Gas Analyzer Theory of Operation

Gas Analyzer Theory of Operation

Gaseous Pollutants mini-courseTAMS 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.

UV

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.

By the way, don’t smoke around any gas analyzer

Ozone analyzers are photometers. They use the fact that ozone absorbs UV light to measure ozone.

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.

Illustration is from Teledyne API 400E Operator’s manual

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.

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 + NO2NOy = NOx + all other reactive oxides of nitrogenNO reacts with ozone driving ozone concentrations downNO2 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).

Power Plant

Monitoring SiteOver 2 miles

Wind Direction

Power Plant

Ozone Concentrations

Wind Direction

NO2 + VOCozone produced

Ozone titration downwind of NO source

NOx Analyzer Theory of Operation: Chemiluminescence

Illustration is from Teledyne API 200E Operator’s manual

Sample gas w/NOO3

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

MolyConMolybdenum Converter

NO2 Converter

NO

NO2 NO2 converted to NO NO

NO

Rx Cell

NOx Cycle goes through converterOnly NO reacts with ozone

NO Cycle passes sample air to reaction chamberOzone reacts with NOOnly NO reacts – NO2 isn’t measuredNO2 is calculated: NOx – NO = NO2

Sample AirNOx = NO + NO2

NOx

NO

NO Cycle

Valves

Dark Cycle bypasses RX Cell

Ozone generator

Dry filtered air

hv in reaction cell is measured by PMT

PMT

SO2 Analyzer Theory of Operation:Fluorescence

(UV Light)

Illustration is from Teledyne API 200E Operator’s manual

Dark period every 30 minutes for PMT offset (API)

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