Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
2
1.1. Theory of Operation
Figure 1.1 is a simplified schematic diagram of the Model 405 nm NO
2
/NO/NO
x
Monitor.
Figure 1.1.
Schematic Diagram of the Model 405 nm NO
2
/NO/NO
x
Monitor.
Sample air is continuously drawn through the instrument by the Pump at a flow rate of
~1.5 L/min. The NO
2
Scrubber Valve alternately bypasses and sends the sample air
through a heated NO
2
scrubber to remove all NO
2
in the sample. The NO
2
-scrubbed
or unscrubbed air passes through the Reactor Volume and the DewLine
TM
Nafion
Tubes (to equilibrate humidity), through the Optical Cell and through the Cell Flow
Meter. Alternate switching of the NO
2
Scrubber Valve once every 5 seconds allows
the measurement of a light intensity in the absence (
I
o
) of NO
2
and presence (
I
) of
NO
2
. The Beer-Lambert Law is then used to calculate the concentration of NO
2
from
I
and
I
o
:
[𝑁𝑂
2
] =
1
𝐿𝜎
𝑙𝑛 (
𝐼
𝑜
𝐼
)
Here,
L
is the path length (~2.1 m) and
σ
is the absorption cross section (~6.06 × 10
-19
cm
2
molec
-1
) for NO
2
averaged over the light-emitting diode (LED) emission centered
on 405 nm. The measurement provides an absolute NO
2
concentration in
molecules/cm
3
. In order to convert this concentration to a mixing ratio (fraction of total
air molecules that are NO
2
), we also measure the cell temperature and pressure,
which determines the total concentration of air molecules. From the temperature and
pressure we use the ideal gas law to calculate the concentration of air molecules, M,
in the optical cell. Nitrogen dioxide in units of ppb is then given by:
