 760 mm Hg, m³/h (see Report EPS 1-AP-74-1, Equation B-2)
• 1.88 x 10-6 - conversion factor, ppm to kg/m3 at 25°C and 760 mm Hg

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