keutsch

Overview
Research in the Keutsch group is aimed at improving our understanding of photochemical oxidation processes of volatile organic compounds (VOCs) that produce tropospheric ozone (O₃) and are central to secondary organic aerosol (SOA) formation. O₃ and aerosol affect human health and climate, and uncertainties in the radiative effects of aerosol comprise the largest uncertainties in current estimates of anthropogenic forcing of climate. Our scientific approach builds on enabling new field observations of key VOC oxidation intermediates (OVOCs) via instrumentation and method development. The selected species (formaldehyde [HCHO], α-dicarbonyls) are directly relevant to O₃ and SOA formation, but can also act as powerful indicators for overall VOC oxidation processes. We combine these field observations, taken during collaborative field campaigns, with laboratory studies of kinetics that provide new detailed chemical information, in order to test and improve existing atmospheric chemistry models. The field observations and scientific analysis are then made available to the wider atmospheric sciences community.

VOC Photochemistry and Ozone and SOA Formation
Photochemical oxidation of biogenic (e.g., isoprene, terpenes) and anthropogenic (benzene, toluene) VOCs is tied to both tropospheric O₃ and SOA formation. Although much research has been conducted in this area the ability to model ozone (O₃) away from urban centers and SOA formation in general is limited. The measurement of higher generation OVOCs, such as α-dicarbonyls, together with formaldehyde, an important first generation OVOC, allows a more detailed means of studying a larger part of the oxidative fate of VOCs. Comparison of field measurements of VOC oxidation chemistry with model results provides a means for testing and improving models of O₃ and SOA formation, as these are a result of this oxidative chemistry. Improved models are necessary for regulators and policy makers to make informed decisions as they seek to reduce pollution levels, especially including O₃ and SOA.

Field Campaigns
Collaborative field campaigns, in which a number of groups make coordinated measurement of key species such as VOCs, OVOCs, NO_x and ozone, are the backbone of atmospheric chemistry as they provide the only means of testing the accuracy of models. The scientific goal of the Keutsch Group in such campaigns is to test and improve our understanding of VOC oxidation processes and SOA formation. We are particularly interested in (1) rapid VOC oxidation in forest canopies, for which formaldehyde flux measurements can provide valuable insight, (2) contribution of biogenic vs. anthropogenic VOC precursors to α-dicarbonyl concentrations, and (3) influence of other anthropogenic emissions (e.g. NO_x) on biogenic VOC oxidative chemistry and α-dicarbonyl concentrations. Our interests also extend to aircraft studies and studies in urban and maritime regions. In addition, we are interested in providing validation of satellite retrievals of glyoxal, the smallest α-dicarbonyl, and formaldehyde.

Laboratory Studies
The Keutsch Group conducts laboratory studies of processes that are central to the uptake and fate of α-dicarbonyls in aerosol. These studies address the uptake mechanism, rate, reversibility, dependence on photo-chemical processes, and products formed in the aerosol phase. Our studies are the first to observe formation of imidazoles from reaction of glyoxal with ammonium sulfate aerosol and first to show that formation of organosulfates during glyoxal uptake only occurs under irradiated conditions. Chamber studies of glyoxal uptake on aerosol seed are undertaken in collaboration with John Seinfeld's Group at Caltech; in our laboratory we study the kinetics of formation of α-dicarbonyl reaction products (including especially those bearing nitrogen) and synthesize organosulfate standards. Our goal is to implement the results of these studies into models of SOA formation.

Instrument Development
Instrument development in the Keutsch Group employs ultra high sensitivity spectroscopic techniques, such as Laser-Induced-Fluorescence (LIF) and cavity-ringdown spectroscopy, combined with cutting edge technology to enable new and improved measurements of key atmospheric species.

LIF is an established atmospheric field measurement technique for NO₂ and OH.  The Keutsch Group has successfully applied LIF to measure formaldehyde (HCHO)  with sensitivity as good as 15 ppt/min. The instrument also takes advantage of a novel pulsed fiber laser, which is lighter and more compact and requires only a fraction of the power drawn by a Ti:Sapphire-all highly desirable in any field instrument, especially for aircraft based measurements. This instrument is also the first to have the capability of HCHO flux measurements via eddy correlation.

We have also developed the first instrument for direct, highly specific, in situ measurement of glyoxal. The instrument is the first atmospheric field instrument to use laser induced phosphorescence (LIP), taking advantage of the unusually efficient intersystem crossing in α-dicarbonyls. The long timescale of phosphorescence compared to scattering and fluorescence from possible interferants makes it virtually background free, and the precision of 1 ppt/min allows fast measurement of glyoxal even in clean environments. We are extending the LIP technique to other α-dicarbonyls and have successfully observed methylglyoxal. We are also developing measurement capabilities for aerosol phase α-dicarbonyls as well as the uptake rate of glyoxal into aerosols under ambient conditions.