Model 202 Ozone Monitor Manual Rev. J-4
2
where
l
is the path length (15 cm) and
is the absorption cross section for ozone at
254 nm (1.15 x 10
-17
cm
2
molecule
-1
or 308 atm
-1
cm
-1
), which is known with an
accuracy of approximately 1%. The 2B Technologies instrument uses the same
absorption cross section (extinction coefficient) as used in other commercial
instruments.
The logarithm of equation 1 is calculated in the microprocessor of the instrument with
sufficient accuracy to provide five orders of dynamic range; ozone mixing ratios are
measured up to 250 ppmv. The shorter path length of the 2B Tech Model 202 Ozone
Monitor also contributes to the wide dynamic range, which is limited at the high end by
the absorption beginning to become optically thick (base 10 optical absorbance = 0.2).
The pressure and temperature within the absorption cell are measured so that the
ozone concentration can be expressed as a volume mixing ratio in ppbv, ppmv, or
pphmv. The mixing ratio can also be expressed as micrograms per cubic meter (µg
m
-3
) or milligrams by cubic meter (mg m
-3
). The instrument displays and records the
cell temperature and pressure in addition to the ozone mixing ratio. The cell pressure
is displayed and logged in units of either torr or mbar and the cell temperature in units
of either °C or K.
FIGURE 1-1. Schematic diagram of the ozone instrument.
