Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol
Equivalent black carbon (EBC) measured by a multi-wavelength Aethalometer can be apportioned to traffic and wood burning. The method is based on the differences in the dependence of aerosol absorption on the wavelength of light used to investigate the sample, parameterized by the source-specific abs...
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/4229/2017/acp-17-4229-2017.pdf |
| _version_ | 1817992327641169920 |
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| author | P. Zotter H. Herich M. Gysel I. El-Haddad Y. Zhang G. Močnik C. Hüglin U. Baltensperger S. Szidat A. S. H. Prévôt |
| author_facet | P. Zotter H. Herich M. Gysel I. El-Haddad Y. Zhang G. Močnik C. Hüglin U. Baltensperger S. Szidat A. S. H. Prévôt |
| author_sort | P. Zotter |
| collection | DOAJ |
| description | Equivalent black carbon (EBC) measured by a
multi-wavelength Aethalometer can be apportioned to traffic and wood
burning. The method is based on the differences in the dependence of aerosol
absorption on the wavelength of light used to investigate the sample,
parameterized by the source-specific absorption Ångström exponent
(<i>α</i>). While the spectral dependence (defined as <i>α</i> values) of
the traffic-related EBC light absorption is low, wood smoke particles
feature enhanced light absorption in the blue and near ultraviolet. Source
apportionment results using this methodology are hence strongly dependent on
the <i>α</i> values assumed for both types of emissions: traffic <i>α</i><sub>TR</sub>, and wood burning <i>α</i><sub>WB</sub>. Most studies use a single
<i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> pair in the Aethalometer model,
derived from previous work. However, an accurate determination of the source
specific <i>α</i> values is currently lacking and in some recent
publications the applicability of the Aethalometer model was questioned.<br><br>Here we present an indirect methodology for the determination of <i>α</i><sub>WB</sub> and <i>α</i><sub>TR</sub> by comparing the source apportionment of EBC
using the Aethalometer model with <sup>14</sup>C measurements of the EC fraction
on 16 to 40 h filter samples from several locations and campaigns across
Switzerland during 2005–2012, mainly in winter. The data obtained at eight
stations with different source characteristics also enabled the evaluation
of the performance and the uncertainties of the Aethalometer model in
different environments. The best combination of <i>α</i><sub>TR</sub> and
<i>α</i><sub>WB</sub> (0.9 and 1.68, respectively) was obtained by fitting the
Aethalometer model outputs (calculated with the absorption coefficients at
470 and 950 nm) against the fossil fraction of EC (EC<sub>F</sub> ∕ EC) derived
from <sup>14</sup>C measurements. Aethalometer and <sup>14</sup>C source apportionment
results are well correlated (<i>r</i> = 0.81) and the fitting residuals exhibit
only a minor positive bias of 1.6 % and an average precision of 9.3 %.
This indicates that the Aethalometer model reproduces reasonably well the
<sup>14</sup>C results for all stations investigated in this study using our best
estimate of a single <i>α</i><sub>WB</sub> and <i>α</i><sub>TR</sub> pair. Combining
the EC, <sup>14</sup>C, and Aethalometer measurements further allowed assessing the
dependence of the mass absorption cross section (MAC) of EBC on its source.
Results indicate no significant difference in MAC at 880 nm between EBC
originating from traffic or wood-burning emissions. Using EC<sub>F</sub> ∕ EC as
reference and constant a priori selected <i>α</i><sub>TR</sub> values, <i>α</i><sub>WB</sub> was also calculated for each individual data point. No clear
station-to-station or season-to-season differences in <i>α</i><sub>WB</sub> were
observed, but <i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> values are
interdependent. For example, an increase in <i>α</i><sub>TR</sub> by 0.1 results
in a decrease in <i>α</i><sub>WB</sub> by 0.1. The fitting residuals of different
<i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> combinations depend on EC<sub>F</sub> ∕ EC
such that a good agreement cannot be obtained over the entire EC<sub>F</sub> ∕ EC
range using other <i>α</i> pairs. Additional combinations of <i>α</i><sub>TR</sub> = 0.8, and 1.0 and <i>α</i><sub>WB</sub> = 1.8 and 1.6,
respectively, are possible but only for EC<sub>F</sub> ∕ EC between ∼ 40 and 85 %. Applying <i>α</i> values previously used in the
literature such as <i>α</i><sub>WB</sub> of ∼ 2 or any <i>α</i><sub>WB</sub> in combination with <i>α</i><sub>TR</sub> = 1.1 to our data set
results in large residuals. Therefore we recommend to use the best <i>α</i>
combination as obtained here (<i>α</i><sub>TR</sub> = 0.9 and <i>α</i><sub>WB</sub> = 1.68) in future studies when no or only limited additional
information like <sup>14</sup>C measurements are available. However, these results
were obtained for locations impacted by black carbon (BC) mainly from
traffic consisting of a modern car fleet and residential wood combustion
with well-constrained combustion efficiencies. For regions of the world with
different combustion conditions, additional BC sources, or fuels used, further
investigations are needed. |
| first_indexed | 2024-04-14T01:23:57Z |
| format | Article |
| id | doaj.art-e110835556d2497ba427f6b17a700b04 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-04-14T01:23:57Z |
| publishDate | 2017-03-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-e110835556d2497ba427f6b17a700b042022-12-22T02:20:31ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242017-03-011764229424910.5194/acp-17-4229-2017Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosolP. Zotter0H. Herich1M. Gysel2I. El-Haddad3Y. Zhang4G. Močnik5C. Hüglin6U. Baltensperger7S. Szidat8A. S. H. Prévôt9Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandLaboratory for Air Pollution and Environmental Technology, Swiss Federal Laboratories for Materials Science and Technology (Empa), Überlandstrasse 129, 8600 Dübendorf, SwitzerlandLaboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandLaboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandLaboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandResearch and Development Department, Aerosol d.o.o., Ljubljana, SloveniaLaboratory for Air Pollution and Environmental Technology, Swiss Federal Laboratories for Materials Science and Technology (Empa), Überlandstrasse 129, 8600 Dübendorf, SwitzerlandLaboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandDepartment of Chemistry and Biochemistry, University of Bern, Bern, SwitzerlandLaboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen PSI, SwitzerlandEquivalent black carbon (EBC) measured by a multi-wavelength Aethalometer can be apportioned to traffic and wood burning. The method is based on the differences in the dependence of aerosol absorption on the wavelength of light used to investigate the sample, parameterized by the source-specific absorption Ångström exponent (<i>α</i>). While the spectral dependence (defined as <i>α</i> values) of the traffic-related EBC light absorption is low, wood smoke particles feature enhanced light absorption in the blue and near ultraviolet. Source apportionment results using this methodology are hence strongly dependent on the <i>α</i> values assumed for both types of emissions: traffic <i>α</i><sub>TR</sub>, and wood burning <i>α</i><sub>WB</sub>. Most studies use a single <i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> pair in the Aethalometer model, derived from previous work. However, an accurate determination of the source specific <i>α</i> values is currently lacking and in some recent publications the applicability of the Aethalometer model was questioned.<br><br>Here we present an indirect methodology for the determination of <i>α</i><sub>WB</sub> and <i>α</i><sub>TR</sub> by comparing the source apportionment of EBC using the Aethalometer model with <sup>14</sup>C measurements of the EC fraction on 16 to 40 h filter samples from several locations and campaigns across Switzerland during 2005–2012, mainly in winter. The data obtained at eight stations with different source characteristics also enabled the evaluation of the performance and the uncertainties of the Aethalometer model in different environments. The best combination of <i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> (0.9 and 1.68, respectively) was obtained by fitting the Aethalometer model outputs (calculated with the absorption coefficients at 470 and 950 nm) against the fossil fraction of EC (EC<sub>F</sub> ∕ EC) derived from <sup>14</sup>C measurements. Aethalometer and <sup>14</sup>C source apportionment results are well correlated (<i>r</i> = 0.81) and the fitting residuals exhibit only a minor positive bias of 1.6 % and an average precision of 9.3 %. This indicates that the Aethalometer model reproduces reasonably well the <sup>14</sup>C results for all stations investigated in this study using our best estimate of a single <i>α</i><sub>WB</sub> and <i>α</i><sub>TR</sub> pair. Combining the EC, <sup>14</sup>C, and Aethalometer measurements further allowed assessing the dependence of the mass absorption cross section (MAC) of EBC on its source. Results indicate no significant difference in MAC at 880 nm between EBC originating from traffic or wood-burning emissions. Using EC<sub>F</sub> ∕ EC as reference and constant a priori selected <i>α</i><sub>TR</sub> values, <i>α</i><sub>WB</sub> was also calculated for each individual data point. No clear station-to-station or season-to-season differences in <i>α</i><sub>WB</sub> were observed, but <i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> values are interdependent. For example, an increase in <i>α</i><sub>TR</sub> by 0.1 results in a decrease in <i>α</i><sub>WB</sub> by 0.1. The fitting residuals of different <i>α</i><sub>TR</sub> and <i>α</i><sub>WB</sub> combinations depend on EC<sub>F</sub> ∕ EC such that a good agreement cannot be obtained over the entire EC<sub>F</sub> ∕ EC range using other <i>α</i> pairs. Additional combinations of <i>α</i><sub>TR</sub> = 0.8, and 1.0 and <i>α</i><sub>WB</sub> = 1.8 and 1.6, respectively, are possible but only for EC<sub>F</sub> ∕ EC between ∼ 40 and 85 %. Applying <i>α</i> values previously used in the literature such as <i>α</i><sub>WB</sub> of ∼ 2 or any <i>α</i><sub>WB</sub> in combination with <i>α</i><sub>TR</sub> = 1.1 to our data set results in large residuals. Therefore we recommend to use the best <i>α</i> combination as obtained here (<i>α</i><sub>TR</sub> = 0.9 and <i>α</i><sub>WB</sub> = 1.68) in future studies when no or only limited additional information like <sup>14</sup>C measurements are available. However, these results were obtained for locations impacted by black carbon (BC) mainly from traffic consisting of a modern car fleet and residential wood combustion with well-constrained combustion efficiencies. For regions of the world with different combustion conditions, additional BC sources, or fuels used, further investigations are needed.http://www.atmos-chem-phys.net/17/4229/2017/acp-17-4229-2017.pdf |
| spellingShingle | P. Zotter H. Herich M. Gysel I. El-Haddad Y. Zhang G. Močnik C. Hüglin U. Baltensperger S. Szidat A. S. H. Prévôt Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol Atmospheric Chemistry and Physics |
| title | Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol |
| title_full | Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol |
| title_fullStr | Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol |
| title_full_unstemmed | Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol |
| title_short | Evaluation of the absorption Ångström exponents for traffic and wood burning in the Aethalometer-based source apportionment using radiocarbon measurements of ambient aerosol |
| title_sort | evaluation of the absorption angstrom exponents for traffic and wood burning in the aethalometer based source apportionment using radiocarbon measurements of ambient aerosol |
| url | http://www.atmos-chem-phys.net/17/4229/2017/acp-17-4229-2017.pdf |
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