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

Abstract

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.

Aging of Atmospheric Brown Carbon Aerosol

“…How important are charge-exchange processes in forming BrC within aerosol? [294][295][296] Studies of BrC material in aqueous solutions are crucial to simulate cloud-processing conditions. However, care should be taken to work with realistic concentrations of water-soluble organic carbon.…”

Photoreaction of biomass burning brown carbon aerosol particles

“…We have previously demonstrated that DMSO is the best solvent to remove colored wood smoke material from the filters. 27 Analysis by UV-Vis (Ocean Optics) was performed in a 1 cm cuvette, with an integration time of 300 ms and 3 scans per average. The AMS was not used for every experiment.…”

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

“…The BBOA samples were produced at the University of Toronto via the pyrolysis of pine wood using the method described by Trofimova et al, which is similar to other methods used to generate proxies of BBOA from the smoldering phase of combustion. ,− The pine wood used was untreated lumber purchased from a hardware store in Toronto, Canada. Approximately 2 g of pine wood was placed in a flow tube inside a tube furnace heated to a temperature of 400 °C.…”

“…21,67−70 Here, we only used the water-soluble component of the organic aerosol generated by the pyrolysis of wood, which corresponds to ∼75% of the total mass of organic aerosol generated in our experiments. 67 The measured diffusion coefficients were used to estimate the mixing times of organic molecules within 200 nm SOA and BBOA particles for RH values typically found in the planetary boundary layer. For β-caryophyllene SOA, the measured diffusion coefficients together with viscosity data in the literature for β-caryophyllene SOA 71 were used to quantify the accuracy of the Stokes−Einstein relation and the fractional Stokes−Einstein relation for predicting diffusion coefficients in more chemically complex samples.…”

“…As a surrogate for BBOA, we used organic aerosol produced via the pyrolysis of pine wood in a tube furnace. This type of organic aerosol was used as a surrogate of BBOA from the smoldering phase of combustion, as done previously. ,− Here, we only used the water-soluble component of the organic aerosol generated by the pyrolysis of wood, which corresponds to ∼75% of the total mass of organic aerosol generated in our experiments …”