Exploring brown carbon analysis: rigorous characterization and source apportionment

Di Lorenzo, Robert Anthony (2016) Exploring brown carbon analysis: rigorous characterization and source apportionment. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Atmospheric particulate matter (PM) represents the greatest factor underlying the total uncertainty of anthropogenically derived radiative forcing. It has been shown that particles containing a large fraction of water soluble carbon have organic species with the ability to absorb solar radiation, referred to as brown carbon (BrC). Preliminary work has shown that BrC is strongly correlated to biomass burning (BB) events, is predominantly composed of extremely low volatility organic compounds (ELVOCs) and is of large molecular weight character. Particle characterization of multiple real-world forest fires and regional background aerosol has been achieved using multiple analytical techniques including size exclusion chromatography, reverse-phase liquid chromatography, ion chromatography, molecular absorption spectroscopy, and multiple combinations of mass spectrometric ionization and detection techniques. Particles were collected both in bulk and by cascade impaction, which size fractionated particles into 13 size bins between 18 μm and 10 nm. This work shows that biomass burning particulate matter composition is highly complex and cannot be characterized mass spectrally using one ionization technique alone. Although comparisons in the literature are often made to the composition of humic acid, the organic aerosol fraction, particularly derived from BB, differs greatly. Further, absorbing species comprising BrC are not accurately represented when commonly employed electrospray ionization is used. These BrC absorbing species have been found to be of large molecular weight character, with a large proportion greater than 500 Da. The molecular weight profile distribution is consistent across multiple forest fire samples, as well as in background aerosol. BB was confirmed to be the dominant source of large molecular weight BrC in the background aerosol samples. The absorbing species were found not to be internally mixed with common BB markers, but the absorbance distribution across particle diameters coincided with the observance of ammonium and dialkyl ammonium species, potentially representing new tracers. The molecular characteristics of these large absorbing molecules would be consistent with the ELVOC-BrC species previously identified in the literature. Large molecular weight BrC absorbers represent a recalcitrant source of absorbing organic molecules to the atmosphere. These large, long-lived absorbing molecules may have important climate impacts and are worthy of further study.

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