Steven Brown

Program Leader

Tropospheric Chemistry

Atmospheric Remote Sensing

NOAA Chemical Sciences Laboratory
325 Broadway, R/CSL7
Boulder, CO 80305 USA

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Dr. Brown received a Ph. D. in physical chemistry 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. He currently leads the Atmospheric Remote Sensing Program, and the Tropospheric Chemistry Program, a group that conducts field measurements to understand atmospheric composition with applications in air quality and climate. His major research theme at NOAA has been the chemistry and impacts of nitrogen oxides in the Earth's atmosphere, with an emphasis on nighttime chemical cycles. He has also led the development of high sensitivity optical instrumentation for laboratory and field studies of atmospheric trace gases and aerosols.


Ph.D., University of Wisconsin-Madison, 1996
B.A., Dartmouth College, 1989


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. Field and laboratory 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. 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. 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 CSL.

Current Topics

Heterogeneous nitrogen oxide chemistry. Gas-particle reactions of nitrogen oxides are key to the regulation of atmospheric oxidant burdens, but they remain rather poorly understood. Recent NOAA led field work led to the discovery of nitryl chloride (ClNO2), a major source of chlorine to the atmosphere, produced by heterogeneous nitrogen oxide chemistry. We have developed key instrumentation and led major field intensives in New England, Texas and the Gulf Coast, New York City, California, Colorado and Hong Kong. Work in this area has been collaborative with the Thornton Group at the University of Washington, including the recent WINTER field campaign on the NSF C-130 aircraft on the U.S. East Coast. The recent Utah Winter Fine Particulate Matter Study (UWFPS) took place in Salt Lake City and surrounding mountain valleys to investigate coupling between wintertime atmospheric chemistry and mountain basin meteorology leading to high levels of ammonium nitrate air pollution in that region. A collaborative project is currently underway to construct a lightweight, autonomous instrument for the CARIBIC program with Professor Andy Ruth at University College Cork, Ireland.

Nocturnal biogenic VOC oxidation. Biogenic VOC from terrestrial vegetation (e.g., isoprene, monoterpenes) and biogenic marine sulfur compounds (e.g., dimethyl sulfide, DMS) undergo rapid nocturnal degradation in the presence of the nitrate radical, NO3. Since NO3 is derived from NOx, an anthropogenic pollutant, these oxidative processes represent an anthropogenic perturbation of a biogenic atmospheric input. These perturbations can have important consequences, such as the formation of organic and sulfate aerosol that affect Earth's climate. These processes have been studied through field studies from ships (New England 2002), aircraft (New England 2004, Texas 2006), at the SAPHIR chamber in Jülich, and at the 2011 BEACHON-RoMBAS campaign at Manitou Forest in Colorado. Aircraft and ground based measurements have taken place in the U.S. during the SENEX and SOAS campaigns.  Collaborators include the Jimenez Group at the University of Colorado and the Fry Group at Reed College and the Troposphere group at Forschungszentrum Jülich, Germany.

Oil and gas emissions and ozone photochemistry. The environmental impacts of the recent increase in production of oil and natural gas in North America are an important current issue. Shale gas basins in both Utah and Wyoming have recently experienced ozone air quality episodes, but only during the winter season. In more populated regions, oil and gas emissions may be a factor influencing urban air pollution. Ground based field studies in Utah and Colorado's front range, along with the recent SONGNEX aircraft project, have investigated atmospheric chemistry and emissions associated with oil and gas activities.

Winter air quality. Several regions of the United States, particularly in the west, experience poor air quality during winter meteorological conditions that confine local emissions in shallow layers near the surface. However, different regions have widely different responses to local emissions in the winter. Sparsely populated oil and gas producing regions are subject to ozone pollution, while urban regions experience very low ozone but high levels of particulate matter comprised primarily of ammonium nitrate. These disparate issues are related both chemically and meteorologically. Recent field studies investigating processes relevant to winter air quality include WINTER, the Uintah Basin Winter Ozone Studies, and the Utah Winter Fine Particulate Study.

Atmospheric Chemistry of Wildfires. Wildfire frequency and intensity is increasing in western North America, with impacts on regional air quality and climate. The FIREX initiative is a set of laboratory and field studies designed to provide detailed understanding of the complex emissions and subsequent chemical transformations that define these impacts. Recent measurements at the Missoula Fire Sciences Laboratory using the Airborne Cavity Enhanced Spectrometer have provided emission factors for glyoxal (CHOCHO), nitrous acid (HONO) and nitrogen dioxide (NO2), species that define photochemical impacts of fire emissions. The recent FIREX-AQ field study was a comprehensive measurement of emissions and chemical transformations associated with wildfires and agricultural fires.

Ultraviolet Broadband Aerosol Extinction. Aerosols both scatter and absorb incoming solar radiation, influencing Earth's climate. Recent developments of visible cavity ring-down and photoacoustic instruments have enabled new and detailed understanding of ambient aerosol optical properties. We have undertaken the development of broadband aerosol extinction (= absorption + scattering) at short wavelengths using cavity enhanced spectrometers with LED and other light sources. This instrument concept provides spectrally resolved measurements over a wavelength range where some aerosol species, such as "brown carbon", may have wavelength dependent absorption. The instrument has recently participated in measurements at the U.S. Forest Service Fire Lab (FLAME IV), at the SOAS campaign in the southeast U.S., and through a collaborative project with Dr. Yinon Rudich at the Weizmann Institute of Science in Israel. These measurements will be an important part of FIREX.

National Oceanic and Atmospheric Administration

last modified: December 15, 2022