Aerosol black carbon is a unique primary tracer for combustion emissions. It
affects the optical properties of the atmosphere and is recognized as the
second most important anthropogenic forcing agent for climate change. It is
the primary tracer for adverse health effects caused by air pollution. For
the accurate determination of mass equivalent black carbon concentrations in
the air and for source apportionment of the concentrations, optical
measurements by filter-based absorption photometers must take into account
the “filter loading effect”. We present a new real-time loading effect
compensation algorithm based on a two parallel spot measurement of optical
absorption. This algorithm has been incorporated into the new Aethalometer
model AE33. Intercomparison studies show excellent reproducibility of the
AE33 measurements and very good agreement with post-processed data obtained
using earlier Aethalometer models and other filter-based absorption
photometers. The real-time loading effect compensation algorithm provides
the high-quality data necessary for real-time source apportionment and for
determination of the temporal variation of the compensation parameter
Model equations describing the filter loading effect.
The combustion of carbonaceous fuels inevitably results in the emission of
gas and particulate air pollutants. One of the fractions of the emitted
particles are light-absorbing carbonaceous aerosol compounds, in particular
black carbon (BC), an aerosol species exhibiting very large optical absorption
across the visible part of the optical spectrum. Black carbon is a unique
primary tracer for combustion emissions as it has no non-combustion sources.
It is inert and can be transported over great distances (Hansen et al.,
1989; Bodhaine, 1995; Sciare et al., 2009). Black carbon affects the optical
properties of the atmosphere when suspended and is recognized as the second
most important anthropogenic forcing agent for climate change after CO
Optical methods are used for real-time determination of aerosol black carbon concentrations. The sample air stream is drawn through a filter tape, and the aerosol particles are collected on it. Optical filter photometers perform measurements of light transmission (Hansen et al., 1982, 1984; Bond et al., 1999) or a combination of reflection and transmission measurements (Petzold et al., 2005). Transmission of light through the sample-laden filter is measured and the attenuation (ATN) coefficient is calculated from the rate of attenuation change with time. The attenuation coefficient is converted to the absorption coefficient, and the mass equivalent black carbon concentration (Petzold et al., 2013) is calculated by dividing the absorption coefficient with the BC specific mass absorption cross section. Other methods to determine the absorbing and/or refractory and/or thermally stable carbonaceous aerosol include photo-acoustic detection, light-induced incandescence and thermal–optical analysis (Arnott et al., 1999; Stephens et al., 2003; Chow et al., 1993; Birch and Cary, 1996; Cavalli et al., 2010).
All filter-based optical methods exhibit loading effects (Bond et al., 1999;
Weingartner et al., 2003; Virkkula, 2010; Virkkula et al., 2005, 2007;
Collaud Coen et al., 2010; Park et al., 2010; Hyvärinen
et al., 2013). The relationship between attenuation and BC surface loading
is linear for low attenuation values. However, this relationship becomes
nonlinear when the attenuation values are high due to a filter saturation
effect (Gundel et al., 1984). In the presence of saturation, assuming linear
proportionality between attenuation and the BC surface loading would lead to
underestimation of the BC concentrations. The measurements must be
compensated for this loading effect, so that they accurately represent the
ambient BC concentrations. This has been performed in the past with an
algorithm embedded in the filter photometer, assuming fixed compensation
parameters (Bond et al., 1999; Virkkula, 2010; Virkkula et al., 2005, 2007) or with an attempt to compensate the data by performing an
additional measurement of reflection (Petzold and Schönlinner, 2004) or
scattering (Arnott et al., 2005). Alternatively, a post-processing algorithm
can be employed using either fixed parameters (Hyvärinen et al., 2013)
or parameters obtained from the data themselves (Virkkula et al.,
2007; Park et al., 2010; Hyvärinen et al., 2013). Weingartner et al. (2003) and Virkkula et al. (2007) models have been most frequently used for
loading effect compensation of the Aethalometer data (Table 1). The
non-compensated concentration, BC
Post-processing the data allows determination of slowly changing compensation parameters which are representative for a longer measurement period. However, it has been shown that the loading effect differs between the seasons, depending on the aerosol properties (Virkkula et al., 2007). Weingartner et al. (2003) have shown that fresh aerosols exhibit more pronounced loading effects than aged ones. The assumption of fixed or specified aerosol properties will introduce systematic errors in the data when the assumed values are incorrect for the sampled aerosol. Especially aerosols from local sources exhibit great temporal and spatial variability and these variations need to be addressed in the algorithms to accurately compensate the data. Post-processing eliminates the loading effects in the data but it frequently relies upon assumptions of the compensation parameters. It also requires time and resources for data processing. The compensation parameters depend on the wavelength of light used in the analysis (Weingartner et al., 2003). Accurate compensation of absorption measurements is essential if the spectral measurements are used for source apportionment of BC or carbonaceous aerosols (Sandradewi et al., 2008b).
Therefore, it is absolutely crucial that the compensation is validated. In filter photometers, there is an automatic tape change when an attenuation threshold is reached. Thus, one can observe a discontinuity of BC when transitioning from a loaded spot to a fresh one, where loading effects are negligible. Providing that the concentration does not change in the time, during which the filter tape is being advanced (typically 20 min), these discontinuities in the BC concentration at the time of the tape advance display the “goodness” of the compensation. A simpler method, making no assumptions, follows the method described in Park et al. (2010) and is developed into a criterion for the test of the compensation in Segura et al. (2014), examining the nonlinearity by plotting the dependence of the measured BC on the accumulated surface loading on the filter as reported by the measured optical attenuation.
The AE33 flow diagram. During measurement, the inlet air passes through filter spots S1 and S2, each with a different flow rate, as set by the orifice 2. Airflow through S1 is measured by the mass flowmeter 1; flow through S2 is calculated as a difference between the total flow (flowmeter C) and flow through S1. The valves are used for routing of airflow during different modes of operation (see the table above). The bypass mode is used during the tape advance procedure with orifice 1 mimicking the filter flow resistance.
A solution, which is superior to the compensation methods described above, is to measure the nonlinearity with high time resolution by the filter photometer itself. First, we start by introducing the loading effect and the method to measure it from the non-compensated data. Then we describe an approach to show how to measure the loading effect. This is achieved by measuring the attenuation of light on two sample spots with different loading and using this information to extrapolate the measurements to zero loading, thereby eliminating the nonlinearity. We introduce the new Aethalometer model AE33 that performs such a measurement and compensates for the nonlinearity in real time.
The new Aethalometer model AE33 collects aerosol particles on the filter continuously by drawing air containing the aerosol through the filter tape. It measures the transmission of light through the filter tape containing the sample and through an unloaded part (spot) of the filter tape acting as a reference area, at seven different wavelengths. It deduces the optical properties and the instantaneous concentration from the rate of change of the attenuation of light in the particle-laden filter. In the newly developed Aethalometer, two measurements are obtained simultaneously from two sample spots with different rates of accumulation of the sample. Both spots derive their samples from the same input air stream. Consequently, any nonlinearity will have the same fundamental characteristics, but the resulting saturation will be of different magnitude. The two results are then combined to eliminate nonlinearities and provide the compensated particle light absorption and BC mass concentration. The method explicitly does not require any knowledge or assumption about the existence, origin or magnitude of the nonlinearity arising from the properties of the aerosol particles collected on the filter.
The newly developed Aethalometer model AE33 (Magee Scientific) follows the
same basic measurement principle as the older models. Aerosol particles are
continually sampled on the filter and the optical attenuation is measured
with high time resolution 1 s or 1 min. Attenuation is measured on two spots
with different sample flows and on the reference spot without the flow
(Fig. 1). The BC mass concentration is calculated from the
change in optical attenuation at 880 nm in the selected time interval using
the mass absorption cross section 7.77 m
Example of the analysis of the filter loading effect: BC as a
function of ATN.
When the attenuation reaches a certain threshold, a tape advance is induced
so that measurements start on a clean spot. The attenuation threshold, first
reached at the lowest wavelength 370 nm, is called ATN
Two different filter tape materials were tested in the new Aethalometer. Both are composed of filtering fibers attached to the polyester backing. The quartz filter is the same as used in the older Aethalometer types (Pallflex Q250F) and tetrafluoroethylene (TFE)-coated glass filter (Pallflex “Fiberfilm” T60A20). Different filter types influence the loading effect and the multiple scattering in the filter matrix differently. The main reason to introduce a TFE-coated glass filter is the sensitivity of the quartz filter to changes in sample air stream humidity. While the Global Atmospheric Watch recommendation states that aerosol optical properties should be measured at low and stable relative humidity (WMO/GAW, 2003), many instruments for measurement of aerosol absorption and scattering are operated without them.
The attenuation of light in the filter tape loaded with the sample is
calculated as
However, the presence of the loading effect in filter-based absorption photometers causes an ATN-dependent change in the instrumental sensitivity. In the Aethalometers, this is seen as the reduction of measured BC at higher attenuations (Fig. 2a). For the quantitative analysis, low values of attenuation are omitted because of the transients after tape advances. At attenuations above a certain value, the frequency of measurements drops (Fig. 2b) and the assumption of Eq. (2) does not hold any more (for example, tape advances may be triggered by high BC events). Consequently, data with ATN > 45 in Fig. 2a are not used for fitting the dependence of the instrumental sensitivity as the function of ATN.
Comparison of the filter loading effect models. The Weingartner et
al. (2003) model curve (Table 1) was calculated using
In Aethalometers, BC(ATN) can be well approximated using a linear fit (Fig. 2a),
in which the average concentration in the analyzed period (BC
It was shown that the filter loading effect changes with location and time of the year (Virkkula et al., 2007). The BC(ATN) analysis, performed on data from five sites across the globe, shows significantly different values of the loading effect relative slope (Figs. S1–S5 and Table S1 in the Supplement). For the development of the real-time compensation algorithm, which will be applicable to aerosol particles with different optical properties, the loading effect must be measured and parameterized with the same time resolution as the measurement itself. Herein, we describe a dual-spot approach, where the data from two filter spots with different loading are used to measure the loading effect, calculate the compensation parameter and apply it to the data to obtain compensated results.
Gundel et al. (1984) showed that the optical attenuation measured at 633 nm (Rosen et al., 1978) for samples collected on quartz filter started to
deviate from the linear relationship as a function of surface loading (
The loading effect is a cumulative property of the cumulative deposit of
particle material on the filter rather than the instantaneous black carbon
concentration. Consequently, it has to be calculated from the total filter
loading
The calculation of
The calculation of BC is based on Eq. (6). Because the airflow is measured
after the air passes the filter, lateral airflow in the filter matrix under
the optical chamber
The dual-spot compensation algorithm was evaluated in the field during a measurement campaign in Klagenfurt (Austria) in March 2012. The performance of the new algorithm was tested for the Aethalometer model AE33 and compared to the offline compensation (Weingartner et al., 2003) used to post-process the Aethalometer model AE31 data manually.
Comparison of the uncompensated and compensated data from the
Klagenfurt campaign:
The Klagenfurt campaign took place between 1 and 12 March 2012 at the air
quality station of the Carinthian regional government at Völkermarkter
Strasse, located in the middle of the busy intersection and strongly
influenced by local traffic. Measurements were made using one Aethalometer
model AE31 instrument (with the regular Q250 quartz filter tape) and two
Aethalometer model AE33 instruments (one with the Q250 quartz filter tape,
the other with T60A20 TFE-coated glass fiber filter tape) during the whole
campaign. Each of the instruments was operated at an airflow of 5 L min
A set of measurements was made in Leipzig as part of the ACTRIS
intercomparison workshop at TROPOS (Leibniz Institute for Tropospheric
Research, Germany) from 18 to 21 February 2013 (ACTRIS, 2014a). Three
Aethalometers AE33, several AE31 (average age 7 years,
range 1 to 14 years), one AE22 and a MAAP (Multiangle Absorption
Photometer, model 5012, Thermo Scientific) were connected to a common inlet;
not necessarily all of the instruments were operational at all times during
the campaign. The aerosol was dried using a silica-gel diffusion drier.
During the campaign, the weather was cloudy and windy with temperatures
slightly below 0
Source apportionment of ambient BC concentrations is based on the Sandradewi
et al. (2008b) model. Briefly, the two-component model considers the aerosol
optical absorption coefficient as a sum of biomass burning and fossil fuel
combustion fractions and takes advantage of the difference in the
wavelength dependence of absorption: since fossil fuel and biomass
contributions to aerosol absorption feature specific values of the
absorption Ångström exponent, it is possible to construct a source
specific two-component model. We assumed the two sources follow
Summary of the characterization of the filter loading effects
on the data for AE31 and AE33 (two different filter materials)
during the Klagenfurt campaign.
During the Klagenfurt campaign, we observed filter loading effects in all
tested Aethalometer data as expected. The effect is most pronounced for the
370 nm channel, which has highest attenuation (Fig. 4). The loading effect
can be analyzed either by checking the magnitude of discontinuities at tape
advances (Fig. 4a) or by performing
Data from Aethalometer AE31 were compensated manually using the Weingartner
model (Weingartner et al., 2003). The compensation parameter
Filter loading effect characterization for AE31 using a quartz filter before (raw data) and after loading effect compensation during the Klagenfurt campaign. The compensation was performed using the Weingartner et al. (2003) model.
The real-time loading effect compensation algorithm was tested in the
Aethalometer model AE33 using both the quartz Q250 and the new TFE-coated
glass filter. As mentioned before, we introduced the TFE-coated glass filter
to reduce changes in the sample air stream humidity which influence the
correct determination of the compensation parameter
The real-time compensation algorithm was evaluated during the winter
measurement campaign in Klagenfurt. Figure 6 shows the relative slope of
Filter loading effect characterization in the AE33 using
quartz (left panel) and TFE-coated glass fiber filters (right panel) during
the Klagenfurt campaign. The loading effect compensation was performed using
the real-time dual-spot algorithm. Relative slopes of
The effectiveness of the compensation algorithm can be tested in two
different ways. The first is the comparison of the parameter value at low
attenuations with the campaign average for the same parameter – for
example, the intersect of the BC concentration, the absorption coefficient
or the absorption Ångström exponent (BC
Comparison of the determined parameters for quartz and TFE-coated
glass fiber filters in the AE33 (Klagenfurt campaign):
The loading effect is stronger at lower wavelengths than at higher ones,
influencing the measurement of absorption to a higher degree in blue than in
the infrared (IR) part of the spectrum. The loading effect therefore reduces the
determined absorption Ångström exponent
Measurements using the AE33 have been performed with two filter types: the
quartz Q250 and the new TFE-coated glass filter, allowing a determination of
the value of the absorption enhancement parameter
Comparison of the average compensation parameter
The parameter
The maximum surface loading should be calculated from the measured parameters in the infrared to avoid any influence of the additional absorbing compounds, such as the ones present in biomass smoke and sometimes called “Brown Carbon”. These absorb heavily in the ultraviolet (UV) and blue but not in the IR part of the spectrum. By calculating the loading in the IR, absorption can be attributed to BC alone, and the surface loading can be calculated as the black carbon loading. In the IR, the attenuation reaches typically values around 50 for a fully loaded spot.
The maximal loading is approximately 1.43 times higher for the TFE-coated
glass fiber filter, similar to the average difference in values of parameter
A comparison of the parameter
Temporal variability of the compensation parameter
The real-time calculation of the compensation parameter
The slope and
Another advantage of using a real-time method for loading effect
compensation is the possibility to perform real-time source apportionment of
BC using a two-component model (Sandradewi et al., 2008b). A diurnal plot of
the biomass burning contribution to BC concentration shows a trend typical
for Alpine cities, where wood is used for the residential heating (compare,
for example, Favez et al., 2010); there is a large contribution of biomass
burning to BC concentration, with a maximum at 32 % during the night
(Fig. 9). The morning peak and the onset of the afternoon peak in BC
concentration are consistent with the increased traffic. The midday dip in
BC is most probably caused by the thermally induced vertical mixing, because
the campaign took place during the period of sunny weather. The reduction of
During the Klagenfurt campaign, data from the AE31 with the quartz filter
and AE33 with TFE-coated glass filter were compared (Fig. 10). The AE31
data were compensated for the loading effects, using the Weingartner
post-processing algorithm, while the AE33 data were compensated using the
real-time “dual-spot” algorithm. The results show excellent agreement
between the instruments across a wide dynamic range, with
During the ACTRIS campaign in Leipzig the Aethalometers AE31, AE22, AE33 and a MAAP were compared. These instruments are all filter-based absorption photometers but use different filter materials and differ in the measurement technique; Aethalometer measurements are based on the measurement of transmission of light (880 nm) through one sample spot (AE31, AE22) or two spots (AE33), whereas the MAAP uses measurement of transmission and reflection of light (637 nm) at two different angles to derive the absorption coefficient and the BC concentration using a radiative transfer model.
The instruments were compared in controlled conditions with all instruments
connected to the same sampling line, using a diffusion drier. Measurements
were compared with a time resolution 5 min – this is the native time
resolution of the AE31 and AE22, while AE33 and MAAP data were averaged to
this resolution (ACTRIS, 2014a). The AE31 and AE22 data in this study have
not been compensated, because the instruments were run at low filter loading
settings. We show the regression between the AE33 instruments and the MAAP
in Fig. 11 (the time series is shown in Fig. S9). BC concentrations were
higher during the day, showing the contribution of local traffic. During the
2.5 days of the experiment, we observed spectral signatures of
Diurnal plots of
Correlation of BC measurements (2 weeks, Klagenfurt, Austria) between the Aethalometers AE31 (using quartz filter) and AE33 (using TFE-coated glass fiber filter), showing 5 min averages of data compensated for the loading effect.
Regression: BC measurements by AE33 and MAAP. The data were resampled to 5 min for both
instruments. The Aethalometer AE33 uses measurements at 880 nm and
parameters
The AE33 instrumental cross-sensitivity to scattering – the ratio of the apparent absorption coefficient and the scattering coefficient as a function of attenuation (ATN).
The cross-sensitivity of the Aethalometer AE33 to the scattering particles
collected in the filter matrix was tested in a laboratory experiment.
Ammonium sulfate aerosols were used as model scattering aerosols. Ammonium
sulfate was aerosolized in an atomizer from a solution. The airstream was
dried to relative humidity below 30 % by mixing with dry clean air and
additionally using diffusion driers. The particles were injected in the
mixing chamber (volume 0.5 m
The sensitivity of the determination of the absorption coefficient from the AE33 to the scattering is shown in Fig. 12, where we plot the ratio of apparent absorption coefficient and the scattering coefficient as a function of ATN. We see two features: firstly, the cross-sensitivity of absorption to scattering is small in the range of below 1 to 1.5 %, smaller than estimated for older Aethalometers (Rosen and Novakov, 1983). Secondly, the cross-sensitivity is almost constant over the whole experimental range and does not depend significantly on ATN. Cross-sensitivity does have some dependence on the size of the absorbing particles. For the PSAP, Nakayama et al. (2010) found a size-dependent sensitivity. We have used absorbing particles of different size to challenge the response of the Aethalometers AE33 and found that C does not depend greatly on the absorbing particle size within the measured range (ACTRIS, 2014b).
The filter loading effect hinders optical measurements of black carbon
performed by filter-based absorption photometers. This effect can be
analyzed using the BC(ATN) method, in which BC as a function of ATN is analyzed.
Measurements at various locations and times of the year show big differences
in the filter loading effect. In Aethalometers, the loading of the filter
spot results in a linear reduction of the instrumental sensitivity. This
allows the determination of the amplitude of the loading effect using
attenuation measurements from two differently loaded spots – a dual-spot
approach. A new real-time loading effect compensation algorithm based on the
dual-spot measurements was developed and incorporated in the new
Aethalometer model AE33. During a winter measurement campaign in Klagenfurt,
Austria, the performance of the new algorithm was compared with the
Aethalometer AE31 measurements, which were compensated using the Weingartner
et al. (2003) compensation method. The BC(ATN) analyses of the compensated data
indicate excellent performance of the new algorithm for all seven measurement
wavelengths. Improvements in the determination of the spectral dependence of
absorption, as described by the Ångström exponent, allow for
real-time high time resolution source apportionment by the AE33. The
dual-spot compensation algorithm determines the value of the compensation
parameter
The work described herein was financed in part by the EUROSTARS grant E!4825 FC Aeth and JR-KROP grant 3211-11-000519. The Klagenfurt campaign was made possible by the Office of the provincial Carinthian government and the Municipality of Klagenfurt (Austria) as a part of project PMinter. The Leipzig campaign was conducted as a part of the ACTRIS WP3 nephelometer and absorption photometer workshop (ACTRIS project – EU grant agreement no. 262254). The participation of the AE33 from CIEMAT in this workshop was supported through the AEROCLIMA project (CIVP16A1811, Fundación Ramón Areces) and the complementary action of the Spanish national R&D&i plan (Ref. CGL2011-16124-E). We thank E. Swietlicki for the use of the Lund University AE33 during the ACTRIS intercomparison workshop; A. Polidori, South Coast Air Quality Management District, for the data showing the loading effects in Los Angeles, USA; and G. Schauer, Sonnblick Observatory and ZAMG, for the extensive efforts required to run instrumentation on the high-altitude observatory Sonnblick, Austria. L. Drinovec, G. Močnik and A. D. A. Hansen have been/are employed by Aerosol d.o.o. and/or Magee Scientific Corp. during the Aethalometer AE33 development and manufacture. The dual-spot loading compensation algorithm introduced here is in the process of being patented. Edited by: W. Maenhaut