Night-Time Evolution and Vertical Distribution of Atmospheric Aerosols from the Largest Open Biomass Burning “Experiment” in Central Europe

Abstract Open biomass burning (OBB) emits large amounts of air pollutants, significantly impacting air quality and climate change. Herein, chemical (carbonaceous content, metals, NR-PM 1 : organics, SO 4 2− , NO 3 − , NH 4 + , Cl − ) and physical (number size distribution, absorption and scattering) properties of OBB-derived aerosols, and their vertical distribution, and night-time evolution were investigated during the country-wide burning of the witches (BoW) in the Czech Republic and neighboring states. The mass concentrations of most of the aerosol components sharply increased during the BoW, and a fast change in their chemical composition, size distribution, and optical properties was observed. The BoW led to significantly higher concentrations of carbonaceous aerosols, with a concentration enhancement ratio ranging from 4.5 to 11.4. The vertical distribution of equivalent black carbon (eBC) was also altered during the BoW due to OBB emissions (eBC BB ) contributing up to > 90% and 67% of eBC at 4 m and 230 m a.g.l., respectively. At 4 m, eBC BB was affected by local OBB plumes trapped within the mixing layer, whereas eBC BB at 230 m was most probably affected by longer distance transported plumes. Cl − and K, Zn, Pb and Cu were also significantly enhanced (2.6–10.8) due to OBB and coexisting sources (combustion of old tires and trash and fireworks activities). The freshest OBB emissions, made of 73% organics, with mobility diameter of 20 nm and 80 nm, rapidly grew during the first hours of the event with a single peak of ~ 100 nm. The higher concentrations of brown carbon led to an elevated absorption Ångström exponent (2.13 ± 0.26) and may also explain the enhanced scattering coefficients observed during this event. The BoW may significantly affect air quality and represents an excellent context to provide valuable background information on the physicochemical properties and night-time aging of non-heating OBB-derived aerosols in temperate climates. Graphical Abstract