VIEWS: 79 PAGES: 29 POSTED ON: 10/19/2011
Gas Analyzer Theory of Operation Gaseous Pollutants mini-course TAMS Center February 2009 Course materials can be found at: http://itep68.itep.nau.edu/itep_downloads/ in the gaseouspollutants folder Ozone Analyzer Theory of Operation The ozone layer in the stratosphere absorbs the vast majority of ultraviolet light entering Earth’s atmosphere. We definitely need ozone in the stratosphere. Ground level ozone, however, is known to have significant adverse impacts on the health of humans and ecosystems. Ground level ozone is what we monitor. Ozone isn’t typically emitted directly; it is formed by atmospheric chemical reactions in the presence of sunlight (photochemical reactions). Major precursors are reactive oxides of nitrogen and volatile organic compounds. Ozone analyzers are photometers. They use the fact that ozone absorbs UV light to measure ozone. By the way, don’t smoke around any gas analyzer Ozone is most efficient absorbing UV light near the 254 nm wavelength. Sample gas fills the absorption tube (measurement cell) and ultraviolet light at near 254 nm is sent through the sample gas; absorbance is measured. Illustration is from Teledyne API 400E Operator’s manual One problem – other gases absorb at 254 nm. Solution: Add a reference cycle to the sample cycle - use a manganese dioxide scrubber to selectively remove ozone from the sample gas, measure absorbance due to interferants, and subtract interferants from sample cycle. Illustration is from Teledyne API 400E Operator’s manual The above text is from Teledyne API 400E Operator’s manual The above text is from Teledyne API 400E Operator’s manual Manganese dioxide scrubbers can adsorb interferants and rapid humidity changes can cause release of interferants impacting measurement . Ozone conditioning Nitrogen oxide titration A very important chemical reaction: Notes: NOx = NO + NO2 NOy = NOx + all other reactive oxides of nitrogen NO reacts with ozone driving ozone concentrations down NO2 reacts with VOCs in sunlight and produces ozone Ozone, NOx, NO2 and NO data, Quapaw Nation Cherokee Nation Tahlequah Site 7/19/06 through 7/21/06 The 5th and 6th is Saturday, Sunday Why does a remote location have higher 24-hour averages? It has few NO sources to titrate ozone down at night. Remote areas typically have a much less pronounced diurnal pattern (daytime and nighttime concentrations are much closer). Wind Direction Power Plant Over 2 miles Monitoring Site Wind Direction Ozone titration downwind of NO source Ozone Concentrations NO2 + VOC ozone produced Power Plant NOx Analyzer Theory of Operation: Chemiluminescence Sample O3 gas w/NO Light leaks can impact measurement PMTs are sensitive and can be damaged by light Illustration is from Teledyne API 200E Operator’s manual Illustration is from Teledyne API 200E Operator’s manual Illustration is from Teledyne API 200E Operator’s manual Illustration is from Teledyne API 200E Operator’s manual Dark Cycle NOx Cycle goes through converter Only NO reacts bypasses RX Cell with ozone NO NO P MolyCon M Sample Air NOx Molybdenum Converter Rx Cell NOx = NO + NO2 T NO2 Converter NO2 NO2 converted to NO NO NO Valves E O x z NO Cycle c o e n s NO Cycle passes sample air to reaction chamber s e Ozone reacts with NO Ozone generator Only NO reacts – NO2 isn’t measured NO2 is calculated: NOx – NO = NO2 Dry filtered air hv in reaction cell is measured by PMT SO2 Analyzer Theory of Operation: Fluorescence (UV Light) Dark period every 30 minutes for PMT offset (API) Illustration is from Teledyne API 200E Operator’s manual Illustration is from Teledyne API 200E Operator’s manual CO Analyzer Theory of Operation: Gas Correlation Illustration is from Teledyne API 360U Operator’s manual
"Gas Analyzer Theory of Operation"