Introduction
The Earth's atmosphere can be viewed as an enormous reaction
vessel where thousands of different physicochemical processes
take place in a highly inhomogeneous and dynamic environment.
Some of these processes are beautifully simple, such as splitting
of oxygen molecules into individual atoms by ultraviolet photons.
Others are rather involved chains of reactions, such as formation
of photochemical smog in oxidation of volatile organics emitted
by traffic and industrial sources. As life on our planet hinges
on the stability of the delicate atmospheric environment, our
group, together with other atmospheric scientists around the
world, strives to understand both the fundamental mechanisms
of atmospheric reactions and the adverse impacts of anthropogenic
activities on the atmosphere.
We are especially interested in the mechanisms of photochemical
interactions between the solar radiation and atmospheric aerosol
particles. Can aerosol particles serve as efficient catalysts
of photochemical processes? What sort of chemistry happens inside
these particles as they drift through the atmosphere exposed
to solar radiation? Can photochemical reactions on particle
surfaces make the particles more toxic? How do these reactions
affect cloud condensation properties of aerosol particles? In
our laboratory, we try to find answers to these and to many
other intriguing problems using modern analytical techniques
based on laser spectroscopy, chromatography, and mass-spectrometry.
Secondary Organic Aerosol Photochemistry
Secondary Organic Aerosol (SOA) particles are produced in the
atmosphere as a result of oxidation of volatile organic compounds
by O3, OH and NO3. One especially interesting
group of organic compounds that has been shown to efficiently
form SOA particles is terpenes, a class of
hydrocarbons emitted by coniferous plants. Our group is currently
investigating the photochemical properties of SOA formed from
the reaction between atmospheric oxidants and selected monoterpenes.
Specifically, we seek to answer the following questions:
- Are SOA particles formed from oxidized terpenes photochemically
active in the actinic region of the solar spectrum? If so,
what are the relevant photochemical reaction mechanisms?
- Does aerosol aging significantly accelerate as a result
of photochemical processes occurring in the SOA particles?
What is the effect of aging on SOA particle properties?
- Do such photochemical processes result in a feedback mechanism
on gas-phase chemistry, e.g. by acting as a source of small
volatile organics?
To date we have answered many of the questions posed above
for the monoterpene + ozone SOA system. We have
found that particles formed from the ozone-initiated oxidation
of limonene do indeed absorb light at atmospherically relevant
wavelengths in the actinic region (wavelength > 295 nm).
This photoactivity leads to the production of measurable amounts
of formaldehyde and formic acid. We also found that humidity
has little to no effect on the photochemistry of the resulting
particles. Currently we are investigating the photochemistry
of SOA formed from NO3 oxidation.
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Photochemistry
of Aged Primary Organic Aerosol
As opposed to SOA, primary organic aerosol (POA) particles
are injected in the atmopshere directly by their sources. They
include sea-salt aerosol particles generated by wave-breaking,
soot particles produced by internal combustion engines, smoke
particles produced by biomass burning, dust particles produced
by re-suspension, and smelly particles emitted in the atmosphere
by the cooking industry. Such particles are often decorated
by an outer layer of fairly hydrophobic organic material such
as phospholipids, fatty acids, and heavy aromatics. This layer
is slowly oxidized by OH, ozone, and nitrogen oxides in a process
known as "chemical aging". The goal of our research
is to understand the role of direct photochemical processes
in processing fresh and aged POA particles.
 We
have studied photodegradation of partially oxidized organic
films and self-assembled
monolayers (SAM) as a model of photochemical processes occurring
in POA particles. We discovered that reactions taking place
during the oxidation transform alkene-terminated SAM into a
photochemically active state capable of photolysis in the lower
atmosphere. Our mechanistic study of ozonolysis and subsequent
photolysis of thin films of undecylenic acid represented demonstrated
that: (1) oxidation of unsaturated organic molecules in aerosol
particles makes them absorb radiation in the tropospheric actinic
window, with organic peroxides and carbonyls being the most
important absorbers; (2) photochemistry occurring in the oxidized
aerosol particles is expected to contribute significantly to
the atmospheric processing of organic aerosols; (3) prolonged
solar photolysis is likely to significantly affect the chemical
composition and properties of organic aerosol particles. The
key result of this work is discovering the significance of previously
overlooked photochemical processes occurring in organic aerosol
particles. This observation is of great interest to atmospheric
chemists, climate scientists, and air pollution researchers
because such processes change physicochemical properties of
atmospheric aerosol particles, potentially making them more
toxic and increasing their cloud condensation efficiencies.
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Aerosol
Photochemistry Techniques
SOA Samples. We generate artificial
SOA particles in the lab by dark oxidation of monoterpenes in
a Teflon reaction chamber. Monoterpenes are fairly volatile,
but some of their oxidation products are not, and they readily
condense into SOA particles. Normally, either ozone (O3)
or nitrogen trioxide (NO3) is used as the oxidant.
We then collect the aerosol particles on filters and use two
different home-built instruments to study their photochemistry.
A host of analytical tools is also used to gain valuable information
on the chemical composition and optical properties of the aerosol
particles.
POA Samples. In the POA case, we do
not work with actual aerosol particles. Instead, we are using
representative organic films, self-assembled monolayers, and
suitable organic liquids as surrogates for the POA organic material.
The sample is appropriately processed to simulate chemical aging
of organic aerosol particles by atmospheric oxidants. For example,
the figure on the left shows how plasma-generated free radicals
are mixed with molecular oxygen and directed into a flow cell
containing a thin film of an unsaturated fatty acid deposited
on a quartz tube. The photochemistry of the resulting aged (oxidized)
sample is then studied in one of our aerosol photochemistry
instruments. The effect of radiation on the chemical composition
of the film is probed with standard analytical techniques including
chemical ionization mass-spectrometry, gas chromatography, and
FTIR spectroscopy.
CRDS. The figure below shows the
IR-CRDS (infrared cavity ringdown spectroscopy) apparatus designed
in our laboratory to study the photochemistry of oxidized organic
films and SOA samples. It is a photodissociation action spectrometer
with a wavelength-tunable UV lamp for an excitation source,
and IR-CRDS cell for a detector. This apparatus was optimized
to sensitively detect small molecular weight products of photolysis
via their rovibrational infrared transitions.
CIMS. To monitor larger volatile
and semi-volatile SOA photolysis products with mass-to-charge
ratios (m/z) up to 500 u, we have built a Chemical
Ionization Mass Spectrometer
(CIMS). In our apparatus, H3O+ ions are
produced by a betta-source (63Ni) and serve as ionizing
agents. Because of the lower proton affinity (PA) of H3O+,
protons are transferred from H3O+ to the
organic molecules. Volatile and semi-volatile photolysis products
are incorporated into a flow of nitrogen gas and transported
to the ionization region where they collide with H3O+.
The protonated photolysis products are detected with a quadrupole
mass spectrometer (QMS). A major advantage of CIMS over other
ionization processes such as electron impact is that there is
little fragmentation of organic molecules during the protonation
process. These experiments complement the experiments done with
the IR-CRDS apparatus by detecting larger mass products and
monitoring multiple products simultaneously.
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Hygroscopicity
of Nanoparticles Containing Surfactants
Ultrafine particles with diameters less than 100 nm have rather
unusual physical and chemical properties because of their large
surface-to-volume ratio. Our research focuses on the hygroscopic
properties of atmospherically relevant multi-component ultrafine
particles. Both chemical content and particle diameter are critical
factors that govern aerosol particle interaction with water;
however, there is a lack of measurements of the hygroscopicity
of model multi-component aerosol systems. Aging processes such
as oxidation and photochemical reactions can lead to changes
in the morphology of the particle surface and aerosol chemical
content and may play a role in converting them to efficient
cloud condensation nuclei. We combine experimental studies and
modeling of hygroscopic properties of ultrafine aerosol composed
of mixtures of soluble salts and organic surfactants. In fact,
we are the only research group that currently possesses the
technology to generate and study ultrafine particles containing
a predetermined amount of surfactants. Hygroscopicity of such
particles is of great fundamental interest. Water uptake studies
measure the deliquescence relative humidity of the aerosol of
interest using a tandem differential mobility analyzer. A solution
containing the salt and organic surfactant of interest is electrosprayed
to generate dry mixed-content aerosol particles. Particles with
diameters of interest are selected with a differential mobility
analyzer. To determine the growth factor, particles are exposed
to humid air and the resulting change in diameter is measured
with a second differential mobility analyzer. Modeling of growth
factors yields insight into the role of both chemical content
and aerosol size in the water uptake properties of multi-component
ultrafine aerosol. The figure on the right displays sample data
showing how organic surfactants affect the deliquescence point
and growth factors of soluble salts.
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Microwave
Plasma Torch Aerosol Particle Counter
 We
have developed a microwave plasma torch instrument for
composition-sensitive detection of aerosol particles containing
metals. The instrument works by injecting size-selected
aerosol particles in the middle of an atmospheric-pressure
microwave plasma. The plasma excitation completely atomizes
the particles and heats them to about 4000 K making it
possible to detect element-specific emission from particle
constituents with an optical spectrometer. We have characterized
the response of the instrument with respect to the particle
size and composition for several kinds of aerosol particles.
Our results demonstrate that microwave plasma torch is
a viable tool for single particle counting and sizing
with chemical information for both field and laboratory
applications. |
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Indoor Air Chemistry
 In
spite of serious concerns
about ozone-generating air purifiers raised
by scientists, consumer unions, and government officials, the
general public continues to use these devices in homes, offices,
and schools. The main goal of our indoor air chemistry project
is to educate the public on the health dangers of indoor ozone
and particulate matter and on proper strategies for indoor air
purification. Our own measurements
in actual offices and residential areas convincingly demonstrated
that ionization and ozonolysis air purifiers can produce levels
of ozone in indoor environments well in excess of health-based
standards. For example, the figure below shows that an ionic
air purifier can build up ozone above the EPA
NAAQS standards in a small bathroom, and an ozonolysis air
purifier violates not only the EPA NAAQS but also the much higher
OSHA STEL level in a larger
bathroom. Similar results were obtained for office rooms, with
the ozonolysis air purifiers generating steady-state ozone levels
as high as 700 ppb.
Furthermore, a substantial amount or
ultrafine particles can be generated by such ozone-generating
air “purifiers” as a result of chemical reactions
between the emitted ozone and volatile organics in the room.
The particle number concentration increases occur as sudden
bursts reflecting the inherent complexity of the underlying
chemistry (an example is shown in the figure above). We have
investigated ultrafine "particle explosion" events
in indoor environments resulting from reactions of ozone emitted
by ozone-generating air purifiers with terpenoid organic molecules
from air-freshening devices.
Our publications in this area generated very strong interest
from the media worldwide (newspapers, radio, and television),
from the California Air Resources Board, and from a number of
private persons. It had a direct effect on the recent California
Assembly Bill 2276 (2006), which authorized the California Air
Resources Board to develop a regulation to limit the ozone emissions
from indoor air cleaning devices by December 31, 2008.
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