Gusto - an open path air pollution sensor

3x puncte

categorie: Geografie

nota: 9.88

nivel: Liceu

The volume mixing ratios of certain trace atmospheric gases may be determined using differential optical absorption spectroscopy (DOAS). This is a well-known method of retrieval. The custom developed DUVAS method makes use of the characteristic narrow band absorption of the gas under study in the UV spectral range 200-270nm. These include SO2, NO, NO2, O3, NH3 and Benzene (all are governed by stri[...]
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The volume mixing ratios of certain trace atmospheric gases may be determined using differential optical absorption spectroscopy (DOAS). This is a well-known method of retrieval. The custom developed DUVAS method makes use of the characteristic narrow band absorption of the gas under study in the UV spectral range 200-270nm. These include SO2, NO, NO2, O3, NH3 and Benzene (all are governed by strict legislative guidelines with respect to acceptable limits of ambient concentration).

The Beer-Lambert law: describes the absorption over a path length x (m) of photons by a gas with number density n ( ) and absorption cross-section ( and is usually written as: I( )= (λ) F(λ)exp[- (λ)nx- α(λ)nx]where:- I(λ)=measured intensity( W ); ( W ); (λ)=lamp intensity (W );F(λ)=wavelength dependence of instrument;= absorption cross-section ( ;n = number density ( ;x = path length (m);
α=scattering cross-section ( If we consider the resulting intensity spectrum to be comprised of broad ( ) and narrow (dI) features, then we may write (dropping the λ),I=( X dI)

The narrow features (arising from tracemolecular absorption) are then de-convolved from the spectrum and the resulting differentials are used to calculate the concentration (numberdensity) of each absorber.
3.3 The Sensor Hardware:When vehicles pass within the vicinity of the sensor, key pollutants (SO2, NOX, O3, etc) emitted by such vehicles absorb UV light at characteristic frequencies and then detected by the GUSTO sensors as illustrated.

Briefly, light from the deuterium light source is retroreflected back towards the source via the focussing optics. The returning light is collected using a fibre optic 'light pipe' coupled to a spectrometer unit. The spectral output is then imaged onto the surface of the CCD detector and intensity values are obtained via a 12-bit ADC to produce an atmospheric spectrum of diode number versus ADC counts (equivalent to wavelength versus intensity) over the GUSTO range.
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