DST ObservAir Manual de instrucciones - Página 3

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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.
4
ObservAir Operating Manual
LED (880 nm)
Glass
BC
deposit
Inlet
Glass
Photodiode
(Signal)
: Sample flow
: Filtered flow
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).
= 100 × ln *
I
= Light intensity through reference filter (ADC count)
ref
I
= Light intensity through signal filter (ADC count)
sig
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:
Δ
(
) =
∙
=
∙ (
)
Δ
∙ ()
%
%
) = Black carbon at time t
(µg/m
BC(t
i
i
A = Filter collection area (D = 3mm) = 7.07x10
MAC = Mass absorption coefficient of BC at 880 nm = 12.5x10
) = Flow rate at time t
(m
/sec)
Q(t
3
i
i
∆ATN = Difference of two ATN measurements = ATN(t
∆t = Measurement interval (seconds) = t
Exhaust
Aerosol
filter
(to pump etc.)
Filter Support
Material (tab)
Photodiode
(Reference)
, (1)
!"#
$%&
(
) − (
)
∙
(2)
%
%'(
−
%
%'(
)
3
m
-7
2
m
/µg
-6
2
)- ATN(t
)
i
i-1
- t
i
i-1
ObservAir
Operating Manual
®
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