Continuous Monitoring of Sulfur Dioxide Removal Using K-Band Molecular Rotational Resonance Spectroscopy

Sulfur dioxide (SO₂), an air pollutant, poses significant threats to both public health and the environment. It is one of the six air pollutants regulated by the U.S. Environmental Protection Agency (EPA) under the Clean Air Act. In efforts to determine the application of molecular rotational resonance (MRR) spectroscopy for monitoring SO₂ and its removal from point sources, a K-band MRR technique was evaluated. This method was applied to measure the products of heated mixtures of SO₂ and oxygen (O₂) in the presence of ammonium metavanadate (NH₄VO₃) as a catalyst. The observed MRR spectrum revealed the presence of SO₂, water vapor (H₂O), and ammonia (NH₃) due to the sensitivity of MRR to only polar species. SO₂ removal was further confirmed by the disappearance of SO₂ as NH₃ formed. The work presented here analyzed the measurements of SO₂ and validated K-band MRR for monitoring SO₂ removal. It was observed that the K-band MRR maintains its linearity and other polar species in the mixture did not interfere with MRR signature of SO₂. The limit of detection, better than 1%, was determined by evaluating targeted K-band MRR signal response of SO₂ removal obtained at varying partial pressures of SO₂ in the mixture and using the MRR signal of pure SO₂ at 3 mTorr as a reference (100%). Additionally, the results showed that the accuracy and precision of K-band MRR for measuring SO₂ partial pressure were satisfactory.