Chemical characterization of pellet stove ashes: Elemental fractionation of ashes results in volatile particles with high concentrations of cadmium and lead.

Use of wood-based biomass for home heating has increased as a more sustainable and economical alternative to fossil fuel heating sources. However, concerns remain regarding particulate emissions and potential human health effects. Pellet stoves are very efficient combustion sources but still emit high particle numbers of nanoparticles into the environment and generate ash within the stove that could be an exposure source during cleaning. In this study we chemically characterize pellet ash collected from zones (upper and lower) from three stoves and determine the chemical composition of room air during cleaning events. Ash was acid digested using reverse aqua regia and microwave digestion and analyzed by ICP-MS. Indoor air was sampled using impingers before and during stove cleaning to assess ultrafine particulate release and exposure. Within the pellet stove different fractions of ash could be identified by color and macro-scale morphology. Ash fractions collected lower in the stove were predominantly calcium-based, 25% by weight, and high in refractory trace elements, Fe, Cr, Mn and Ni. In contrast, ash collected higher in the stove was predominantly K and S based (25 and 5% respectively) and had high concentrations of volatile elements Zn (0.4%), Cd and Pb. Maximum Cd and Pb ash concentrations were 274 mg/kg and 807 mg/kg respectively corresponding to enrichment factors of 1581 and 870 respectively compared to the respective pellets. Electron microscopy of the particles suggested that ash particles were agglomerates of smaller primary particles and EDS identified areas of high K and S concentration on the particles. Impinger studies of room air during cleaning showed an increase in K, S, Zn, Cd and Pb suggesting a mobilization of upper ash particles into room air upon stove cleaning.