assembly that is identical to the first. Since the air is filtered (devoid of
PM), the intensity of light transmitted through the reference filter is
unaffected by BC concentrations. By comparing the reference light
intensity to that measured at the signal filter, it is possible to isolate the
light attenuation resulting from BC absorption alone, while largely
eliminating other factors.
Figure 2. Schematic of the aerosol absorption photometer
Optical attenuation (ATN) is defined in terms of the two light intensity
measurements, as shown below. In the ObservAir, both measurements are
reported as the bit count from the photodiodes’ Analog to Digital
Converter (ADC), ranging from 0 to 8388607 (full scale 23-bit output).
I
ref
= Light intensity through reference filter (ADC count)
I
sig
= Light intensity through signal filter (ADC count)
Using the ATN measurements, BC concentrations are calculated using the
fundamental equation:
BC(t
i
) = Black carbon at time t
i
(µg/m
3
)
A = Filter collection area (D = 3mm) = 7.07x10
-7
m
2
MAC = Mass absorption coefficient of BC at 880 nm = 7.8x10
-6
m
2
/µg
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