SO2 (sulphur dioxide)

SO2 is the chemical formula for sulphur dioxide, an acidic, pungent gas made of one sulphur and two oxygen atoms. It forms whenever sulphur present in a fuel or material is burned, and whenever sulphide ores are smelted. In Indian air regulation SO2 is a NAAQS criteria pollutant with a 24-hour ambient limit of 80 µg/m³, an annual limit of 50 µg/m³, and a tighter 20 µg/m³ annual limit in ecologically sensitive areas.

The harm from SO2 is well documented: it irritates the respiratory tract and aggravates asthma and bronchitis; it oxidises in the atmosphere to sulphuric acid and falls as acid rain, damaging soil, water bodies, crops and buildings; and it forms secondary sulphate particulates that add to PM2.5. Because of this last link, SO2 control and particulate control are connected.

For recyclers, SO2 is most relevant to tyre pyrolysis and rubber processing (rubber's sulphur content carries into char and pyro-oil), and to any combustion equipment — boilers, gensets, furnaces — burning high-sulphur diesel or furnace oil. The amount of SO2 emitted is governed directly by the sulphur content of what is burned, so fuel choice is the primary lever.

Control measures: switch to low-sulphur fuels (BS-VI diesel at 10 ppm), install alkaline scrubbing (lime, limestone or caustic) where the stack load is high, and for pyrolysis, desulphurise the pyro-oil before it is sold or burned. SO2 is a routine stack-test parameter and, above threshold capacity, is monitored continuously by OCEMS that reports to the SPCB server.