Characterization and dark oxidation of the emissions of a pellet stove

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1 (particulate matter with aerodynamic diameter less than 1 µm) emissions consisted mainly of OA (93 ± 4 %), and black carbon (5 ± 3 %). The primary OA (POA) oxygen-to-carbon ratio (O:C) was 0.58 ± 0.04, higher than fresh logwood emissions. The emitted OA at a concentration of 70 μg m-3 mainly consisted of semi-volatile (68%) and intermediate-volatility (16%) compounds. The oxidation of the pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) in the chamber, at different (10-80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3 radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA formed was up to 30% of the initial OA, accompanied by a moderate change in both O:C levels (7−8 % increase) and OA spectrum. Aging under higher RH conditions (60−80%) led to a more oxygenated aerosol (increase in O:C of 11−18 %), but only a minor (1−10%) increase in OA mass. The increase in O:C at high RH, indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change of the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g. wintertime period) with important enhancement in SOA mass under certain conditions.