A two year's source apportionment study of wood burning and traffic aerosols for urban and rural sites in Switzerland

Abstract

The contributions of fossil fuel (FF) and wood burning (WB) emissions to black carbon (BC) have been investigated in the past by analysis of multi-wavelength aethalometer data. This approach utilize the stronger light absorption of WB aerosols in the near ultraviolet compared to the light absorption of aerosols from FF combustion. Here we present two years of seven-wavelength aethalometer data from one urban and two rural background sites in Switzerland measured from 2008–2010. The contribution of WB and FF to BC was directly determined from the absorption coefficients of FF and WB aerosols which were calculated by using confirmed absorption exponents and aerosol light absorption cross-sections that were determined for all sites. Reasonable separation of total BC into contributions from FF and WB was achieved for all sites and seasons. The obtained WB contributions to BC are well correlated with measured concentrations of levoglucosan and potassium while FF contributions to BC correlate nicely with NOx. These findings support our approach and show that the applied source apportionment of BC is well applicable for long-term data sets. During winter, we found that BC from WB contributes on average 24–29% to total BC at the considered measurement sites. This is a noticeable high fraction as the contribution of wood burning to the total final energy consumption is in Switzerland less than 4%.

Full text

Page 1

AMTD
3, 5313–5342, 2010
Source
apportionment of
wood burning and
traffic aerosols
H. Herich et al.
Title Page
AbstractIntroduction
ConclusionsReferences
Tables Figures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Atmos. Meas. Tech. Discuss., 3, 5313–5342, 2010
www.atmos-meas-tech-discuss.net/3/5313/2010/
doi:10.5194/amtd-3-5313-2010
© Author(s) 2010. CC Attribution 3.0 License.
Atmospheric
Measurement
Techniques
Discussions
This discussion paper is/has been under review for the journal Atmospheric Measure-
ment Techniques (AMT). Please refer to the corresponding final paper in AMT
if available.
A two year’s source apportionment study
of wood burning and traffic aerosols
for urban and rural sites in Switzerland
H. Herich, C. Hueglin, and B. Buchmann
Empa, Swiss Federal Laboratories for Materials Science and Technology,
Laboratory for Air Pollution and Environmental Technology, Duebendorf, Switzerland
Received: 7 October 2010 – Accepted: 12 November 2010 – Published: 24 November 2010
Correspondence to: H. Herich (hanna.herich@empa.ch)
Published by Copernicus Publications on behalf of the European Geosciences Union.
5313

Page 2

AMTD
3, 5313–5342, 2010
Source
apportionment of
wood burning and
traffic aerosols
H. Herich et al.
Title Page
AbstractIntroduction
ConclusionsReferences
Tables Figures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Abstract
The contributions of fossil fuel (FF) and wood burning (WB) emissions to black carbon
(BC) have been investigated in the past by analysis of multi-wavelength aethalometer
data. This approach utilize the stronger light absorption of WB aerosols in the near
ultraviolet compared to the light absorption of aerosols from FF combustion.
Here we present two years of seven-wavelength aethalometer data from one
urban and two rural background sites in Switzerland measured from 2008–2010.
The contribution of WB and FF to BC was directly determined from the absorption
coefficients of FF and WB aerosols which were calculated by using confirmed
absorption exponents and aerosol light absorption cross-sections that were determined
for all sites. Reasonable separation of total BC into contributions from FF and WB
was achieved for all sites and seasons. The obtained WB contributions to BC are
well correlated with measured concentrations of levoglucosan and potassium while FF
contributions to BC correlate nicely with NOx. These findings support our approach
and show that the applied source apportionment of BC is well applicable for long-term
data sets.
During winter, we found that BC from WB contributes on average 24–29% to total
BC at the considered measurement sites. This is a noticeable high fraction as the
contribution of wood burning to the total final energy consumption is in Switzerland
less than 4%.
5
10
15
20
1Introduction
Atmospheric aerosol particles affect chemical, microphysical, and radiative atmo-
spheric processes. They are important when considering both the natural and the
anthropogenic climate forcing (Forster et al., 2007).
atmospheric aerosols is carbonaceous matter (CM), which is composed of black
carbon (BC) and organic carbon (OC). BC is the light absorbing part of carbonaceous
An abundant constituent in
25
5314

Page 3

AMTD
3, 5313–5342, 2010
Source
apportionment of
wood burning and
traffic aerosols
H. Herich et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
material, commonly referred to as soot. Compared to other aerosol constituents BC
has very different optical and radiative properties, contributing significantly to current
global warming (Jacobson, 2001; Forster et al., 2007; Ramanathan and Carmichael,
2008; Jacobson, 2010).
Besides their effects on the Earth’s radiation budget, fine carbonaceous particles
have been found to cause serious health effects as they penetrate into the human
respiratory system (Oberdorster et al., 2002; Jerrett, et al., 2005; Kennedy, 2007).
Epidemiologic studies associated BC in particular with respiratory health effects in
children and with cardiovascular diseases (Peters et al., 2000; Gauderman et al.,
2004).
The major emission sources of soot particles in Europe and Switzerland are diesel
engines and incomplete biomass burning. Especially in winter, wood combustion from
domestic heating has been found to be a major contributor to air pollution in residential
areas (Szidat et al., 2007; Lanz et al., 2008). Given that wood burning (WB) is a
CO2neutral energy source its impact on air quality is likely to become more and more
relevant in the coming years.
Several commercial instruments are available for the determination of BC in
particulate matter (PM). In this study, the aethalometer (AE, Hansen et al., 1984)
was used for continuous measurement of BC. The working principle of an AE is the
following: aerosols are collected on a quartz fibre filter. Subsequently the attenuation of
an illuminating light beam is determined. The aerosol absorption coefficient babsresults
after correcting the measured light attenuation for artefacts as multiple scattering,
“shadowing” and filter-loading (Weingartner et al., 2003; Collaud Coen et al., 2010
and references therein). The BC mass concentration is obtained from babsdivided by
the mass specific aerosol light absorption cross section σabs. Newer AE instruments
operate at several wavelengths ranging from the near-ultraviolet (UV) to the near-
infrared (IR). The wavelength dependence of the aerosol absorption coefficient babs
can be described by the power law babs(λ)∼λ−α, where λ is the wavelength of the light
beam and α is the˚Angstrom exponent. The spectral dependence allows distinguishing
5
10
15
20
25
5315

Page 4

AMTD
3, 5313–5342, 2010
Source
apportionment of
wood burning and
traffic aerosols
H. Herich et al.
Title Page
AbstractIntroduction
ConclusionsReferences
Tables Figures
??
??
Back Close
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
carbonaceous aerosols from different sources. This is because of light absorbing OC
which in contrast to BC exhibits a stronger absorption at shorter wavelengths (Andreae
and Gelencser, 2006; Lukacs et al., 2007; Moosm¨ uller et al., 2009). For example,
biomass burning aerosols are known to contain a significant number of light absorbing
organic substances or brown-carbon and have a strong spectral dependence (α >2)
while emissions from diesel engines contain primarily BC and have a weak spectral
dependence (α ∼1) (e.g. Kirchstetter et al., 2004; Clarke et al., 2007). Under certain
conditions the range of˚Angstrom exponents for brown carbon and BC may possibly be
wider (Lack and Cappa, 2010).
In the past multiple wavelength AEs were deployed to determine the contributions of
traffic and WB to total CM. This was accomplished with a source apportionment model
introduced by Sandradewi et al. (2008a). Sandradewi et al. (2008a) collected AE data
in winter during a measurement campaign in an alpine valley where WB from domestic
heating and traffic emissions were the dominating sources of CM. Linear regression of
CM against the light absorption coefficient of FF combustion aerosols in the infrared
(950nm) and the light absorption coefficient of WB aerosols in the UV (470nm) was
proposed for source apportionment. The authors estimated an average contribution
of WB to total CM of 88%. In a recent study, Favez et al. (2009) applied the same
approach to data sampled in urban Paris during a winter field campaign. The authors
determined regression coefficients similar to Sandradewi et al. (2008a) and estimated
the average contribution of WB to total CM to be 46%.
In this study we applied the AE model to data collected from 2008 to 2010 at three
measurement sites in Switzerland. To our knowledge this is the first long-term source
apportionment study using this modelling approach. In a first step we focused on
CM. Sensitivity tests for different regression models and for various light absorption
exponents were performed. It was found that the regression modelling approach is not
suitable for long-term datasets because of significant fractions of CM resulting from
sources and processes other than FF and WB. Thus in a second step we focused
on the contributions of FF combustion and WB to BC which was calculated directly
5
10
15
20
25
5316

Page 5

AMTD
3, 5313–5342, 2010
Source
apportionment of
wood burning and
traffic aerosols
H. Herich et al.
Title Page
Abstract Introduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
Discussion Paper
|
from babsby the use of site specific σabsvalues. We determined the fractions of BC
resulting from emissions of FF combustion and WB on a seasonal and a daily basis and
compared our findings with measured concentrations of tracers for WB (levoglucosan
and potassium) and with estimated elemental carbon emitted by WB as obtained from
a receptor modelling study.
5
2Experimental procedure
2.1 Sampling sites and instrumentation
Measurements were performed at one urban and two rural stations of the Swiss
National Air Pollution Monitoring Network (NABEL) (EMPA, 2010) from 2008 to 2010.
The NABEL station Zurich-Kaserne (ZUE) is an urban background site located in a
courtyard in the city centre of Zurich (47◦22?N, 8◦32?E, 410ma.s.l.). The location is
surrounded by roads with rather low traffic and is not affected by major emissions
from industries. The stations Payerne (PAY) and Magadino-Cadenazzo (MAG) are
rural-agricultural sites. PAY is located in the western part of the Swiss Plateau one
kilometre outside of Payerne, a small city with 8000inhabitants. The site is surrounded
by agricultural land (grassland and crops), forests and small villages (46◦48?N, 6◦56?E,
489ma.s.l.). The MAG site is located south of the Alps in the Magadino plane close to
the Lago Maggiore (46◦09?N, 8◦56?E, 204ma.s.l.)and about two kilometres outside of
Cadenazzo, a village with 2000inhabitants.
Multiple-wavelength AEs (Magee Scientific, USA, model AE31) were deployed at all
measurement sites for determination of BC. The instrument continuously detects the
aerosol light absorption coefficient at seven wavelengths (370, 470, 520, 590, 660,
880 and 950nm) with a time resolution of 5min. Data correction of babs(λ) was done
according to the procedure by Weingartner et al. (2003). All AEs were equipped with
PM2.5inlets.
10
15
20
25
5317