Characterization and night-time oxidation of pellet stove emissions

A growing energy demand has shifted interest towards alternative energy sources such as biomass combustion. Wood has become an important fuel even in the developed world, frequently used in households for heating purposes mostly as logs but also as pellets. During winter biomass burning is one of the most important air pollution sources emitting both primary organic aerosol (POA) and organic vapours that can be oxidized producing secondary organic aerosol (SOA). Pellet stoves are considered as low-emitting combustion sources, but a growing influence of their emissions on air quality is expected. For example, during 2018 global wood pellet consumption increased by 130% compared to its 2013 levels, reaching 53 million tons (EPC, 2019). Half of this consumption took place in Europe (27 million tons; 60% increase in 5 years). Recently, there has been increasing interest in exploring the extent of night-time chemical processing of biomass burning emissions. Hartikainen et al. (2018) reported substantial SOA production in laboratory experiments under dark conditions. Kodros et al. (2020) suggested that dark oxidation of biomass burning plumes by NO3 radicals may be an additional formation pathway of oxygenated OA and may lead to secondary inorganic and organic aerosol nitrate formation. Environmental simulation chamber experiments were performed in the Foundation of Research and Technology-Hellas atmospheric simulation chamber (FORTH-ASC), to characterize fresh and aged pellet stove emissions. The fresh PM1 (particulate matter with diameter less than 1 μm) emissions consisted mainly of organics (93 ± 4 %), followed by black carbon (5 ± 3 %), nitrates and sulfate (1 %). The emission rates of fresh OA were in the range of 2.6 to 12 g kg-1 of pellets depending on burning conditions, while the POA oxygen-to-carbon ratio (O:C) was 0.58 ± 0.04. The pH for the fresh emissions estimated to be 3.2 ± 0.3. The oxidation of the pellet emissions was investigated under dark conditions by injecting nitrogen dioxide (NO2) and ozone (O3), at different relative humidity (RH) levels. In all experiments SOA was formed (1-32 μg m-3), increasing the OA levels by 2-28 % after a few hours of exposure to NO3 radicals in the chamber (3 - 5 hours of equivalent atmospheric dark oxidation) (Figure 1). An increase in the O:C ratio of the OA by 7-21 % was also observed. Figure 1. SOA (red bars) and organic nitrate (black bars) mass concentrations for the dark ageing experiments. These results suggest that dark oxidation of pellet emissions is an additional biomass burning SOA source that should be considered in atmospheric models. This work was supported by the European Research Council PyroTRACH project (grant 726165) and the EU Horizon-2020 project REMEDIA (grant 874753).