- Email: [email protected]
The hydrogen-sulfur correlation, by PIXE plus PESA, and aerosol source identification
Nuclear
296
Instruments
and Methods
in Physics
Rescarch
B22 (1087)
North-Holland,
THE HYDROGEN-SULFUR IDENTIFICATION Thomas Cracker
A. CAHILL, Nuclear
L&oratory.
Robert University
CORRELATION,
A.
ELDRED,
of Culifornia,
BY PIXE PLUS PESA,
Don Davis,
WALLACE Culiforniu
and Bruce
AND AEROSOL
296-300
Amsterdam
SOURCE
H. KUSKO
4.5616, USA
The use of hydrogen-free thin t&on filters for particulate sampling has allowed us to simultaneously measure sulfur (and other elements) by PIXE and hydrogen by PESA. Particulate hydrogen in nonvolatile forms (since all analyses arc in vacuum) is an important component of aerosols, totalling typically about f of all atoms. The hydrogen is measured at the same time as PIXE by placing a surface barrier detector at 30” in the forward direction, allowing the (p, p) kinematic energy shift to safely resolve hydrogen from the unresolved peaks of C. N, 0. and heavier elements. The method is absolute and simple, with no important corrections. Sensitivity on 400 fig/cm’ teflon filters is about 5 ngicm’ H in 100 s. This technique was introduced to the National Park Service 31 station network in June, 1984, and immediately proved enormously valuable in separating natural aerosols from anthropogenic aerosols. For example. at Great Smoky NP, the H/S correlation was excellent, D 0.90, while the molar ratio was 10:l (note: (NH,),SO, has a ratio 8:1), while at North Cascades NP. there was essentially no correlation and the H/S molar ratio was 37:l. In the former. most hydrogen is tied to presumably anthropogcnic sulfur species, while at North Cascades NP, the converse is true. Evidence of H,SO, aerosols from Arctic studies will also be shown.
1. Introduction PIXE has played a central role in studies of atmospheric particles during the past 15 years. Much of this success has come from the development or adaption of innovative air sampling instrumentation to maximize aerosol information by composition, size. and time at a reasonable cost. However, another aspect of PIXE’s success has been the use of complementary accelerator-based techniques to extend elemental range, calibrate sample thicknesses, correct for absorption, and so on. Yet, in most cases, these techniques are not routinely used in conjunction with PIXE, but rather called up to aid in special situations or difficult problems. In this paper we will discuss an elastic scattering technique to be used concurrently with the PIXE analysis on a routine basis. The choice of PIXE as the primary analytical method for large nonurban particulate networks of the U.S. Environmental Protection Agency and the National Park Service since 1979 has encouraged the widest use of PIXE-compatible methods to gain yet more information on samples. Methods were developed to measure the mass concentration [l], and the coefficient of optical absorption [2] on every sample, but only selected samples could be analyzed for the very light elements. H to F, by forward alpha scattering techniques (FAST) (31, necessary to fully explain particulate composition. Yet. a great need existed to gain information on light elements, particularly since at our remote sites, a significant fraction of
aerosol mass could be due to natural sources of hydrocarbons. Measurement of particulate hydrogen appeared to be one way to gather some information on the very light aerosol component on every fine air filter. In this effort, we were encouraged by the role hydrogen measurements played in our studies of arctic haze using Mass, FAST, and PIXE [4]. Since the fine particle filters in the NPS network were of hydrogenfree teflon, it was possible to measure the hydrogen concentration for particles of aerodynamic diameter below 2.5 Km using proton elastic scattering analysis (PESA). Starting in June 1984, routine measurements were made of hydrogen on all fine filters, concurrently with the PIXE analyses. Our experience to date shows that nothing that we have done to extend our PIXE program has been so simple to achieve but so valuable to our goals in understanding nonurban aerosols.
2. Experimental
arrangements
For measurement of hydrogen by proton-proton scattering, one must operate with forward scattering, allowing the beam and scattered particles to pass through the thin filter. Thus, the key to hydrogen measurements was to position the proton peak away from the unresolved peak for all heavier elements, primarily carbon, oxygen and fluorine. Fig. 1 shows the spectrum resulting from B,Zgh = 30” (0,, = 60”). using a single surface barrier detector, for a clean Membrana “M stretched tellon filter and a loaded filter.
T. A.
Cahill
et al.
I N-S
correlation
and aerosol
source
identijication
297
network from June 1084 through November 1985. for particles smaller than 2.5 pm (table 1). Mass concentrations were measured gravimetrically and coefficients of optical absorption were obtained by the integrating plate method. The overall resuits are discussed elsewhere in these proceedings [.5/. In this paper we will examine the relationship between hydrogen and sulfur and then the relationship between the remaining hydrogen after removing the fraction contributed by sulfate with the remaining mass after removing sulfate soil and soot from the measured fine mass. (The soil concentrations are estimated from PIXE using typical oxide forms. The soot concentrations are calculated from the coefficients of optical absorption.) The pattern of average concentrations for hydrogen and sulfur for the 18-month period are clearly different, as seen in fig. 2. The major similarity is that those
Fig. 1. Proton spectra for clean teflon (upper spectrum) and loaded filter (loaded spectrum). The left spectrum is hydrogen and the right peak is due to all other elements, primary carbon and fluorine from the teflon. The equivalent blank value for these clean teflon filters was 1Ongicm’ hydrogen, yielding aerosol concentration detectable limits below 1 ngim”. Thus, hydrogen became immediately one of our most sensitive elements. Calibration was achieved by using a series of weighed thin plastic foils of mylar (C,,,H,O,), Kapton (C,,H,,,N,O,), polyethylene (CH2), and Kimfoil (effectively C,H,O). Great care had to be taken not to stretch the standard foils on mounting, and beaminduced shrinkage required regular replacement. Thickness ranged from 220 to 1700 *g/cm’. Absolute accuracy of t 3% was achieved in the region below IO00 @g/cm’, the mass region encountered for almost all loaded filters. We must note, however, that the hydrogen measurements are made in vacuum, and thus we expect unbound water and some other volatiles to leave the filter.
3. Results and discussion 3100 hydrogen concentrations were measured concurrently with FIXE in the National Park Service
Fig. 2. Average concentrations of hydrogen and sulfur and their correlation from June 1984 to November 1985. The concentrations are in ngim” and the coefficients in percent. III. ATMOSPHERIC
APPLICATIONS
T. A. Cahill et al. I H-S correlation and aerosol source identification
298
Table 1. Comparison of hydrogen with other fine variablcb June 1984 to February 1985. Site
Correlation
(National
coefficient
Park Service
Mean
S
Soot
K
0.94 O.YS 0.90 0.77 0.87 0.81 0.86 0.83
0.56 0.83 0.58 0.77 0.77 0.94 0.83 0.59
0.62 0.84 0.62 0.80 0.83 0.86 0.79 0.64
0.76 0.77 0.54 0.78 0.71 0.49 0.45 0.65
Grand Teton
0.93 0.83
0.85 0.23
0.67 0.65
0.34 0.02
Th. Roosevelt Wind Cave Rocky Mountain
0.83 0.84 0.71
0.81 0.78 0.77
0.63 0.41 0.82
0.42 0.59 0.49
Dinosaur Lehman Caves Canyonlands Bryce Canyon Grand Canyon Mesa Verde Chaco Culture Bandelier Capulin Mountain Chiricahua Guadalupe Mountains Big Bend Buffalo River Great Smoky Mountains Shenandoah
0.89 0.81 0.73 0.68 0.56 0.43 0.76 0.75 0.89 0.80 0.73 0.67 0.74 0.92 0.90
0.65 0.79 0.76 0.76 0.77 0.71 0.76 0.78 0.91 0.77 0.81 0.90 0.79 0.00 0.95
0.68 0.58 0.48 0.45 0.47 0.25 0.23 0.37 0.68 0.56 0.45 0.69 0.51 0.20 0.05
Median
0.82
0.78
0.64
values
‘) Coefficient
uncertain:
The difference
between
monitoring
with
Mass North Cascades Mount Rainier Crater Lake Lava beds Yosemite Death Valley Joshua Tree Glacier Craters of Moon
particulate
H
Sampling
is done from
c/c of mass
Molar
H:S
Fe
0.09”’ 0.45 0.27 0.20 0.25 0.23 0. IO 0.23 0.39 0.20“’ 0.26 0.05”’ 0.25 0.44 0.23
0.55 0.66 0.57 0.51 0.71 0.41 0.27 0.15 0.04 0.26 0.18 0.36 0.36 0.28 0.59 0.16 0.38 0.24 0.26 0.22 0.22 0.29 0.35 0.35 0.00“’ 0.12 0.23 0.28
283 166 136 138 198 135 194 237 172 208 165 142 151 173 93 I20 115 102 12s I40 157 I40 164 17’) 182 432 484 471
5.0 4.3 4.8 4.3 4.3 3.6 3.9 4.3 4.4 4.4 4.2 4.6 4.7 4.5 4.5 4.2 4.6 4.0 4.2 4.3 4.5 4.5 4.6 4.3 3.6 4.4 3.9 4.5
37:1 1x:1 34: I 33:1 25:1 1S:l 15:l 30: 1 24:1 38:1 13:l 17:l 17:l 2O:l 16:l 13:l 14:l 13:l 12:1 l2:l 13:l 12:l ll:l 12:l 1l:l 12:l 10: 1 IO:1
0.37
0.28
160
4.3
15:l
the 95% confidence
areas with high sulfur also have high hydrogen. However, in the northwest there are sites with low sulfur and moderately high hydrogen. The two elements correlate at around 0.80 except in the northwest. Fig. 3 shows time plots of hydrogen and sulfur for a site with high correlation and one with low correlation. Shenandoah is in the east where sulfates account for 60% of the measured fine mass, while Crater Lake is in the northwest where sulfates contribute only 20%. The correlation coefficients for hydrogen versus sulfur are 0.86 at Shenandoah and 0.49 at Crater Lake. Note that at Shenandoah the ratio of S:H concentrations is close to the ratio of 4:l obtained for (NH,),SO,. At Crater Lake the two variables have similar average values, indicating that most of the hydrogen comes from sources other than ammonium sulfate. The contribution of ammonium sulfate to the measured hydrogen can be estimated for all sites in
network).
limits exceeds
0.50.
Fig. 3. Time plots of sulfur and hydrogen at a site with high correlation and one with low correlation.
T. A.
Cahill
Percent 50
et al.
I H-S
of
associated
H
correlution
with
and mrosol
(NH41Z
xource
299
identification
SO4
I\
60
'30 O64 \
1067
V Fig. 4. Percent
of hydrogen
contributed
by ammonium
\I sulfate
from June
1984 to November
1985
Fig. 5. Average concentrations of remaining nonsulfate hydrogen (H ~ S/4) and remaining mass (less sulfate, soil and soot), and the correlation and average ratios for the two variables, from June 1984 to November 1985. The concentrations are in ngim’ for remaining hydrogen and pglm’ for remaining mass. The correlations and ratios are in percent. III.
ATMOSPHERIC
APPLICATIONS
300
T. A. Cahill et al. I H-S
correlation and aerosol source identification
the network if the sulfur is assumed to be present as (NH,),SO,. Fig. 4 shows the percent of hydrogen associated with sulfate. It is largest for the sites in the east and parts of the southwest, ranging from SO% to 950/c, and smallest in the northwest, where the contribution is around 20%. An interesting relationship results if the remaining nonsulfate hydrogen is compared to the “fine remaining mass”, obtained by subtracting ammonium sulfate, soil and soot from the measured mass. Fig. 5 shows that the two residual variables follow similar patterns with high values in the east, northwest and at Yosemite, and low values in the southwest. The two variables correlate well. with correlation coefficients of 0.86 for all western sites combined and of 0.76 for all sites combined. This correlation is surprisingly high since the two residual variables are obtained by subtracting results by one method from those by others. The ratios of the two variables are extremely constant over all sites in the network, with remaining hydrogen contributing 5 ? 1% of the remaining mass. Since the hydrogen content of ammonium nitrate is 5% and that of organic molecules range from 5% to 14%, it is evident that these two species must contribute most of the remaining mass not measureable by PIXE. Since the hydrogen content of ammonium sulfate (6%) is similar to that for remaining mass, it is not surprising that hydrogen (from PESA) correlates with mass (measured gravimetrically) and has a ratio of around 5%. The relationship can be very useful for checking the results of both mass and hydrogen. A quality assurance check based on the ratio permitted us to detect several errors among the 3000 calculations
of particulate mass, and thus to improve of the data set.
the reliability
4. Conclusions The addition of (p, p) scattering to our existing PIXE system has allowed us to measure hydrogen concurrently with our PIXE analysis. Because they operate simultaneously, the additional cost is very small. The hydrogen variable can be extremely valuable in two areas. First, it furthers our understanding of the portion of fine mass primarily not measureable by PIXE, by showing that this remaining mass consists of hydrogen-bearing material such as nitrates and organics. Secondly, the hydrogen measurements can be used as a quality assurance check of individual calculations of fine mass.
References
[ll
P. Feeney, J. Olivera and R. Guidara, J. Air Pollution Control Assoc. 34 (1984) 376. PI T.A. Cahill, R.A. Eldred, D. Shadoan, P.J. Feeney, B.H. Kusko, and Y. Matsuda, Nucl. Instr. and Meth. B3 (1984) 291. D. Shadoan, R.A. Eldred and [31 T.A. Cahill, Y. Matsuda, B.H. Kusko, Nucl. Instr. and Meth. B3 (1984) 263. Res. Lett. 11 I41T.A. Cahill and R.A. Eldred, Geophys. (1984) 413. [51R.A. Eldred, T.A. Cahill and P.J. Feeney, these Proceedings (4th Int. Conf. on PIXE, Tallahasse. FL) Nucl. Instr.
and Meth.
B22 (1987)
289.
296
Instruments
and Methods
in Physics
Rescarch
B22 (1087)
North-Holland,
THE HYDROGEN-SULFUR IDENTIFICATION Thomas Cracker
A. CAHILL, Nuclear
L&oratory.
Robert University
CORRELATION,
A.
ELDRED,
of Culifornia,
BY PIXE PLUS PESA,
Don Davis,
WALLACE Culiforniu
and Bruce
AND AEROSOL
296-300
Amsterdam
SOURCE
H. KUSKO
4.5616, USA
The use of hydrogen-free thin t&on filters for particulate sampling has allowed us to simultaneously measure sulfur (and other elements) by PIXE and hydrogen by PESA. Particulate hydrogen in nonvolatile forms (since all analyses arc in vacuum) is an important component of aerosols, totalling typically about f of all atoms. The hydrogen is measured at the same time as PIXE by placing a surface barrier detector at 30” in the forward direction, allowing the (p, p) kinematic energy shift to safely resolve hydrogen from the unresolved peaks of C. N, 0. and heavier elements. The method is absolute and simple, with no important corrections. Sensitivity on 400 fig/cm’ teflon filters is about 5 ngicm’ H in 100 s. This technique was introduced to the National Park Service 31 station network in June, 1984, and immediately proved enormously valuable in separating natural aerosols from anthropogenic aerosols. For example. at Great Smoky NP, the H/S correlation was excellent, D 0.90, while the molar ratio was 10:l (note: (NH,),SO, has a ratio 8:1), while at North Cascades NP. there was essentially no correlation and the H/S molar ratio was 37:l. In the former. most hydrogen is tied to presumably anthropogcnic sulfur species, while at North Cascades NP, the converse is true. Evidence of H,SO, aerosols from Arctic studies will also be shown.
1. Introduction PIXE has played a central role in studies of atmospheric particles during the past 15 years. Much of this success has come from the development or adaption of innovative air sampling instrumentation to maximize aerosol information by composition, size. and time at a reasonable cost. However, another aspect of PIXE’s success has been the use of complementary accelerator-based techniques to extend elemental range, calibrate sample thicknesses, correct for absorption, and so on. Yet, in most cases, these techniques are not routinely used in conjunction with PIXE, but rather called up to aid in special situations or difficult problems. In this paper we will discuss an elastic scattering technique to be used concurrently with the PIXE analysis on a routine basis. The choice of PIXE as the primary analytical method for large nonurban particulate networks of the U.S. Environmental Protection Agency and the National Park Service since 1979 has encouraged the widest use of PIXE-compatible methods to gain yet more information on samples. Methods were developed to measure the mass concentration [l], and the coefficient of optical absorption [2] on every sample, but only selected samples could be analyzed for the very light elements. H to F, by forward alpha scattering techniques (FAST) (31, necessary to fully explain particulate composition. Yet. a great need existed to gain information on light elements, particularly since at our remote sites, a significant fraction of
aerosol mass could be due to natural sources of hydrocarbons. Measurement of particulate hydrogen appeared to be one way to gather some information on the very light aerosol component on every fine air filter. In this effort, we were encouraged by the role hydrogen measurements played in our studies of arctic haze using Mass, FAST, and PIXE [4]. Since the fine particle filters in the NPS network were of hydrogenfree teflon, it was possible to measure the hydrogen concentration for particles of aerodynamic diameter below 2.5 Km using proton elastic scattering analysis (PESA). Starting in June 1984, routine measurements were made of hydrogen on all fine filters, concurrently with the PIXE analyses. Our experience to date shows that nothing that we have done to extend our PIXE program has been so simple to achieve but so valuable to our goals in understanding nonurban aerosols.
2. Experimental
arrangements
For measurement of hydrogen by proton-proton scattering, one must operate with forward scattering, allowing the beam and scattered particles to pass through the thin filter. Thus, the key to hydrogen measurements was to position the proton peak away from the unresolved peak for all heavier elements, primarily carbon, oxygen and fluorine. Fig. 1 shows the spectrum resulting from B,Zgh = 30” (0,, = 60”). using a single surface barrier detector, for a clean Membrana “M stretched tellon filter and a loaded filter.
T. A.
Cahill
et al.
I N-S
correlation
and aerosol
source
identijication
297
network from June 1084 through November 1985. for particles smaller than 2.5 pm (table 1). Mass concentrations were measured gravimetrically and coefficients of optical absorption were obtained by the integrating plate method. The overall resuits are discussed elsewhere in these proceedings [.5/. In this paper we will examine the relationship between hydrogen and sulfur and then the relationship between the remaining hydrogen after removing the fraction contributed by sulfate with the remaining mass after removing sulfate soil and soot from the measured fine mass. (The soil concentrations are estimated from PIXE using typical oxide forms. The soot concentrations are calculated from the coefficients of optical absorption.) The pattern of average concentrations for hydrogen and sulfur for the 18-month period are clearly different, as seen in fig. 2. The major similarity is that those
Fig. 1. Proton spectra for clean teflon (upper spectrum) and loaded filter (loaded spectrum). The left spectrum is hydrogen and the right peak is due to all other elements, primary carbon and fluorine from the teflon. The equivalent blank value for these clean teflon filters was 1Ongicm’ hydrogen, yielding aerosol concentration detectable limits below 1 ngim”. Thus, hydrogen became immediately one of our most sensitive elements. Calibration was achieved by using a series of weighed thin plastic foils of mylar (C,,,H,O,), Kapton (C,,H,,,N,O,), polyethylene (CH2), and Kimfoil (effectively C,H,O). Great care had to be taken not to stretch the standard foils on mounting, and beaminduced shrinkage required regular replacement. Thickness ranged from 220 to 1700 *g/cm’. Absolute accuracy of t 3% was achieved in the region below IO00 @g/cm’, the mass region encountered for almost all loaded filters. We must note, however, that the hydrogen measurements are made in vacuum, and thus we expect unbound water and some other volatiles to leave the filter.
3. Results and discussion 3100 hydrogen concentrations were measured concurrently with FIXE in the National Park Service
Fig. 2. Average concentrations of hydrogen and sulfur and their correlation from June 1984 to November 1985. The concentrations are in ngim” and the coefficients in percent. III. ATMOSPHERIC
APPLICATIONS
T. A. Cahill et al. I H-S correlation and aerosol source identification
298
Table 1. Comparison of hydrogen with other fine variablcb June 1984 to February 1985. Site
Correlation
(National
coefficient
Park Service
Mean
S
Soot
K
0.94 O.YS 0.90 0.77 0.87 0.81 0.86 0.83
0.56 0.83 0.58 0.77 0.77 0.94 0.83 0.59
0.62 0.84 0.62 0.80 0.83 0.86 0.79 0.64
0.76 0.77 0.54 0.78 0.71 0.49 0.45 0.65
Grand Teton
0.93 0.83
0.85 0.23
0.67 0.65
0.34 0.02
Th. Roosevelt Wind Cave Rocky Mountain
0.83 0.84 0.71
0.81 0.78 0.77
0.63 0.41 0.82
0.42 0.59 0.49
Dinosaur Lehman Caves Canyonlands Bryce Canyon Grand Canyon Mesa Verde Chaco Culture Bandelier Capulin Mountain Chiricahua Guadalupe Mountains Big Bend Buffalo River Great Smoky Mountains Shenandoah
0.89 0.81 0.73 0.68 0.56 0.43 0.76 0.75 0.89 0.80 0.73 0.67 0.74 0.92 0.90
0.65 0.79 0.76 0.76 0.77 0.71 0.76 0.78 0.91 0.77 0.81 0.90 0.79 0.00 0.95
0.68 0.58 0.48 0.45 0.47 0.25 0.23 0.37 0.68 0.56 0.45 0.69 0.51 0.20 0.05
Median
0.82
0.78
0.64
values
‘) Coefficient
uncertain:
The difference
between
monitoring
with
Mass North Cascades Mount Rainier Crater Lake Lava beds Yosemite Death Valley Joshua Tree Glacier Craters of Moon
particulate
H
Sampling
is done from
c/c of mass
Molar
H:S
Fe
0.09”’ 0.45 0.27 0.20 0.25 0.23 0. IO 0.23 0.39 0.20“’ 0.26 0.05”’ 0.25 0.44 0.23
0.55 0.66 0.57 0.51 0.71 0.41 0.27 0.15 0.04 0.26 0.18 0.36 0.36 0.28 0.59 0.16 0.38 0.24 0.26 0.22 0.22 0.29 0.35 0.35 0.00“’ 0.12 0.23 0.28
283 166 136 138 198 135 194 237 172 208 165 142 151 173 93 I20 115 102 12s I40 157 I40 164 17’) 182 432 484 471
5.0 4.3 4.8 4.3 4.3 3.6 3.9 4.3 4.4 4.4 4.2 4.6 4.7 4.5 4.5 4.2 4.6 4.0 4.2 4.3 4.5 4.5 4.6 4.3 3.6 4.4 3.9 4.5
37:1 1x:1 34: I 33:1 25:1 1S:l 15:l 30: 1 24:1 38:1 13:l 17:l 17:l 2O:l 16:l 13:l 14:l 13:l 12:1 l2:l 13:l 12:l ll:l 12:l 1l:l 12:l 10: 1 IO:1
0.37
0.28
160
4.3
15:l
the 95% confidence
areas with high sulfur also have high hydrogen. However, in the northwest there are sites with low sulfur and moderately high hydrogen. The two elements correlate at around 0.80 except in the northwest. Fig. 3 shows time plots of hydrogen and sulfur for a site with high correlation and one with low correlation. Shenandoah is in the east where sulfates account for 60% of the measured fine mass, while Crater Lake is in the northwest where sulfates contribute only 20%. The correlation coefficients for hydrogen versus sulfur are 0.86 at Shenandoah and 0.49 at Crater Lake. Note that at Shenandoah the ratio of S:H concentrations is close to the ratio of 4:l obtained for (NH,),SO,. At Crater Lake the two variables have similar average values, indicating that most of the hydrogen comes from sources other than ammonium sulfate. The contribution of ammonium sulfate to the measured hydrogen can be estimated for all sites in
network).
limits exceeds
0.50.
Fig. 3. Time plots of sulfur and hydrogen at a site with high correlation and one with low correlation.
T. A.
Cahill
Percent 50
et al.
I H-S
of
associated
H
correlution
with
and mrosol
(NH41Z
xource
299
identification
SO4
I\
60
'30 O64 \
1067
V Fig. 4. Percent
of hydrogen
contributed
by ammonium
\I sulfate
from June
1984 to November
1985
Fig. 5. Average concentrations of remaining nonsulfate hydrogen (H ~ S/4) and remaining mass (less sulfate, soil and soot), and the correlation and average ratios for the two variables, from June 1984 to November 1985. The concentrations are in ngim’ for remaining hydrogen and pglm’ for remaining mass. The correlations and ratios are in percent. III.
ATMOSPHERIC
APPLICATIONS
300
T. A. Cahill et al. I H-S
correlation and aerosol source identification
the network if the sulfur is assumed to be present as (NH,),SO,. Fig. 4 shows the percent of hydrogen associated with sulfate. It is largest for the sites in the east and parts of the southwest, ranging from SO% to 950/c, and smallest in the northwest, where the contribution is around 20%. An interesting relationship results if the remaining nonsulfate hydrogen is compared to the “fine remaining mass”, obtained by subtracting ammonium sulfate, soil and soot from the measured mass. Fig. 5 shows that the two residual variables follow similar patterns with high values in the east, northwest and at Yosemite, and low values in the southwest. The two variables correlate well. with correlation coefficients of 0.86 for all western sites combined and of 0.76 for all sites combined. This correlation is surprisingly high since the two residual variables are obtained by subtracting results by one method from those by others. The ratios of the two variables are extremely constant over all sites in the network, with remaining hydrogen contributing 5 ? 1% of the remaining mass. Since the hydrogen content of ammonium nitrate is 5% and that of organic molecules range from 5% to 14%, it is evident that these two species must contribute most of the remaining mass not measureable by PIXE. Since the hydrogen content of ammonium sulfate (6%) is similar to that for remaining mass, it is not surprising that hydrogen (from PESA) correlates with mass (measured gravimetrically) and has a ratio of around 5%. The relationship can be very useful for checking the results of both mass and hydrogen. A quality assurance check based on the ratio permitted us to detect several errors among the 3000 calculations
of particulate mass, and thus to improve of the data set.
the reliability
4. Conclusions The addition of (p, p) scattering to our existing PIXE system has allowed us to measure hydrogen concurrently with our PIXE analysis. Because they operate simultaneously, the additional cost is very small. The hydrogen variable can be extremely valuable in two areas. First, it furthers our understanding of the portion of fine mass primarily not measureable by PIXE, by showing that this remaining mass consists of hydrogen-bearing material such as nitrates and organics. Secondly, the hydrogen measurements can be used as a quality assurance check of individual calculations of fine mass.
References
[ll
P. Feeney, J. Olivera and R. Guidara, J. Air Pollution Control Assoc. 34 (1984) 376. PI T.A. Cahill, R.A. Eldred, D. Shadoan, P.J. Feeney, B.H. Kusko, and Y. Matsuda, Nucl. Instr. and Meth. B3 (1984) 291. D. Shadoan, R.A. Eldred and [31 T.A. Cahill, Y. Matsuda, B.H. Kusko, Nucl. Instr. and Meth. B3 (1984) 263. Res. Lett. 11 I41T.A. Cahill and R.A. Eldred, Geophys. (1984) 413. [51R.A. Eldred, T.A. Cahill and P.J. Feeney, these Proceedings (4th Int. Conf. on PIXE, Tallahasse. FL) Nucl. Instr.
and Meth.
B22 (1987)
289.