MECH6019 Chap.1 Pollutant Sources and Concentration Units
Pollutant Sources and Concentration Units
Primary pollutants leave a source directly, while secondary pollutants form through later atmospheric reactions. That classification identifies where a control can act. A direct stack contaminant can be reduced by changing the process, fuel or exhaust treatment. An ambient secondary pollutant must be traced to its precursors and the chemistry that converts them after release.
Gases and particles also need separate treatment because gas concentration follows mixture composition while particle behaviour depends strongly on size, density and shape.
The quantitative language starts with a fraction. Mole fraction is moles of one gas divided by total gas moles on a stated basis. For ideal gases at common temperature and pressure, it is also the volume fraction.
Multiplying a fraction by one million gives ppmv; multiplying by one hundred gives percent. These are ratio units. Mass concentration requires molecular weight and gas molar volume, so the conversion must state temperature and pressure. A correct calculation carries units through every line instead of relying on a memorised shortcut whose reference conditions may differ.
Moisture changes the denominator.
A wet-basis composition retains water vapour, while a dry-basis value removes it. The pollutant amount stays fixed during this conversion, so the dry fraction is higher. Reference oxygen creates another normalisation by accounting for dilution with excess air. Apply it only after the wet-or-dry basis is resolved, and use oxygen measured on the same basis.
Otherwise the final number combines conditions that no physical sample possessed.
A comparison also needs the same sampling boundary and averaging time. Raw process gas, treated stack gas and ambient air after dispersion answer different engineering questions. Control efficiency uses paired inlet and outlet mass rates. Exposure criteria use ambient concentration over a published averaging period.
Emission inventories use mass per time. Converting between these requires flow, dispersion or time information; a concentration alone cannot supply it.
The safest workflow is to write a basis box before calculating: species definition, sample location, wet or dry, temperature, pressure and averaging period. Convert the pollutant amount and denominator together, then check direction and magnitude.
Dry concentration should exceed wet concentration for unchanged pollutant moles. A trace mole fraction can still yield hundreds of milligrams per cubic metre for a heavy molecule because milligrams are small units. If two routes disagree, return to the basis rather than averaging results.
Finally, distinguish a concentration decrease from an emission decrease.
Adding clean air can reduce ppm while leaving pollutant mass per hour unchanged. Multiply compatible mass concentration and volumetric flow to obtain mass rate when performance is being judged. That single boundary check carries forward into every collector, absorber and treatment calculation later in the course.
What this chapter covers
- 01
Primary and secondary pollutants
- 02
Gas mole and volume fractions
- 03
Parts per million by volume
- 04
Mass concentration
- 05
Wet and dry reporting
- 06
Reference conditions
Worked example · free
Convert mole fraction to ppmv
- 1Multiply by 10⁶.
- 1Report 25 ppmv on the stated basis.
Key terms
- Mole fraction
- Moles of one species divided by total moles.
- Primary pollutant
- A pollutant emitted directly from a source.
- Secondary pollutant
- A pollutant formed after emission.
- Reference condition
- The temperature, pressure and moisture basis attached to a gas value.
Pollutant Sources and Concentration Units FAQ
Why must the gas basis be stated?
Wet or dry reporting changes the denominator, while temperature and pressure change the volume occupied by a fixed amount of gas. A bare concentration cannot be reconstructed or compared safely. The sampling location and averaging period also remain part of the quantity, so matching units alone does not make two readings comparable.
Are mole fraction and volume fraction equivalent?
For ideal gases at common temperature and pressure, mole fraction equals volume fraction. That equivalence supports ppmv conversion but does not remove the need to state reference conditions. Mass concentration still needs molecular weight and molar volume, and a wet-to-dry conversion still changes the mixture denominator.
Why can mass and volume trends differ?
Mass concentration also depends on molecular weight and molar volume. Two species at equal ppmv need not have equal mass concentration because their molecules have different masses. Temperature and pressure also affect the reference volume, so preserve them when comparing mass-based readings.
State whether the sample is wet or dry, and confirm that the chemical species definition matches the molecular weight used in the conversion. Keep the averaging time and sampling boundary beside the result so the converted value is not compared with an unlike criterion.
Exam move
Practise each conversion as a unit-cancellation chain. Keep the original basis in the first line and finish with an order-of-magnitude check. Build a comparison sheet with columns for pollutant species, source or ambient boundary, averaging time, wet or dry condition, reference temperature, reference pressure and unit.
Take one value at a time and convert it to fraction, percent, ppmv and mass concentration only when the required molecular weight and molar volume are available. Say aloud which information is added at each step. Work direction checks separately: remove water from a sample and predict that the dry fraction rises; add clean carrier gas and predict that concentration falls while pollutant mass stays fixed.
Then attach flow and calculate mass per hour so the difference between dilution and removal becomes visible. Use fresh values rather than memorising a lecture example. On review, mark any line where the denominator changed without a new label. Finish by explaining whether the result belongs to source control, ambient exposure or an inventory.
This verbal classification prevents a numerically correct conversion from answering the wrong engineering question. In the final revision pass, cover the equations and reconstruct them from units: a dimensionless fraction needs multiplication by one million for ppmv, while mass per volume needs molecular weight divided by molar volume.
If reference conditions differ, normalise before comparison and write the transformed conditions beside the answer.
