United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assesment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/048 Sept. 1990
EPA Project Summary
Development of a Sampling
Procedure for Large Nitrogen-
and Sulfur-Bearing Aerosols
Stephen J. Randtke, Dennis p. Lane, and Terry E. Baxter
The objective of this research project
was to develop a procedure that would
permit accurate sampling and analysis
of large (>2.5 urn) aerosols, especially
those containing nitrate, ammonium, and
sulfate, without interfering with the
sampling and analysis of the gases and
fine particle species of interest In dry
deposition and acid aerosol sampling.
Although other sampling devices were
initially considered, the experimental
work focused exclusively on a single-
stage Impactor modified to utilize a re-
movable impaction surface mounted on
the end of an annular denuder.
The cut point of the modified Impac-
tor, used with a polycarbonate (PC) filter
coated with 5 |iL of silicone oil, was 2.5
\an, with less than 1% particle bounce
occurring for 8-^.m particles. Significant
particle bounce occurred when using
uncoated or grease-coated impaction
surfaces composed of PTFE or porous
glass. Saturating a porous glass impac-
tion disk with oil reduced particle bounce
to about 2%, but the impacted particles
could not be easily removed from the
porous glass impaction surface.
Non-isoaxial sampling resulted in
substantial undersampling. Isoaxial
sampling of ambient air requires either a
horizontally mounted sampling system
or a vertically mounted system with an
inlet bend. Horizontal configurations
performed poorly; greased and lightly
oiled surfaces exhibited a high degree of
bounce and oil flowed sideways off oil-
saturated disks. Vertically mounted
systems employing a bent-tube inlet
performed reasonably well, but inlet
losses were quite high for very large
particles and for blunt-tipped Inlets. Inlet
losses averaged only 2.7% for 7.9-(im
uranine particles sampled using a sharp-
edged 90° bent-tube Inlet.
Losses of solid particles within the
combined impactor/denuder system
were greatest for particles close in size
to the cut point. For particles having an
aerodynamic diameter of 2.0-2.8 |im,
losses in the denuder ranged from 4-8%
of the mass sampled, while losses on the
spacer between the impactor and de-
nuder ranged from 10-17% of the mass
sampled. Losses for particles of other
sizes (larger and smaller) were generally
negligible (i.e., <1%).
A number of oils and greases were
found to be compatible with the analysis
of impacted particles by ion chromatog-
raphy. PC filters were found to be free of
nitrate, nitrite, and sulfate. PC filters,
silicone oil, and Vaseline exposed to
high concentrations of HNO3 and SO2
were found to adsorb little, if any, nitrate
or sulfate. TFE impaction disks may
adsorb traces of HNO3, but it is not nec-
essary to include them in the analysis if
they are overlain by a PC filter.
Impacted particles of varying com-
position were exposed to high concen-
trations of potentially reactive gases
(HNO3, SO2, or NH) for four hours at a
nominal flowrate of 16.7 Lpm. NH4CI and
NH4NO3 impacted on an uncoated PC
filter did not volatilize when exposed to
purified air. NH Cl and NaCI particles
Impacted on a sllicone-greased PC filter
did not react with SO2. NaCI and NH4NO,
particles impacted on a PC filter coated
-------
with flvauL of silicons oil did not react to
a significant extent with HNO or SO.,
respectively. Acidic NaHSO3 particles
Impacted on a greased surface reacted
to a limited extent with NH, (i.e., they wore
1.7 to 3.5% neutralized).
In a 24-hour field test employing
four collocated sampling systems, the
concentrations of coarse-particle nitrate-
N and sulfate were determined to be
0.939 ± 0.032 u,g/m*(± 3.4%) and 0.222
± 0.007 u.g/m3 (± 3.3%), respectively.
However, substantial percentages of the
nitrate, sulfate, and ammonium (36%,
44%, and 50%, respectively) were col-
lected on the bent-tube Inlets rather than
on the Impactlon disks.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to an-
nounce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
introduction
So that reliable estimates of the loads
of nitrogen and sulfur associated with dry
deposition can be made, scientists at the
U.S. EPA and elsewhere have been work-
ing to develop quantitative methods to
sample and analyze the major nitrogen-
and sulfur-containing species present in
gaseous and aerosol form in ambient air.
These include HNO3, HNO,, NO, NO,, NH,.
n'rtrate, nitrite, ammonium, SO,, SO, H,SO.,
sulfate, and sulfite.
Progress in developing sampling
methods for dry deposition has been ham-
pered by the chemical complexity of the
system; several of the chemical species of
interest are quite reactive and there are
interactions between the solid, liquid, and
gaseous components. As a result, a con-
siderable number of artifacts have been
encountered, including: 1) adsorption of
gaseous species (e.g., HNO3 and SO2) on
thefittermediaorthe sampling device used
to collect aerosols; 2) adsorption of HNO3
and other gaseous species by particles
deposited on the filter media; 3) volatiliza-
tion of paniculate species such as NH.NO3
and NH4CI; 4) and formation and subse-
quent volatilization of HNO3 as a result of
metathetfcal or displacement reactions. A
number of investigators have been working
to develop sampling devices and procedures
able to circumvent these problems.
One system currently under develop-
ment and potentially able to circumvent
these problems consists of a filter pack
preceded by one or more annular denuders
and a Teflon-coated glass impactor (Figure
1). The filter pack is designed to capture
fine particles and any gases that may vola-
tilize from them, while the denuders capture
reactive gases and allow fine particles to
pass through unaltered into the filter pack.
. The impactor is designed to remove large
particles (i.e., those having an aerodynamic
diameter greater than 2 u.m), which might
otherwise be trapped in the denuder or
which, if they were alkaline, might neutralize
acidic aerosols captured by the filter pack.
Initial attempts to circumvent the prob-
lems associated with large particles have
focused on the use of either a Teflon-coated
glass impactororaTef Ion cycloneto remove
large particles ahead of the first denuder
tube. However, there are potential problems
with each of these devices, including: 1)
particle resuspensipn or bounce, especially
at high mass loadings; 2) volatilization of
constituents of interest from the surfaces of
large particles; and 3) reactions of gaseous
constituents with impacted particles, with
thesampling device itself, orwith any grease
oroilusedtotraptheparticles. Furthermore,
neither device is inherently well suited to
analysis of the large particle fraction, which
would be highly desirable in view of its
potential significance. Although particulate
sulfate is "primarily associated with fine
particles, large particles can be very sig-
nificant in terms of total particle mass, par-
ticulate nitrate, and particulate ammonium.
The objective of this investigation was
to develop a sampling procedure for large
aerosols that would: 1) permit accurate de-
termination of the mass and chemical com-
position of large particles present in ambi-
ent air over a 24-hour period; 2) employ a
device that could be used in conjunction
with an annular denuder and filter pack
Couplers
PTFE/FEP-Coated Bent-Tube Inlet
TFE Sleeve Coupler
PTFE/FEP-Coated Glass-lmpactor Nozzle (4.0 mm)
Silicone-Rubber-Filled TFE Spacer
TFE Impaction Disk
Annular Denuder with Pedestal
(Coated with Sodium Carbonate)
2nd Annular Denuder (Coated with Citric Acid)
3-Stage TFE Filter Pack
Mass Flow Controller
To Vacuum
Figure 1. Glass-impactor annular-denuder filter-pack sampling system.
2
-------
without interference in the determination of
the gaseous and fine-particle constituents;
3) avoid the formation of chemical artifacts;
and 4) be simple enough to deploy in a
nationwide monitoring network.
Although other sampling devices were
initially considered, the experimental work
focused exclusively on a single-stage im-
pactor modified to utilize a removable im-
paction surface mounted on the end of the
first annular denuder (Figure 1). This
modified design was conceived by re-
searchers at the University of Kansas and
designed and constructed (in cooperation
with University Research Glassware,
Carrboro, NC) as part of another project
previously funded by the U.S. EPA. Al-
though it had not yet been fully character-
ized, it was subsequently incorporated into
sampling systems being used by several
other teams of investigators.
Experiments were conducted to exam-
ine: the cut point of the modified impactor;
impactor inlet losses; particle bounce utiliz-
ing various impaction surfaces and impac-
tion-surface coatings; horizontal versus
vertical mounting; collection efficiency as a
function of mass loading; particle deposi-
tion patterns; the compatibility of various
oils and greases with chemical analysis of
impacted particles; the reactivity of various
oils, greases, and impaction surfaces with
gases (SO,,, HNO3, and NH3); the volatility
and reactivity of impacted particles on coated
and uncoated surfaces; and the precision of
the method underfield sampling conditions.
Procedure
A. Sampling System Components
The sampling system (Figure 1) con-
sisted of the modified impactor coupled to a
bent-tube inlet (ASTM method D3685-78)
and the annular-denuderfilter-pack assem-
bly previously described by Vossler et al.
(Atmos. Environ., 22,8,1729-1736,1988).
Several types of impaction disks were
used. The most successful impaction disk
(Figure 2) was machined from virgin TFE
and featured a small (77-u.L) "well" 14 mm
wide and 0.5 mm deep. This well was
designed to accept either a small pool of oil
(underlain by a 13-mm PC membrane filter)
or an oiled or greased 13-mm membrane or
glass-fiber -filter. l| was most frequently
used with a silicone-oiled PC (Nuclepore)
filter, which was found to migrate if placed
on a flat impaction surface. Disks fitted with
a coarse 10-mm fritted (porous) glass disk
were also used.
B. Oils and Greases
Four greases and three oils were se-
lected for study: petroleum jelly (Vaseline);
high-vacuum silicone grease (Dow Corn-
16 mm
i 14 mm
0.5 mm
p
5.75 mm-
Figure 2.' TFE impaction disk with well.
ing); Halocarbon grease, type 25-5S (Halo-
carbon Products Corp.); Fluorolube grease,
type GR-290 (Hooker Chemical Co.); sili-
.cone diffusion-pump oil (type 704, Dow
Corning Corp.); and two mineral oils
(Fisherbrand 19 and Duo Seal).
C. Analytical Methods
Anions were determined by ion chro-
matography; ammonium was determined
using an Alpkem continuous flow analyzer;
and uranine was determined using a flu-
orometer. Oiled and greased impaction
surfaces were extracted with pentane and
then reagent water; and the aqueous phase
was subsequently analyzed for the con-
stituents of interest.
D. Particle Generation and Impac-
tion
Particles were generated using a Ber-
glund-Liu Monodisperse Aerosol Genera-
tor equipped with a Krypton-85 charge neu-
tralizer. The size, shape, and uniformity of
the particles were verified by scanning elec-
tron microscopy (SEM). Particles spanning
asize range of < 1 u.m to 30 u.m and particles
composed of uranine, NHLCI, NH4NO3,
(NH4)2SO4, NaCI, and NaHSO4 were used
2.5 mm
t i
5 mm
1.5 mm
10 mm
over the course of the investigation. Par-
ticles generated in the laboratory were im-
pacted by inserting the impactor inlet (with
or without the bent-tube inlet) into either a
wind tunnel or a small-diameter plastic pipe
into which the particles had been introduced.
The wind tunnel was used for low mass
loadings and the pipe was used for high
mass loadings.
E. Preparation of Impaction Surfaces
After scrupulous cleaning, impaction
disks were prepared for use in one of the
following ways: 1) oiled PC filters were
prepared by carefully placing a clean 13-
mm, 0.4-u.m-pore filter in the well and then
placing the desired amount of oil (usually 5
u.L of silicone oil) on top of the filter; 2)
uncoated PC filters were held in place by
first adding a very small dab of silicone
grease to the top of the impaction disk; 3)
greased membrane filters were prepared
by placing ten 50-u.L portions of a 1% solu-
tion of grease (usually silicone) in pentane
on top of the filter, allowing the solvent to
evaporate after adding each 50-u.L portion;
4) greased impaction disks were prepared
as described in item 3, omitting the filter;
and 5) oiled fritted glass disks were pre-
-------
Bell Connectors
(To Mount Optional Denude~r)
Norprene
Tubing
Air
Purifier
47-mm
Filter Filter
Ambient
="-< Mr
Air «Red«
Purifier Coupler
1/4" FEP
1/8" TFE
TFE Sleeve
frnpactor Nozzle
TFE Spacer
Impaction Disk
Annular Denuder
Bell Connector
2-Way Valve (SS)
Bypass Line
(Norprene Tubing)
3-Way
Valve (SS)
TEE(SS)
1/4"
~FEP~
2 5-mm
Filter
Microvalve
Reservoir
A/2@ 5 PSI
Vacuum
Figure 3. Impaction-surface exposure system.
-------
pared by adding oil dropwiseto saturate the
disk or adding a fixed amount of oil in a
pentane solution as described in item 3
above.
F. Tests for Chemical Artifact
Formation
Oils, greases, impaction surfaces, and
impacted particles were exposed to various
gases (SO2, HNO?, NH3, or purified air) by
placing an appropriately prepared specimen
into an impaction-surface exposure system
(Figure 3) or an impacted-particle expo-
sure system (similar to the system shown in
Figure 3, except that a control sample
without particles was exposed simulta-
neously with the particles). A side-stream of
nitrogen, carrying the reactive gas of inter-
est, was introduced through a stainless-
steel Swagelock tee. A solution of HNO3
and H2SO4 was used to generate HNO3
vapor; a solution of NH4OH was used to
generate NH3 vapor; and a solution of H2SO3
and KH2PO4 was used to generate SO2
vapor.
G. Field Test
A 24-hr field test was conducted to
determine the precision of the method. Four
identical sampling systems were mounted
on the roof of afour-story building (Learned
Hall) on the campus of Kansas University.
The bent-tube inlets were turned in the
direction of the prevailing wind (north to
northwest) on the day of sampling. Mass
flow controllers having an accuracy of ±1 %
and a precision of ± 0.2%, were used to
maintain a flow of 12.5 Lpm (STP: 25°C and
1 atm); prior to use they were cross-cali-
brated and found to agree within 0.5%.
Results and Discussion
A. Physical Performance
Characteistics of the Modified
Impactor
The use of an oil-saturated impaction
surface dictates a vertical impactor orien-
tation (to prevent oil from dripping off),
whereas the use of greaseorathin film of oil
allows either a vertical or a horizontal ori-
entation to be used. In the latter case,
particle bounce may occur at high mass
loadings. Since ambient winds are gener-
ally horizontal or nearly so, non-isoaxial
sampling conditions will exist when the im-
pactor is used in the vertical position, and
particles may be elutriated. A 90° bend can
be used with a vertically positioned impac-
tor to sample isoaxially and thus eliminate
the elutriation effect, but the inlet bend
presents an additional surface where par-
ticle deposition and chemical artifact forma-
tion may occur. Thus, regardless of which
impaction surface coating is used, its influ-
ence on the orientation, performance, and
operation of the impactor must be consid-
ered.
To gain insight into certain aspects of
the impactor's physical performance char-
acteristics, experiments were conducted to
examine: 1) the cut-point of the modified
impactor; 2) particle collection efficiency
under non-isoaxial sampling conditions; 3)
inlet losses as a function of particle size;
and 4) particle bounce as afunction of mass
loading and impaction surface type. These
experiments were not intended to provide a
comprehensive solution to the age-old large-
particle sampling problem. Rather, they
were intended to characterize the physical
performance (and limitations) of the im-
pactor, to aid in the conduct and interpre-
tation of subsequent experiments address-
ing chemical artifact formation, to reveal
problems requiring further investigation, and
most of all to determine the size of particles
that could be successfully sampled.
The cut point of the modified impactor,
when used with a PC membranef iltercoated
with 5 |o.L of silicone oil, was experimentally
determined to be 2.5 u.m, with less than 1%
particle bounce for 8-u.m particles. Signifi-
cant particle bounce was found to occur
when using uncoated or grease-coated im-
paction surfaces or when using a porous
glass impaction surface (Figure 4). Satu-
rating a porous glass impaction disk with oil
reduced particle bounce to about 2%, but
the impacted particles could not be easily
removed from the porous glass impaction
surface. Sudden increases in flowrate
caused impacted particles to be dislodged
from uncoated or greased surfaces.
Non-isoaxial sampling resulted in sub-
stantial undersampling of large particles, so
isoaxial sampling (perhaps using a vane-
mounted system) appears to be a necessity.
Iso'axial sampling of ambient air requires
either a horizontally mounted sampling sys-
tem or a vertically mounted system with an
inlet bend. The horizontal configurations
tested performed poorly; greased and lightly
oiled surfaces exhibited a high degree of
particle bounce and oil tended to flow side-
ways off oil-saturated disks. Vertically
mounted systems employing a 90° bent-
100
90
80
70
T
T
T
=T
=T
Upright, Membrane Filter, 5 \iL Si Oil
Inverted, Porous Glass Dish, Mineral Oil (Sat'd)
Inverted, Porous Glass Disk, 50 pi. SI Oil
Inverted, Porous Glass Disk, 20 \iL Si Oil
Upright, Porous Glass Disk, 20 pi. SI Oil
WOO 2000
Mass of Uranine Sampled, micrograrns
3000
Figure 4. Impactor collection efficiency as a function of mass loading for solid 7.9-y.m
uranine particles.
-------
tuba \n\Q\ performed reasonably well, but
Inlet losses were quite high for very large
particles (e.g., 91 to 96%for26.5-u.muranine
particles) and for blunt-tipped inlets (e.g., 7-
14%for8-u,muranineparticles). Inletlosses
averaged only 2.7% for 7.9 urn uranine
particles sampled using asharp-edged bent-
tube Inlet in an upright position; the pattern
of the deposited particles indicated the
presence of a double vortexing flow condi-
tion, which did not appear to adversely
affect impactor performance.
Losses of solid particles within the
combined impactor denuder system (ex-
cluding the inlet) were greatest for particles
aboutthesizeofthecut point (Figures). For
particles having an aerodynamic diameter
of 2.0-2.8 urn, losses in the denuder ranged
from 4-8% of the mass sampled, while losses
on the spacer between the impactor and
denuder ranged from 10-17% of the mass
sampled. However, losses for particles of
othersizes (largerand smaller) weregener-
ally negligible (i.e., <1%).
B. Tests for Chemical Artifact
Formation
Selected oils and greases that might
be used to prevent particle bounce were
examined.and experiments were conducted
on the most promising candidates to de-
termine: 1) their purity; 2) their compatibility
with analysis of trace anions by ion chro-
matography; 3) their gravimetric stability; 4)
their ability to react with gaseous constitu-
ents to form chemical artifacts; and 5) their
100
80
70
60
50
30
20
+ Passing Impaction Disk
Loss on Spacer
A. Loss In Denuder
2345 678
Particle Aerodynamic Diameter, microns
to
ability to prevent reactions that might occur
between reactive gasses and impacted
particles.
A number of oils and greases were
found to be compatible with the analysis of
impacted particles using ion chromatogra-
phy, i.e., the anions of interest were not
leached or sorbed, impacted particles ap-
peared to be completely extracted, and no
interfering peaks were observed. PC filters
were found to be free of nitrate, nitrite, and
sulfate. PC filters, siliconeoil, and Vaseline
exposed to high concentrations of HNO3
and SO2 were found to adsorb little, if any,
nitrate or sulfate. TFE impaction disks may
adsorb HNO3; but there is no need to ana-
lyze them, since the overlying impaction
filters can readily be removed for analysis.
NH4CI and NH NO3 particles impacted
on an uncoated PC filter did not volatilize
when exposed to purified air. Impacted
particles of varying composition were im-
pacted on uncoated, greased, or oiled sur-
faces and subsequently exposed to high
concentrations of potentially reactive gases
(HNO3, SO2, or NH3) at room temperature
and low relative humidity for four hours at a
nominal flowrate of 16.7 Lpm. Table 1
summarizes the results.
It should be noted that there may have
been particles on the rims of the impaction
disks exposed prior to August 18; and par-
ticles may have been physically dislodged
from the disks by the force of the jet prior to
August 23, after which a "soft-start" proce-
dure was implemented. Considering only
those samples exposed after August 23, it
can be seen that: 1) NaCI particles impacted
on oiled orgreased filters reacted only slightly
with HNO3; 2) NH4CI and NaCI particles
impacted on uncoated and greased filters,
respectively, did not react with SO2> nor did
NH4NO3 particles impacted on an uncoated
filter; 3) NH Cl particles impacted on greased
filters reacted to only a very limited extent
with HNO3; and 4) acidic NaHSO4 particles
impacted on greased filters were partially
(1.7 to 3.5%) converted to NaNH4SO4 by
reaction with NH3.
C. Field Test
In a 24-hour field test employing four
collocated sampling systems, the concen-
trations of coarse-particle nitrate-N and sul-
fate were determined to be 0.939 ± 0.032
u.g/m3 (± 3.4%) and 0.222 ± 0.007 u.g/m3 (±
3.3%), respectively. However, substantial
percentages of the nitrate, sulfate, and
ammonium (36%, 44%, and 50%, respec-
tively) were collected on the bent-tube inlets
rather than on the impaction disks.
Figure 5. Particle tosses on the spacer and in the denuder as a function of particle size.
6
-------
Conclusions
1. The cut point of the modified im-
pactorwas experimentally deter-
mined to be 2.5 u.m, identical to
its theoretical outpoint.
2. Non-isoaxialsampling resulted in
a substantial reduction in sam-
pling efficiency, with less than
10% of 8-(4.m particles entering
an inlet orthogonal to the air
stream. Accurate sampling in
the field will require isoaxial sam-
pling.
When the samplers were
mounted horizontally, minus the
bent-tube inlets, significant frac-
4.
tions (2.0-9.0%) of 8.34-jim
uranine particles were collected
in the impactor inlets, presum-
ably as a result of sedimentation
or turbulent diffusion.
Extensive particle bounce oc-
curred when using uncoated
impaction surfaces. Hence, oil or
Table 1. Summary of Particle Reactivity Experiments*
Date
(1988^
8/01
8/02
8/03
8/04
8/05
8/09
8/09
8/12
8/15
8/16
8/18
8/19
8/22
8/23
8/31
9/07
9/14
9/21
9/28
ft /Ort
9/30
10/03
10/07
10/07
10/12
10/12
10/21
10/28
11/04
11/11
11/18
11/25
12/01
Particle
Iwaa
NH4CI
NH4CI
NH4CI
NH4CI
NH4CI
NH4CI
NH4CI
NaCI
NaCI
NaCI
After this date,
NaCI
NaCI
Particle Oil or
Mass. u.g Grease
62.2 None
87.0 None
129.4
106.8
74.0
83.4
32.7
26.8
148.4
70.9
impacted
239.7
389.8
Oil
Oil
Oil
Grease
Grease
None
Oil
Grease
Test Gas
Type ng/m3
HNO3 338
HNO3 318
HN03317
HN03 301
HNO3 343
HNO3 328
HN03114
HN03 341
HNO3 340
HN03 320
particles were transferred to a fresh TFE
Oil
Oil
After this date, the "soft start" procedure was
NaCI
NACI
NACI
NaCI
10.9
41.6
58.9
94.1
Oil
Oil
Oil
Oil
HNO3 304
HNO3 307
used.
HN03 30.1
HN03 60.0
HN03 71.0
HNO3 58.1
Mass Found
on Disk, ng
2.57 (N03-N)
3.13(NO3-N)
0.17(N03-N)
0.48 (NO3-N)
0.00 (NO3-N)
0.93 (NO3-N)
2.88 (NO3-N)
1.57(NO3-N)
0.72 (NO3-N)
2.83 (N03-N)
impaction disk prior to
3.08 (N03-N)
2.94 (N03-N)
0.03 (NO3-N)
0.10 (NO3-N)
0.11 (NO3-N)
0.26 (N03-N)
Displacement to Denuder
JPJ1 %. B.
Chloride 27.5 1.74
Chloride 24.6 1.79
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
exposure.
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
1.5
3.2
0.9
63.9
48.5
43.2
2.3
14.7
5.6
2.7
0.00
1.0
0.58
1.3
3.10
1.86
15.07
1.44
1.77
1.14
1.04
1.11
0.88
0.00
1.03
0.93
1.11
After this date, afresh citric-acid-coated denuder was used to scrub the inlet air when N-containing particles were exposed.
NH.NO,
4 J
NH4NO3
NH4NO3
NH4N03
NH4CI
NH4CI
NaCI
NaCI
NaCI
NH4CI
NH4CI
NaHSO4
NaHS04
199.9
20.1
208.6
298.8
78.4
46.8
159.7
207.6
190.6
72.6
163.0
461.9
495.0
Oil
None
None
None
None
None
Grease
Grease
Grease
Grease
Grease
Grease
Grease
SO2 371
SO2 337
SO2 486
Air
SO2 452
Air
SO2 375
HN03101
HN03118
HN03 139
HN03 125
NH3 39.8
NH3 114
0.11(SO4)
0.00 (SO4)
0.00 (SO4)
NA
0.00 (SO4)
NA
0.00 (S04)
0.33 (NO3-N)
0.73 (NO3-N)
1.48(N03-N)
1.06(N03-N)
1.90{NH4-N)
1.00(NH4-N)
Nitrate
Nitrate
Nitrate
Nitrate
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
Chloride
0.00
0.00
0.67
0.11
4.8
3.6
0.50
0.45
1.34
7.43
0.00
(3.5%toNaNH4S04)
(1.7%toNaNH4So4)
0.00
NA
NA
0.66
0.84
0.96
0.00
Particle mass = initial mass on particles on the impaction disk; oil =silicone oil; grease = high-vacuum silicone grease; R is the control-
corrected raiio of moles of ion displaced to moles of contaminant ion found on the impaction filter; NA = not applicable
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grease must be used to prevent
particle bounce.
5. When using a silicone-greased
impaction surface, significant
particle bounce occurred at a
uranine-particle mass loading of
only 38 u.g, about 70% of a mono-
layer. Hence, use of silicone
grease will limit mass loadingsto
very low levels unless a rotating
surf ace or other means of reduc-
ing the mass of particles per unit
area is developed.
6. Using a porous glass impaction
surface coated with 50 uL of
silicone oil, 27-29% of the par-
ticle mass bounced off the im-
paction surface when sampling
7.9-u.m uranine particles at a
mass loading of only 4-5 |ig.
Saturating the porous glass sur-
face with mineral oil reduced
particle bounce to approximately
2% of the particle mass sam-
pled. Under severe mass load-
ing conditions (126-2,920 u.g),
only about 1% bounce was ob-
served using a PC filter coated
with only 5 \iL of silicone oil.
7. Since excessive bounce oc-
curred with uncoated, greased,
or lightly oiled impaction sur-
faces, and since the oil tended to
flow sideways off oil-saturated
porous glass impactions disks
when the sampling system was
horizontally mounted, it was de-
termined that isoaxial sampling
could best be accomplished us-
ing a vertically mounted system
with a bent-tube inlet.
8. Inlet losses were observed to be
91 to 96% for 26.5-u.m uranine
particles sampled using sharp-
edged 90° bent-tube inlets, 48-
52% for 15.2-u.m ammonium-
suifate particles sampled using
bent-tube inlets constructed from
thin-walled glass tubing, 7-14%
for 8-u,m uranine particles
sampled using blunt 90° bent-
tube inlets, 3.7-9.5% for 7.9-u.m
uranine particles sampled using
sharp-edged 90° bent-tube inlets
in an inverted position, and 0.5-
5.3% (averaging 2.7%) for 7.9-
]im uranine particles sampled
using sharp-edged 90° bent-tube
inlets in an upright position.
9. For 7.9-u.m uranine particles, at
least 12-15% of the total particle
mass sampled using an oiled
porous glass impaction surface
became entrapped within the
pores and was not readily ex-
tractable for subsequent chemi-
cal analysis. Uranine particles
collected on oiled membrane fil-
ters were readily extracted.
10. Deposition of 7.9-u.m uranine
particles collected on a silicone-
oiled PC filter indicated the pres-
ence of a double-vortexing flow
condition, presumably resulting
from the use of the bent-tube in-
let. The particle collection char-
acteristics of the impactordonot
appear to be adversely affected
by this condition.
11. Particle losses in the combined
impactor-denuder system (ex-
clusive of the inlet losses) were
found to occur predominantly
with particles about the size of
the impactor's cut-point diam-
eter. Within the denuder, losses
of particles having an aerody-
namic diameter of 2.0-2.8 u.m
ranged from 4-8% of the total
mass sampled. Particles were
also lost on the TFE spacer be-
tween the impactor and the de-
nuder, representing 10-17% of
the mass sampled for 2.0-2.8 u.m
particles. Losses of particles
having other aerodynamic diam-
eters were negligible.
12. Analysisoffourgreasesand three
oils revealed that they were of
suitable purity and were compat-
ible with analysis of trace con-
centrations of anions by ion
chromatography after extraction
with pentane and water. They did
not generally leach or sorb sig-
nificant amounts of the anions of
interest. The mineral oils were
found to contain detectable
amounts of extractable sulfate,
but not enough to be significant
when a small quantity of oil is
used in a 24-hour test.
13. Silicone oil and Vaseline ap-
peared to be compatible with
gravimetric analysis of impacted
particle mass. However, gravi-
metric analysis would only be
useful for high mass loadings
(i.e., those for which weighing
would give reasonably accurate
results), a dust-free weighing
room might be needed, and addi-
tional testing is needed to de-
termine how long the samples
must be equilibrated prior to
weighing.
8
14. PC filters were found to be free
of nitrate, nitrite, and sulfate; but
mixed-fiber (cellulose acetate/
cellulose nitrate) filters contained
measurable amounts of leach-
able nitrate.
15. Silicone oil, PC filters, and
Vaseline exposed to HNO3 and
SO2 were found to sorb little if
any nitrate or sulfate. The traces
of nitrate and sulfate which were
often detected in exposed speci-
mens may very well have been
due to handling rather than to
chemical artifact formation; and
the amounts detected would
generally not be significant for
samples collected over a 24-
hour period.
16. TFE impaction disks may adsorb
traces of HNO ; but it is not nec-
essary to analyze the TFE disk
when it is overlain by a PC filter.
17. NH4CI and NH4NO particles im-
pacted on uncoated PC filters did
not volatilize from the collection
surface when exposed to purified
air.
18. NH4CI, NaCI, and NH4NO3 par-
ticles impacted on uncoated or
greased PC filters did not react
with SO2.
19. NaCI and NH NO3 particles im-
pacted on a silicone-oiled PC fil-
ter did not react to a significant
extent with HNO3 or SO2, re-
spectively.
20. Acidic NaHSO4 particles im-
pacted on a silicone-greased PC
filter reacted to a limited extent
(1.7 to 3.5%) with NH3.
21. In a 24-hour field test employing
four collocated sampling sys-
tems incorporating the modified
impactors, the concentrations of
coarse-particle nitrate-N and sul-
fate were determined to be 0.939
±0.032 ug/m3 (±3.4%) and 0.222
± 0.007 ng/m3 (± 3.3%), respec-
tively. However, substantial per-
centages of the nitrate, sul-
fate, and ammonium (36%, 44%,
and 50%, respectively) were col-
lected on the bent-tube inlets
rather than the impaction disks.
Recommendations
1. Impaction surfaces used to
sample particles (or to prevent
their entry into downstream de-
nuders and filter packs) should
be coated with a suitable oil or
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grease to prevent bounce. At
high mass loadings, excess oil
should be used.
2. If an oiled porous glass disk is
used to collect particles for sub-
sequent chemical analysis, the
analyst should make sure that
the extraction procedure used to
remove the particles from the
disk is adequate.
3. The nature of the nitrate and
sulfate collected on the impactor
inlet during collection of field
samples should be investigated,
and experiments should be con-
ducted to ascertain whether
jarge particles deposited in the
inlet will react to a significant
extent with HNO3, SO.,, NH3, or
other gaseous pollutants.
4. A small vane system able to turn
a bent-tube inlet mounted on a
rotary union and orient it into the
wind should be developed and
field tested.
5. The efficiency with which both
anions and cations associated
with various types of particles can
be extracted from oils and
greases into water should be
thoroughly examined and docu-
mented.
6. The procedure proposed herein
for gravimetric determination of
large particles should be perfected
and field tested.
7. The reactivity of gases (not only
HNO SO and NH,, but also
O3, NO, NO,,, PAN, and espe-
cially SO3) with oils and greases,
with various types of Teflon and
Teflon coatings, with various
types of particles (especially
acidic and basic particles) col-
lected on oiled and greased sur-
faces, and with denuders and
their coatings should be explored
in much more detail. The effects
of temperature and relative hu-
midity on such reactions also
need to be thoroughly examined.
8. Additional field tests should be
conducted to determine the pre-
cision of the procedure under a
variety of ambient conditions and
to explore ways to improve the
precision and to simplify the pro-
cedure for use by individuals with
minimal experience.
9. Research should continue toward
the development of a sampling
system capable of quantitatively
sampling very large (i.e., 15-20
u.m) particles.
10. The sampling system herein de-
scribed should be thoroughly ex-
amined vis-a-vis its potential use
in sampling acid aerosols. Par-
ticular attention should be given
to: interactions between gases
and impacted particles; proce-
dures for quantifying the acidity
or basicity of impacted particles;
and loss or alteration of submi-
cron particles as they pass
through the denuders.
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SJ. Randtke, D.D. Lane, and T. E. Baxter are with the University of Kansas,
Lawrence, KS 66045.
Russell Wiener Is the EPA Project Officer (see below).
The complete-report, entitled "Development of a Sampling Procedure for Large
Nitrogen- and Sulfur-Bearing Aerosols," (Order No. PB90-235 789/AS; Cost:
$23.00 cost subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Off her can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/048
.
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