Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
3
[𝑁𝑂
2
]
𝑝𝑝𝑏
= 10
9 [𝑂
3
]
[𝐴𝑖𝑟]
= 10
9
𝑁
𝐴
𝑅𝑇
𝑃𝐿𝜎
𝑙𝑛 (
𝐼
𝑜
𝐼
)
where
N
A
is Avo
gadro’s number (6.02214129 x 10
23
molec/mol),
R
is the gas constant
(82.05746 cm
3
atm K
−1
mol
−1
),
T
is the absolute temperature in K, and
P
is the cell
pressure in atmospheres.
Nitric oxide is measured by bypassing the NO
2
Scrubber and measuring the light
intensity while adding (
I
) or not adding (
I
o
) ozone to convert NO to NO
2
according to
the well known reaction:
NO + O
3
→ NO
2
+ O
2
(1)
As in all our instruments, a DewLine
TM
Nafion
®
tube is used to equilibrate humidity
during
I
and
I
o
measurements, so that any water vapor interference due to refractive
effects on light transmission through the optical cell is eliminated.
NO
x
is obtained by adding the measurements of NO
2
and NO. The instrument may be
operated in continuous NO
2
or NO modes, or in a mode where NO
2
and NO are
alternately measured, once every 5 seconds.
As discussed above, the pressure and temperature within the absorption cell is
measured so that the NO
2
concentration can be expressed as a mixing ratio in parts-
per-billion by volume (ppb). The instrument displays and records the cell temperature
and pressure in addition to the NO
2
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.
In principle, the measurement of NO
2
by absorbance at 405 nm requires no external
calibration; it is an absolute method. However, factors such as variability in the LED
peak emission wavelength and band width, and non-linearity of the photodiode and
amplifier response, can result in a small measurement error. Therefore, each
instrument is calibrated against NIST-traceable standards of NO and NO
2
. These
results are used to calibrate the Model 405 nm with respect to an offset and slope
(gain or sensitivity). The corrections for offset and slope are recorded in the
instrument Birth Certificate and on a calibration sticker that can be viewed by
removing the top cover of the instrument. These calibration parameters are entered
into the microprocessor memory prior to shipment. The user may change the slope
and offset calibration parameters from the front panel by entering the Menu if desired.
It is recommended that the instrument be recalibrated at least once annually and
preferably more frequently. The offset may drift on time scales of hours to days due to
temperature change or chemical contamination of the absorption cell. As described
below, an Auto Zero function is available to periodically adjust the baseline
measurement to zero using an internal zero scrubber.
