Teledyne API Model 401 O
3
Photometric Calibrator Instruction Manual, 01124, Rev. J1
Appendix B RECOMMENDED ZERO AIR
SYSTEM
The following is from the TECHNICAL ASSISTANCE DOCUMENT FOR THE
CALIBRATION OF AMBIENT OZONE MONITORS - Richard J. Paur and Frank F. McElroy
(1979).
Zero air must be free of ozone and any other substance that might react with ozone (e.g. NO,
NO2, various hydrocarbons, and particulates). Air from any source must be purified to remove
such substances. Very dirty air may require a pre-cleaning process to remove large particles, oil
mist, liquid water, etc. The primary purification process is based on mechanical and chemical
filtering. While various schemes may be acceptable, systems similar to the following have been
used successfully:
The air is first dried with a Perma-Pure type dryer followed by a column of indicating silica gel.
The air is then irradiated with a UV lamp to generate ozone that converts existing NO to NO2,
and a large column of activated charcoal removes NO2, O3, hydrocarbons, and various other
substances. If desired, molecular sieve can be included for good measure.
A final particulate filter removes particulates which can originate in the scrubber columns. The
removal of moisture may not be necessary, but fewer problems seem to be encountered when dry
air is used.
The particulars of air purification are not well known, so some experimentation may be
necessary to determine the appropriate size of volume for scrubber columns. Also, the capacity
of the scrubber materials may not be accurately known, so frequent replacement of renewal of
these materials is advisable. Additional information on air purification is available (APHA
Intersociety Committee 1977, Section 20, Part I).
A very important requirement in photometer operation is the need for the zero air supplied to the
photometer during the I
o
measurement to be obtained from the same source as that used for
generation of ozone. The impurities present in zero air from different sources can significantly
affect the transmittance of an air sample.
B-1
