Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
This thesis advances our understanding of secondary organic aerosol (SOA) formation from small, water soluble molecules like glyoxal and methyl glyoxal that partition to the aerosol aqueous phase according to Henry's law. Their partitioning behavior can be altered by the presence of inorganic salts, which are abundant in aerosols. If the molecules become more soluble this is called "salting in" and the reverse is "salting out". We present the first measurements of salting constants of glyoxal and methyl glyoxal in the aerosol-relevant salts ammonium sulfate, ammonium nitrate, sodium chloride, and sodium nitrate, as well as additional salts for theory-measurement comparison purposes: tetramethylammonium bromide, tetrabutylammonium bromide, and sodium oxalate. We find that glyoxal "salts in" to all salts tested, while methyl glyoxal "salts out". Explanations for their salting behavior are presented using quantum-mechanical calculations to quantify molecule-ion attractions and proposed literature mechanisms to predict repulsive forces (solution volume decrease upon salt addition, dielectric decrement). We calculate a ΔG of interaction (the enthalpy for replacing a water molecule in the hydration shell of an ion with an organic molecule) for isoprene epoxydiol (IEPOX) and its ring-opening product methyl tetrol with sulfate, and given the structural similarities of IEPOX and methyl tetrol with methyl glyoxal, we find that the values are similar to the value for methyl glyoxal. Therefore we predict that trans-Β-IEPOX and its methyl tetrol likely salt out with a salting constant similar to methyl glyoxal.
Partitioning also influences reactivity. In a series of simulation chamber experiments we have studied the effect of NH3 on the rate of gly-SOA formation and find that in the presence of added NH3 we observe a significant enhancement in total SOA mass formed from glyoxal, especially in the formation of nitrogen-containing products (imidazoles) and the overall aerosol N/C. We present the results of simulation chamber experiments that focus on the chemical composition of gly-SOA. We are able to identify a number of imidazoles resulting from the reaction of glyoxal + NH4+ . We also use time-resolved data to propose a mechanism for their formation. By comparing imidazole formation under a variety of different chamber conditions we are able to determine under what conditions they will contribute most strongly to brown carbon formation.
We have further used a box model to simulate SOA formation in Mexico City during the MCMA-2003 campaign using an explicit aqueous-phase mechanism for glyoxal processing (gly-SOA). This gly-SOA was compared to SOA from semi/intermediate volatility gases (S/I VOC) using a near explicit gas-phase oxidation mechanism, and two different volatility basis sets to characterize
their partitioning. We also have applied our measured salting constants to predict SOA formation over the continental United States using the Community Multi-scale Air Quality Model (CMAQ). Significant SOA forms from the further reactions of these precursors in the aerosol, but the effect of salt is most important. The small Henry's law constant for methyl glyoxal combined with the fact that it salts out means that even though methyl glyoxal is far more abundant in the gas phase than glyoxal, methyl glyoxal forms significantly less SOA than glyoxal. Our results suggest that salts also play an important role in modulating the SOA formation from isoprene.
Waxman, Eleanor M., "Secondary Organic Aerosol Formation from alpha-dicarbonyls in Aerosol Water: Salt Interactions and Multi-Phase Chemistry" (2015). Chemistry & Biochemistry Graduate Theses & Dissertations. 162.