A modern production method of nitric acid may lead to a generation of several different gaseous emission components, such as nitrogen oxides (NOx), ammonia NH3 and nitrous oxide N2O. Nitrogen monoxide is an intermediate gaseous product in the manufacturing of nitric acid HNO3. NO is formed in a catalytic reaction between ammonia and molecular oxygen O2. But undesirable side reactions may result in a formation of nitrogen dioxide NO2 and nitrous oxide N2O. The NO and NO2 emissions can be reduced by use of Selective Catalytic Reduction unit (SCR), which deploys ammonia gas NH3 to limit emissions of NO and NO2. A modern nitric acid plant needs to monitor the emissions of these gases.