Chemical composition, sources and atmospheric processing of submicron aerosol during wintertime smog episodes

Wintertime pollution episodes are frequently driven by intense biomass burning for residential heating; yet detailed insights into the chemical composition and processing of submicron aerosols from residential heating remain limited. To address this, a field campaign was conducted at an urban site in northern Greece during December 2021-January 2022. The site is located in a valley surrounded by mountains that favor pollution accumulation and is characterized by intense residential biomass burning. To assess the chemical composition, sources, and processing of the high PM 1 levels (reaching approximately 300 μg m -3 on an hourly basis), we employed an Aerosol Chemical Speciation Monitor together with an aethalometer and a Proton Transfer Reaction–Time of Flight–Mass Spectrometer for real-time monitoring of aerosol constituents and volatile organic compounds. Biomass burning was determined as the dominant pollution source, especially for organic aerosol and black carbon. Positive Matrix Factorization analysis identified five organic sources, with biomass burning-related organic aerosol accounting for 58% of total organic aerosol mass. In addition, exceptionally high absorption Ångström exponents (AAE > 4) were observed during nighttime, indicating a strong influence from residential heating. Thermodynamic modeling estimated moderately acidic aerosol conditions (pH 4.2±1.2), similar to other locations globally affected by strong biomass burning. The substantial liquid water content (up to 400 μg m -3 ), was found to enhance secondary aerosol formation through aqueous-phase processing. Based on the measured volatile organic compounds, we find that biomass burning could account for half of the locally produced secondary organic aerosol in winter, driven by fast chemical processing via heterogeneous reactions. The deterioration of local air quality by both direct emissions and secondary production of organic aerosol highlights the urgent need for targeted air quality policies addressing residential biomass combustion in southeastern European cities facing similar climatic and socioeconomic conditions. • Severe air quality deterioration due to both direct emissions but also secondary aerosol production. • Biomass burning-related organic aerosol accounts for 58% of total organic aerosol mass. • Strong influence to absorption Angstrom exponents from residential heating. • Enhanced secondary organic aerosol formation by substantial amounts of liquid water. • Based on volatile organic compound precursors, biomass burning could account for half of the locally produced secondary organic aerosol in winter.