Crop-Residue Burning and Air Pollution

Crop-residue burning is the open-field combustion of straw and stalks left after harvesting. It produces particulate matter directly and also releases gases that form additional particles through atmospheric reactions, thereby raising PM2.5 and PM10 concentrations.

How particulate matter is formed

Open burning is usually incomplete, so residue is converted into a complex mixture of particles and gases rather than being fully oxidised.

• Smoke directly contains fine soot, black carbon and organic carbon, while ash and mineral fragments contribute to coarser particulate matter.
• Fine combustion particles largely fall within the PM2.5 range; ash and resuspended material also add to PM10.
• Emitted nitrogen oxides and volatile organic compounds undergo oxidation and other atmospheric reactions, producing secondary nitrate and organic aerosols.

Transport and accumulation

Smoke can travel beyond the field, making residue burning a regional airshed problem rather than only a rural or local source.

• Low wind, a shallow mixing layer and temperature inversion can restrict vertical dispersion and concentrate pollutants near the surface.
• Fine particles remain suspended longer than coarse ash and can be transported to downwind towns and cities.
• The resulting pollution episode combines with emissions from transport, industry, construction dust and household fuel use; the contribution of burning therefore varies across places and days.

Environmental significance and prevention

Elevated particulate concentrations reduce visibility and expose populations to particles capable of entering the respiratory system.

• Preventing open burning avoids a concentrated, short-duration emission pulse at its source.
• In-situ management, including mulching and incorporation into soil, retains residue on the farm.
• Ex-situ management collects residue for uses such as composting, bioenergy and industrial raw material.