Ambient Air quality Monitoring Guidlines. - Maharashtra Pollution ...

11. CONTINUOUS MEASUREMENT OF OZONE IN THE
ATMOSPHERE BY ULTRAVIOLET PHOTOMETRIC
INSTRUMENTS
1.0 TITLE
Method for continuous measurement of Ozone in the atmosphere by Ultraviolet
Phtometric Instruments.
2.0 PURPOSE
The purpose is to lay down a uniform and reliable method for measurement of
ozone in ambient air.
3.0 PRINCIPLE
3.2 The method is based on the photometric assay of ozone (O3) concentrations in a
dynamic flow system. The concentration of O3 is determined in an absorption
cell from the measurement of the amount of light absorbed at a wavelength of
254 nm. The method is based on the absorption coefficient of O3 at 254 nm, the
optical path length through the sample, and the transmittance, temperature and
pressure of the sample (1,2,3,4,5). The quantities above are related by the Beer-
Lambert absorption law.
I
Transmittance = --- = e -αc1
Io
Where :
α = absorption coefficient of O3 at 254 nm = 310 atm -1 cm -1 at 0 o C and
101.3 kPa
e = O3 concentration in units of atmosphere
1 = optical path of absorption cell length in cm
I = intensity of light passing through cell with an ozone sample
Io = intensity of light passing through cell with zero air
Typically, an air sample is first directed through a scrubber that removes any O3
present, but otherwise does not affect the sample. The ozone-free sample then
flows through the absorption cell, and its transmittance is measured. This
constitutes the zero cycle. At a present time, solenoid switches and another air
sample flows directly into the absorption cell, bypassing the scrubber and its
transmittance is measured. This constitutes the ozone measurement cycle. The
difference in transmittance between the two cycles is a measure of the O3
concentration. The complete measurement cycle takes about 20 to 30 s.
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