value). This measurement degradation is known as the ‘filter loading
artifact’. In the ObservAir it is corrected using the equation below,
developed by Novakov and Kirchstetter (See References 1 and 2 at the end
of this section).
BC
corr
(t) = Filter loading corrected BC at time ‘t’ (µg/m
3
)
BC
raw
(t) = Raw, uncorrected BC at time ‘t’ (µg/m
3
)
ATN(t) = Optical attenuation at time ‘t’
k = Filter loading correction factor
The filter loading artifact is not static – it depends on the emissions source
(e.g. biomass vs. diesel), atmospheric conditions, seasonality, and other
factors. Therefore, the filter loading correction factor should be
periodically derived at the deployment site. This is done by collocating two
ObservAir units, each operating at a different flow rate. Typically, one unit
is operated at 50 ccm, while the other is operated at 100 or 125 ccm. As
the two filters load unevenly, the two sensors’ BC data will diverge: BC
measurements from the high flow unit will be lower than the BC
measurements from the low flow unit. Using this collocated data, a filter
loading correction factor can be iteratively derived that best reconciles the
two sensors’ BC measurements. Methods for field derivation of the filter
loading correction factor are currently being published. Please contact
DSTech for more details.
By default, the filter loading correction factor is 0.66 – this factor
was derived for urban sampling conditions in California, and has been
found to be applicable in similar sampling environments.
The filter loading artifact can be largely eliminated by operating
the sensor at low optical attenuation levels ( ATN < ~40). However, this
requires frequent replacement of the filter which may not always be
practical and convenient, and some BC measurement error necessarily
remains. Therefore, filter loading correction algorithms should be
calibrated and validated for each particular application of the ObservAir,
and periodically updated over long-term deployments.
The filter loading artifact is a well-known issue with absorption
photometers, and many algorithms have developed and evaluated. Some
useful resources have been provided below, and there are many more in
the literature.
Filter loading correction references:
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