Dynamic Changes of the Aerosol Composition and Concentration during Different Burning Phases of Wood Combustion

The presented wood combustion emission study employing a logwood stove showed that four burning phases of different aerosol compositions and different amounts of emitted particulate matter (PM) occurred during a combustion batch. As a novel approach, in this study, the burning phases were defined by chemical changes in the aerosol gas phase during combustion, instead of being linked to predefined time periods or the amount of PM emissions. This deeper view into the aerosol chemistry of the different burning phases was possible by employing online mass spectrometry techniques with high time resolution. A special soft ionization technique enabled the selective detection of polyaromatic hydrocarbons (PAHs) in the gas phase, whereas changes in the particle phase were observed by aerosol mass spectrometer (AMS). The use of AMS allowed the description of changes in the particle phase and in the amounts of PM emitted during the burning phases, as well as verification of the observed burning phases. Finally, it was shown that the organic fraction and the amount of particles emitted during the ignition phase were main contributors to the emitted PM. The definition of these four burning phases was supported by a more detailed investigation of the chemistry of the organic matter using a Van Krevelen description (average H/C ratios of 1.32-1.64 and O/C ratios of 0.25-0.44, with individual values up to 1.4) along with our novel approach of employing positive matrix factorization (PMF) as a source apportionment tool for the separation of one emission source into different combustion-dominated processes. In addition, the highly dynamic and complex nature of wood combustion emissions was revealed by these analysis methods. It was shown that different combustion conditions have a strong impact on the amount of emitted PM. For instance, an experiment with an overloaded stove emitted a roughly 4-fold higher mass of PM compared to a stove run under the manufacturer's recommended (normal) combustion conditions. This experiment showed a much higher amount of PAHs, which are very harmful for human health.