6
ObservAir
Operating Manual
The Mass Absorption Coefficient (MAC) is a calibration factor that relates
the time differential of ATN to BC concentrations in the flow. The MAC varies
depending on the air pollution source, PM composition and other factors.
By default, the ObservAir uses a MAC of 12.5 m
2
/g, but this value may be
adjusted following cross-calibration with a reference instrument. For the
factors specific to the ObservAir, Equation (2) simplifies to:
𝐵𝐶(𝑡
%
) =
𝐾
𝑄(𝑡
%
) ∙
Δ𝐴𝑇𝑁
Δ𝑡 (3)
K =
ObservAir constant = 339,292
µ
g · ccm · sec/m
3
Q(t
i
)
= Flow rate at time
t
i
(ccm)
Note:
In Equation (3) above, the flow rate (Q) is input in units of cubic
centimeters per minute (ccm), as reported by the ObservAir.
1.1.2.
Electrochemical cells (Gaseous pollutants)
The ObservAir can be optionally outfitted with electrochemical cells to
measure gaseous pollutants. Six electrochemical cells are available, to
monitor concentrations of carbon monoxide (CO), nitrogen dioxide (NO
2
),
ozone (O
3
), sulfur dioxide (SO
2
), hydrogen sulfide (H
2
S), and ethanol. The
ObservAir can be configured to measure up to two (2) of these six species.
Electrochemical cells contain a chemical reagent that creates a
small electrical current when exposed to the gaseous analyte of interest
(e.g., CO). This electrical current is amplified and converted to a voltage
signal (the ‘gas voltage’) for digital acquisition. Each electrochemical cell
also outputs a reference voltage to compensate for drift and environmental
sensitivity. Using these two voltage signals, the gas concentration is
calculated as follows:
𝐶
&)$
(𝑡) =
𝑉
&)$
(𝑡) − 𝑉
!"#
(𝑡) − 𝑉
*"!+
𝐶𝑜𝑑𝑒 ∙ 𝐺𝑎𝑖𝑛 ∙ 10
',
(4)
C
gas
(t)
= Gas concentration at time ‘t’ (ppm)
V
gas
(t) =
Gas voltage at time ‘t’ (V)
V
ref
(t) =
Reference voltage at time ‘t’ (V)
V
zero
= Offset voltage differential =
V
gas
– V
ref
when
C
gas
= 0 ppm (V)
Code
= Calibration code (nA/ppm). See Section 3.5.1.
Gain
= Voltage gain (kV/nA). See Table 1.
An offset voltage differential (
V
zero
)
value is determined for each ObservAir
during DST’s zero-calibration procedure, but this value should also be
measured and logged prior to your monitoring application. Operate the
7
ObservAir
®
Operating Manual
ObservAir for 12 to 24 hours in an environment that is free of the analyte
gas(es) and average the voltage differential data collected over the entire
zero calibration period.
Code
is the factory calibration factor and is specific
to each individual cell. The code for each cell is automatically logged in the
SD card’s Settings file, as outlined in Section 3.5.1.
Gas
Gain (kV/nA)
Carbon Monoxide (CO)
100
Hydrogen Sulfide (H
2
S)
49.9
Nitrogen Dioxide (NO
2
)
499
Sulfur Dioxide (SO
2
)
100
Ozone (O
3
)
299
Ethanol
249
Table 1.
Gain settings for each type of electrochemical cell
1.2.
Environmental compensation
All air quality instruments are susceptible to environmental fluctuations. For
example, the temperature sensitivity of the aerosol absorption photometer’s
LEDs, photodiodes, and other electronic components results in erroneous
or inaccurate BC measurements during rapid environmental changes, such
as may be expected diurnally when the sensor is deployed outdoors. The
ObservAir incorporates proprietary hardware and software features to
minimize the sensor’s environmental dependence.
Hardware compensation features include the aerosol absorption
photometer’s active reference filter. Since the sampled air flow is actively
drawn through the reference filter, the transmitted light intensity is largely
dependent on the flow’s temperature and humidity content. By passing the
same air through both filters and monitoring each intensity measurement
independently, the ObservAir corrects for the photometer’s environmental
sensitivity and other measurement artifacts (e.g., water absorption in the
filter). Similarly, the electrochemical cells’ reference voltage outputs are
logged independently and used to compensate gaseous concentration
measurements. The sensor is also outfitted with temperature control
hardware and other proprietary design elements that preserve
measurement accuracy in harsh environments.
While hardware features contribute significantly to correcting the
ObservAir’s environmental dependence, some sensitivity remains that must
be corrected by software. This software compensation centers on DST’s