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Current practices involve wrapping organic aerosol (OA) filter samples with aluminum foil and storing in a freezer to avoid sample degradation prior to analysis. However, there is a lack of evidence supporting these practices. Here, we investigate the effect of thermal and photodegradation on toluene-combustion OA, pine-combustion OA, and ambient OA samples by storing them in petri dishes for one month at three different conditions: covered with aluminum foil and in a freezer (F), covered with aluminum foil at room temperature (RC), and uncovered at room temperature (RUC). We performed three types of analyses: thermal-optical measurements, electrospray ionization mass spectrometry, and UV-Vis spectroscopy. For all samples, we observed a mild reduction in the relatively volatile organic carbon fraction (OC1) in the RC and RUC conditions. We did not account for positive artifacts in this study, and therefore, this reduction was at least partially due to the evaporation of adsorbed vapors rather than particles. There was no evidence of significant systematic change in chemical composition relative to control for any of the storage conditions, except for loss of some small-molecular-size compounds in conditions RC and RUC for pine-combustion OA, likely due to evaporation. There was no evidence that photobleaching occurred in the RUC condition for any of the samples. On the contrary, we observed an increase in absorption in the RC and RUC conditions for pine-combustion OA, likely due to evaporation of small-molecular-size species with relatively weak absorption. Copyright © 2023 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The chemical composition of particulate matter (PM) in biomass-burning smoke evolves upon aging in the atmosphere. The effect of this evolution on the toxicity of biomass-burning PM is understudied. Here, we burned oak foliage, pine needles, and hickory twigs in an environmental chamber. We used UV radiation to initiate photochemical aging of the emissions leading to the production of secondary organic aerosol (SOA), quantified using online particle size distribution measurements, and an overall increase in the PM oxygenation and decrease in the relative abundance of aromatic and condensed aromatic structures, obtained using ultra-high-resolution electrospray ionization mass spectrometry. In vitro exposure of human lung epithelial cells to PM from hickory combustion led to the strongest reduction in metabolic activity, followed by pine and oak, which was associated with the heavy metal content of the PM from the three fuels, quantified using induction-coupled plasma mass spectrometry. Furthermore, exposure to the fresh PM led to more reduction in metabolic activity than the aged PM for all fuels, whereas the aged PM induced more cell death by apoptosis. The differential cellular response to the fresh and aged PM indicates that the increase in oxygenation and decrease in aromaticity associated with photochemical aging alters the toxicity mechanisms exhibited by the PM, with a possible role of decreasing the heavy metal content (gram-metals per gram-PM) due to SOA formation. Together, these findings highlight the complex effect of photochemical aging on biomass-burning PM toxicity and motivate further studies to elucidate the underlying differences in toxicity mechanisms between fresh and aged PM. [Display omitted] • Biomass-burning organic aerosol is toxic toward human lung epithelial cells. • Heavy metals potentially play an important role in the observed toxicity. • Photochemical aging renders the aerosol more potent at inducing cell death. • Photochemical aging renders the aerosol less potent at reducing metabolic activity. [ABSTRACT FROM AUTHOR]

Light-absorbing organic particulate matter (PM), or brown carbon (BrC), may constitute an important fraction of combustion PM. Here, we investigate the effect of combustion conditions on the molecular sizes of BrC, their light-absorption properties, and their cytotoxicity. We used toluene in a combustion reactor with highly controlled conditions to produce two different types of BrC under two conditions corresponding to smoldering and near-flaming combustion, with temperatures of 670 °C and 1035 °C, respectively. We performed online measurements of the size distributions and light-absorption properties of the BrC. The BrC produced at 1035 °C was more light absorbing, with an imaginary component of the refractive index at 532 nm (k 532) an order of magnitude larger than that of the BrC produced at 670 °C. We also collected samples for offline chemical characterization using laser desorption ionization (LDI) mass spectrometry. The LDI mass spectra showed that the BrC produced at 1035 °C was composed of species with significantly larger molecular sizes than the BrC produced at 670 °C. Using human lung epithelial cells, we conducted in vitro cytotoxicity analysis on the two types of BrC with doses ranging from 3.5 to 136.0 μg of BrC/ml. After 24-h exposure, the viability of the cells was assessed using a WST-8 assay. The cytotoxicity analysis showed that, for both BrC samples, the cells exhibited a clear dose-dependent response with significant BrC cytotoxicity that plateaued at the higher doses. However, while the viability of cells exposed to the BrC produced at 1035 °C reached a minimum of around 65% at the highest dose, the BrC produced at 670 °C proved to be significantly more toxic, with the viability dropping asymptotically to 25%. The results presented here suggest that organic PM of smaller molecular sizes produced under lower temperature, smoldering combustion could be significantly more toxic than that of larger molecular sizes produced under higher temperature, flaming conditions. The use of a single-molecule fuel in a highly controlled combustion setup distinguishes this work from experiments that rely on real-life sources and combustion setups, where different combustion conditions could be occurring simultaneously and clouding the conclusions. • Brown carbon particles produced from highly controlled combustion setup. • Starkly different molecular composition of particles under low and high temperatures. • Lower temperature particles comprise smaller molecules. • Smaller molecules significantly more cytotoxic to lung epithelial cells in vitro. [ABSTRACT FROM AUTHOR]