Overview of the impact of wood burning emissions on carbonaceous aerosols and PM in large parts of the Alpine region
Introduction
It has been largely demonstrated that ambient airborne particulate matter (PM) in general and carbonaceous particles in particular can have a serious impact on human health (Jerrett et al., 2005, Pope and Dockery, 2006, Kennedy, 2007). In addition, chemical, microphysical, and radiative atmospheric processes are affected by airborne particles, making them important factors in both the natural and the anthropogenic climate forcing (Forster et al., 2007). Currently, the limit value for the annual (daily) average mass concentration of particles with aerodynamic diameters smaller than 10 μm (PM10) is 40 (50) μg m−3 in the EU and 20 (50) μg m−3 in Switzerland. Carbonaceous matter (i.e. organic carbon, OC, and light absorbing or black carbon, BC) represents a major fraction of ambient PM. Globally, a major source of PM and an important source of carbonaceous aerosols is biomass combustion (Crutzen and Andreae, 1990). In residential areas in various regions around the world, most notably wood fuel is being burned in fireplaces and stoves during winter. The importance of wood burning (WB) for domestic heating as a source of particulate air pollutants has been shown in many studies (e.g. Schauer et al., 1996, Khalil and Rasmussen, 2003, Lanz et al., 2010, Maenhaut et al., 2012).
The Alpine region is frequently influenced by pollution events and high PM concentrations, due to a combination of emissions, topography (e.g. deep valleys) and local meteorology (e.g. inversion layers during winter). For example, daily PM10 concentrations in Zurich can reach 120 μg m−3 during winter (Lanz et al., 2008). Recent studies show that at many locations in the Alps, WB can be the dominating source of carbonaceous aerosols during the cold season (Szidat et al., 2007, Gilardoni et al., 2011, Piot, 2011).
During the past years, actions implemented for the reduction of PM emissions have in many European countries focused on road traffic emissions. Much less attention has been paid to emissions from domestic WB. As shown recently by Gianini et al. (2012), average road traffic contributions to PM10 have declined in Switzerland during the past ten years, whereas the contributions from WB remained unchanged. As a consequence, the contribution of WB to PM10 is today on annual average at many locations in Switzerland comparable to (or even higher than) the contributions from road traffic (Gianini et al., 2012). Despite WB being a CO2 neutral and a renewable energy source, its impact on air quality might become even more important in the coming years.
Specific tracers can be associated with wood burning emissions. Examples for such tracers are the anhydrosugars levoglucosan, mannosan, and galactosan. Levoglucosan is known to be an important product in the pyrolysis of cellulose (Shafizadeh, 1984, Simoneit et al., 1999) and is a reasonably stable molecule in the atmosphere (Simoneit, 2002, Schkolnik and Rudich, 2006). Recent laboratory studies report some extent of reaction of levoglucosan with photo-oxidation (Kessler et al., 2010, Hennigan et al., 2011 and references therein) suggesting that levoglucosan may not be as inert as previously thought. However, these findings are only partly relevant for field measurements close to the sources and during winter when temperatures are low and photo-oxidation is reduced by meteorological conditions.
Levoglucosan has previously been recommended and used as a single tracer for PM emissions from wood combustion (Simoneit et al., 1999, Jordan et al., 2006, Caseiro et al., 2009). Moreover, the ratio between levoglucosan and mannosan in WB emissions has been determined and used to distinguish the contributions of hard and softwood to wood combustion related PM (Schmidl et al., 2008).
In this study, we provide a comprehensive overview of the impact of WB emissions on carbonaceous aerosols and PM in the Alpine region. This overview is based on results of recently published source apportionment studies using different methods, such as multivariate statistical models (Positive Matrix Factorization, PMF), Chemical Mass Balance (CMB) modelling, a 14C-method or by evaluation of the wavelength dependence of the aerosol optical absorption (the so-called Aethalometer model). In all the studies considered here, the corresponding concentrations of levoglucosan and mannosan are provided, allowing the calculation of the ratios of wood burning related aerosols and these wood burning tracers in ambient air. The obtained ratios and associated uncertainties are then used in a mono-tracer approach applied to the available ambient concentrations of levoglucosan and mannosan at Alpine sites for the estimation of the WB emission contributions to carbonaceous aerosol concentration and PM at these sites.
Section snippets
Source apportionment models in literature: a short overview
Several studies have recently focused on the source apportionment of PM and carbonaceous material (CM) in ambient air. Although the contribution of WB to PM and CM cannot be determined directly by measurements, various methods for source apportionment are available and have been applied in the past. A problem that is common to all of them is the validation of the results that is not possible due to the absence of a reference method. Plausibility checks and comparisons of results obtained with
Measurement sites and data
In the last decade, many publications reported elevated concentrations of PM10 and PM2.5 in the Alpine region during the cold season due to the impact of wood combustion emissions. Table 2 gives an overview of results from a literature review and from known other measurement campaigns. A map of the sites is shown in Fig. 2.
The type of measurements and the sampling intervals can be very different for the studies included in Table 2. Some of the studies are based on single short-term campaigns,
Results and discussion
In this section, an overview of average wood combustion contributions to carbonaceous aerosols and PM for the measurement sites in the Alpine region introduced in Section 3 is given. The overview is on the one hand based on published results using different methods as mentioned in Section 2. On the other hand, the available concentrations of the wood combustion tracers levoglucosan and mannosan are used together with the relationship between ECwb, OCwb and PMwb and these tracers in the ambient
Conclusions
Different methods for the determination of ECwb, OCwb and PMwb are available. However, none of these approaches can be considered as a standard method. Additional assumptions, interpretation of data and subjective choices are usually required for the determination of the wood burning contributions. Here, we used an ambient mono-tracer approach combining data from different source apportionment studies performed in the Alpine area. The approach is very simple and robust and allows the estimation
Acknowledgements
This study represents part of the research project IMBALANCE funded by the Competence Center Environment and Sustainability of the ETH Domain (CCES). Support from the Swiss Federal Office for the Environment (FOEN) is gratefully acknowledged. The sampling in the French Alps was conducted by Air APS and Air Rhône Alpes; support for the studies in France came from Primequal, ADEME, CNRS, Université de Grenoble, and Université de Savoie. We are grateful for the support from the Nova Gorica
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