PNNL-13501 - Pacific Northwest National Laboratory

Oxidation of Volatile Organics Leading to Tropospheric Aerosol Formation
Study Control Number: PN00072/1479
Michel Dupuis, Shawn Kathmann, Robert Disselkamp
Because atmospheric aerosols have been shown to have important effects on human health, climate, and visibility they are
currently the focus of intense research activity. Among the atmospheric aerosols, organic aerosols are the least well
understood. An important mechanism leading to the formation of organic aerosols is the oxidation of volatile organic
compounds. The reaction pathways are not well understood and the oxidation products can serve as condensation nuclei
leading to organic droplets.
Project Description
Much of the uncertainty in volatile organic
compounds/NOx atmospheric chemistry lies in knowledge
about the mechanisms of degradation of the organic
species. The oxidation degradation products act as
condensation seeds to organic aerosol droplets. Improved
atmospheric models are in need of more detailed
elucidation of gas-phase photo-oxidation pathways and
rates of organics degradation. This project is targeted at
providing mechanistic and kinetic information about a
key class of volatile organic compounds, the alkenes, and
to illustrate the possible impact of computational
chemical studies on atmospheric models. Data to be
calculated include enthalpies of formation, vibrational
spectra, electronic excitation spectra, and rate constants
from the electronic structure calculations of transition
state structures together with transition state and
unimolecular rate theories. An important aspect of this
project is identifying the atmospheric modeling group that
will make use of molecular kinetics data.
Introduction
Volatile organic compounds play central roles in
tropospheric oxidation chemistry that lead to ozone and
secondary aerosol formation. Ozone is a critical pollutant
for which the U.S. Environmental Protection Agency
(EPA) has set regulatory thresholds for violating national
tropospheric air quality standards. Urban ozone is of
concern primarily because of its impact on health
(breathing and eye irritation). Rural ozone is of concern
because of damage to crops and forests. Ozone levels are
controlled by NOx and by volatile organic compounds in
the lower troposphere. The volatile organic compounds
can be from either natural emissions from such sources as
vegetation and phytoplankton or from anthropogenic
sources such as automobiles and oil-fueled power
production plants. It is of critical importance to DOE in
developing national energy use policies to understand the
role of volatile organic compounds in determining ozone
levels and how volatile organic compounds emission or
NOx emission control strategies should be designed.
The initial means of classifying hydrocarbons and their
impact on air quality focused on the initial OH rate
constants just as had been done for the CFC alternatives.
However, just as found for the CFC alternatives such as
HFC-134a, it has become critical to know what products
are formed and what is the ultimate fate of all of the
various products resulting from the oxidation. Indeed a
number of reviews have summarized the uncertainties that
limit our ability to understand the impact of volatile
organic compounds on air quality. As noted by Paulson
in JR Baker, Ed. (1994, Chap 4, 111-144), “the ability of
a compound to contribute to oxidant formation can only
be derived from the detailed oxidation mechanism for
each hydrocarbon.” Thus, one must examine the
complete oxidation mechanism including the primary step
and subsequent radical and oxidation reactions of the
products in order to develop more accurate measures of
how a compound contributes to oxidant formation. The
farther one moves from the emission source, the more
important the mixture of partially oxidized hydrocarbons
becomes as many of these compounds can be transported
for long distances. Thus, the rates of the initial reactions
of the hydrocarbons with OH, O3, and NO3 and the type
of partially oxidized products as well as the numbers of
free radicals that the reactions produce along the way, all
combine to determine how much an individual
hydrocarbon contributes to oxidant formation.
Furthermore, Paulson notes that major uncertainties still
exist in the 1990s even after three decades of research.
For the homogeneous gas phase processes, these
uncertainties fall into the area of degradation mechanisms
as opposed to kinetic measurements, although there are
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