(BRAVO) Study: Final Report. - Desert Research Institute

Final Report — September 2004
which was a critical orifice. The rotation rate was 6 mm/day, which over four weeks
produced eight Apiezon-greased Mylar strips each 168 mm long. The strips, representing the
finest particles, 0.26 > D p > 0.09 µm, were analyzed on the University of California at Davis
DELTA Group synchrotron x-ray fluorescence (S-XRF) microprobe (Bench et al., 2002).
The extremely high x-ray flux and 100% polarization allowed highly sensitive measurements
of all elements sodium and heavier, to a few pg/m 3 , although elements heavier than
molybdenum and lighter than gold are often interfered with by more abundant light elements.
The seven larger-particle DRUM strips were archived and have not been analyzed.
The results for two of the most abundant very fine elements, silicon and sulfur, are
shown in Figure 5-9 for August 1999. Generally, the average concentrations of these
particles are very low, (Si = 0.14 ng/m 3 , S = 1.88 ng/m 3 ) with the very fine sulfur
(presumably sulfate) a small fraction of PM 2.5 sulfate. However, on occasion, sharp increases
in both very fine Si and S occur, always in correlation (r 2 = 0.95 for August, for example).
Comparison with high resolution PM 2.5 sulfate data show that on one occasion, August 18,
there were three hours in which almost all of the PM 2.5 sulfate was in the very fine size range.
The silicon is not accompanied by other crustal elements (enrichment factor vs. aluminum is
∼20) but is probably derived from the same processes as the “large glassy plerospheres” long
associated with coal-fired power plants. (The postulated formation mechanism is SiO 2 →
SiO in the reducing fuel rich region of combustion, then oxidation to SiO 2 in the flame,
followed by rapid condensation to very fine glassy mist that can form very fine particles,
some of which agglomerate into the large plerospheres.) The sulfur in the largest peaks is
probably H 2 SO 4 , based on hydrated droplets seen even after vacuum, which argues against
long-range transport.
Silicon x 25
Sulfur
Nanograms/m3
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
11 13 15 17 19 21 23 25
10 12 14 16 18 20 22 24
August, 1999
Figure 5-9. Timeline of very fine (0.26 µm > D p > 0.09 µm) silicon (x 25) and sulfur concentrations
at Big Bend National Park during August 1999.
HYSPLIT4 isentropic backward trajectory analyses at 500, 1,000, and 2,000 m AGL
for these events show that the largest of these events in early and late August occurred with
5-14