Light-absorbing organic material, or "brown carbon" (BrC), can significantly influence the effect that aerosols have on climate. Here, we investigate how changing pH affects the absorption spectra of water-soluble BrC from ambient particulate matter smaller than 2.5 μm collected in Athens, Georgia, in the spring and fall of 2016, including samples from nearby wildfires. We find that absorption increases 10% per pH unit from pH 2 to pH 12 with a broad, featureless tail at visible wavelengths, where the largest fractional increase is also observed. The resulting change in the spectral shape causes the absorption Ångström exponent to decrease by 0.18 per unit increase in pH. Similar behavior with humic substances suggests that they and BrC share a common link between pH and absorption, which we propose could be a consequence of conformational changes in supramolecular assemblies thought to exist in humic substances. Specifically, we hypothesize that a wider variety and larger number of absorbing charge transfer complexes are formed as functional groups in these molecules, such as carboxylic acid and phenol moieties, become deprotonated. These findings suggest that (1) the pH of ambient particulate matter samples should be measured or controlled and (2) radiative forcing by BrC aerosols could be overestimated if their pH-dependent BrC absorption is not accounted for in models.

By integrating organic synthesis, secondary organic aerosol synthesis and collection, density functional theory (DFT) calculations, and vibrational sum frequency generation (SFG) spectroscopy, we identify close spectral matches between the surface vibrational spectra of β-caryophyllene-derived secondary organic material (SOM) and those of β-caryophyllene aldehyde and β-caryophyllonic acid at various interfaces. Combined with the record high surface tension depression described previously for these same oxidation products, we discuss possibilities for an intrinsically chemical origin for cloud activation by terpene-derived surfactants. Although the present study does not unequivocally identify the synthesized and analyzed oxidation products on the β-caryophyllene-derived SOM surfaces, these two compounds appear to be the most surface active out of the series and have also been foci of previous β-caryophyllene field and laboratory studies. An orientation analysis by phase-resolved SFG spectroscopy reveals a “pincer-like” configuration of the β-caryophyllene oxidation products, albeit on a model quartz surface, that somewhat resembles the orientation of inverse double-tailed surfactants at the surfaces of biological systems. The structural information suggests that the less polar moiety of a surface-localized oxidation product, such as those studied here, may be the first site-of-contact for a gas-phase molecule approaching an SOA particle containing surface-active β-caryophyllene oxidation products.

The surface activity of ten atmospherically relevant α-pinene-derived dimers having varying terminal functional groups and backbone stereochemistry is reported. We find ∼10% differences in surface activity between diastereomers of the same dimer, demonstrating that surface activity depends upon backbone stereochemistry. Octanol–water (K OW ) and octanol–ammonium sulfate partitioning coefficient (K OAS ) measurements of our standards align well with the surface activity measurements, with the more surface-active dimers exhibiting increased hydrophobicity. Our findings establish a link between molecular chirality and cloud activation potential of secondary organic aerosol particles. Given the diurnal variations in enantiomeric excess of biogenic emissions, possible contributions of such a link to biosphere:atmosphere feedbacks as well as aerosol particle viscosity and phase separation are discussed.

We examine synthesized standards of structurally related β-caryophyllene ozonolysis products at the air/water and air/ammonium sulfate (aq) interfaces using polarization-resolved standard- and high-resolution vibrational sum frequency generation (SFG) spectroscopy in the C–H and O–H stretching regions. β-Caryophyllene aldehyde, the most surface-active molecule in the suite, appears to undergo a change in its molecular orientation distribution when transitioning from low to high relative surface coverage. We also report that the water molecules present at the outermost layer of the aqueous interface are displaced by or hydrogen-bonded to β-caryophyllene aldehyde on both the ammonium sulfate (aq) and water subphases. Additionally, the more weakly hydrogen-bonded water network observed around ∼3400 cm–1 seems to be perturbed to a greater extent by β-caryophyllene aldehyde than the more strongly hydrogen-bonded population observed at ∼3200 cm–1 on the ammonium sulfate (aq) subphase, while this effect is not observed on the water subphase. We provide surface spectroscopic evidence for the “salting out” effect of ammonium sulfate in aqueous environments and discuss the implications for understanding the organization of surface-localized molecules present in aqueous cloud-forming droplets containing dissolved aerosol cloud condensation nuclei (CCN) in the atmosphere. Despite the importance of elucidating the surface activity and interfacial organization of individual molecules in aqueous media, future work will focus on how the combined effects of several surface-active constituents dictate overall cloud activation potentials of aerosol particles.

The surface activity of ten atmospherically relevant α-pinene-derived dimers having varying terminal functional groups and backbone stereochemistry is reported. We find ~10 percent differences in surface activity between diastereomers of the same dimer, demonstrating that surface activity depends upon backbone stereochemistry. Octanol¬–water (KOW) and octanol–ammonium sulfate partitioning coefficient (KOAS) measurements of our standards align well with the surface activity measurements, with the more surface-active dimers exhibiting increased hydrophobicity. Our findings establish a link between molecular chirality and cloud activation potential of secondary organic aerosol particles. Given the diurnal variations in enantiomeric excess of biogenic emissions, possible contributions of such a link to biosphere:atmosphere feedbacks as well as aerosol particle viscosity and phase separation are discussed.