Chemical Reactions in the Atmosphere: How Smog Forms
Smog is a visible sign of complex atmospheric chemistry. It develops when pollutants released by vehicles, industries, household fuels, and open burning react with sunlight, oxygen, water vapour, and one another. The resulting mixture can reduce visibility, irritate the lungs, and damage plants and materials.
The word smog originally described a mixture of smoke and fog. Modern urban smog is often photochemical: nitrogen oxides and volatile organic compounds react under sunlight to produce ground-level ozone and other secondary pollutants. Understanding these reactions helps explain why air pollution can become severe even when the original emissions are invisible.
For Nepali students and researchers, smog chemistry connects textbook concepts with familiar environmental concerns. Traffic emissions, brick kilns, dust, biomass burning, and valley geography can all influence air quality in places such as Kathmandu and other rapidly growing urban areas.
Primary pollutants begin the process
Primary pollutants enter the atmosphere directly. Motor vehicles release nitrogen monoxide (NO), nitrogen dioxide (NO₂), carbon monoxide (CO), and unburned hydrocarbons. Industries and fuel combustion can add sulfur dioxide (SO₂), particulate matter, and additional organic vapours.
These substances are not always the most harmful products of pollution. Many participate in atmospheric reactions and form secondary pollutants. Ground-level ozone, nitric acid, sulfate particles, nitrate particles, and peroxyacetyl nitrate (PAN) are examples of compounds created after emission.
The composition of smog depends on fuel type, sunlight intensity, temperature, humidity, and the time available for chemical reactions. This is why pollution can change during the day and vary greatly between cities.
Sunlight drives photochemical smog
A central reaction begins when nitrogen dioxide absorbs ultraviolet light:
[ \mathrm{NO_2 + sunlight \rightarrow NO + O} ]
The oxygen atom formed in this step reacts rapidly with molecular oxygen:
[ \mathrm{O + O_2 \rightarrow O_3} ]
This produces ozone near the ground. Ozone is useful in the stratosphere because it absorbs harmful ultraviolet radiation, but it is a respiratory irritant at street level.
In clean air, ozone can react with nitric oxide and regenerate nitrogen dioxide:
[ \mathrm{O_3 + NO \rightarrow NO_2 + O_2} ]
Volatile organic compounds change this balance. Sunlight breaks down organic molecules, producing reactive radicals such as hydroperoxy (HO₂) and organic peroxy radicals (RO₂). These radicals convert NO into NO₂ without consuming ozone, allowing ozone concentrations to rise.
Secondary pollutants multiply the effects
Oxidation of volatile organic compounds creates aldehydes, ketones, organic acids, and other oxygen-containing molecules. Some of these products react with nitrogen oxides to form PAN, a powerful eye irritant and plant-damaging pollutant.
Sulfur chemistry can produce acidic aerosols. In the atmosphere, SO₂ is oxidized by hydroxyl radicals, hydrogen peroxide, ozone, and cloud-water reactions. The final products may include sulfuric acid and sulfate particles. These fine particles can enter deep into the respiratory system.
Nitrogen oxides also form nitric acid and nitrate aerosols. Together with sulfate, ammonium, organic compounds, and water, these substances contribute to fine particulate matter such as PM₂.₅. Thus, smog is a mixture rather than a single chemical substance.
| Atmospheric component | Main source | Important reaction or process | Main concern |
|---|---|---|---|
| NO and NO₂ | Vehicles, generators, combustion | Photolysis and radical reactions | Ozone and nitrate formation |
| Volatile organic compounds | Fuel vapours, solvents, plants, burning | Oxidation by OH, O₃, and radicals | Ozone, aldehydes, PAN |
| Ground-level ozone | Secondary pollutant | Formation from NOx and VOCs in sunlight | Lung irritation and crop damage |
| SO₂ | Coal, oil, industries, brick kilns | Oxidation to sulfate and sulfuric acid | Fine particles and acid deposition |
| PM₂.₅ | Combustion and secondary formation | Nitrate, sulfate, and organic aerosol production | Deep respiratory and cardiovascular effects |
Weather controls the concentration
Strong sunlight and warm temperatures generally accelerate photochemical reactions. Smog often becomes worse from late morning to afternoon, after emissions have had time to react. However, the exact pattern depends on local traffic, wind, cloud cover, and pollutant levels.
Temperature inversions are especially important. Normally, warm air near the ground rises and disperses pollutants. During an inversion, a layer of warmer air sits above cooler surface air, trapping emissions close to the ground. Calm conditions can intensify this effect.
Valleys and basins are vulnerable because surrounding hills restrict air movement. Kathmandu Valley, for example, can experience stagnant conditions that allow vehicle exhaust, dust, smoke, and industrial emissions to accumulate. Monitoring both pollutants and meteorological conditions is therefore essential.
Health and environmental consequences
Ozone can inflame airways, worsen asthma, and reduce lung function. Fine particles can travel deep into the lungs and are associated with cardiovascular and respiratory disease. Sensitive groups include children, older adults, people with asthma, and outdoor workers.
Plants are also affected. Ozone enters leaves through small openings called stomata, where it can damage cellular tissues and reduce photosynthesis. PAN and acidic particles may injure leaves, while sulfate and nitrate deposition can alter soil and water chemistry.
Visibility reduction is another clear sign of atmospheric pollution. Light scatters from fine particles and droplets, producing a grey or hazy appearance. This haze may contain pollutants that are chemically active even when they cannot be seen individually.
Applying smog chemistry in Nepal
Chemistry education can support better air-quality decisions. Students can compare daily patterns in ozone, nitrogen dioxide, PM₂.₅, temperature, wind speed, and sunlight. Simple data analysis can reveal whether pollution peaks during traffic hours, stagnant weather, or regional smoke events.
Laboratory and field projects should use safe, low-cost approaches. Portable sensors, passive samplers, publicly available monitoring data, and basic meteorological observations can help learners investigate local air pollution without generating hazardous gases.
Useful priorities for communities and institutions include:
- Reduce vehicle idling and improve public transport to lower nitrogen oxide emissions.
- Control dust from roads and construction sites through covering, watering, and proper material handling.
- Improve combustion efficiency in brick kilns, industries, and household stoves.
- Avoid open burning of plastics, waste, and crop residue.
- Expand local monitoring and share results in clear, accessible formats.
Connecting atmospheric chemistry with action
Smog formation shows how emissions can change after they enter the air. A pollutant released in one location may participate in reactions and affect communities several kilometres away. Effective air-quality management therefore requires cooperation among chemists, engineers, health professionals, policymakers, and residents.
NepaChem can help strengthen this connection by making atmospheric chemistry understandable and relevant to Nepali learners. Share reliable measurements, classroom observations, research findings, and community experiences through the chemistry network, and help turn chemical knowledge into cleaner air decisions.