Chemical composition and morphological analysis of atmospheric particles from an intensive bonfire burning festival

Atmospheric particles were sampled in Rehovot, Israel during a national Lag Ba'Omer bonfire festival as a case study to investigate the physical and chemical transformations of mixed mineral dust and biomass burning (BB) aerosols. Aerosol mass spectrometry was used in situ to characterize aging and chemical evolution of BB aerosols in real time throughout the event. During this dynamic period of BB emissions, particle samples were collected for chemical imaging using spectromicroscopy techniques. Computer-controlled scanning electron microscopy with energy dispersive X-ray analysis identified multiple particle types including highly carbonaceous (54-83%) particles, aged mineral dust (1-6%), and sulfur-containing particles (17-41%). Synchrotron-based scanning transmission X-ray microscopy coupled with near edge X-ray absorption fine structure (STXM/NEXAFS) was used to assess the internal chemical heterogeneity of individual BB particles and the morphology of soot inclusions. The observed higher contribution of mixed component particles along with an increase in particle organic volume fraction suggests an atmospheric aging process, consistent with in situ measurements. An estimation method for particle component masses (i.e., organics, elemental carbon, and inorganics) inferred from STXM measurements was used to determine quantitative mixing state metrics of particles based on entropy-derived diversity measures for different periods of the BB event. In general, there was a small difference in the particle-specific diversity among the samples (D-alpha = 1.3-1.8). However, the disparity from the bulk population diversity observed during the intense periods was found to have high values of D-gamma = 2.5-2.9, while particles collected outside of the burning event displayed lower bulk diversity of D-gamma = 1.5-2.0. Quantitative methods obtained from chemical imaging measurements presented here will serve to accurately characterize the evolution of mixed BB aerosols within urban environments.