Nitrogen and Sulfur Oxides: How Transport, Energy, and Industry Affect the Atmosphere

What the Term NOₓ Includes

NOₓ commonly refers to nitric oxide (NO) plus nitrogen dioxide (NO₂). During high-temperature combustion, atmospheric nitrogen and oxygen react to form NO, which then partly oxidises to NO₂. Ambient monitoring focuses on NO₂, while source inventories may report total NOₓ expressed as NO₂ — these are not interchangeable unless the method and units are clearly stated.

Sources of Nitrogen Oxides

Major human sources include road and marine transport, power plants, boilers, industrial furnaces, cement production, metallurgy, and other high-temperature processes. The contribution varies by region according to energy, heating, and mobility patterns. Natural sources include lightning, microbial soil processes, and fires — but in urban and roadside environments, short-term levels are dominated by human activity.

Sulfur Dioxide and Its Sources

SO₂ forms when sulfur in fuel or raw material is oxidised. Sources include coal and heavy-oil combustion, marine fuel, smelting of sulfide ores, oil refining, sulfuric acid production, and other industrial processes. Low-sulfur fuels and flue-gas cleaning have substantially reduced SO₂ in many countries, but local problems can remain near industrial complexes, ports, and accidental releases.

Smog and Ground-Level Ozone Formation

NOₓ and volatile organic compounds participate in photochemical reactions that form ground-level ozone. The process depends on sunlight, temperature, precursor ratios, and transport time — so ozone episodes are often regional. In areas with very high fresh NO emissions, ozone can be temporarily consumed, while it increases downwind. Control plans therefore require inventories and atmospheric modelling, not just simple comparisons between two monitoring stations.

Acid Deposition and Secondary Particles

Sulfur and nitrogen oxides oxidise into sulfuric and nitric acids or their salts. These are deposited in rain and snow or as dry material. In sensitive ecosystems this changes soil and water acidity, removes nutrients, and can increase metal mobility. Sulfates and nitrates are also key components of PM2.5 — nitrate formation often involves ammonia, so controlling NOₓ without agricultural measures may be insufficient. The gas–particle link makes air management a cross-sector task.

Health and Environmental Effects

NO₂ and SO₂ irritate airways, increase bronchial responsiveness, and worsen asthma and other respiratory conditions. Additional effects arise through ozone and secondary PM2.5 formation. Excess nitrogen changes ecosystem composition; acidification affects aquatic organisms and soil biota; ozone formed from NOₓ damages leaves and reduces plant productivity. Acidic compounds also accelerate corrosion of metals, limestone, concrete, and cultural heritage.

NOₓ Reduction Technologies

The first step is lower fuel demand through efficiency and electrification. Combustion controls include low-NOₓ burners, staged air and fuel delivery, flue-gas recirculation, and temperature optimisation. Further treatment uses selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR), where a reagent converts NOₓ mainly into nitrogen and water. Performance depends on temperature range, mixing quality, catalyst condition, and reagent control.

SO₂ Reduction Technologies

SO₂ is reduced through low-sulfur fuel, fuel switching, raw-material treatment, and flue-gas desulfurisation (FGD). Wet scrubbers use an alkaline reagent to bind SO₂; dry and semi-dry systems inject sorbent and capture reaction products together with dust. Technology should be assessed across its full life cycle — not only by stack removal efficiency — since treatment creates by-products and consumes water and energy.