Short Lived Climate Pollutants - method development for emission inventories of Black Carbon
There is an increasing attention and interest in the role of Black Carbon (BC) in climate change and the possibilities to slow down the on-going temperature increase by reducing emissions of BC and other so called Short Lived Climate Pollutants (SLCP). Reduced emissions of SLCP, and in particular BC, will give a more rapid response in climate change in comparison to reducing CO2 emissions, due to the shorter life time of these species in the atmosphere. Furthermore, BC is a component of emitted particulate matter (PM), and recently national reduction targets for emissions of PM2.5 in 2020 were included in the amended CLRTAP Gothenburg protocol, agreed in May 2012. In the amended protocol text, a general recommendation is given that sources with the largest emitted fractions of BC should be prioritised when implementing emission reduction actions for PM2.5. Currently PM2.5, but not BC, is included in the emission reporting program under the UNECE Convention on Long Range Transboundary Air Pollution (CLRTAP). The national Swedish inventory of PM2.5, and other air pollutants, is performed and reported on an annual basis. The current project builds on present knowledge and established structures regarding emission inventory work and the aim is to improve earlier preliminary estimates of total Swedish BC emissions, which for 2005 were estimated to 5.1 kton. Available estimates of BC emissions for Sweden indicate that stationary biomass combustion and emissions from diesel vehicles and machinery are the most important sources, together contributing in the order of 75-80% of national BC emissions. As the emissions of BC are calculated as a fuel- and technology specific fraction of emitted PM2.5, the quality and accuracy of the PM2.5 emission estimates are crucial for reliable estimates of BC emissions. This study includes and presents, for important Swedish BC sources, the results of a literature review of emission factors and emissions of PM2.5 and of published data on the BC content in PM2.5. Example calculations of emissions of PM2.5 and of BC for stationary biomass combustion and for mobile diesel combustion are made based on the different sets of emission factors reviewed. The review of emission factors for PM2.5 for stationary biomass combustion shows that the emission factors are highly variable, particularly for residential sources. The variability is due to operational factors but importantly also due to the sampling method applied for the PM emission measurements that are used in developing emission factors. The differences in emissions of PM between hot flue gas measurements and measurements performed after cooling of the flue gases in a dilution tunnel have been reported to be between 2-10 times. This of course also affects the subsequent estimates of BC as a fraction of PM2.5. A comparison of emission factors for PM2.5 and for BC between the Swedish national factors, factors from the EMEP/EEA Air Pollutant Emission Inventory Guidebook and from IIASA shows large differences, especially for small scale biomass combustion. The Swedish emission factors are based on hot flue gas measurements, which give lower results, while the emission factor data in the EMEP/EEA Guidebook generally are based on measurements in dilution tunnel, resulting in higher numbers. Example calculations of Swedish emissions, using different sets of PM2.5 and BC factors for stationary biomass combustion and for diesel vehicles and machinery, show that this can result in substantially different estimates. For stationary biomass combustion the differences are most pronounced for residential/small scale technologies, but also for power plants. For mobile diesel combustion the estimated BC emissions can differ about a factor of 2 for heavy duty vehicles and for off-road vehicles and machinery depending on choice of factors. The review of available emission factors for PM2.5 and BC, as well as the differences in the examples calculated for Swedish emissions, show that further work is needed to assess the representativeness of published factors for Swedish conditions, in order that a future national total emission inventory of BC will be as accurate and with as low uncertainty as possible. Reliable source specific emission data for BC are essential as background information when assessing and prioritising sources for implementing emission reduction actions.