PNNL-13501 - Pacific Northwest National Laboratory

expected to result in significant advances in our ability to
develop predictive tropospheric air quality models.
Approach
The particle collector was largely developed under
another LDRD project (Cowin et al. LDRD program in
Ultrasensitive Studies of Reactive Aerosols, 1999-2000),
and is only briefly described here. Figure 2 shows it in its
present form. Air at 1 liter per minute is pumped through
a nozzle, which deposits on substrates most of the
particles larger than 0.25 micron, and a useful fraction of
those down to 0.1 micron. The substrate tray shown holds
177 samples. The tray is scanned by a laptop computercontrolled
drive at preplanned intervals. When deployed,
we typically run a commercial laser-particle counter in
parallel with it, to help recognize priority times to
analyze.
Figure 2. Time-tagged particle collector, shown with
chamber open. In the chamber is a sample tray with 177
samples, in the door at left is a particle deposition nozzle.
Results and Accomplishments
Automated Analysis
Many thousands of particles need be examined. For this
we installed commercial software (DiskInspector for
Oxford Instruments) to run a scanning electron
microscope. This permits fully automated analysis of
about 10,000 particles every 24 hours. Typically this kind
of analysis has been considered feasible only for the
largest particles (> 1 micron). Through various
innovations, we are able to achieve excellent results down
to 0.2 micron, and useful results to 0.1 micron. A typical
x-ray analysis of a small particle from our automated
analysis is shown in Figure 3.
Figure 3. Typical x-ray of small atmospheric particle,
showing excellent elemental analytical results and small
background
Validation of Analysis
We demonstrated the validity of the analysis by
examining a dozen or so different kinds of laboratory
generated aerosols of known composition. Figure 4
shows results for a particular sulfur containing organic
aerosol. Generally the results are excellent down to
0.2 micron and useful to 0.1 micron (Laskin 2000).
Mobile Platform
To demonstrate the mobility of our collector, we made
two, 3-hour field trips with the collector mounted in a
standard car, powered from the cigarette lighter outlet,
taking samples every 2 minutes. We drove on
exceedingly rough dirt roads, on highways, and up
mountains, and found the device adequate to the task.
The collector also can be flown in an airplane.
Houston Field Study
A large atmospheric pollution study was conducted from
August 15 to September 15, in Houston, Texas. This
Texas 2000 Air Quality Study was an excellent
opportunity to test the device and allow for comparisons
against other particle methods. We collected over 3000
samples at the William’s Tower site.
Analytical and Physical Chemistry 21