Sources of wintertime PM2.5 at a major city in an Alpine Valley: The role of atmospheric dispersion and inversion dynamics

Urban areas in mountain environments are generally located on valley floors surrounded by slopes, where mountain orography drives peculiar meteorology and atmospheric circulation. Also, persistent inversion dynamics may occur strongly affecting air pollution. This study characterizes the PM2.5 pollution in a major city located in an Alpine valley (Belluno, Northeastern Italy) during the cold season (Autumn-Winter). Major particulate species (elemental and organic carbon, major inorganic ions) and minor/trace elements conventionally used as tracers for source apportionment were analyzed, including oxalate and specific PM2.5-bound tracers for biomass burning (K+, levoglucosan, mannosan, galactosan) and for primary biogenic organic aerosol (arabitol, mannitol, glucose). PM2.5 sources are identified through positive matrix factorization and a series of post-processing tools. Results indicate that biomass burning, mostly emitted by residential wood combustion for domestic heating, is the major source (52% PM2.5 mass concentration), followed by secondary aerosol (21%), biogenic aerosol (20%), traffic (4%), and dust resuspension (3%). The source contributions are assessed by accounting for the local meteorology. Insights into the dispersion or buildup of PM2.5 sources were then investigated by dispersion normalization. In addition, the possible effects of persistent thermal inversion events were evaluated by assessing the inversion strength from temperature profiles measured at multiple weather stations at different elevations with respect to the source contributions. Beyond the identification of those emission sources requiring further mitigation actions, this study also analyses the potential effects of local meteorology on PM2.5 pollution. The methodologies applied in this study can be easily adopted to other mountain environments for successful management of air quality.