4
ObservAir
Operating Manual
1.1.
Principle of operation
Figure 1.
Functional diagram of the ObservAir
The ObservAir is centered around an aerosol absorption photometer
configured to measure concentrations of black carbon (BC). BC is a type of
particulate matter (PM) pollution generated by the incomplete combustion
of fossil fuels or biomass. For most purposes, BC is functionally defined as
the light-absorbing component of PM pollution. The ObservAir's
micropump first draws air into the inlet and through a fibrous aerosol filter.
that is mounted on black supporting material (the disposable filter tab). As
light absorbing PM collects on the fibrous filter, calculates BC
concentrations in real time. Downstream of the photometer, a relative
humidity and temperature sensor records environmental conditions, and
optional electrochemical cells measure up to two gaseous pollutants. Air
then passes through the flow rate sensor and is exhausted by the pump.
1.1.1.
Aerosol absorption photometer (Black carbon)
A schematic of the ObservAir's aerosol absorption photometer is provided
below. Photodiodes continuously monitor the intensity of 880 nm light
transmitted from an LED source through two aerosol filters. As polluted air
is drawn through the photometer, light absorbing BC accumulates on the
first ‘signal’ filter and the transmitted light intensity attenuates predictably
over time. The filter collection area is 3 mm in diameter. After the first filter,
the air flow passes through a second ‘reference’ filter 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.
5
ObservAir
®
Operating Manual
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).
𝐴𝑇𝑁 = 100 × ln *
𝐼
!"#
𝐼
$%&
, (1)
I
ref
= Light intensity through reference filter (ADC count)
I
sig
= Light intensity through signal filter (ADC count)
In this way, the optical ATN through the photometer is monitored in real
time (note that ATN is unitless) and BC concentrations are calculated using
the fundamental equation:
𝐵𝐶(𝑡
%
) =
𝐴
𝑀𝐴𝐶 ∙ 𝑄(𝑡
%
) ∙
Δ𝐴𝑇𝑁
Δ𝑡 =
𝐴
𝑀𝐴𝐶 ∙ 𝑄(𝑡) ∙
𝐴𝑇𝑁(𝑡
%
) − 𝐴𝑇𝑁(𝑡
%'(
)
𝑡
%
− 𝑡
%'(
(2)
BC(t
i
)
= Black carbon at time
t
i
(
µ
g/m
3
)
A
= Filter collection area (D = 3mm) = 7.07x10
-7
m
2
MAC
= Mass absorption coefficient of BC at 880 nm = 12.5x10
-6
m
2
/
µ
g
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
Photodiode
(Signal)
Photodiode
(Reference)
LED (880 nm)
Inlet
Exhaust
(to pump etc.)
Glass
Glass
BC
deposit
Aerosol
filter
Filter Support
Material (tab)
: Sample flow
: Filtered flow