Q(t
i
) = Flow rate at time t
i
(m
3
/sec)
∆ATN = Difference of two ATN measurements = ATN(t
i
)- ATN(t
i-1
)
∆t = Measurement interval (seconds) = t
i
- t
i-1
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 7.8 m
2
/g, but this value
may be adjusted following cross-calibration with a reference instrument.
For the default factors specific to the ObservAir, Equation (2) simplifies to:
K = ObservAir default constant = 5438461.5 µg ∙ ccm ∙ sec/m
3
Q(t
i
) = Flow rate at time t
i
(ccm)
Note: Flow rate (Q) is input in units of cubic centimeters per minute (ccm).
1.1.2. Electrochemical cells (Gaseous pollutants)
The ObservAir uses interchangeable electrochemical cells to monitor up
to two toxic gases: Carbon monoxide (CO), nitric oxide (NO), nitrogen
dioxide (NO
2
), ozone (O
3
), sulfur dioxide (SO
2
), hydrogen sulfide (H
2
S), and
volatile organic compounds (VOC). O
3
cells must always be paired with
NO
2
.
Air diffuses through a membrane and comes into a contact with
the cells working electrode, as shown in Figure 3. The analyte gas oxidizes
or reduces the working electrode, and generates a small electrical current
that is proportional to the analyte gas concentration in the air sample. The
reference electrode does not contact air and generates a baseline current.
By comparing the electrical currents generated by the working and
reference electrodes, gas concentrations are logged in real time.
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