Temporal variation and impact of wood smoke pollution on a residential area in southern Germany
Introduction
This study is a continuation of our previous investigation about wood smoke pollution in residential areas in southern Germany (Bari et al., 2009). In that study, we determined particle-phase organic compounds emitted from wood stoves, investigated source fingerprints of polycyclic aromatic hydrocarbons (PAHs) of wood combustion and other emissions, characterised ambient levels of PAHs and methoxyphenols in detail and finally implemented the positive matrix factorization model to find out emission sources as well as to quantify the wood smoke contribution to the ambient PM10-bound organic compounds in the residential area. In this continued study, we made a detailed characterisation of the identified wood smoke tracer compounds, including the ambient levels of wood smoke compounds, the size distribution and composition of ambient air particles, and the temporal variation of wood smoke tracer compounds. Furthermore, we determined the impact of wood-fired domestic heating on ambient PM10 pollution in the investigated residential site of southern Germany.
In residential villages of Germany, located in forest-rich areas, several people use log wood boilers with and without a heat storage tank for central heating. For additional heating, commonly used combustion appliances are manually fed chimney stoves, tiled stoves, and open fire places. Depending on the operational parameters (e.g., fuel seasoning, fuel distribution inside chamber, kindling approaches etc.), such wood-firings can cause high particulate emissions (Tissari et al., 2008, Johannson et al., 2004). Wood combustion causes regional haze with high PM10 concentrations in the ambient air, resulting in considerable annoyance and nuisance with complaints among the inhabitants. Even the inhabitants of such residential villages can often smell a distinct ‘wood smoke odor’ in the air. Thus, it is evident that residential wood combustion has a direct influence on ambient air quality. So, due to public concern about the need of emission reduction, it was necessary to find out the impact of wood-fired heating on ambient PM10 pollution in such residential areas. One common approach is to characterise the organic composition of wood smoke emissions and using some of these compounds as potential tracers (Rogge et al., 1993, Schauer et al., 1996, Zheng et al., 2002). In our previous study (Bari et al., 2009), we determined the organic composition of wood smoke emissions and identified wood smoke tracer compounds. Here in this study, we characterise different identified tracer compounds and try to find out the impact of wood smoke on ambient air quality.
In the previous study, we considered wood smoke compounds from the thermal degradation of lignin, cellulose and hemicellulose. As pyrolysis products of wood lignin, methoxylated phenolic compounds (methoxyphenols) have been suggested as potential wood smoke tracers in multivariate source apportionment models to determine the contribution of urban particulate matter (PM) derived from wood combustion (Schauer and Cass, 2000). Smoke from residential wood stoves has been reported to contain syringol (2,6-dimethoxyphenol) and its derivatives (syringaldehyde, acetosyringone, propionylsyringol, sinapylaldehyde) in large amounts in hardwood combustion but no significant quantities are detected in softwood emissions. Whereas guaiacol (2-methoxyphenol) and its derivatives are emitted from both softwood and hardwood combustion but the emission rates of individual guaiacol derivatives (e.g., vanillin, acetovanillone, guaiacylacetone, coniferylaldehyde) are very different for the two types of wood (Schauer et al., 2001, Rogge et al., 1998, Hawthorne et al., 1989). We found similar results in our previous study (Bari et al., 2009).
Monosaccharide anhydrides (MA) such as levoglucosan, mannosan and galactosan are the predominant organic compounds in wood smoke emissions, which are emitted in high quantities from the smouldering stage of combustion and formed from the thermal degradation of cellulose and hemicellulose and have been considered as the candidate tracers for residential wood combustion (Shafizadeh, 1968, Hornig et al., 1985, Locker, 1988, Simoneit et al., 1999, Jordan et al., 2006). Due to certain chemical properties i.e., low vapour pressure with high molecular weight, the partition of levoglucosan is in the particulate phase (Fine et al., 2001, Locker, 1988). Another important property that enables the use of levoglucosan as a potential tracer for particle emissions from wood burning is its resistance to chemical degradation. It is found stable in the atmosphere over a period of 10 days in aqueous solutions (Locker, 1988, Fraser and Lakshmanan, 2000). The levoglucosan to particle emission fraction may vary due to different types of wood species, different wood combustion appliances, burn rate, different air flow settings and moisture content in the fuel (Gullett et al., 2003, Fine et al., 2001, Fine et al., 2004a, Hedberg et al., 2006, Jordan and Seen, 2005). Levoglucosan emitted from wood stoves and fireplace combustion, ranged 0.8–32% of PM2.5 emissions (Fine et al., 2001, Schauer et al., 2001, Jordan and Seen, 2005). In central Europe comparatively similar results were found from domestic tiled stoves ranging 4–15% levoglucosan in PM10 emissions (Schmidl et al., 2008). Levoglucosan has been used extensively to address the impact of wood combustion on local air quality in the USA not only from the wood-fired winter heating, but also from other forms of biomass burning such as wild fires especially in the west and prescribed burning for wide spread forest land management in the east. It is found in high concentrations in the ambient air in several US cities (Simoneit et al., 1999, Fraser and Lakshmanan, 2000, Nolte et al., 2001, Nolte et al., 2002) as well as in Europe (Zdráhal et al., 2002, Carvalho et al., 2003, Yttri et al., 2005, Puxbaum et al., 2007, Saarikoski et al., 2008, Caseiro et al., 2009).
To distinguish between softwood and hardwood emissions, resin acids and retene have been suggested as tracers for softwood smoke (Fine et al., 2001, Fine et al., 2002, Standley and Simoneit, 1994, Simoneit et al., 1993, Ramdahl, 1983). During the combustion of coniferous wood, tricyclic resin acids are released due to volatilisation by steam in either their unaltered form, partially altered or completely combusted, where the common altered resin acid is dehydroabietic acid. Dehydroabietic acid may be unstable in the ambient air and it degrades in water after treatment with ultraviolet light (Corin et al., 2000). In our study, we characterised the identified wood smoke tracer compounds to assess the impact of wood-fired domestic heating on ambient PM10 pollution in a residential site of southern Germany.
Section snippets
Site description
Ambient PM10 samples were collected at the residential site in Dettenhausen as we did in our first investigation (Bari et al., 2009), which is located at the northern edge of the nature park area “Schoenbuch”. The word “Schoenbuch” means “nice beeches” i.e., mainly beeches are growing in this forest and hardwood from these trees is the main solid biofuel used for domestic heating. Since people in this village can get hardwood easily from the forest, more than 90% of firewood used in this site
Emission levels of wood smoke tracers
The emission factors in terms of mass in total PM (mg g−1 PM) and concentrations of wood smoke tracer compounds in the flue gases (μg m−3), as well as their ambient concentrations (ng m−3) found in the investigated residential site are presented in Table 1. The emission factors and emission concentrations of all syringol and guaiacol derivatives are described in detail in Bari et al. (2009). In our study, the average emission factor of levoglucosan for hardwood (beech) was 22.87 mg g−1 PM and for
Summary and conclusion
In this study, different wood smoke organic tracer compounds were characterised to assess the relative contribution of wood-fired heating to ambient PM10 loadings in the residential area. In ambient PM10 samples, significant amounts of hardwood tracers (syringaldehyde, acetosyringone, propionylsyringol, sinapylaldehyde) were found as well as lower concentrations of softwood tracers (vanillin, acetovanillone, coniferylaldehyde, dehydroabietic acid, retene) were obtained, which may indicate the
Acknowledgements
The authors would like to thank the Institute of Combustion and Power Plant Technology and the Institute of Sanitary Engineering, Water Quality and Solid Waste Management, Universitaet Stuttgart, for the possibility to carry out this research work. Aynul Bari received a doctoral grant from the Ministry of Baden-Wuerttemberg state, which is gratefully acknowledged.
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