Oxidation mechanisms in the troposphere.
The problem of
photochemical air pollution remains a serious threat to human health and welfare
today. Ozone, the primary component of photochemical smog, forms at ground-level
by the interaction of nitrogen oxides and volatile organic compounds emitted
from transportation and industrial activities. Ozone is highly toxic to both
plants and animals, impairing respiratory functions by causing inflammation and
scarring of lung tissue. The difficulty associated with control of
photochemical smog on any scale lies in the fact that ozone is a secondary
pollutant. Control strategies thus require an understanding the chemistry
leading to its production. Our research involves both laboratory studies
and field
measurements designed to improve our understanding of the complex chemistry of
the atmosphere.
Field Measurements of Tropospheric OH and HO2.
The hydroxyl
radical (OH) plays a central role in the chemistry of the atmosphere. Reactions
with OH are the primary removal process for many trace gases important to local
and regional air quality and global climate change, such as carbon monoxide,
methane, and the alternative chlorofluorocarbons. Because of its high
reactivity, ambient concentrations of OH in the troposphere are extremely small
(less than 1 pptv) and its chemical lifetime is very short (less than 1 second).
As a result, ambient concentrations of OH are difficult to measure accurately.
We have constructed an instrument capable of detecting OH
and HO2 radicals in the
troposphere with high sensitivity using laser-induced fluorescence techniques.
This instrument is used for both ground-based field measurements of ambient
OH and laboratory experiments of OH radical chemistry. Recent field
deployments include the
MCMA-2006 (Mexico City
Metropolitan Area - 2006 ) component of
MILAGRO (Megacity
Initiative: Local and Global Research
Observations), as well as the
PROPHET (Program for
Research
on Oxidants: Photochemistry, Emission and Transport) tower in northern Michigan.
MCMA-2006
PROPHET
Laboratory measurements of biogenic VOC chemistry. Our current research involves
the oxidation chemistry
of biogenic volatile organic compounds (VOCs) that play an important role in the production
of tropospheric ozone such as isoprene and a- and
b-pinene. These reactions can be
isolated and studied in the laboratory using high-pressure gas flow techniques
coupled with resonance fluorescence, laser-induced fluorescence, and mass
spectrometric detection of reactants, intermediates and products. These
experiments are part of a collaborative research effort with
Professor
Ronald Hites' Analytical
Environmental Chemistry Laboratory.
Theoretical studies of OH radical reactions. There
is still considerable uncertainty associated with the detailed
chemical mechanism of the reactions of OH radicals with alkenes such
as isoprene (2-methyl-1,3-butadiene) and oxygenated compounds such
as hydroxyacetone, and the subtle interactions that control the
reactivity of these systems. Recent studies have suggested
that the mechanisms for these reactions may proceed through
pre-reactive stable intermediates.
We are using ab initio electronic
structure and state-of-the-art ab initio dynamics
calculations to determine the potential energy surface for these
reactions, including the structures and energetics of the transition
states leading to each product. These results are used in
conjunction with reaction rate theories and compared to experimental
measurements to help elucidate the oxidation mechanism for these
compounds in the atmosphere, and to give fundamental information on
the factors controlling the kinetics of radical-molecule reactivity.
This work is part of a collaborative research effort with
Professor
Srinivasan S. Iyengar's
research group, and
Professor
Caroline Jarrold's
research group.
These projects are supported by grants from the National Science Foundation. Any
opinions, findings and conclusions or recommendations expressed in this material
are those of the author(s) and do not necessarily reflect the views of the
National Science Foundation (NSF).
