Chemical composition of atmospheric organic matter
Many studies report the presence of complex macromolecular and high molecular weight species in atmospheric aerosols. Some studies compare these to humic and fulvic acids described in the oceanography and freshwater literature, while others compare the complex mixture to oligomers. The common theme is that these compounds lack chemical characterization due to analytical constraints, and their formation is not well understood. My identification of oligomer formation in simple laboratory experiments designed to characterize cloud chemistry (Altieri et al., 2006, 2008) motivated the analysis of local rainwater samples using the same ultra-high resolution MS to investigate whether the laboratory-formed oligomers are present in the environment, and to elucidate any connections between the rapid radical chemistry in the laboratory and the complexity observed in atmospheric organics.
The rain samples contained >500 unique elemental formulas, and since the number of possible structures increases as the molecular weight increases, there are likely far more than 500 organic compounds in the rain (Altieri et al., 2009a). All nine oligomer series identified in the laboratory experiments (Altieri et al., 2008) were identified in the rainwater (Altieri et al., 2009a), verifying that in-cloud chemical reactions that occur in the environment were well-simulated in our laboratory experiments. Many of the identified compounds had characteristics similar to known products of atmospheric reactions that form SOA through gas-phase, aerosol-phase, and in-cloud reactions including oligomers, organosulfates, and nitrooxy-organosulfates.
Oligomers formed through in-cloud processing
During my Ph.D., my lab conducted experiments to simulate in-cloud chemistry, demonstrating that in-cloud processing ultimately leads to SOA (Carlton et al., 2006, 2007). As part of this effort, I used electrospray ionization mass spectrometry (ESI-MS), a soft ionization technique that does not fragment molecules, to analyse the cloud chemistry experiments, and found that in addition to low volatility organic acids, small polymers (i.e., oligomers) form within minutes (Altieri et al., 2006). Oligomers have lower vapour pressures than organic acids, such that they remain to a larger extent in the particle phase after cloud droplet evaporation, potentially contributing more SOA mass. Depending on the oligomer properties, the more complex molecules can affect the absorptive and scattering properties of clouds and particles, influencing radiative forcing. This provides a source of SOA mass that is chemically complex and formed at multiple elevations with both temporal and spatial variability, thus the addition of cloud produced SOA to models results in improved predictions (Carlton et al., 2008).
To elucidate the mechanism(s) of oligomer formation, and the properties of the oligomers formed, I employed ultra-high resolution ESI Fourier transform ion cyclotron resonance MS (ESI FT-ICR MS), a technique with sub-ppm resolution that allows for the identification and separation of thousands of compounds. For each mass detected, the exact elemental composition (CaHbNcOdPeSf), the number of rings and double bonds, and the elemental ratios (e.g., O:C, N:C) can be calculated. This work resulted in the first detailed description of the chemical mechanism of aqueous-phase oligomerization, esterification of an organic acid with an α- or β- hydroxy acid that generates nine series of oligoesters related by regular differences in mass and elemental composition (Altieri et al., 2008). The elemental ratios of the oligomers are consistent with aged atmospheric aerosols, suggesting that oligomers are a larger contributor to aged organic aerosol mass than the simple organic acids that are typically measured.