Air Pollution and Noise

Units of air pollutant concentration

Air pollutant concentrations are reported either as a mass per unit volume of air, such as µg/m³ or mg/m³, or as a volume-based mixing ratio, such as ppmv or ppbv. A mixing ratio is a volume (mole) fraction: the volume of the pollutant divided by the total volume of the sample, both measured at the same temperature and pressure. Because it is a ratio, the mixing ratio does not change when the sample is heated or cooled at constant pressure, provided the pollutant is not adsorbed, reacted or lost. For an ideal gas the volume fraction equals the mole fraction, so a sample analysed at a higher temperature still gives the same ppmv value.

Two quantities must not be confused. Mass concentration changes with temperature because the molar volume changes, while the mixing ratio does not. The ideal gas law gives the molar volume as V_m = RT/P, and at 25 °C and 1 atm it is about 24.45 L/mol, the standard value used in most conversions.

To convert a mixing ratio to a mass concentration at 25 °C and 1 atm, use C\,(\mu\text{g/m}^3) = \dfrac{\text{ppb} \times M}{24.45}, where M is the molar mass in g/mol. For ppm the result is 1000 times larger, so C\,(\mu\text{g/m}^3) \approx 40.9 \times \text{ppm} \times M. At other temperatures, replace 24.45 L/mol by RT/P at that temperature. For example, at 1 atm and 300 K the air holds about 40.6 mol/m³, so 1 ppmv of CO_2 is about 1.8 mg/m³.

Working backwards, the molar mass of an unknown gas can be found from a measured mass concentration and mixing ratio using M = 24.45\,C/\text{ppb}, with C in µg/m³. Comparing the result with the molar masses of SO_2, NO_2, O_3 and CO identifies the pollutant.

Sound pressure level (dB)

Sound is measured on a logarithmic decibel scale because the range of audible pressures is very wide. The sound pressure level is \mathrm{SPL} = 20\log_{10}(p/p_0) dB, where p is the root-mean-square sound pressure and p_0 = 20\ \mu\text{Pa} is the reference pressure at the threshold of hearing in air. The factor is 20 rather than 10 because sound intensity, and hence power, is proportional to p^2, so 10\log_{10}(p^2/p_0^2) = 20\log_{10}(p/p_0).

Sound power level uses its own reference, L_W = 10\log_{10}(W/W_0) with W_0 = 10^{-12} W. Sound power is an intrinsic property of the source, whereas sound pressure at a receiver decreases with distance from the source. Sound intensity is power per unit area perpendicular to the direction of propagation, not power per unit volume.

Decibels cannot be added arithmetically. Each level must be converted to power or energy, the terms summed, and the result converted back: L = 10\log_{10}\sum_i 10^{L_i/10} for incoherent sources. Two equal sources add about 3 dB, while a source that is 10 dB quieter than another adds only about 0.4 dB to the louder one. Background sound is added in exactly the same way.

Averages of sound levels must also be taken on an energy basis: L_{avg} = 10\log_{10}\left[\dfrac{1}{n}\sum_i 10^{L_i/10}\right]. The arithmetic mean of the decibel values underestimates the true average because the loudest measurements dominate the energy sum. Levels with frequency weighting that follows human hearing are written in dBA.

CPCB ambient air quality standards 2009

The National Ambient Air Quality Standards notified by the Central Pollution Control Board in 2009 set limits for each pollutant over a stated averaging period, and one pollutant may carry more than one limit. The key values are: SO_2 annual 50 µg/m³ and 24-hour 80 µg/m³; NO_2 annual 40 µg/m³ and 24-hour 80 µg/m³; O_3 8-hour 100 µg/m³ and 1-hour 180 µg/m³; CO 8-hour 2 mg/m³ and 1-hour 4 mg/m³. CO has no annual standard, and SO_2 has no 8-hour standard, so the averaging period must be matched to each gas.

Particulate matter standards apply to ambient air only. PM_{10} has an annual standard of 60 µg/m³ and a 24-hour standard of 100 µg/m³, while PM_{2.5} has an annual standard of 40 µg/m³ and a 24-hour standard of 60 µg/m³. The 2009 notification does not prescribe indoor-air limits for either size fraction.

Since the annual PM_{10} standard (60 µg/m³) is higher than the annual PM_{2.5} standard (40 µg/m³), the statement that the annual PM_{10} standard exceeds the PM_{2.5} standard is true. This 2009 standard is separate from the sub-index scheme of the National Air Quality Index.

Primary and secondary air pollutants

Air pollutants are classified by how they reach the atmosphere. Primary pollutants are emitted directly from a source such as combustion, traffic or industry. Secondary pollutants are not emitted as such; they form in the atmosphere through chemical reactions among primary pollutants and natural constituents, often driven by sunlight or water vapour.

• Primary pollutant — emitted directly from a source. e.g. Carbon monoxide from incomplete combustion is primary.
• Secondary pollutant — formed in the air by reaction. e.g. Ozone formed photochemically from nitrogen oxides and hydrocarbons is secondary.
• Sulphur dioxide and nitrogen oxides — emitted from fuel burning, so they are primary, even though they also undergo further reactions.

Ozone, nitric acid and sulphuric acid are secondary. Ozone forms photochemically from NOx and volatile organic compounds, nitric acid forms when NO_2 is oxidised and taken up by water, and sulphuric acid forms when SO_2 is oxidised. Carbon dioxide is emitted directly by combustion and is classed as primary, even though it is a greenhouse gas.

PM2.5 measurement methods

PM_{2.5} is particulate matter with an aerodynamic diameter of 2.5 µm or less, and it is reported as a mass concentration in µg/m³. Its reference measurement is the gravimetric method: particles are collected on a pre-weighed filter, the filter is conditioned and reweighed, and the mass gain divided by the sampled air volume gives the concentration.

The beta attenuation method is an automated equivalent. Particles deposited on a filter tape attenuate a beta-radiation beam, and the attenuation is converted to particulate mass per unit volume of air. Gas-analysis techniques do not measure particulate mass: chemiluminescence is used for nitrogen oxides and ozone, and non-dispersive infrared spectroscopy is used for gases such as CO and CO_2.

National Air Quality Index (NAQI)

The National Air Quality Index of 2015 converts each pollutant's concentration into a sub-index on a 0 to 500 scale using pollutant-specific breakpoint tables. The overall index is not an average or a sum of the sub-indices. It equals the highest sub-index among the pollutants, and the pollutant that gives this maximum is called the critical pollutant.

Because each pollutant has its own breakpoint bands, the same numerical concentration can give different sub-indices. The PM_{2.5} bands are narrower than the PM_{10} bands, so for the same concentration in µg/m³ the PM_{2.5} sub-index can be higher than the PM_{10} sub-index. It is therefore possible for the PM_{10} sub-index to be lower than the PM_{2.5} sub-index.

Gas laws and transport phenomena

Several gas and transport laws recur in air-pollution problems, and each describes a distinct process. Each law should be learned together with the process it governs.

• Dalton's law — the total pressure of a gas mixture equals the sum of the partial pressures, and p_i = x_i P. e.g. A mixture of 0.2 atm O_2 and 0.8 atm N_2 has a total pressure of 1 atm.
• Fick's law — the diffusive flux is proportional to the concentration gradient, J = -D\,\dfrac{dC}{dx}. It describes diffusion.
• Henry's law — at equilibrium the partial pressure of a gas above a liquid is proportional to its dissolved concentration, p = H\,C. It governs gas-liquid phase transfer, such as SO_2 uptake into water droplets.
• Stokes' law — the terminal settling velocity of a small sphere is v_s = \dfrac{g(\rho_p - \rho_f)d^2}{18\mu}. It describes gravitational settling of particles.