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Emissions from large-scale fires significantly contribute to the atmospheric burden of primary organic aerosol (OA). 

Previous studies have suggested that the photochemistry of nitroaromatics in organic solvents can vary significantly from the photochemistry in aqueous solutions. This work compares the photodegradation of 2-nitrophenol (2NP), 4-nitrophenol (4NP), 2,4-dinitrophenol (24DNP), and 2,4,6-trinitrophenol (246TNP) in 2-propanol and water to better understand the photochemical loss of nitrophenols in atmospheric organic particles and aqueous droplets. Polychromatic quantum yields were determined by monitoring the loss of absorbance of each nitrophenol with UV/vis spectroscopy in the presence of an acid (undissociated nitrophenol) or base (nitrophenolate). There was no orderly variation between loss rates in the organic and aqueous phases: 2NP and 4NP had similar yields in the two solvents. 246TNP was an outlier in these results as it dissociated in both acidified 2-propanol and water due to its exceptionally strong acidity. A notable result is that only for 24DNP was a dramatically increased reactivity found in 2-propanol compared to that in water. Time-dependent density functional theory calculations were carried out to characterize the excited state energies and absorption spectra with a conductor-like polarizable continuum model or explicit solvation by a few solvent molecules. Explicit solvent calculations suggest the enhanced reactivity of 24DNP in 2-propanol is due to the strong interaction between a 2-propanol molecule and an –NO 2 group in the excited state. For the other nitrophenols, the solvent effects on electronic structure were minimal. Overall, the observations in this work suggest that solvent effects on the electronic structure and condensed-phase photochemistry of nitrophenols are minimal, with the exception of 24DNP. 

Wildfires, which have been occurring increasingly in the era of climate change, emit massive amounts of particulate matter (PM) into the atmosphere, strongly affecting air quality and public health. Biomass burning aerosols may contain environmentally persistent free radicals (EPFRs, such as semiquinone radicals) and redox-active compounds that can generate reactive oxygen species (ROS, including ·OH, superoxide and organic radicals) in the aqueous phase. However, there is a lack of data on EPFRs and ROS associated with size-segregated wildfire PM, which limits our understanding of their climate and health impacts. We collected size-segregated ambient PM in Southern California during two wildfire events to measure EPFRs and ROS using electron paramagnetic resonance spectroscopy. EPFRs are likely associated with soot particles as they are predominantly observed in submicron particles (PM 1 , aerodynamic diameter ≤ 1 μm). Upon extraction in water, wildfire PM mainly generates ·OH (28–49%) and carbon-centered radicals (∼50%) with minor contributions from superoxide and oxygen-centered organic radicals (2–15%). Oxidative potential measured with the dithiothreitol assay (OP-DTT) is found to be high in wildfire PM 1 , exhibiting little correlation with the radical forms of ROS ( r 2 ≤ 0.02). These results are in stark contrast with PM collected at highway and urban sites, which generates predominantly ·OH (84–88%) that correlates well with OP-DTT ( r 2 ∼ 0.6). We also found that PM generated by flaming combustion generates more radicals with higher OP-DTT compared to those by smoldering or pyrolysis. 

Biomass burning organic aerosol (BBOA) is one of the largest sources of organics in the atmosphere. Mineral dust and biomass burning smoke frequently co-exist in the same atmospheric environment. Common biomass burning compounds, such as dihydroxybenzenes and their derivatives, are known to produce light-absorbing, water-insoluble polymeric particles upon reaction with soluble Fe( iii ) under conditions characteristic of aerosol liquid water. However, such reactions have not been tested in realistic mixtures of BBOA compounds. In this study, model organic aerosol (OA), meant to replicate BBOA from smoldering fires, was generated through the pyrolysis of Canary Island pine needles in a tube furnace at 300, 400, 500, 600, 700, and 800 °C in nitrogen gas, and the water-soluble fractions were reacted with iron chloride under dark, acidic conditions. We utilized spectrophotometry to monitor the reaction progress. For OA samples produced at lower temperatures (300 and 400 °C), particles (P300 and P400) formed in solution, were syringe filtered, and extracted in organic solvents. Analysis was conducted with ultrahigh pressure liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (UHPLC-PDA-HRMS). For OA samples formed at higher pyrolysis temperatures (500–800 °C), water-insoluble, black particles (P500–800) formed in solution. In contrast to P300 and P400, P500–800 were not soluble in common solvents. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) were used to image P600 and determine bulk elemental composition. Electron microscopy revealed that P600 had fractal morphology, reminiscent of soot particles, and contained no detectable iron. These results suggest that light-absorbing aerosol particles can be produced from Fe( iii )-catalyzed reactions in aging BBOA plumes produced from smoldering combustion in the absence of any photochemistry. This result has important implications for understanding the direct and indirect effects of aged BBOA on climate. 

Relative humidity, NO_x, and NH₃can all alter the molecular, optical, and hygroscopic properties of naphthalene SOAs, with a complex synergy between these factors.