Air pollution

Air pollution is the presence in the outdoor atmosphere of one or more substances (gases, particles, vapours) in quantities and for durations that are harmful to human health, plants, animals, materials or the environment.

Classification of pollutants

• Primary pollutants are emitted directly from a source, e.g. $\text{SO}_2$, CO, NO, hydrocarbons, fly ash, soot.
• Secondary pollutants are formed in the atmosphere by reactions between primary pollutants, e.g. ozone, PAN, sulphuric acid mist, $\text{NO}_2$ from NO.

Natural sources

• Volcanic eruptions (ash, $\text{SO}_2$, $\text{H}_2\text{S}$)
• Forest fires (smoke, CO, $\text{CO}_2$)
• Dust storms, sea spray, pollen and spores
• Decay of organic matter (methane, $\text{H}_2\text{S}$) and lightning (NO)

Anthropogenic (man-made) sources

• Combustion of fuels in power plants, industries and homes: $\text{SO}_2$, $\text{NO}_x$, fly ash, CO.
• Transport: CO, hydrocarbons, $\text{NO}_x$, lead (where leaded fuel is used), PM from diesel engines.
• Industrial processes: cement, brick kilns, steel, chemical plants, mining (dust, fumes, acid gases).
• Agriculture and waste: burning of crop residue, open burning of waste, methane and ammonia.
• Domestic: cooking with biomass, kerosene and coal.

Anthropogenic sources are the main concern because they are concentrated in cities and are continuous.

Particulate matter (PM) is a mixture of tiny solid particles and liquid droplets suspended in air. PM10 means particles with aerodynamic diameter $\le 10\ \mu\text{m}$ and PM2.5 means $\le 2.5\ \mu\text{m}$.

Types

Sources

• Fuel combustion (coal, wood, diesel), brick kilns, cement and stone crushers
• Road dust, construction and mining activity
• Open burning of waste and crop residue
• Natural: wind-blown soil, volcanoes, pollen, sea salt
• Secondary particles formed from $\text{SO}_2$, $\text{NO}_x$ and ammonia

Effects

• Health: PM10 is deposited in the nose and upper airways and causes irritation and cough. PM2.5 reaches the alveoli and enters the blood. It causes asthma, bronchitis, lung cancer, heart disease and early death. Children and the elderly are most affected. Toxic particles (lead, asbestos, silica) cause specific diseases such as lead poisoning, asbestosis and silicosis.
• Plants: A dust layer on leaves blocks sunlight and clogs stomata, which reduces photosynthesis and yield.
• Materials and buildings: Soiling, abrasion and corrosion; acid particles attack marble and metals.
• Atmosphere: Fine particles scatter light and reduce visibility (haze) and can alter rainfall and climate.

Sulphur dioxide ($\text{SO}_2$)

Sources: burning of coal and fuel oil containing sulphur ($\text{S} + \text{O}_2 \rightarrow \text{SO}_2$), smelting of sulphide ores, oil refineries, and volcanoes.

Effects:

• A colourless gas with a sharp smell that irritates the eyes and respiratory tract and aggravates asthma and bronchitis.
• Oxidises to $\text{SO}_3$ and then to sulphuric acid, causing acid rain that damages lakes, forests and buildings.
• Bleaches and damages leaves (chlorosis); corrodes metals and attacks limestone and marble.

Oxides of nitrogen ($\text{NO}_x$ = NO + $\text{NO}_2$)

Sources: high-temperature combustion in vehicle engines and power plants, where atmospheric $\text{N}_2$ and $\text{O}_2$ combine ($\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$); also fertiliser plants and lightning.

Effects:

• $\text{NO}_2$ is a reddish-brown toxic gas that damages lung tissue and lowers resistance to infection.
• Takes part in photochemical smog and ozone formation.
• Forms nitric acid, which adds to acid rain.
• Damages plants and fades fabrics.

Carbon monoxide (CO)

Sources: incomplete combustion of carbon fuels, mainly petrol vehicles, also biomass burning, forest fires and industries.

Effects:

• Colourless and odourless. It combines with haemoglobin about 200-250 times more strongly than oxygen, forming carboxyhaemoglobin (COHb) and reducing oxygen transport.
• Causes headache, dizziness, drowsiness, and at high levels unconsciousness and death.
• It is also slowly oxidised to $\text{CO}_2$ in the atmosphere and is harmful in closed garages and poorly ventilated kitchens.

Photochemical smog is a brownish haze formed when sunlight acts on nitrogen oxides and volatile organic compounds (VOCs) from vehicle exhaust. It was first observed in Los Angeles, so it is also called Los Angeles smog. It contains ozone, PAN and aldehydes (an oxidising mixture).

Favourable conditions

• Heavy traffic (source of $\text{NO}_x$ and hydrocarbons)
• Strong sunlight and warm weather
• Low wind speed and a temperature inversion that traps pollutants

Reactions

1. NO is formed in engines: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$, and some oxidises to $\text{NO}_2$:

$$2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$$

2. Photolysis of $\text{NO}_2$ by sunlight ($\lambda < 400$ nm):

$$\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$$

3. Atomic oxygen reacts with oxygen to form ozone:

$$\text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M$$

4. Ozone reacts back with NO:

$$\text{O}_3 + \text{NO} \rightarrow \text{NO}_2 + \text{O}_2$$

Steps 2-4 alone give a steady state with no net ozone build-up. 5. Hydrocarbons (RH) react with OH and O radicals to form peroxy radicals ($\text{RO}_2$):

$$\text{RH} + \text{OH} \rightarrow \text{R} + \text{H}_2\text{O}, \qquad \text{R} + \text{O}_2 \rightarrow \text{RO}_2$$

6. Peroxy radicals oxidise NO to $\text{NO}_2$ without destroying ozone, so ozone accumulates:

$$\text{RO}_2 + \text{NO} \rightarrow \text{RO} + \text{NO}_2$$

7. Organic radicals combine with $\text{NO}_2$ to give peroxyacyl nitrates (PAN) and aldehydes (HCHO).

Daily variation

conc.
 |  NO        NO2          O3
 |  /\      /    \       /  \
 | /  \____/      \____/    \___
 +--------------------------------> time
   6am     10am      1pm     6pm

NO peaks in the morning rush hour, then $\text{NO}_2$ rises and ozone peaks around noon to early afternoon.

Effects

• Eye irritation (PAN, aldehydes), cough, chest pain, and reduced lung function.
• Plant damage (leaf spotting) and crop loss from ozone and PAN.
• Cracking of rubber, and damage to paints and fabrics.
• Reduced visibility.

(a) High-volume sampler

Principle: a large volume of air is drawn through a pre-weighed filter. Particles are retained on the filter, and the mass gained divided by the air volume gives the concentration.

        rain-proof roof
       ________________
      /  air inlet gap \
     /__________________\
        | filter paper |  (20 x 25 cm glass fibre)
        |______________|
        |   support    |
        |______________|
        |  blower +    |--> flow meter / recorder
        |  motor       |
        |______________|

Working:

1. A conditioned glass-fibre filter is weighed ($W_1$) and placed in the holder.
2. A motor-driven blower draws air at about 1.1-1.7 $\text{m}^3$/min through the filter. The roof keeps out rain and large particles (about > 100 $\mu$m).
3. The flow rate is recorded at the start and end (or continuously) and the mean value used.
4. After 24 h the filter is conditioned and re-weighed ($W_2$).

Concentration $= \dfrac{(W_2 - W_1)\times 10^6}{V}\ \mu\text{g/m}^3$. It measures total suspended particulates (TSP); a PM10 inlet can be fitted for PM10.

(b) Numerical

Volume of air sampled:

$$V = 1.5\ \text{m}^3/\text{min} \times 24 \times 60\ \text{min} = 2160\ \text{m}^3$$

Mass of particulates:

$$m = 3.412 - 3.215 = 0.197\ \text{g} = 197\,000\ \mu\text{g}$$ $$C = \frac{197\,000}{2160} = 91.2\ \mu\text{g/m}^3$$

SO$_2$ conversion:

$$C = \frac{0.05 \times 64.06 \times 1000}{24.45} = 131.0\ \mu\text{g/m}^3$$

(0.05 ppm = $0.05\times10^{-6}$ mol per mol of air; 1 ppm of a gas = $\text{MW}\times 1000/24.45\ \mu\text{g/m}^3$.)

Answer: TSP = 91.2 $\mu$g/m$^3$; 0.05 ppm SO$_2$ = 131 $\mu$g/m$^3$.