(3cu) The Thermodynamics and Chemistry of Atmospheric Organic Compounds

Postdoctoral Work, University of California, Irvine, Chemistry.  Advisor:  Sergey Nizkorodov

The ubiquity of atmospheric particulate matter (PM) is a global problem that contributes to decreased life expectancies, respiratory effects, and may lead to premature cognitive decline.  PM also effects visibility and influences climate by altering the radiative balance of the earth and changing cloud properties.  The pervasiveness of particulate matter remains one of the most significant environmental problems; in the United States 74 million people live in counties that do not meet Environmental Protection Agency standards for PM less than 2.5 nm in diameter.  PM contains a diverse mixture of chemical species, both inorganic and organic in nature.  The inorganic fraction is relatively well-characterized; however, due to its complexity, the organic fraction (25 to 75% of the total particulate mass) remains to be adequately described. 

The formation and evolution of atmospheric particulate matter is an ideal problem for chemical engineers.  The organic fraction (organic aerosol, OA) consists of thousands of distinct chemical species in equilibrium with the gas phase.  A large fraction of the species in OA have vapor pressures such that they will repartition between the particle and gas phase in response to changes in temperature or total condensed organic mass.  I will present a semi-empirical relationship designed to correlate the heat of vaporization of an aerosol mixture with its distribution of volatilities¹.  This relationship has been used to help atmospheric modelers predict organic aerosol concentrations as a function of ambient temperature. 

OA is either directly emitted by anthropogenic and biogenic sources or is formed by reactions of volatile organic precursors.  The chemistry of these volatile organic precursors governs their eventual fate.  If reactions with atmospheric oxidants such as O₃, NOx, and OH radical sufficiently reduce their volatility, the oxidation products will partition to the particle phase and contribute to the total PM mass.  Ozonolysis oxidation reactions are complex, containing several intermediates and branching points.  I will summarize experiments designed to elucidate the reaction barrier governing the reaction of the first intermediate, the primary ozonide, for a series of reactive alkenes^{2, 3}.  These experiments, performed at cryogenic temperatures under vacuum to isolate the short-lived intermediates, are the first measurements of these reaction barriers and enhance our understanding of ozonolysis. 

In the presence of clouds or fog, gases or aerosol may partition into the aqueous phase and undergo cloud processing by dissolved oxidants or actinic radiation.  The significance of aqueous photolysis is not well understood due to limited experimental measurements.  I will show photolysis rate measurements of aqueous methyl peroxide⁴ (the simplest organic peroxide) and aqueous glyceraldehyde⁵ (an oxidized water-soluble photolabile compound), which are used to calculate atmospheric lifetimes.  I will also present a theoretical framework that uses Henry’s law constants and hydration equilibrium to determine the significance of aqueous photolysis for a large group of atmospherically relevant organic compounds⁶.  This framework will help guide researchers in the selection of compounds for experimental measurements of aqueous rate constants. 

My proposed research will focus on enhancing our understanding of the sources, composition, and evolution of particulate matter through laboratory studies and parameterizations designed to bridge the gap between experimentalists and chemical transport modelers. 

REFERENCES

1.  Epstein, S. A.; Riipinen, I.; Donahue, N. M., A Semiempirical Correlation between Enthalpy of Vaporization and Saturation Concentration for Organic Aerosol. Environ. Sci. & Technol. 2009, 44, (2), 743-748.

2.  Epstein, S. A.; Donahue, N. M., The Kinetics of Tetramethylethene Ozonolysis: Decomposition of the Primary Ozonide and Subsequent Product Formation in the Condensed Phase. J. Phys. Chem. A 2008, 112, (51), 13535-13541.

3.  Epstein, S. A.; Donahue, N. M., Ozonolysis of Cyclic Alkenes as Surrogates for Biogenic Terpenes: Primary Ozonide Formation and Decomposition. J. Phys. Chem. A 2010, 114, (28), 7509-7515.

4.  Epstein, S. A.; Shemesh, D.; Tran, V. T.; Nizkorodov, S. A.; Gerber, R. B., Absorption Spectra and Photolysis of Methyl Peroxide in Liquid and Frozen Water. J. Phys. Chem. A 2011.

5.  Epstein, S. A.; Nizkorodov, S. A., Direct Aqueous Photolysis of Carbonyls in the Atmosphere. In preparation. 2012.

6.  Epstein, S. A.; Nizkorodov, S. A., A Comparison of the Chemical Sinks of Atmospheric Organics in the Gas and Aqueous Phase. Atmos. Chem. Phys. Discuss. 2012, 12, (4), 10015-10058.