PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN99073/1401
Ultrasensitive Studies of Reactive Aerosols
James P. Cowin, Stephen A. Joyce
Atmospheric particles are potentially harmful to human health, and to play a major role in the regional pollution
chemistry of gases like ozone. In this project, we developed a field device to better collect, characterize, and maintain
these particles and track their sources. Three laboratory methods for analyzing the collected particles with single aerosol
particle sensitivity were also refined and improved.
Project Description
Reactive aerosols, particles ranging from 0.01 micron to
5 microns in diameter, are important in the atmosphere, in
nanoparticle composite materials, in medicine, and in
chemical processes. Characterizing their composition and
reactivity is very difficult for several reasons: 1) their
heterogeneity requires studying the individual reactivity
of many classes of particles; 2) the samples can be too
sparse for most analytical methods to deal with (a few
thousand 0.25 micron particles, for example, for timeresolved,
field-collected atmospheric samples); 3) usable
analytical methods need automation to examine thousands
of individual particles; and 4) reactive aerosols are not
sufficiently protected between collection and analysis. To
address these needs, we developed a new field collector
for particles, the “Time-Tagged Particle Sampler,” that
can take thousands of samples of particles in the field, and
preserve this archive for intensive, single-particle analysis
in the laboratory. We also further developed three
methods for conducting such laboratory analyses as
follows:
• Automated analysis of particles via EDAX (energy
dispersed analysis of x-rays) using a scanning
electron microscope was developed, so that we could
examine 10,000 particles per day in particles ranging
from 0.1 to 5 microns for elemental composition
(carbon and beyond in the periodic table).
• The “Enviroscan” environmental microscope at
EMSL was used to study hydration properties of
laboratory-generated NaCl/NaBr mixed aerosols, in
which a surface film of dissolved NaBr composition
was formed over a wide range of humidities. This
microscope will also be used to study hydration
properties of field-collected aerosols.
• We demonstrated the utility of time-of-flight
secondary ion mass spectrometry to study the
molecular speciation of field aerosols, both at the
surface and as a depth profile.
Introduction
Existing methods to collect and analyze atmospheric
particles are not satisfactory (McMurry 2000; Chow
1998), because the information typically is obtained on
too coarse a time frame, and is usually an average
composition of all the particles present. This deficiency
has spurred many recent new approaches. Time-resolved
single particle analysis in the field using a mass
spectrometer with an aerosol inlet is one such new
development (Seuss 1999), but these “field instruments”
are relatively large, expensive, and difficult to operate.
We chose instead to attempt to refine field-sampling
approaches and optimize single particle analysis in the
laboratory. The new aerosol characterization technology
developed in this project allows field collection and
analysis of aerosol properties with unprecedented time
resolution (1 minute) and the ability (through individual
particle analysis) to determine the specific mix of
materials making up the particles. This new approach
will better determine the source of the particle, and allow
better prediction of their chemistry and health effects.
These are revolutionary capabilities that are expected to
result in significant advances in our ability to develop
predictive tropospheric air quality models.
Results and Accomplishments
Field Portable Instrument. The laboratory prototype unit
was repackaged and redesigned to make a field portable
unit as shown in Figure 1. It weighs about 50 kg, and is
about 20 x 20 x 16 inches, although it could be built much
Analytical and Physical Chemistry 25