ARB Research Seminar

This page updated June 19, 2013

Observations of NO₂, Total Peroxynitrates, Total Alkyl nitrates and HNO₃ on the Western Slopes of the Sierra: Implications for Transport of Nitrates to Lake Tahoe

Ronald C. Cohen, Ph.D., University of California, Berkeley, Department of Chemistry and Department of Earth and Planetary Sciences, Berkeley, California

March 08, 2004
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Research Project


Observations of NO₂, total peroxynitrates (SPNs), total alkyl nitrates (SANs) and HNO₃ have been ongoing at Big Hill (since March 2003) and at the UC Blodgett Forest Research Station (since October 2000) on the Western slopes of the Sierra Nevada. The measurements are made using thermal dissociation coupled to LIF detection of NO₂. Analysis of the daily, monthly, and annual variations in the observed mixing ratios, and their relationships to O₃ and H₂O, are used to understand the sources of nitrogen oxides and to determine the relative role of transport and chemistry on nitrogen oxide concentrations at the two sites. We find that transport of a recognizable plume with a source in the Sacramento metropolitan area is a strong influence in the late afternoon at both sites. The influence of the plume is much weaker at other times of the day and the plume is less important as air gets closer to Lake Tahoe.

Speaker Biography

Ronald C. Cohen Professor (1996-present), UC Berkeley Department of Chemistry and Department of Earth and Planetary Sciences, Faculty Scientist, Lawrence Berkeley National Laboratory, Division of Energy and Environment. Cohen received his Ph.D. in Physical Chemistry from UC Berkeley in 1991. From 1991-1996, he was a postdoctoral fellow and on the research staff at Harvard University in the laboratory of J.G. Anderson where he developed instruments to study stratospheric ozone depletion from high altitude research aircraft.

Professor Cohen's current research aims to use observations of the chemicals present in the atmosphere to understand natural and human induced contributions to climate change, ozone depletion, and urban smog. Cohen and his research group have developed novel, sensitive, laser based techniques for observing NO₂, NO₃, N₂O₅, HO₂NO₂, the sum total of all PAN analogues, the sum total of all alkyl nitrate analogues and HNO₃. Their instruments for observing NO₂ are the world's most sensitive. They have developed inexpensive methods for specific detection of NO₂ and for detection of N₂O₅. Observations using these instruments on the ground at various sites in California, Houston, Texas, Nashville, Tennessee, and from aircraft over the Arctic and within the stratosphere have been used to establish unique tests of atmospheric photochemistry that point to weaknesses in our current understanding of peroxy radical reactions and of the abundance of VOC precursors to O₃. Prior to the Cohen group's observations at the UC Blodgett Forest Research Station, alkyl nitrates were largely ignored and thought to be unimportant, but are now becoming recognized as crucial players in atmospheric chemistry, especially urban and regional air quality. Cohen also works on understanding evaporation of water so that we can build a proper model of climate history using information from 400,000 year old ice in the Antarctic glaciers. He and his students have shown that the evaporation model used to describe isotopes in current climate simulations is incorrect and have developed a new parameterization that captures the variability seen in experiment and properly represents the molecular scale processes.

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