Model 202 Ozone Monitor Manual Rev. J-4
1
1.
OZONE MONITOR INTRODUCTION
The 2B Technologies Ozone Monitor
™ is designed to enable accurate measurements
of atmospheric ozone over a wide dynamic range extending from a limit of detection of
3.0 parts-per-billion by volume (ppbv) to an upper limit of 250 parts-per-million (ppmv)
based on the well-established technique of absorption of ultraviolet light at 254 nm.
The Ozone Monitor
™ is light weight (5.5 lb, 2.5 kg) and has a low power consumption
(~7.2 watt) relative to conventional instruments and is therefore well suited for
applications such as:
•
vertical profiling using balloons, kites, remotely piloted aircraft, and other aircraft
where space and weight are highly limited
•
long-term monitoring at remote locations where power is highly limited
•
urban arrays of ground-based detectors
•
personal exposure monitoring for studies of health effects of air pollutants
1.1.
Theory of Operation
Absorption of UV light has long been used for measurements of atmospheric ozone
with high precision and accuracy. The ozone molecule has an absorption maximum
at 254 nm, coincident with the principal emission wavelength of a low-pressure
mercury lamp. Fortunately, few molecules found at significant concentrations in the
atmosphere absorb at this wavelength. However, interferences, such as organic
compounds containing aromatic rings, can occur in highly polluted air. Mercury vapor
can be a significant interference inside buildings where mercury spills have occurred
in the past and in the vicinity of certain mining operations.
Figure 1-1 is a schematic diagram of the ozone monitor. Ozone is measured based
on the attenuation of light passing through a 15-cm long absorption cell fitted with
quartz windows. A low-pressure mercury lamp is located on one side of the
absorption cell, and a photodiode is located on the opposite side of the absorption
cell. The photodiode has a built-in interference filter centered on 254 nm, the principal
wavelength of light emitted by the mercury lamp. An air pump draws sample air into
the instrument at a flow rate of approximately 1 L/min. A solenoid valve switches so
as to alternately send this air directly into the absorption cell or through an ozone
scrubber and then into the absorption cell. The intensity of light at the photodiode is
measured in air that has passed
through the ozone scrubber (
I
o
) and air that has not
passed through the scrubber (
I
). Ozone concentration is calculated from the
measurements of
I
o
and
I
according to the Beer-Lambert Law:
=
I
I
l
C
o
O
ln
1
3
(1)
