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Atmospheric Chemistry, Air Pollution, and Climate

Air pollution poses a growing problem in both developed and developing urban centers. Anthropogenic emissions and their products have strong impacts on human health, leading to an estimated 4 million premature deaths in 2015 alone, predicted to grow to 6.5 million annually by 2040. The Okumura group investigates the chemical kinetics and mechanisms of pollutant-forming oxidation of volatile organic compounds and related atmospheric reactions via spectroscopic, mass spectrometric, and quantum chemical methods.

Anthropogenic emissions impact climate both directly via radiative forcing and indirectly through subsequent oxidation and aerosol-forming aggregation processes. The Okumura group performs laboratory studies in support of remote and direct sensing projects providing spatially and temporally resolved information on sources and sinks of atmospherically relevant species. In addition, we study the formation of secondary organic aerosols, which have complex impacts on climate via scattering, absorption, and modulation of cloud nucleation rates and lifetimes.

In urban areas, the primary contributor to aerosol formation is volatile chemical products (VCPs). These compounds common in pesticides, personal care products, and cleaning agents oxidize in the atmosphere. The condensed products of these VCPs are called secondary organic aerosol. In collaboration with the Seinfeld lab, we are studying VCP oxidation mechanisms so we can improve air quality models and influence environmental policy related to VCP emissions. The Linde Atmospheric Chambers, each about 20 m3, provide large enough reaction vessels to study these systems over several hours.

Related Projects
  • Direct detection of low temperature combustion intermediates
  • Kinetics and branching ratios of peroxy radical self reactions
  • Generalized peroxy radical self-reaction mechanism
  • HOONO formation
  • Kinetic isotope effect
  • Clumped isotopes
  • O2 A-band
  • Aerosol PAS

Developing Spectroscopic Techniques 

The study of chemical systems of ever-increasing complexity has placed tremendous demands on spectroscopic sensitivity and resolution. In order to study these systems, the Okumura group has been developing and extending techniques in cavity ringdown spectroscopy, photoacoustic spectroscopy, and frequency combs. In collaboration with the NASA Jet Propulsion Laboratory, the group has also developed compact, chip-scale spectrometers. Future interests include the application of spectroscopic techniques for the search for life in our solar system.

Technique Summaries
Related Projects

Planetary and Astrochemistry 

The evolution of the chemical composition of planetary atmospheres is an active area of research. There is abundant evidence that Mars used to have flowing water; however, present-day Mars has no liquid water. One key to understanding the mechanism of water loss is the atmospheric D/H ratio, which is 6x enriched in deuterium relative to Earth's standard mean ocean water. This enrichment is consistent with preferential escape of lighter H atoms over D atoms from the top of the Martian atmosphere.

The process of Martian water loss is outlined above. Water is photolyzed, chemically transformed to long-lived H2, then transported to the top of the atmosphere, after which it can escape. While the chemistry underpinning the transformation of water to H2 is thought to be well understood, the effect of isotopic substitution on reaction rates is poorly characterized, or in many cases, entirely unstudied!

In the Okumura group, we are working on characterizing the kinetic isotope effect (KIE) of various HOx reactions in the Martian atmosphere, using both quantum chemical/in silico methods and direct experimental measurement.

Future interests include study of reactions relevant to other planetary atmospheres and the application of spectroscopic techniques for the search for life in our solar system.

Collaborators: Danica Adams, Yuk Yung

Related Projects
  • Kinetic Isotope Effect
  • HOx chemistry of the Martian atmosphere