# 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:**

- V
_{c}- corrected volume of sample gas, ml - V
_{f}- volume of sample collection flask, ml - V
_{a}- volume of absorbing solution used, 25 ml - P
_{i}- initial absolute pressure in flask prior to sampling, mm Hg - P
_{f}- final absolute pressure in flask after absorption period, mm Hg - T
_{i}- initial absolute temperature of flask prior to sampling, degrees K (Celsius + 273) - T
_{f}- final absolute temperature of flask after absorption period, degrees K (Celsius + 273) - T
_{ref}- reference temperature, 298°K - P
_{ref}- reference pressure, 760 mm Hg

## Description of Equation (1)

V_{c}, the corrected volume of sample gas, V_{c}, in mililitres, equals V_{f} the difference between the volume of sample collection flask, in millilitres, minus V_{a} the volume of absorbing solution used, 25 millilitres, times P_{f}, the difference between the initial absolute pressure in flask prior to sampling, in mm Hg, divided by T_{f} , the final absolute temperature of flask after absorption period, in degrees K, minus P_{i}, the initial absolute pressure in flask prior to sampling, in mm Hg, divided by T_{i}, 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 NO_{2}) as follows:

(2)

**Where:**

- C
_{NO2}- concentration of NO_{2}in ppm on a dry basis at 25°C and 760 mm Hg - V
_{C}- corrected volume of gas sample, ml - W - weight of NO
_{2}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 NO
_{2}

## Description of Equation (2)

Concentration of NO_{2} equals 24.45 × 10³ divided by 46.01 multiplied by the weight of NO_{2} in the gas sample, in micrograms, divided by the corrected volume of gas sample, in millilitres.

**Note 8** - Parts per million by volume of NO_{2} 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) ER_{NO2} = Q_{S} x C_{NO2} x 1.88 x 10^{-6}

**Where:**

- ER
_{NO2}- emission rate of NO_{x}(as NO_{2}) on a dry basis at 25°C and 760 mm Hg, kg/h - C
_{NO2}- concentration of NO_{2}in ppm on a dry basis at 25°C and 760 mm Hg - Q
_{S}- 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/m^{3}at 25°C and 760 mm Hg

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