Light Absorption by Cinnamaldehyde Constituents of Biomass Burning Organic Aerosol Modeled Using Time-Dependent Density Functional Theory

Organic aerosol emitted from biomass burning absorbs visible radiation. However, the impact of this light absorption on the overall climate effects of atmospheric aerosol is not well known, partly due to variability in particle composition and absorptivity. Cinnamaldehydes, which consist of an aromatic ring with an unsaturated aldehyde substituent, are an important class of chromophores in light-absorbing organic aerosol, or brown carbon. Here, light absorption by a homologous series of three cinnamaldehydescoumaraldehyde, coniferaldehyde, and sinapaldehydeis modeled with time-dependent density functional theory (TD-DFT) calculations, in the gas and aqueous phases. Based on a survey of hydration and acid dissociation equilibria, the neutral aldehyde is expected to be the predominant form of each species in the atmospheric aqueous phase. These species have complicated conformational landscapes compared to many other brown carbon constituents, like rigid polycyclic aromatic hydrocarbons. For coumaraldehyde, coniferaldehyde, and sinapaldehyde, a total of 8, 26, and 18 conformers were located, respectively. For each species, most of the total population is accounted for by the four most-populated conformers. The relative contributions of the conformers to the total light absorption of the respective species are dictated more by differences in the relative free energies than by differences in the molar absorption coefficients. As the functionalization increases, the absorption is red-shifted. The peaks predicted in water agree well with experimental spectra of coniferaldehyde and sinapaldehyde. No conformers have vertical transitions in the visible spectral range, so absorption above 380 nm is due to the shoulders of transitions of major conformers at ultraviolet wavelengths. These results demonstrate the importance of exploring potential energy landscapes, determining conformer stability and absorptivity, to predict the light absorption of chromophores in brown carbon

Hygroscopicity of Secondary Brown Carbon Aerosol from Aqueous Photo-Oxidation of Phenolic Precursors

To understand the impact of light-absorbing organic aerosol, also called brown carbon (BrC), it is necessary to determine the extent to which the direct effect through aerosol–radiation interactions and the indirect effect through aerosol–cloud interactions change during its atmospheric residence time. Toward addressing this need, the light absorption and water uptake of secondary BrC aerosol produced from phenolic compounds, abundant biomass burning emissions, were measured. Phenol, catechol, and pyrogallol were selected to form a homologous series, varying in the number of hydroxyl substituents, and they were exposed to aqueous hydroxyl radical in a photoreactor, leading to the formation of secondary BrC. The absorptivity of the BrC was monitored by UV–vis spectroscopy; the hygroscopicity was determined using a hygroscopic tandem differential mobility analyzer. The absorptivity of the secondary BrC increased within 8 h of photo-oxidation and then began decreasing. After 24 h of photo-oxidation, at an atmospherically relevant OH exposure of 2.2 × 10–10 mol s L–1, the hygroscopicity parameters for BrC from phenol, catechol, and pyrogallol were similar, i.e., 0.13 ± 0.02, 0.10 ± 0.02, and 0.13 ± 0.02, respectively, so BrC from phenolic compounds exhibits similar water uptake regardless of the functionalization of the precursor. After 36 and 48 h of continued photo-oxidation, during which the product mixture exhibited further whitening, the hygroscopicity parameter of secondary BrC from catechol did not change. These observations suggest that the changes in absorptivity (related to the direct effect) of secondary BrC produced from phenolic precursors are greater than the changes in hygroscopicity (related to the indirect effect) upon atmospheric aging

Contribution of Charge-Transfer Complexes to Absorptivity of Primary Brown Carbon Aerosol

Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate. A large fraction of the absorptivity of ambient BrC is unassigned, and organic charge-transfer (CT) complexes have the potential to contribute to this fraction. Here, the contributions of CT complexes to the absorptivity of laboratory-generated BrC and ambient aerosol material influenced by biomass burning have been investigated, using a wide range of chemical, spectroscopic, and physical analyses. Chemical functionalization experiments are inconclusive about the role of CT complexes, whereas fluorescence spectra exhibit distinct spectral features indicative of individual chromophores. Determinations of the concentration and temperature dependences of absorbance are more conclusive. In particular, for laboratory-generated BrC extracted in either water or methanol, absorbance scaled linearly with orders-of-magnitude changes in concentration, indicating that intermolecular complexes do not contribute to the absorptivity. Furthermore, whereas the absorbance of BrC extracts in dimethyl sulfoxide exhibited a slight temperature dependence, consistent with a 15% contribution from intramolecular CT complexes at 15 °C, the complete temperature independence of absorbance of water-soluble extracts from surrogate and ambient BrC indicates a negligible role for CT complexes. Overall, our results find little evidence for CT complexes in the primary BrC studied, suggesting that they do not contribute significantly to the missing absorptivity of ambient BrC

Ultraviolet Irradiation Can Increase the Light Absorption and Viscosity of Primary Brown Carbon from Biomass Burning

The light absorption of brown carbon (BrC) constituents of biomass burning organic aerosol (BBOA) changes in the atmosphere, in part due to multiphase oxidation. For example, ozonolysis leads to the whitening of primary BrC constituents. Irradiation can also change the properties of BrC. Here, we investigate the interplay between irradiation and multiphase processing by measuring the reactive uptake of ozone to thin films of BBOA before and after exposure to UV radiation in a photoreactor. Thin films were prepared from the lower volatility fraction of BBOA from eastern red cedar, a species associated with wildfires and prescribed fires in the southern Great Plains, United States. Irradiation increased the mass absorption coefficient of the BrC at near-UV and visible wavelengths. It also significantly decreased the reactive uptake of ozone, which was attributed to increased viscosity of the BBOA material. These changes in absorptivity and viscosity are consistent with results of mass spectrometry and volatility tandem differential mobility analysis, which show that high-molecular-weight species constitute a greater fraction of the total mass after irradiation. Our results may have significant implications on the warming effect of BrC, since UV irradiation here both darkens this BBOA material and makes it more resistant to multiphase processing and whitening by ozone under the conditions investigated

Diffusion Coefficients and Mixing Times of Organic Molecules in β‑Caryophyllene Secondary Organic Aerosol (SOA) and Biomass Burning Organic Aerosol (BBOA)

Information on the diffusion rates of organic molecules within secondary organic aerosol (SOA) and biomass burning organic aerosol (BBOA) is needed to predict the impact of these aerosols on atmospheric chemistry, air quality, and climate. Nevertheless, no studies have measured diffusion rates of organics within SOA generated from β-caryophyllene or within BBOA. Here, we measured diffusion rates of organic molecules in laboratory-generated SOA and BBOA as a function of water activity (aw) using fluorescence recovery after photobleaching. The SOA was generated by the ozonolysis of β-caryophyllene, and the BBOA was generated by the pyrolysis of pine wood. Only the water-soluble component of the BBOA was studied. The measured diffusion coefficients of organic molecules in β-caryophyllene range from 1.1 × 10–16 to 1.3 × 10–14 m2 s–1 for aw values ranging from 0.23 to 0.86. For BBOA, the diffusion coefficients range from 7.3 × 10–17 to 6.6 × 10–16 m2 s–1 for aw values ranging from 0.23 to 0.43. Based on these values, the mixing times of organic molecules within a 200 nm SOA or BBOA are less than 1 min for aw values >0.23. Since aw values are often greater than 0.23 in the planetary boundary layer and temperatures in the planetary boundary are often within 5 K of our experimental temperatures, mixing times are likely often short in that part of the atmosphere for the types of aerosols studied here. For β-caryophyllene SOA, we compared the measured diffusion coefficients with predictions based on the Stokes–Einstein relation and the fractional Stokes–Einstein relation. For both the Stokes–Einstein and the fractional Stokes–Einstein relations, the measured diffusion coefficients agree with the predicted diffusion coefficients. This work illustrates that when the radius of the diffusing molecules is greater than the average radius of the matrix molecules, the Stokes–Einstein equation is able to predict diffusion coefficients in β-caryophyllene SOA with reasonable accuracy

Phase Behavior and Viscosity in Biomass Burning Organic Aerosol and Climatic Impacts

Smoke particles generated by burning biomass consist mainly of organic aerosol termed biomass burning organic aerosol (BBOA). BBOA influences the climate by scattering and absorbing solar radiation or acting as nuclei for cloud formation. The viscosity and the phase behavior (i.e., the number and type of phases present in a particle) are properties of BBOA that are expected to impact several climate-relevant processes but remain highly uncertain. We studied the phase behavior of BBOA using fluorescence microscopy and showed that BBOA particles comprise two organic phases (a hydrophobic and a hydrophilic phase) across a wide range of atmospheric relative humidity (RH). We determined the viscosity of the two phases at room temperature using a photobleaching method and showed that the two phases possess different RH-dependent viscosities. The viscosity of the hydrophobic phase is largely independent of the RH from 0 to 95%. We use the Vogel–Fulcher–Tamman equation to extrapolate our results to colder and warmer temperatures, and based on the extrapolation, the hydrophobic phase is predicted to be glassy (viscosity >1012 Pa s) for temperatures less than 230 K and RHs below 95%, with possible implications for heterogeneous reaction kinetics and cloud formation in the atmosphere. Using a kinetic multilayer model (KM-GAP), we investigated the effect of two phases on the atmospheric lifetime of brown carbon within BBOA, which is a climate-warming agent. We showed that the presence of two phases can increase the lifetime of brown carbon in the planetary boundary layer and polar regions compared to previous modeling studies. Hence, the presence of two phases can lead to an increase in the predicted warming effect of BBOA on the climate