The Chemistry of Smog in Kathmandu Valley
Kathmandu Valley’s hazy air is a complex chemical mixture rather than a single substance. Vehicle exhaust, road dust, construction activity, brick kilns, household fuel use, waste burning, and regional pollution can all contribute to the atmosphere people breathe. The resulting smog contains suspended particles and reactive gases that change as they move through the valley.
The most important pollutant is often fine particulate matter, especially particles smaller than 2.5 micrometres, known as PM2.5. These particles can travel deep into the lungs. Other components include nitrogen oxides, ozone, carbon monoxide, sulfur compounds, volatile organic compounds, and secondary particles formed in the air itself.
Understanding this chemistry helps explain why air pollution can remain severe even when one visible source is absent. A clear-looking morning may still contain invisible gases and microscopic particles, while a visibly smoky day may reflect a mixture of dust, soot, and chemically aged aerosol.
What smog means in a valley city
Kathmandu Valley is surrounded by hills, which can limit the movement of polluted air. During calm weather, emissions accumulate near the ground instead of dispersing quickly. This effect is particularly strong when a temperature inversion develops: cooler air remains trapped beneath warmer air, preventing vertical mixing.
Smog is therefore shaped by both emissions and atmospheric conditions. Traffic may release pollutants throughout the day, while smoke from cooking, waste burning, or industrial activity adds further gases and particles. Wind, rainfall, sunlight, and humidity then determine how long these pollutants remain and how they transform.
The particles entering our lungs
Primary particles are emitted directly. Diesel engines and biomass burning produce black carbon, ash, and organic particles. Construction sites, unpaved roads, and resuspended dust release larger mineral particles, including silicates and carbonates. These coarse particles irritate the nose and throat, while smaller particles can penetrate farther into the respiratory system.
Secondary particles form after gases react in the atmosphere. Sulfur dioxide can be oxidized into sulfuric acid, while nitrogen oxides can eventually produce nitric acid. Ammonia from agriculture, waste, and other sources reacts with these acids to form ammonium sulfate and ammonium nitrate. Organic vapors can also oxidize into low-volatility compounds that condense into secondary organic aerosol.
| Pollutant | Common sources | Important chemistry or effect |
|---|---|---|
| PM2.5 | Diesel, biomass burning, industry, secondary aerosol | Penetrates deep into the lungs; may contain soot, metals, salts, and organic compounds |
| PM10 | Road dust, construction, soil, resuspension | Irritates the eyes and airways; includes larger mineral particles |
| Nitrogen oxides | Vehicle engines, generators, combustion | Participate in ozone and nitrate-particle formation |
| Ozone | Formed from NOx and volatile organic compounds in sunlight | Irritates airways and damages plant tissue |
| Carbon monoxide | Incomplete combustion, traffic, burning | Reduces oxygen delivery by binding to hemoglobin |
| Sulfur dioxide | Sulfur-containing fuels and industrial combustion | Irritates airways and contributes to sulfate aerosol |
How gases react in sunlight
Nitrogen dioxide is central to photochemical smog. Sunlight can split NO₂ into nitric oxide and an oxygen atom. The oxygen atom then combines with molecular oxygen to form ozone. In simplified form:
NO₂ + sunlight → NO + O
O + O₂ → O₃
Near busy roads, freshly emitted nitric oxide can react with ozone and convert back to nitrogen dioxide. This means ozone concentrations may vary sharply between traffic corridors and less congested areas. The chemical balance depends on sunlight, wind, emissions, and the amount of reactive nitrogen present.
Volatile organic compounds from fuel evaporation, solvents, vehicle exhaust, and burning make the chemistry more complicated. In sunlight, they react with hydroxyl radicals and oxygen to produce peroxy radicals. These radicals help convert nitric oxide into nitrogen dioxide without consuming ozone immediately, allowing ozone levels to rise. This is why photochemical smog can become more intense downwind of emission sources.
Why winter mornings can be worse
Cold, dry, and calm conditions encourage pollutant accumulation. A shallow atmospheric mixing layer leaves less air available to dilute emissions, while overnight cooling can strengthen temperature inversions. Morning traffic then adds fresh nitrogen oxides, carbon monoxide, and soot to air that may already contain aged particles.
Humidity also changes particle behavior. Many inorganic aerosols absorb water, increasing their size and altering how they scatter light. Moist conditions can make haze appear thicker and support chemical reactions on particle surfaces. Rain, by contrast, can temporarily remove some suspended particles through wet deposition, although pollution may return when emissions continue.
Health risks from the mixture
The health effects of smog depend on concentration, exposure time, particle size, and chemical composition. PM2.5 can reach the alveoli, where it may trigger inflammation and oxidative stress. Repeated exposure is associated with increased risks of respiratory and cardiovascular disease. Children, older adults, pregnant people, and those with asthma or heart conditions may be especially vulnerable.
Ozone is a strong oxidant that can inflame airway tissue and worsen breathing symptoms. Nitrogen dioxide irritates the respiratory tract and may contribute to airway sensitivity. Carbon monoxide is especially dangerous in enclosed or poorly ventilated spaces because it interferes with the blood’s ability to transport oxygen. Pollutant mixtures can also contain toxic metals and carcinogenic organic compounds, depending on their sources.
Using air-quality information wisely
An air-quality index compresses several measurements into a public health message, but it does not describe every chemical component. Two locations with the same index may have different proportions of dust, traffic soot, ozone, or secondary aerosol. Monitoring stations, sensor placement, calibration, and local weather also affect reported values.
Residents can reduce exposure by paying attention to both pollution data and daily conditions. Practical steps include:
- Check PM2.5 and ozone readings before prolonged outdoor exercise.
- Avoid heavy traffic corridors during pollution peaks when possible.
- Improve indoor ventilation when outdoor air is cleaner, and limit smoke from candles, waste, and solid fuels.
- Use a well-fitted particulate respirator during severe pollution episodes, especially for sensitive individuals.
- Support cleaner transport, dust control, waste management, and transparent air-quality monitoring.
Kathmandu’s smog is a living chemical system shaped by combustion, sunlight, moisture, and mountain meteorology. Learning how particles and gases interact makes air-quality readings more meaningful and strengthens public discussion about cleaner energy and healthier cities. Follow NepaChem for accessible chemistry explainers, environmental research, and resources that connect Nepal’s scientific community.