Steven Brown
Research Chemist
Mailing address:
NOAA ESRL Chemical Sciences Division
325 Broadway, R/CSD7
Boulder, CO 80305 USA
Phone: (303) 497-6306
Fax: (303) 497-5126
Email: Steven.S.Brown@noaa.gov
Dr. Brown received a Ph. D. in chemical physics with Professor Fleming Crim at the University of Wisconsin-Madison. He came to NOAA in 1997 as an NRC post-doctoral fellow working with Dr. A. R. Ravishankara, was a Research Scientist with CIRES at the University of Colorado from 2000 - 2005, and has served as a federal Research Chemist since then. His major research theme at NOAA has been the chemistry and impacts of nitrogen oxides in the Earth's atmosphere. His initial research was on laboratory studies of stratospheric nitrogen oxide kinetics; more recently his focus has been on field measurements of tropospheric nitrogen oxides, particularly those that occur in the dark ("nighttime chemistry"). His other main research interest has been the development of high sensitivity optical instrumentation for laboratory and field studies of atmospheric trace gases and aerosols.
Education
Ph.D., University of Wisconsin-Madison, 1996
B.A., Dartmouth College, 1989
Research
Atmospheric nitrogen oxides. Nitrogen oxides (NOx = NO + NO2) have both natural and anthropogenic sources, although in the troposphere they are primarily a pollutant derived from fossil fuel combustion. They regulate the abundance of ozone in all regions of the atmosphere and as such are important to air quality, climate and the stratospheric ozone layer. Laboratory and field measurements quantify their ambient concentrations and characterize the processes that govern their atmospheric chemistry.
Nighttime tropospheric chemistry. Atmospheric chemical transformation is driven by sunlight, which acts as a photolytic source of radicals to initiate chemical reactions. In the dark, however, a different set of chemical cycles involving species that are unstable in sunlight can become important. For example, the nocturnal nitrogen oxides, NO3 and N2O5, regulate the nocturnal lifetime of both NOx and ozone, initiate the oxidation of reactive VOC such as biogenic hydrocarbons, release active gas-phase halogen compounds from sea salt, and participate in heterogeneous chemistry and formation of secondary aerosol.
High sensitivity optical instrumentation. Direct absorption spectroscopy is an absolute technique for measurement of atmospheric trace gases and aerosol extinction that has been traditionally limited in atmospheric science applications because of its low sensitivity. Recent advances in optical cavities (e.g., two mirrors aligned such that light makes multiple reflections between them) have greatly enhanced the sensitivity and utility of direct absorption methods. Multiple field and laboratory instruments based on cavity ring-down and cavity enhanced absorption spectroscopies are currently in use and / or under development at ESRL / CSD.
Current Topics
Halogen activation. Recent development of instruments for the measurements of nitryl chloride (ClNO2) have shown that the nighttime reactions of NOx with chloride-containing aerosol is a surprisingly efficient source of halogens and may in fact be a substantial fraction of tropospheric active halogen budgets. Ship-based campaigns have investigated this mechanism in the Gulf of Mexico in summer (TexAQS 2006), wintertime outflow from New York City (ICEALOT 2008), mid continental sites near Boulder, CO (ACCRONIM 2009, NACHTT 2011) and the Los Angeles Basin (CalNex 2010).
Heterogeneous nitrogen oxide chemistry. Reactive uptake of N2O5 aerosol is one of the most important reactions governing the NOx lifetime and oxidant burdens in the troposphere. Aircraft measurements of N2O5 from the NOAA P-3 during the ICARTT and TexAQS campaigns in 2004 and 2006 have shown that this uptake is variable and often much slower than suggested by laboratory studies on model aerosol substrates and the atmospheric models that are based on these studies. These field results may point to the need for substantial revisions of NOx chemistry in global and regional atmospheric chemical models. Analysis of nighttime P-3 flights and measurements of N2O5 from the CalNex field study in 2010 are currently underway.
last modified: April 17, 2012