Source apportionment
Also called: Pollution source apportionment · Topic: Geographical Issues: Pollution, Urban Waste, Slums and Land Degradation · NCERT: Beyond NCERT
Meaning
Source apportionment means working out what share of a place's air pollution comes from each source, such as vehicles, dust, industry or biomass burning (burning crop waste, wood and other plant matter). Scientists collect air samples and study the particles in them. Different sources leave different chemical "fingerprints", so each share can be worked out. The results often change from season to season, which makes the study important for planning. It lets the government aim its curbs at the biggest sources instead of using blanket bans.
Example
In Delhi, the TERI-ARAI study (2018) looked at PM2.5, the very fine particles that can get deep into the lungs. It found that dust made up 38% of PM2.5 in summer but only 17% in winter, while transport made up 28% in winter. This shows why a construction ban in winter hits a source that is then at its smallest share. Different studies of Delhi give quite different shares for industry, so there is still no single agreed figure.
Don't confuse with
- National Air Quality Index (AQI): AQI tells you how bad the air is, as one number on a 0–500 scale. Source apportionment tells you where the pollution comes from.
Recent developments
- 2025–26: A parliamentary standing committee (a group of MPs that checks the work of ministries) said Delhi's source apportionment studies, mainly TERI–ARAI (2018), are out of date. It asked for fresh studies and a system to update them regularly. This matters because if the shares are old, rules like GRAP stages, bans on old vehicles and truck entry limits may be aimed at the wrong sources, and nobody can measure how much each rule actually cleans the air.
- December 2025: CAQM set up an Expert Committee on vehicular emissions in Delhi-NCR. One of its jobs is to work out how much each type of vehicle adds to pollution. This takes source apportionment a step further: instead of one share for all "transport", it breaks that share down by vehicle type, so curbs can target the vehicles that pollute most.