(4ae) The Thermodynamics and Chemistry of Organic Atmospheric Particulate Matter

Epstein, S. A., Carnegie Mellon University

Research Interests

Atmospheric particulate matter (PM) is a global pollutant with adverse effects on human health, visibility, and climate.  In the United States, 75 million people live in counties that fail to meet government standards for concentrations of fine PM (<2.5 nm in diameter).  In order to design effective and efficient regulation, we must understand the sources, formation, and evolution of PM in the atmosphere.  This is a complex problem that involves modeling upwards of 100,000 compounds in three phases (gas, aerosol, and aqueous) under constant attack by atmospheric oxidants and sunlight.  PM is composed of inorganic and organic compounds.  The inorganic fraction is relatively well-characterized.   However, due to its complexity, the organic fraction (25 to 75% of the total mass) remains to be adequately described.  My work is aimed at understanding the formation and evolution of organic aerosol (OA) in the atmosphere.

OA is either directly emitted by anthropogenic and biogenic sources or is formed by reactions of volatile organic precursors in the atmosphere.  The chemistry of these volatile organic precursors governs their eventual fate.  My research experience focuses on the formation of organic particulates from oxidation by atmospheric oxidants.  Ozonolysis oxidation reactions—a significant source of biogenic PM—are complex, containing several intermediates and branching points.  I will summarize Temperature Programmed Reaction Spectroscopy experiments, performed at cryogenic temperatures to elucidate previously unknown barrier heights that govern the fundamental behavior of these reactions.   

Organic aerosol consists of thousands of distinct chemical species in equilibrium with the gas phase.  A large fraction of the species found in OA have vapor pressures that lead to repartitioning between the particle and gas phase in response to changes in temperature or total condensed organic mass.  An aspect of my research experience and interests involves characterizing the thermodynamics that govern the interactions between gas and particulate phase species.

Along with oxidation, photolysis by sunlight is an important process governing the evolution and fate of organic compounds.  The majority of photolysis studies have been focused towards gaseous compounds.  However, photolysis can occur inside cloud and fog droplets, where hydrophilic gas and aerosol compounds may partition.  In addition, particulate phase compounds can be photolized directly.  Another major aspect of my research is aimed at determining the significance and quantifying the rates of these non-gaseous photolysis reactions with novel experiments and theoretical models.       

Teaching Experience

I have had many opportunities to acquire teaching experience during my tenure at Carnegie Mellon.  For several years I served as the AIChE Chemical Engineering Car team advisor for undergraduate students at Carnegie Mellon.  I had the opportunity to mentor several student groups as they developed and tested their vehicles.  I was also a chemical engineering instructor for the Carnegie Mellon Summer Academy for Math and Science.  In this position, I gave lectures on fundamental chemical engineering concepts and led discussions sessions as students pursued their own design projects.  I also had the opportunity to serve as a teaching assistant for four courses at Carnegie Mellon and give several guest lectures for both undergraduate and graduate students.


I am actively involved within and outside the University community.  I currently serve as the Vice-Chairman of the Laguna Beach Environmental Sustainability Committee, established to advise the city council on environmental policy decisions.  I also regularly give educational air quality seminars to school children and adults throughout Orange County and coordinate the weekly AirUCI Institute seminar series.  I am also an active member of the Heavenly Volunteer Ski Patrol in South Lake Tahoe, CA.

Educational Background

  • Bachelors of Chemical Engineering from University of Delaware
  • Ph.D. in Chemical Engineering from Carnegie Mellon University working with Prof. Neil Donahue
  • Currently a postdoctoral researcher at the University of California, Irvine working with Prof. Sergey Nizkorodov in the Department of Chemistry.

Selected Publications

Epstein, S.A. and N.M. Donahue, The Kinetics of Tetramethylethene Ozonolysis: Decomposition of the Primary Ozonide and Subsequent Product Formation in the Condensed Phase. The Journal of Physical Chemistry A, 2008. 112(51): p. 13535-13541

Epstein, S.A., I. Riipinen, and N.M. Donahue, A Semiempirical Correlation between Enthalpy of Vaporization and Saturation Concentration for Organic Aerosol. Environmental Science & Technology, 2009. 44(2): p. 743-748

Epstein, S.A. and N.M. Donahue, Ozonolysis of Cyclic Alkenes as Surrogates for Biogenic Terpenes: Primary Ozonide Formation and Decomposition. The Journal of Physical Chemistry A, 2010. 114(28): p. 7509-7515

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. The Journal of Physical Chemistry A, 2011. 116(24): p. 6068-6077

Epstein, S.A. and S.A. Nizkorodov, A Comparison of the Chemical Sinks of Atmospheric Organics in the Gas and Aqueous Phase. Atmospheric Chemistry and Physics, 2012. 12(17): p. 8205-8222

Epstein, S.A., Tapavicza, E., Furche, F., and Nizkorodov, S.A, Direct Photolysis of Carbonyl Compounds Dissolved in Cloud and Fog Droplets. Atmospheric Chemistry and Physics Discussions, 2013. 13(4): p. 10905-10937