Introduction
The National Park Service (NPS) and other Federal Land Managers are required by
the Clean Air Act to protect visibility at Class I areas, which include most
national parks and wilderness areas. This is being accomplished through the
Interagency Monitoring of Protected Visual Environments (IMPROVE) program, which
has representatives from the NPS, the Forest Service (USFS), the Bureau of Land
Management, the Fish and Wildlife Service (FWS), the Environmental Protection
Agency, and regional-state organizations. The IMPROVE program includes the
characterization of the haze by photography, the measurement of optical
extinction with transmissometers and nephelometers, and the measurement of the
composition and concentration of the fine particles that produce the extinction
and the tracers that identify emission sources.
Figure 1 shows the locations all particulate monitoring sites using IMPROVE
samplers through August 1995. Funding agencies include the IMPROVE committee,
the NPS, the USFS, the FSW, the Tahoe Regional Planning Agency, the Department
of Energy, the Northeast States Cooperative Air Use Management, the state of
Vermont, and the Regional District of Fraser Cheam (British Columbia). All of
all sites are operated by the University of California, Davis. Table 1 gives the
start and end months for each site.
Figure
1. Particulate sampling sites using IMPROVE samplers through August 1995.
Site Name
START
END
Site Name
START
END
Abbotsford, BC
4/94
6/95
Lone Peak WA
11/93
Acadia NP
3/88
Lye Brook WA
3/91
Arches NP
3/88
5/92
Mammoth Cave NP
3/91
Badlands NP
3/88
Meadview NRA
9/91
9/92
Bandelier NM
3/88
Mesa Verde NP
3/88
Big Bend NP
3/88
Mohawk Mountain, CT
9/88
11/93
Bliss State Park, CA
11/90
Moosehorn NWR
12/94
Boundary Waters Canoe Area
3/91
Mount Rainier NP
3/88
Bridger Wilderness
4/94
Mount Zirkel WA
11/93
Bridgton, Maine
9/88
11/93
Okefenokee NWR
3/91
Brigantine NWR
3/91
Petrified Forest NP
3/88
Brooklyn Lake, WY
4/94
Pinnacles NM
3/88
Bryce Canyon NP
3/88
Point Reyes NS
3/88
Canyonlands NP
3/88
Proctor Maple Research Farm, VT
9/88
Cape Romain NWR
8/94
Quabbin Reservoir, MA
12/88
11/93
Chassahowitzka NWR
3/93
Redwood NP
3/88
Chilliwack, BC
4/94
6/95
Ringwood State Park, NJ
9/88
11/93
Chiricahua NM
3/88
Rocky Mountain NP
3/88
Columbia River Gorge NSA
6/93
Saguaro NM
6/88
Crater Lake NP
3/88
Salmon NF
11/93
Craters of the Moon NM
5/92
San Gorgonio WA
3/88
Death Valley NM
10/93
Sawtooth NF
1/94
Denali NP
3/88
Scoville, Idaho
5/92
Dolly Sods/Otter Creek WA
3/91
Sequoia NP
9/92
Dome Lands WA
8/94
Shenandoah NP
3/88
Everglades NP
9/88
Shining Rock WA
8/94
Gila WA
4/94
Sipsy WA
2/92
Glacier NP
3/88
Snoqualamie NF
7/93
Grand Canyon NP
South Lake Tahoe, CA
3/89
Hopi Point
3/88
Sula (Selway Bitteroot WA)
8/94
Indian Gardens
10/89
Sunapee Mountain, NH
12/88
11/93
Great Basin National Park
5/92
Sycamore Canyon WA
9/91
9/92
Great Gulf WA
6/95
Three Sisters WA
7/93
Great Sand Dunes NM
5/88
Tonto NM
3/88
Great Smoky Mountains NP
3/88
Upper Buffalo WA
6/91
Guadalupe Mountains NP
3/88
Virgin Islands NP
10/90
Haleakala NP
2/91
Voyageurs NP
3/88
Hawaii Volcanoes NP
3/88
4/93
Washington D.C.
3/88
Jarbridge WA
3/88
8/91
Weminuche WA
3/88
Jefferson/James River Face WA
8/94
Whiteface Mountain , NY
9/88
11/93
Joshua Tree NM
9/91
9/92
White River NF
7/93
Lassen Volcanic NP
3/88
Yellowstone NP
3/88
Yosemite NP
3/88
Table 1: Start and end dates of IMPROVE particulate sampling.
Sample
Collection and Analyses
The standard IMPROVE sampler has four sampling modules, listed in the Table 2:
A, B, and C collect fine particles (0-2.5 µm), and D collects PM10 particles
(0-10 µm). Module A Teflon is the primary filter, providing most of the fine
particle data. Module B, with a denuder before the nylon filter to remove acidic
gases, is used primarily for nitrate. Module C, with tandem quartz filters,
measures carbon in eight temperature fractions. At many sites, the Module A or D
Teflon filter is followed by a quartz filter impregnated with K2CO2 that
converts SO2 gas to sulfate on the filter. Some sites have a single Module A
Teflon.
Module:
A
B
C
D
A2 or D2
Size:
fine
fine
fine
PM10
gas
Filter:
Teflon
nylon
quartz
Teflon
impregnated
Analysis:
gravimetric
PIXE/PESA
XRF
Absorbtion
IC
TOR combustion
gravimetric
IC
Variables:
mass
H, Na-Pb
babs
nitrate
sulfate
chloride
carbon in 8 temperature fractions
PM10 mass
SO2
Table 2: Measurements by full IMPROVE sampler.
Each module is independent, with separate inlet, sizing device, flow measurement
system, critical orifice flow controller, and pump. All modules have a common
controller clock. The flow rate is measured before and after the collection by a
primary method using an orifice meter system and a secondary method using the
pressure drop across the filter and the equation of flow rate through a critical
orifice. The particle sizing depends on the flow rate; the standard deviation of
annual flow rates is 2% to 3%. The average particle cut point for the fine
modules has averaged 2.6 µm, with a standard deviation of 0.2 µm. All
concentrations are based on local volumes. Two 24-hour samples are collected
each week, on Wednesday and Saturday from midnight to midnight local time. The filter cassettes are changed weekly by
on-site personnel and shipped to Davis for processing and analysis. All filter
handling is done in clean laboratory conditions. The recovery rate for validated
data since 1991 has been 96%.
Teflon A and D: The five analytical methods used at Davis to
analyze the Teflon A filters are listed in Table 3. All PM10 (Teflon D) filters
were analyzed by gravimetric analysis; 4% were analyzed by all five methods. The
elemental concentrations (H, Na-Pb) are obtained by PIXE, PESA, XRF. XRF was
added for samples collected after May 1992; this affected the precision, minimum
detectable limits and fraction found for elements between Fe and Pb.
The coefficient of absorption (babs) was measured either by an integrating plate
or an integrating sphere system. Comparisons between the two methods verify that
they accurately determine the absorption for the filter. However, because of
shielding by other particles, this is less than the atmospheric coefficient.
Based on separate experiments, an empirical equation has been derived using the
areal density of all particles on the filter that corrects for the effect. The
reported babs and the precision include this correction factor. Collocated
samplers with differing collection areas verify that the expression is
reasonable. The coefficient of absorption is an optical measurement with units
of 10-8 m-1 in the database. To convert to inverse megameters (10-6 m-1), divide
the value by 100. (For the seasonal summaries, the units are written in inverse
megameters.)
Because of volatilization of nitrate and organics during sampling, the
gravimetric mass measurements on Teflon filters may be slightly less than the
actual mass. Studies comparing nitrate collected on Teflon filters with that
collected on nylon indicate that one-half to three-quarters of the nitrate
volatilizes from the Teflon filter during sampling. At most sites and seasons,
ammonium nitrate is approximately 5% of the fine mass, so this loss is only a
small fraction of the mass. At some western sites near major cities, such as San
Gorgonio, the ammonium nitrate may be one-half of the fine mass in summer,
resulting in major underestimates of fine mass.
Table 3. Analytical methods used for A and D Teflon filters.
Nylon B: The nylon filters were analyzed by ion chromatography
(IC) at Research Triangle Institute or Global GeoChemical for nitrate (NO3-),
chloride (CL-), sulfate (BSO4), and nitrite (NO2-). Nitrate vapors are removed
prior to collection, so that the measured nitrate concentration represents only
particulate nitrate. Chloride ion (CL-) is useful for sites near marine sources,
but elsewhere the ambient concentrations are below than the minimum detectable
limit. Sulfate on nylon (BSO4) is used as a quality assurance check of the
sulfur measured by PIXE on the Teflon A filter. However, we strongly recommend
using the Teflon sulfur as the measurement of ambient sulfate, because of
possible adsorption of SO2 on the nylon filter. The nitrite concentrations are
generally below the minimum detectable limit.
Quartz C: The quartz filters were analyzed at Desert Research
Institute for carbon using the Thermal Optical Reflectance (TOR) combustion
method. The sample is heated in steps and the evolved CO2 measured. The
atmosphere is 100% He until part way through the 550°C step, when 2% O2 is
introduced. The reflectance of the sample is monitored throughout. It decreases
at 120°C and returns to the initial value during the 550°C step after oxygen
is added. All carbon before this return of initial reflectance is considered
organic carbon and the remainder elemental carbon. The eight carbon fractions in
the database are defined in Table 4. OP is the portion of E1, E2, or E3 before
the reflectance returns to the initial value.
Fraction
Pyrolized Fraction
Temperature Range
Atmosphere
Reflectance Vs. Initial
O1
Ambient to 120 deg C
At initial
O2
120-250 deg C
100% He
Under Initial
O3
250-450 deg C
O4
450-550 deg C
E1
OP
remains at 550 deg C
98% He
Over Initial
E2
550-700 deg C
2% O2
E3
700-800 deg C
Table 4. Carbon components as a function of temperature and added oxygen.
The primary interest is in two fractions, organic carbon and elemental or
light-absorbing carbon (LAC). The equations are:
total organic carbon = OC1+OC2+OC3+OC4+OP
total elemental carbon = EC1+EC2+EC3-OP
Preliminary statistical comparisons between the coefficient of absorption and
the carbon measured by TOR suggest that the carbon evolved at 550°C without
added oxygen (OC4) may be light-absorbing. The comparison also suggests that
much of the OP may not be pyrolized organic. The carbon in question (OC4+OP)
could be either light-absorbing organic carbon or elemental carbon. If it is
organic, then the current organic and elemental measurements are correct, but
there is approximately three times as much absorbing carbon than would be
estimated by elemental carbon alone. If it is elemental, then the current
organic carbon concentrations are approximately 30% too large. Until we
determine otherwise, we will assume that the equations above correctly determine
the organic and elemental fractions.
SO2 gas: The sulfate on the impregnated quartz filter following a
Teflon filters were analyzed by ion chromatography at Desert Research Institute
or Research Triangle Institute to give the concentration of SO2. Concentration and
Precision of Measured Variables
The general equation for the concentration of a given variable is
,
where A is the measured mass of the variable, B is the artifact mass determined
from field blanks or secondary filters , and V is the volume determined from the
average flow rate and the sample duration. The artifact B may be produced by
contamination in the filter material, and in handling and analysis, and by
adsorption of gas during collection. The artifact is negligible for all Teflon
measurements, including gravimetric analysis. It is determined from designated
field blanks for ions and from secondary filters for carbon.
The precision in each concentration is included in the data base. The overall
precision is a quadratic sum of four components of precision. These are:
Fractional
volume precision, fv, primarily from the flow rate measurement. A value of
3% is used, based on third-party audits.
Fractional
analytical precision associated with calibration or other factors, fa. This
is zero for gravimetric analysis. The values for all other methods are
determined from replicate analyses. Most variables have an fractional
analytical precision of around 4%, so that the combined volume and
analytical precision is around 5%. For the eight carbon fractions, the
primary source of fractional uncertainty is the separation into temperature
fractions. This may be associated with temperature regulation, but it may
also be from inherent variability of the species involved. The fractional
uncertainty of the sum of all carbon species is around 3% to 4%. The
fractional uncertainty for the fractions range from 11% to 40%, averaging
22%. Thus for sums of fractions, such as total organic, the uncertainties
are less than would be estimated from the individual fractions. This will be
discussed in the section of carbon composites.
Constant
mass per filter precision, σa, from either the analysis or
artifact subtraction. These are determined from the standard deviations in
the designated field blanks, secondary filters, or system control filters.
For large concentrations, this is small compared to the fractional terms.
This is zero for XRF, PIXE, and PESA.
Statistical
precision based on the number of counts in the spectrum, σ stat. This is used for XRF, PIXE,
and PESA. For large concentrations, this is small compared to the fractional
terms.
The
equation for the total precision is:
The relative precision depends on the concentrations. For large concentrations,
only the fractional terms (1 and 2) are important, so the relative precision is
around 5%. For small concentrations, the constant analysis/artifact term (3) or
the statistical term (4) is important. At the mdl, the precision increases to
50%. Table 5 separates the relative precisions of key measured variables
into three groups. This is defined as the ratio of the mean precision from all
sources divided by the mean concentration. Most variables are in the most
precise group, 4% to 7%.
The average minimum detectable limits (mdl) are provided with each concentration
in the database. A concentration is assumed to be statistically significant only
is if is larger than the mdl. For ion chromatography and carbon the mdl
corresponds to twice the precision of the field blanks or secondary filters. For
mass and absorption, the minimum detectable limit corresponds to twice the
analytical precision determined by controls. For PIXE, XRF, and PESA, the
minimum detectable limit is based on the background under the peaks in the
spectrum and is calculated separately for each case. The assumption for all
elements except As is that there are no interference from other elements.
Because the measurement for arsenic requires subtracting the value for lead, the
mdl for As depends on the Pb concentration, and is generally larger than the
value estimated from the background. When calculating averages, if the value is
below the minimum detectable limit, we use one-half of the minimum detectable
limit as the concentration and the precision in the concentration. In all cases,
the relative precisions are around 50% at the mdl.
Table 5: Relative precision of key measured variables. Ratio of mean precision
divided by mean concentration.
The minimum detectable limits of many elements changed in June 1992, with the
addition of XRF. Figure 2 shows the mdl's for each season for sulfur and
selenium. The minimum detection limits for Fe decreased by a a factor of nearly
10, The minimum detection limits for elements below Fe increased slightly,
because of a reduction in cyclotron time to compensate for the extra cost of XRF
analysis.
The minimum detectable limits of standard network samples for elements measured
by PIXE and XRF are given in Table 6. Arsenic is not included because the mdl
depends on the lead concentration. Also important is the fraction of cases with
statistically significant concentrations (above the mdl). This depends on the
relationship between the mdl and the ambient concentrations. Table 7 separates
these into four ranges. A significant change for aluminum occurred with samples
beginning 2/93. Because of detector problems, Al, which is on the shoulder of
the Si peak, was often not detected. Before this date, Al was observed on 65% of
all samples; afterwards it was found on almost every sample. Sodium, chlorine,
and chloride ion were observe in significant amounts only at sites with marine
influences.
Figure 2: Minimum detectable limits of sulfur and selenium by season.
Dates
Na
Mg
Al
Si
P
S
Cl
K
Ca
Ti
V
Cr
Mn
6/88-5/92
8.7
2.9
1.8
1.4
1.3
1.2
1.3
0.83
0.64
0.57
0.50
0.41
0.39
6/92-5/94
13
4.8
3.0
2.2
1.9
1.9
2.0
1.2
0.90
0.81
0.69
0.57
0.52
Fe
Ni
Cu
Zn
Ga
Se
Br
Rb
Sr
Zr
Pb
6/88-5/92
0.34
0.24
0.24
0.21
0.20
0.22
0.25
0.37
0.42
0.65
0.57
6/92-5/94
0.11
0.05
0.05
0.05
0.03
0.03
0.03
0.06
0.07
0.11
0.06
Table 6: Minimum detectable limits of elements in ng/m3.
Range
Before 6/1/92
After 6/1/92
90%-100%
PM2.5,PM10, S, H, Si, K, Ca, Ti, Fe, Zn,
PM2.5, PM10, S, H, Si, K, Ca, Fe, Cu, Zn,
Br, SO4=, NO3-, SO2, OP, E1,
Br, Pb, SO4=, NO3-, SO2, O4, OP, E1
70%-90%
Cu, Pb, O2, O3, O4, E2
Ti, Se, Sr, O2, O3, E2
60%-70%
Mn
Mn, As, Rb
Less than 40%
P, V, Ni, Se, As, Rb, Sr, Zr, O1, E3
P, V, Ni, Zr, O1, E3
Table 7: Fraction of cases with statistically significant concentrations.
Level I validation procedures for sample collection include comparison of the
two measurements of flow rate. Level I validation procedures for sample analysis
include comparison to recognized standards and periodic replicate measurements.
Level II validation procedures include comparison of selected variables measured
by different methods. This includes comparison of the PIXE and XRF measurements,
comparison of sulfur by PIXE on Teflon with sulfate by ion chromatography on
nylon, comparison of OMC and OMH, comparison of LAC and BABS, and comparison of
MF with RCMA and RCMC.
Collocated sampling is an important part of the quality assurance program. These
are conducted routinely at Davis and periodically at field locations. All
collocated sampling has indicated that the precision estimates in the database
are accurate representations of the actual differences.
Composite Variables
The database contains only measured variables. The composite variables listed in
Table 8 can be derived from the measured variables based on reasonable
assumptions.
NHSO ammonium
sulfate, (NH4)2SO2: 4.125*S
NHNO ammonium
nitrate, (NH4)NO3: 1.29*NO3-
OC
total organic carbon (quartz): OC1+OC2+OC3+OC4+OP
OMC
organic mass by carbon (quartz): 1.4* OC
OMH
organic mass by hydrogen (Teflon): assumes all sulfur is
ammonium sulfate and there is no hydrogen from
TC
total carbon (quartz): OC1+OC2+OC3+OC4+EC1+EC2+EC3
SOOT
light absorbing carbon from optical measurement: If BABS in 10-8 m-1, and
SOOT and SOIL in ng/m3,
SOOT=BABS-0.11*SOIL
SOIL
soil: 2.20* Al + 2.49* Si + 1.63* Ca + 2.42* Fe + 1.94* Ti
KNON nonsoil
potassium: K-0.6*Fe
RCMC
reconstructed mass without nitrate, carbon from quartz filter C:
NHSO+SOIL+1.4*KNON+2.5*Na+LAC+OMC
RCMA
reconstructed mass without nitrate, carbon from Teflon filter A:
NHSO+SOIL+1.4*KNON+2.5*Na+BABS/2+OMH
Table 8: Composite Variables
For the uncertainty in all composites except for the four involving the quartz
measurements, we recommend quadratically adding the uncertainties of the
constituent terms times the appropriate multiplicative constant. For example,
the uncertainty for soil would be:
Because of the fact that temperature separation plays a much larger role for
carbon fractions than for the composites, and because the factions are not
independent, we cannot follow the above method for OC, OMC, LAC, and TC. For
these we recommend the following equations for 24-hour samples:
The constant terms (120, 168, 34, 133) are appropriate for volumes near 32.4 m3,
which is typical for 24-hour samples. For other volumes they should be
multiplied by (32.4/V). For typical 12-hour samples, the constant terms should
be multiplied by 2.
ammonium sulfate ((NH4)2SO4): The sulfur on the Teflon filter is always
present as sulfate. In most cases the sulfate is fully neutralized ammonium
sulfate, which is 4.125 times the sulfur concentration. The sulfate at eastern
sites during the summer is not always fully neutralized, but overall the
occurrences are rare. If 100% of the sulfur were sulfuric acid, the correct
sulfate mass would be 74% of the calculated NHSO. The uncertainty in NHSO is 1.4
times the uncertainty in S. The calculate sulfate ion from sulfur, multiply by
3.0.
ammonium nitrate (NH4NO3): As with sulfate, the nitrate is
expected to be fully neutralized ammonium nitrate. This is 1.29 times the
nitrate ion concentration. The uncertainty in NHNO is 2.9 times the uncertainty
in NO3-.
total organic carbon (OC) and organic mass by carbon (OMC): The total organic
carbon concentration is assumed to be the sum of the four organic fractions plus
the pyrolized fraction, OP. To obtain organic mass, we recommend multiplying the
total carbon by 1.4, which assumes that carbon accounts for 71% of the organic
mass. The ratios for various typical compounds range from 1.2 to 1.8.
organic mass by hydrogen (OMH): The hydrogen on the Teflon filter
is associated with sulfate, organics, nitrate, and water. Since the analysis is
done in vacuum, all water will volatilize. We also assume that no significant
hydrogen from nitrate remains. If we assume that the sulfate is fully
neutralized ammonium sulfate, we can estimate the organic concentration by
subtracting the hydrogen from sulfate and multiplying the difference by a
constant representing the fraction of hydrogen. (We suggest a constant of 13.75.
This gives the best comparison with OMC for the network samples. However, a
value near 10 is suggested by various typical organic compounds.) The OMH
variable is defined only when both H and S are valid measurements.
The OMH calculation is invalid when (1) there is high nitrate relative to
sulfate, as at sites near Los Angeles and San Francisco, and (2) the sulfur is
not present as ammonium sulfate. This latter includes sites with marine sulfur,
and sites in the eastern United States with unneutralized sulfate. For the
western sites except San Gorgonio, Sequoia, Pinnacles, Point Reyes, Redwoods,
and Hawaii Volcanoes, the correlation coefficient (r2) between OMH and OMC for
the first two years was 0.89 and the slope was 0.98 ± 0.02. For 1992, r2 was
0.87 and the slope was 1.07±0.01. The main advantage of using OMH at these
sites is that its precision is better than that for OMC during periods of low
organic, as winter in the West. At sites in the East, OMH is often low because
of unneutralized sulfate, and imprecise because of the high sulfate relative to
organic. For 10 eastern sites in 1992, the average OMH was one-half the average
OMC, and one-half of the OMH values were less than the minimum quantifiable
limit.
An organic artifact was found on a batch of Teflon filters used between
September 1990 and November 1991. Approximately 7% of the samples had OMH
significantly larger than OMC. The artifact was apparently completely organic
(there was no elevated sulfur) and appeared during collection. For these
samples, both H and MF (fine mass) were invalidated. These variables were not
invalidated on the remaining 93%, but flagged as less reliable than normal. No
other variables were invalidated.
light-absorbing carbon (LAC): This is the sum of elemental carbon
fractions. The pyrolized fraction is subtracted. Preliminary analyses indicate
that some of the O4 fraction may absorb light, and that OP may overestimate the
pyrolytic mass.
light-absorbing carbon (SOOT): This is estimated from the
coefficient of absorption assuming absorption efficiencies of 10 m2/g for
elemental carbon and 0.11 m2/g for soil.
soil (SOIL): This is a sum of the soil derived elements (Al, Si,
K, Ca, Ti, Fe) along with their normal oxides. The variable does not depend on
the type of soil, such as sediment, sandstone, or limestone. One fine element,
K, however, may partly derive from smoke as well as soil. We have eliminated
this from the calculation and used Fe as a surrogate. This is discussed in
nonsoil potassium below.
nonsoil potassium (KNON): Fine potassium has two major sources,
soil and smoke, with the smoke potassium on much smaller particles than the soil
potassium. The potassium in coarse particles will be solely produced from soil.
The soil potassium is estimated from the measured concentration of Fe and the
ratio of K/Fe of 0.6 measured on coarse samples ( 2.5 to 15 µm) collected
between 1982 and 1986. This ratio depends on the soil composition and varies
slightly from site to site. If the ratio were slightly smaller (say 0.5), the
KNON values will be negative when there is no smoke influence. The residual
potassium, K - 0.6*Fe, is then assumed to be produced by smoke. The burning of
most organic fuels will produce potassium vapor. During transport, this vapor
will transform into fine particles. The KNON parameter is not a quantitative
measure of the total smoke mass, since the ratio of nonsoil potassium to total
smoke mass will vary widely, depending on the fuel type and the transport time.
However, the KNON parameter can be used as an indicator of a nonsoil
contribution for samples with large KNON. In some situations there may be some
fine Fe from industrial sources which could cause occasional smoke episodes to
be lost.
reconstructed mass (RCMC and RCMA): We use two estimates of
reconstructed mass, which differ only in the estimation of organic mass and
light-absorbing carbon. RCMC uses the quartz C measurements, while the RCMA uses
the Teflon A measurements. The RCMC estimate should be used at sites where the
OMH calculation is invalid, while the RCMA estimate should be used when the
organic and LAC concentrations are small. It can also be used when there is no
quartz measurement, as with a single Module A sampler.
Neither reconstructed mass estimate includes nitrates. The Teflon filter does
not collect any nitrate in the vapor state, and loses one-half to three-quarters
of the particulate nitrate by volatilization during sampling. At most sites this
is a few percent the reconstructed mass.
Precision: The precisions of the composite variables are estimated
by quadratically adding the precisions of the components. This assumes that the
precisions are independent. Since this is not quite valid, the calculated
precisions for composites formed by adding (SOIL, OMC, LAC, RCMC, RCMA) are
slightly smaller than they should be. For example, the average calculated
precision for SOIL of 4% should probably be closer to 5%. The composite formed
by subtraction (OMH) may have a slightly smaller precision than reported.
Major Components of Fine Mass ammonium sulfate: Sulfate is generally the major component of the
fine mass throughout the United States, accounting for 20-40% of the mass in the
West to 45-60% in the East. (It is less than organic at most sites in the
Northwest and less than nitrate at San Gorgonio.) Sulfur primarily enters the
atmosphere as SO2 gas. The SO2 converts in the atmosphere to sulfuric acid,
which reacts with ammonia gas to form ammonium sulfate. There are periods at
some sites when there is too much sulfuric acid to be neutralized by ammonia;
some of it may remain as sulfuric acid. The rate of transformation and the size
of the resulting particle depends on the relative humidity. This has a
significant impact on visibility, because in high humidity the sulfate particles
are larger and scatter light much more efficiently relative to the mass of
sulfur. That is, the scattering per unit mass of sulfur is greater at high
humidity than at low humidity. This growth can occur anytime during the lifetime
of the particle. If the relative humidity later decreases the particle will
shrink, but not immediately. Therefore the particle size and scattering
efficiency depends on the relative humidity of the past as well as the present.
The scattering efficiency for a small sulfate particle is less than that for a
large one, but still significant. Because sulfate is such an efficient scatterer
of light, its contribution to the extinction budget is even larger than its
contribution to the mass budget.
ammonium nitrate: Nitrate is generally a minor component of the
particulate mass and the extinction budget. At half of the sites, ammonium
nitrate is less than 6% of the mass, compared to 32% for ammonium sulfate. The
main exceptions are on the West Coast, where the average nitrate concentration
can be more than the average sulfate concentration. In the east, it is 15% of
ammonium sulfate.
soil: Most of this component is produced by soil dust. At some
sites in the West, soil can be one of the largest components of the mass. Its
effect on visibility is less per unit mass than sulfate, because the particles
are generally larger than the optimum size. Soil emission is significantly
enhanced by disturbances to the soil: off-road and dirt-road vehicular traffic,
agricultural activities, bison stampedes. A smaller source of these elements can
come from industrial and mining activities.
organic: Organic material is the largest components at most sites in the
northwest, and elsewhere the second largest component. Possible sources are
fires (wildfires, controlled burns, slash and field burning, incineration,
household heating), industrial emissions, and biogenic emissions.
elemental carbon or light-absorbing carbon: This component accounts for
5% to 10% of the fine mass, depending one whether LAC or BABS is used.
reconstructed mass: The reconstructed mass by either definition
generally correlates well with the gravimetric mass, accounting for almost all
of the fine mass. About 20% of the unaccounted mass may be nitrate, with the
remainder primarily residual water on the particles.