Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
1
1.
NO
2
/NO/NO
X
MONITOR INTRODUCTION
The 2B Technologies Model 405 nm NO
2
/NO/NO
x
Monitor
™ is designed to enable
accurate measurements of atmospheric nitrogen dioxide (NO
2
), nitric oxide (NO) and
NO
x
(NO + NO
2
) over a dynamic range extending from a few parts per billion by
volume (referred to herein as simply ppb) up to 10 parts-per-million by volume (ppm)
for NO
2
and 2 ppm for NO based on the absorption of visible light at 405 nanometers
(nm).
The Model 405 nm provides an absolute method for measuring NO
2
based on the
Beer-Lambert Law and thus requires only infrequent calibration. The NO
2
measurement is analogous to the measurement of O
3
using a conventional
absorbance-based ozone monitor; the two main differences are the use of 405 nm
light for NO
2
in place of 254 nm light for O
3
, and a much longer path length of ~2
meters (vs. 15-30 cm for ozone) to compensate for the much lower absorption cross
section of NO
2
. The long path length is achieved by use of a cell with a tubular design
that provides low volume and rapid gas exchange.
Nitric oxide is measured by measuring the light intensities with and without ozone
added to oxidize NO to NO
2
. As described in more detail below, the result is a “semi-
direct” measurement of NO in that the NO concentration is output directly and not
based on subtraction of NO
2
concentration from a total NO
x
concentration. Instead,
the NO
x
concentration is computed as the sum of the measurements of NO
2
and NO.
The NO
2
/NO/NO
x
Monitor is provided with a NIST-traceable calibration. Because
detection is based on the absolute method of absorbance, frequent calibration of the
span (sensitivity) is not required.
