CHENNAI: Even as the Vedanta management and the Tamil Nadu government slug it out at the National Green Tribunal after the Sterlite smelter plant shutdown over charges of pollution and air quality in Tuticorin , it has been found that the three significant pollutants - sulphur di-oxide, nitrogen oxide and respirable suspended particulate matter (RSPM) – have remained unchanged since the shutdown of the factory.Moreover, sulphur di-oxide and nitrogen oxide were well below permissible levels, invalidating the state government’s claim of air pollution. RSPM levels oscillated, though it was below the safety threshold.A Tamil Nadu state government order on May 28 endorsed the Tamil Nadu Pollution Control Board’s directive to close the unit citing constitutional provisions to “protect and improve the environment,” and “in larger public interest.”RTI documents accessed by TOI show that there is no change in sulphur di-oxide or nitrogen oxide levels after the smelter was shut down. The air monitoring device at the TNPCB station in SIPCOT, Tuticorin (where the smelter is located) showed SO2 levels were 13.62 micrograms per cubic metre in October 2017, against 13.61 in July. The smelter has not been in operation since March, when the Vedanta management shut it down for maintenance. In fact, the SO2 level rose to 15.04 micrograms per cubic metre in May.The levels of nitrogen oxide, another pollutant, too remained almost constant at 9.52 micrograms per cubic metre in October 2017 and 9.53 in July. The permissible level for both these noxious gases is 80 micrograms per cubic metre.“SO2 and NOx are the primary pollutants. A combination of atmospheric temperature and moisture can impact readings. If the levels are what the PCB data says, it is the duty of the government to make it public and that too real time. If we are getting real time pollution levels of Delhi and commenting on it, why not Tuticorin. If only the state made this public, much of what happened could have been avoided,” said K Karthikeyan, retired TNPCB member-secretary. He did not want to talk about the specifics of Sterlite.The third constituent to the air pollution, RSPM, has dropped from 101 in March (when the smelter was shut) to 85 in July. “RSPM is not a gaseous emission. It can be caused by fly ash due to thermal plants in the region or excessive vehicular movement and dust,” an environment specialist said. He said that since the smelter shut down its operations and a few thermal plants also stopping power generation, truck movement in the coastal town had dropped sharply.Vedanta officials declined to comment, while Tamil Nadu government officials said the matter was sub judice and hence they did not want to say anything. “There is an expert committee looking at this in NGT. Let’s wait,” the official said.