NO2 (nitrogen dioxide)

NO2 is the chemical formula for nitrogen dioxide, a reddish-brown toxic gas and the most regulated of the nitrogen oxides (collectively NOₓ). It forms when atmospheric nitrogen and oxygen combine at the high temperatures inside a flame, so it is generated by every hot combustion process irrespective of the fuel's sulphur content. In India's NAAQS it has a 24-hour ambient limit of 80 µg/m³ and an annual limit of 40 µg/m³ (30 µg/m³ in ecologically sensitive areas).

NO2 is a respiratory irritant that inflames the airways, a precursor to ground-level ozone and photochemical smog when it reacts with VOCs in sunlight, and a contributor to acid rain as nitric acid. It also feeds secondary nitrate particulate, linking it to PM2.5 pollution.

For recyclers, NO2 is emitted by DG sets, boilers, dryers, kilns and pyrolysis burners — anything that burns at high temperature. Crucially, NO2 output depends on combustion conditions: hotter flames and more excess air produce more of it, so the same equipment can be tuned to emit less.

Control is combustion-side: low-NOₓ burners, staged combustion, flue-gas recirculation and careful air-fuel control reduce formation; large sources may add SCR/SNCR with urea or ammonia in the flue gas. NO2/NOₓ is a routine stack-test parameter and an OCEMS-monitored value above threshold capacity, reported in the SPCB consent.