Organosulfates formed from heterogeneous reactions of organic-derived oxidation products with sulfate ions can account for >15% of secondary organic aerosol (SOA) mass, primarily in submicron particles with long atmospheric lifetimes. However, fundamental understanding of organosulfate molecular structures is limited, particularly at atmospherically relevant acidities (pH = 0–6). Herein, for 2-methyltetrol sulfates (2-MTSs), an important group of isoprene-derived organosulfates, protonation state and vibrational modes were studied using Raman and infrared spectroscopy, as well as density functional theory (DFT) calculations of vibrational spectra for neutral (RO–SO3H) and anionic/deprotonated (RO–SO3 –) structures. The calculated sulfate group vibrations differ for the two protonation states due to their different sulfur–oxygen bond orders (1 or 2 versus 12/3 for the neutral and deprotonated forms, respectively). Only vibrations at 1060 and 1041 cm–1, which are associated with symmetric S–O stretches of the 2-MTS anion, were observed experimentally with Raman, while sulfate group vibrations for the neutral form (∼900, 1200, and 1400 cm–1) were not observed. Additional calculations of organosulfates formed from other SOA-precursor gases (α-pinene, β-caryophyllene, and toluene) identified similar symmetric vibrations between 1000 and 1100 cm–1 for RO–SO3 –, consistent with corresponding organosulfates formed during laboratory experiments. These results suggest that organosulfates are primarily deprotonated at atmospheric pH values, which have further implications for aerosol acidity, heterogeneous reactions, and continuing chemistry in atmospheric aerosols.

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In this study, we use a modeling approach to evaluate the potential impact of microbial metabolism on the organic composition of cloud droplets and atmospheric aerosols. Microbial consumption rates for small organic molecules typically found in cloud and aerosol water were incorporated into a 0-D multiphase photochemical atmospheric chemistry model. We then use the model to simulate the evolution of the organic content of individual cloud and aerosol particles, along with the atmospheric gas phase. We find that metabolically active microorganisms may significantly impact organic acid concentrations in the individual aerosols and cloud droplets in which they reside. However, as a result of the low density of metabolically active cells in the atmosphere, the impact of these processes on the chemical composition of the overall population of cloud droplets or aerosols, or on gas-phase chemistry, is likely negligible.

We performed a laboratory investigation of the chemical processing of syringol, a representative model phenolic compound emitted from wood burning, in concentrated aqueous salt solutions mimicking tropospheric aerosol particles. For solutions containing chloride salts, we observed the formation of light-absorbing organic products (“brown carbon”), accompanied by a phase separation, within 10 h under dark conditions. Products were characterized at the molecular level using ultraperformance liquid chromatography interfaced to diode array detection and high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization and matrix-assisted laser desorption/ionization interfaced to high-resolution time-of-flight mass spectrometry. The ultraviolet–visible spectra, together with high-resolution mass spectra results, suggest that syringol can be oxidized by dissolved oxygen, and the presence of Cl– promotes this reaction. Our results provide new insights into the evolution of aerosol optical properties during aging, specifically the formation of aerosol brown carbon in biomass-burning plumes.

Recent studies have shown the potential of the photosensitizer chemistry of humic acid, as a proxy for humic-like substances in atmospheric aerosols, to contribute to secondary organic aerosol mass. The mechanism requires particle-phase humic acid to absorb solar radiation and become photoexcited, then directly or indirectly oxidize a volatile organic compound (VOC), resulting in a lower volatility product in the particle phase. We performed experiments in a photochemical chamber, with aerosol-phase humic acid as the photosensitizer and limonene as the VOC. In the presence of 26 ppb limonene and under atmospherically relevant UV–visible irradiation levels, there is no significant change in particle diameter. Calculations show that SOA production via this pathway is highly sensitive to VOC precursor concentrations. Under the assumption that HULIS is equally or less reactive than the humic acid used in these experiments, the results suggest that the photosensitizer chemistry of HULIS in ambient atmospheric aerosols is unlikely to be a significant source of secondary organic aerosol mass.