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The influence of the baseline drift on the resulting extinction values of a CAPS PMex

13 Nov 2019-Atmospheric Measurement Techniques Discussions (Copernicus GmbH)-pp 1-9

Abstract: . The effect of the baseline drift on the resulting extinction values of three CAPS PMex monitors with different wavelengths and the respective correlation with NO2 was analysed for an urban background station. A drift of more than 0.8 Mm−1 min−1 was observed for ambient air, with high probability caused by traffic emissions driven changes in carrier gas composition. The baseline drift leads to characteristic measurement artefacts for particle extinction. Artificial particle extinction values of approximately 4 Mm−1 where observed using a baseline period of 5 min. These values can be even higher for longer baseline periods. A new method is shown to minimize this effect. Modified continuous baseline values are calculated in a post-processing step using cubic smoothing splines. With this approach the extinction artefacts are diminished and the effective scattering of the resulting extinction values is reduced by about 50 %.

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Atmos. Meas. Tech., 13, 2161–2167, 2020
https://doi.org/10.5194/amt-13-2161-2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
The influence of the baseline drift on the resulting extinction values
of a cavity attenuated phase shift-based extinction monitor (CAPS
PMex)
Sascha Pfeifer
1
, Thomas Müller
1
, Andrew Freedman
2
, and Alfred Wiedensohler
1
1
Leibniz Institute for Tropospheric Research, Permoserstraße 15, 04318 Leipzig, Germany
2
Aerodyne Research Inc., 45 Manning Road, Billerica, MA 01821-3976, USA
Correspondence: Sascha Pfeifer (pfeifer@tropos.de)
Received: 3 September 2019 Discussion started: 13 November 2019
Revised: 2 April 2020 Accepted: 7 April 2020 Published: 5 May 2020
Abstract. The effect of the baseline drift on the resulting ex-
tinction values of three cavity attenuated phase shift-based
extinction monitors (CAPS PMex) with different wave-
lengths and the respective correlation with NO
2
was anal-
ysed for an urban background station. A drift of more than
0.8 Mm
1
min
1
was observed for ambient air, with high
probability caused by traffic-emissions-driven changes in
carrier gas composition.
The baseline drift leads to characteristic measurement
artefacts for particle extinction. Artificial particle extinc-
tion values of approximately 4 Mm
1
were observed using
a baseline period of 5 min. These values can be even higher
for longer baseline periods.
Two methods are shown to minimize this effect. Mod-
ified continuous baseline values are calculated in a post-
processing step using simple linear interpolation and cu-
bic smoothing splines. Both methods are useful to reduce
artefacts, although the use of cubic smoothing splines gives
slightly better results. The extinction artefacts are diminished
and the effective scattering of the resulting extinction values
is reduced by about 50 %.
1 Introduction
Aerosol particles affect the global albedo or radiation balance
of the earth by interacting with solar and thermal radiation
through absorption and scattering processes. In order to esti-
mate the influence on the climate, it is therefore important to
determine the optical properties of the atmospheric aerosol
with sufficient accuracy. In particular, the aerosol scatter-
ing σ
sp
, absorption σ
ap
and extinction σ
ep
coefficients, from
which the single scattering albedo ω = σ
sp
ep
is derived, are
important parameters.
Various in situ measurement techniques exist for the re-
spective parameters. In the past, cavity ring-down technology
was used to measure the σ
ep
directly (Brown, 2003). A very
similar measurement method is the cavity attenuated phase
shift (CAPS) technique. A square wave modulated light of
a light-emitting diode (LED) is injected in an optical cavity,
defined by two high reflectivity mirrors (R > 0.999) in a dis-
tance of 26 cm. The phase shift of the distorted signal caused
by the effective optical path is measured by a vacuum photo
diode on the opposite side. This is a robust, state-of-the-art
and commercially available measurement method, which is
also used as a gas monitor to measure ambient NO
2
concen-
tration (Kebabian et al., 2005, 2008).
The cavity attenuated phase shift-based extinction moni-
tors (CAPS PMex) (Massoli et al., 2010) enable the mea-
surement of the σ
ep
by periodically changing between ambi-
ent air (normal measuring period) and particle-free air (base-
line period). The Rayleigh scattering value for air σ
ea
at a
given temperature and pressure condition is subtracted from
the respective raw signals, called total loss (“loss”). The re-
sulting values are averaged over the period of baseline dura-
tion. This value is called last baseline (“lastbaseline”), which
not only depends on device parameters, in particular the de-
gree of contamination of the cavity mirrors, but also on the
concentration of absorbing gases.
Published by Copernicus Publications on behalf of the European Geosciences Union.

2162 S. Pfeifer et al.: The influence of the baseline drift on the resulting extinction values of a CAPS PMex
The resulting values for particle extinction σ
ep
for the fol-
lowing normal measuring period is calculated as follows:
σ
ep
(T , P ) =
loss(T , P ) σ
ea
(T , p) lastbaseline
g
, (1)
where g is the geometry factor, considering the effect of the
purge air on the effective optical path length. The crucial
point is that the measurement is calculated using the base-
line values, which are assumed to be constant for a certain
period and lag behind in time.
A detailed description of the instrument is given by Mas-
soli et al. (2010). CAPS PMex has already been compared
and characterized in combination with other instruments
(Petzold et al., 2013) and used in various campaigns (Yu
et al., 2013; Perim de Faria et al., 2017).
Although the instrument delivers a satisfying performance,
Massoli et al. (2010) already mentioned deviations due to
baseline drifts. Motivated by this aspect, the aim of this work
is to examine the effect of the baseline drift in more detail.
For this purpose, exemplary measurements at an urban back-
ground station are analysed. In addition, a possible approach
in post-processing is proposed to reduce the influence of the
baseline drift.
2 Experimental set-up
In order to analyse the influence of the baseline drift on
the resulting extinction values, measurements of ambient air
were carried out at the Leibniz Institute for Tropospheric Re-
search (Leipzig, Germany) over a period of 2 weeks. The
measurement site, classified as an urban background station,
is influenced by two main roads and rail traffic, as well as a
small gas power plant.
The measurements were performed with three different
CAPS PMex monitors of different wavelengths: CAPS-blue
(450 nm), CAPS-green (530 nm) and CAPS-red (630 nm).
The sampling rate for all CAPS PMex monitors was set to
1 Hz. The baseline period was set to 5 min with 60 s duration
and 30 s flushing time.
In addition, the concentration of equivalent black carbon
(eBC) was measured with a multi-angle absorption photome-
ter (MAAP) at the same inlet system with a time resolution
of 1 min. Furthermore, the NO
x
concentration was measured
with an APNA-370 Ambient NO
x
Monitor at a separate inlet
at the rooftop with 3 min resolution.
To analyse the influence of the variability of the gas con-
centration of the carrier gas and to rule out the influence of
aerosols on the resulting extinction values, an additional fil-
ter was installed upstream of the three CAPS PMex moni-
tors. According to a zero filter test, values are expected to be
around zero for the whole period. Deviations from this indi-
cate a systematic error.
Before and after measurements the quality of the CAPS
PMex monitors were checked by a comparison with a
Table 1. Correlation coefficients of the loss values of the three
CAPSs (450, 530 and 630 nm), and the eBC and NO
2
concentra-
tions.
Loss Loss Loss eBC NO
2
(blue) (green) (red)
Loss (blue) 1.000 0.845 0.255 0.785 0.945
Loss (green) 1.000 0.201 0.605 0.844
Loss (red) 1.000 0.221 0.175
eBC 1.000 0.773
NO
2
1.000
thoroughly and regularly calibrated reference nephelometer
(Ecotech Aurora 4000) using CO
2
as a high-span gas. For
this purpose non-absorbing ammonium sulfate particles were
used. The truncation error in the nephelometer has been cor-
rected using the method of Müller et al. (2011). The values
were adjusted to the corresponding wavelength of the CAPS
PMex monitors (450, 530 and 630 nm) by using scattering
Ångström exponents.
Nevertheless relatively small particles were generated
(mean size of approx. 50 nm) to minimize the effect of trun-
cation. Analogous to the comparison of the measured and
Mie calculated theoretical values using mono-disperse par-
ticles and a reference CPC (Petzold et al., 2013), correction
factors can be derived by comparing the truncation-corrected
scatter values of the reference nephelometer with the respec-
tive measured extinction values. The factors represent a cor-
rection of internal calibration, which primarily consider the
influence of the variable ratio of purge air and sample air flow
rate. In order to reduce the influence of potential non-linear
effects of CAPS PMex, only values less than 500 were used
for this analysis.
3 Results
3.1 Variability of background signal
Time series for the loss signals of all three CAPS PMex
monitors, as well as the eBC and NO
2
concentrations, are
shown in Fig. 1. CAPS-blue shows a significant variabil-
ity of the loss signal, with background values of 585 Mm
1
and peaks up to 635 Mm
1
. CAPS-green shows identical be-
haviour but with a lower amplitude, with background val-
ues of 380 Mm
1
and peaks up to 400 Mm
1
. The values for
CAPS-red are independent and rather stable, ranging from
480 Mm
1
up to 484 Mm
1
. During the 2-week period the
maximum eBC and NO
2
concentrations were 5 µgm
3
and
45 ppb, respectively.
In Table 1 the corresponding correlation coefficients for
the time series are shown. As already expected from Fig. 1,
both loss values of CAPS-blue and CAPS-green are highly
correlated (R
2
= 0.845). The highest correlation is found be-
Atmos. Meas. Tech., 13, 2161–2167, 2020 www.atmos-meas-tech.net/13/2161/2020/

S. Pfeifer et al.: The influence of the baseline drift on the resulting extinction values of a CAPS PMex 2163
Figure 1. Time series of loss signal measuring particle-free ambient air (450, 530 and 630 nm) and eBC and NO
2
concentration for a 2-week
period.
tween the loss of CAPS-blue and the NO
2
concentration
(R
2
= 0.945), while the correlation of CAPS-blue with eBC
is R
2
= 0.785. The values of loss from CAPS-red was found
to be uncorrelated to the other variables. On average, the time
series for loss (CAPS-blue and CAPS-green) as well as eBC
and NO
2
show increased values in the night with a maximum
in the late evening and another maximum in the morning. A
minimum occurs at noon. This behaviour is repeated every
day, with the exception of the weekend (21–22 September).
In general, these values follow the daily pattern, resembling
traffic rush hours and development of the planetary bound-
ary layer. Because of the total filter upstream of the CAPS
PMex the measured variability of the loss signal is not due
to aerosol particles. This variability can only be explained by
changes of the ambient air, which is likely based on changes
of NO
2
concentration due to traffic-related emissions. The
steady increase in the loss of CAPS-green in the second week
is significant. The reason for this is unknown. Because a par-
ticle filter was used, it can be excluded that this is based on
contamination by aerosol particles on the cavity mirrors.
However, the variability of the loss signal can be quite
high, whereby the ascending flank is steeper than the de-
scending flank. For CAPS-blue the rate of change was in
the range of 0.72 to 0.83 Mm
1
min
1
(99 % percentile).
The values for maximum rate of change were 1.78 and
4.15 Mm
1
min
1
respectively. The influence on CAPS-
green is lower but still noticeable with values in the range
of 0.18 to 0.22 Mm
1
min
1
(99 % percentile).
Before and after the measured time series the comparison
of CAPS PMex and reference nephelometer show a small but
very stable deviation, shown in Fig. 2. The devices show val-
ues that are slightly too high, in the range of 3 %–4 %, 6 %–
8 % and 6 %–7 % for the blue, green and red wavelengths,
respectively.
3.2 Artefacts from internal baseline correction
As previously mentioned, variations in the baseline by rapid
change in the concentration of absorbing gases may occur
with values up to 4 Mm
1
min
1
. Hence, the assumption of
a constant baseline value for internal data processing may
cause uncertainties.
Any changes of the baseline during a normal measuring
period due to changes in gas composition are immediately
www.atmos-meas-tech.net/13/2161/2020/ Atmos. Meas. Tech., 13, 2161–2167, 2020

2164 S. Pfeifer et al.: The influence of the baseline drift on the resulting extinction values of a CAPS PMex
Figure 2. Comparison of measured extinction (450, 530 and 630 nm) and scattering measured with a nephelometer (truncation corrected) for
non-absorbing ammonium sulfate particles after the measured time series.
misinterpreted as aerosol extinction. Furthermore, due to the
forward extrapolation, the internal “lastbaseline” value is
phase shifted to the supposedly correct value.
Figure 3 shows a 1 h excerpt from the time series of CAPS-
blue. A smooth and continuous increase in the loss signal
from 590 to 620 Mm
1
for the measuring period is observed.
The time series for the “lastbaseline” value shows a step-like
function, which is phase-shifted relative to the loss signal.
This results in artificial extinction values of up to 5 Mm
1
with a saw-tooth structure. For a continuously increasing loss
signal the extinction values are strictly positive. The opposite
is true for decreasing loss signals. Due to the stronger in-
crease than decrease for loss signal, the resulting extinction
values are not symmetrically distributed.
It is possible to reduce these artefacts by using interpo-
lation methods. Two different procedures were considered.
The first one is a simple and often used linear interpolation
method. A second, and potentially better, alternative is the
use of cubic smoothing splines.
For this post-processing the loss values for the base-
line period were extracted, subtracted by the corresponding
Rayleigh value and used as predictor variables for interpo-
lation with cubic smoothing splines. The cubic smoothing
spline function (smooth.spline) provided by R (R Core Team,
2020) was used for this purpose. A free smoothing parameter
(“spar”) must be chosen, which depends on many factors, e.g.
on baseline period and duration but also on sampling rate and
device noise. Therefore, a suitable parameter must be found
for each individual device and application. For the case with
1 Hz sampling rate, a baseline period of 5 min and a duration
of 1 min, the smoothing parameters used were 1.1, 1.3 and
1.4 for the blue, green and red, respectively. These values
were determined by minimizing the artefacts of a separate
test dataset. Alternatively it is also possible to automatically
determine a suitable smoothing parameter from the time se-
ries of the baseline using for example the implemented gen-
eralized cross-validation method (GCV). The resulting val-
ues of the automatically calculated smoothing parameters us-
ing the GCV method do not differ significantly from the first
method, with values of 1.06 (blue), 1.25 (green) and 1.30
(red). Furthermore, all distinct data points with 1 Hz sam-
pling rate were used (all.knots=TRUE). All other param-
eters were set to default. A complete description of the func-
tion can be found in the R Documentation (R Core Team,
2020).
It should be emphasized that the use of any interpolation
method to recalculate the baseline has its limits. Only trends
that can be estimated from the baseline data can be repro-
duced for “lastbaseline”. It is impossible to reproduce any
faster fluctuations that are not covered by the selected base-
line period and duration. Furthermore, when using the cu-
bic smoothing splines there is the possibility that under ex-
treme conditions with strongly fluctuating baseline trends the
method can lead to erroneous overshot structures. In these
cases, the first step should be the readjustment the baseline
settings.
If the requirements are fulfilled, these approaches result
in a continuous time series of current baseline values, with-
out phase-shift relative to the loss signal (see Fig. 3). As
expected, the result is slightly better when cubic smoothing
splines are used, as this is a continuously differentiable func-
tion. Another important difference is that with cubic smooth-
ing splines trends during a baseline measurement are con-
sidered and are therefore reproducible. In contrast to this,
the linear interpolation method uses only one average value
per baseline measurement, analogous to the internal proce-
dure. As a result, there are individual cases where the lin-
ear interpolation does not lead to any improvement of the
extinction values, but there are also cases where the im-
provement corresponds to that of the cubic smoothing spline.
However, in both cases the resulting extinction values im-
prove significantly. In Fig. 4 the resulting histograms and
statistical parameters for particle extinction for all instru-
ments and the entire time series are shown. As expected,
the mean value remains almost unchanged at values close to
zero. But the distribution becomes narrower and more sym-
metrical. For CAPS-blue the standard deviation is reduced
by 43 % using the linear interpolation and 50 % using cu-
Atmos. Meas. Tech., 13, 2161–2167, 2020 www.atmos-meas-tech.net/13/2161/2020/

S. Pfeifer et al.: The influence of the baseline drift on the resulting extinction values of a CAPS PMex 2165
Figure 3. Time series of “lastbaseline” (a) and the resulting extinction values (b) for the uncorrected (blue) and corrected methods, using
linear interpolation (orange) and cubic smoothing splines (red), for CAPS-blue (450 nm) measuring particle-free ambient air. In (a) the loss
signal subtracted by the Rayleigh scattering for the measurement (grey) and baseline period (black) are additionally shown. The resulting
extinction value in (b) is the direct consequence of the deviation between these values the used baseline.
Figure 4. Histogram of the extinction values from measurements of particle-free ambient air for all three wavelengths (450, 530 and 630 nm):
(a)(c) show the uncorrected extinction values, while (d)(f) and (g)(i) show corrected values using linear interpolation and cubic smoothing
splines, respectively. The corresponding statistical parameters of the histograms are indicated in the boxes.
www.atmos-meas-tech.net/13/2161/2020/ Atmos. Meas. Tech., 13, 2161–2167, 2020

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References
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Journal ArticleDOI
Steven S. Brown1Institutions (1)

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TL;DR: A greatly improved version of a previously reported nitrogen dioxide monitor that utilizes cavity attenuated phase shift spectroscopy (CAPS) is presented, and the response to nitrogen dioxide (calculated as the cotangent of the phase shift) is demonstrated to be linear.
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"The influence of the baseline drift..." refers methods in this paper

  • ...This is a robust, state-of-the-art and commercially available measurement method, which is also used as gas monitor to measure ambient NO2 concentration (Kebabian et al., 2005, 2008)....

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Journal ArticleDOI
Abstract: We present laboratory and field measurements of aerosol light extinction ( σep ) using an instrument that employs Cavity Attenuated Phase Shift (CAPS) spectroscopy. The CAPS extinction monitor comprises a light emitting diode (LED), an optical cavity that acts as the sample cell, and a vacuum photodiode for light detection. The particle σep is determined from changes in the phase shift of the distorted waveform of the square-wave modulated LED light that is transmitted through the optical cell. The 3-σ detection limit of the CAPS monitor under dry particle-free air is 3 Mm–1 for 1s integration time. Laboratory measurements of absolute particle extinction cross section ( σext ) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ± 3% (2-σ) at both 445 and 632 nm. A comparison with Mie theory scattering calculations indicates that these results are accurate within the 10% uncertainty stated for the particle number density measurements. The CAPS extinction ...

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"The influence of the baseline drift..." refers background or methods in this paper

  • ...A detailed description of the Instrument is given by Massoli et al. (2010)....

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  • ...Although the instrument delivers a satisfying performance, Massoli et al. (2010) already mentioned deviations due to a baseline drifts....

    [...]

  • ...The CAPS PMex (Massoli et al., 2010) enables the measurement of the σep by periodically changing between ambient air (normal measuring period) and particle-free air (baseline period)....

    [...]


Journal ArticleDOI
Abstract: . An evaluation of the Cavity Attenuated Phase Shift particle light extinction monitor (CAPS PMex) using a combination of a 3-wavelength Integrating Nephelometer (NEPH) and a 3-wavelength filter-based Particle Soot Absorption Photometer (PSAP) was carried out using both laboratory-generated test particles and ambient aerosols. An accurate determination of a fixed pathlength correction for the CAPS PMex was made by comparing extinction measurements using monodisperse PSL spheres in combination with Mie scattering calculations to account for the presence of PSL conglomerates. These studies yielded a linear instrument response over the investigated dynamical range from 20 to 450 Mm−1 (10−6 m−1) with a linear correlation coefficient of R2 > 0.98. The adjustment factor was determined to be 1.05 times that previously reported. Correlating CAPS extinction to extinction measured by the NEPH + PSAP combination using laboratory-generated polydisperse mixtures of purely scattering ammonium sulfate and highly absorbing black carbon provided a linear regression line with slope m = 1.00 (R2 = 0.994) for single-scattering albedo values (λ = 630 nm) ranging from 0.35 (black carbon) to 1.00 (ammonium sulfate). For ambient aerosol, light extinction measured by CAPS was highly correlated (R2 = 0.995) to extinction measured by the NEPH + PSAP combination with slope m = 0.95.

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"The influence of the baseline drift..." refers methods in this paper

  • ...CAPS PMex has already been compared and characterized in combination with other instruments (Petzold et al., 2013) and used in various campaigns (Yu et al....

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  • ...CAPS PMex has already been compared and characterized in combination with other instruments (Petzold et al., 2013) and used in various campaigns (Yu et al., 2013;10 Perim de Faria et al., 2017)....

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