Uncertainties in reported emissions

3. Residential combustion: an important source of air pollution

There is currently a growing interest in the use of wood/biomass in residential energy production due to a combination of energy and climate policies, as well as to the public’s perception of biomass as an environmentally friendly fuel option. The objective set by the European Union to achieve a minimum 20% share of its gross energy consumption from renewable sources in 2020 (EU, 2009d), following climate-oriented policies, is a main driver. In addition, in some European regions, the recent economic recession also contributed to the increased use of biomass as a residential fuel (Paraskevopoulou et al., 2015; Saffari et al., 2013).

Wood (and biomass, in general) is a renewable source of energy, with evident advantages from a climate perspective. When combusted efficiently, it may be a (nearly) CO₂-neutral source of energy, even if it does generate emissions of other atmospheric pollutants. However, it has been evidenced that climate-oriented policies may not always work in line with air quality-oriented policies, and vice-versa. For instance, under non-optimal operating conditions, the use of wood as residential fuel entails negative consequences as it contributes considerably to adverse effects on human health. The health-related impacts of residential heating, specifically for the case of wood and coal, have been addressed in detail by the World Health Organization (WHO, 2015, 2014a, 2014d). Epidemiological studies have shown that pollutants originating from solid fuel combustion significantly increase the risk of respiratory disease, chronic obstructive pulmonary disease and cardiovascular disease (Lighty et al., 2000). In Europe, 61 000 premature deaths were attributable to outdoor PM_2.5 pollution originating from residential heating with solid fuels (wood and coal) in 2010 (WHO, 2015).

The main health hazardous pollutants emitted during combustion of fuels such as wood and coal in residential stoves are fine particulate matter (PM_2.5), polycyclic aromatic hydrocarbons (PAHs), especially BaP, and BC. While BaP is a well-known human carcinogen, the health effects of PM_2.5 and BC are also described by WHO (WHO, 2014a, 2014c). Particulate pollutants may be emitted directly from stoves (primary particles) or be formed in the atmosphere after emission (secondary particles), and even undergo transformations resulting in pollutants with a higher toxicity (Nussbaumer et al., 2008).

In addition to containing health hazardous pollutants, emissions from residential combustion are usually released close to people, i.e. at low height (over house roofs) and in populated areas. Furthermore these emissions typically occur in areas (e.g. valleys) and periods of higher atmospheric stability and thus poor dispersion, i.e. during cold periods/winter and in the evening or night. These factors contribute to increased concentrations and higher population exposure.

3.1. Air pollutant emissions from residential combustion

Fuel consumption habits in the residential sector have undergone changes over time due to economic and social aspects, as well as in response to national or EU policies such as banning or incentivising specific fuel types, as mentioned earlier. Because of the intrinsic variability between European countries and regions, the assessment of trends is highly complex.

Solid fuel (mainly, coal) consumption for domestic heating followed a clear decline across EU28 between 1990 and 2002, stabilising in the last decade (Figure 3.1). In some countries, the decrease is probably due to national policies targeting the use of this fuel in residences (e.g., the coal ban in Irish cities gradually implemented since 1990; Clancy et al., 2002). Conversely, biomass consumption increased across EU28. Even though most of the countries showed clear increases (e.g., Norway, Austria, Denmark or Bulgaria), decreases were also detected in certain countries. In countries such as Spain, Greece, Hungary or Croatia a change in consumption habits seemed apparent, with decreasing contributions from biomass since 1990 but increased consumption after 2005. This could be due to the economic recession, to investments in renewable sources prior to the recession based on the environmentally-friendly perception of biomass, and/or the implementation of local policies such as incentivising the installation of biomass stoves in newly-built or refurbished homes. Finally, gaseous fuel consumption remained at similar levels between 1990 and 2012 in EU28, while the consumption of liquid fuels followed a decrease starting approximately in the year 2000. So, since 2000, a shift from liquid to biomass fuels may be observed in the residential combustion sector given that solid and gaseous fuel consumption remained relatively constant.

With regard to BaP emissions, during the last decade (2003-2012) and for certain Member States (Denmark, Netherlands, Romania, Hungary, Croatia, Lithuania, among others; ETC/ACM, 2016a; Figure 3.2) the increase in reported BaP emissions coincides with increased biomass consumption in the residential sector, suggesting a relationship between residential biomass consumption and BaP emissions from this sector. However, this trend was not observed consistently across all Member States (see France, Germany, Czech Rep.; ETC/ACM, 2016a). It should be highlighted here that BaP emission inventories suffer from major uncertainties (see box 3.1). Thus, even though a correlation between BaP emissions and biomass consumption trends is detectable in most cases, a direct relationship between BaP emissions and biomass consumption cannot be extracted with the data available. Different reasons may account for this: (a) changing BaP emission factors over time due to improving combustion technologies; (b) the use of different combustion technologies in different countries, which would also account for different emission factors; and (c) the fact that BaP sources from biomass but also from coal combustion.

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  Incomplete reporting (both in time, e.g., missing years of data, and in space, e.g. missing countries): gap filling strategies are applied, however these distort trends analyses at national or EU scale.
  
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  Emission factors (*): especially for components such as BaP and BC emission factors are highly uncertain. This is evidenced by the large variations in the emission ratios (e.g., BaP/total PAH or BaP/NOx of total emissions or domestic emission over the years or between countries.
  
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  Activity data (*): especially in forested areas large amount of the wood combusted is not recorded in official statistics.
  
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  Maintenance and usage of stoves.