United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-83-005 May 1983
Project Summary
Air Quality Data for Nonmetallic
Inorganic Ions: Nitrate and
Sulfate for 1979 from the
National Air Surveillance
Networks
Gary F. Evans, John C. Puzak, and John E. Frazer
The National Air Surveillance Net-
works provide information on air quality
for many urban and nonurban locations
within the United States. This report
summarizes the network data for two
nonmetallic inorganic ions (nitrate and
sulfate) determined from high volume
samples collected during the year
1979. Concentration values are
presented in the form of cumulative
frequency distributions. Arithmetic and
geometric sample statistics are also
reported, as are measures of the
precision and bias associated with the
analytical methods employed.
Comparison of the 1 979 data with that
from previous years in the decade
shows that network nitrate
concentrations on a nationwide basis
increased substantially during the
1970's. The average annual rate of
increase was 6.2% during the nine-year
period in areas classified as urban and
9.6% in areas classified as nonurban.
Average network sulfate concentra-
tions did not exhibit a consistent trend
through the 1970's, but have
apparently increased since 1976 in
both urban and nonurban areas. Some
of the variability from year to year in the
national frequency distributions may be
attributed to different sets of sites being
used in the summarization, depending
upon the completeness criteria for valid
data and changes in state and local
agency participation in the networks
over the years.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park, NC, to announce 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
The U.S. Environmental Protection
Agency (EPA) conducts a variety of air
sampling activities to obtain information
about the quality of the nation'sair. Often
this work is accomplished with the
assistance and cooperation of state and
local government agencies. One such
program consists of the National Air
Surveillance Networks (NASN) which
have reported air quality data for more
than 20 years. Although the operation of
the sampling sites is decentralized, the
determination and publication of trace
pollutant concentrations remains the
responsibility of EPA's Environmental
Monitoring Systems Laboratory at
Research Triangle Park (EMSL/RTP).
The air quality monitoring stations of
the NASN are located throughout the
country in areas originally classified as
urban or nonurban. Urban-classified
sites were generally located within a city,
town, or adjacent suburb. Nonurban-
classified sites were originally located in
rural or remote areas; but, over time,
many of these areas became more
-------
heavily populated and subjected to the
influences of spreading urbanization.
This report summarizes the 1979
network data for nitrate and sulfate ion
concentrations as measured by
colorimetric analysis of high volume
paniculate samples.
Procedure
Preweighed glass-fiber filters were
distributed to the cooperating local
agencies for sample collection. After
high-volume sampling, the filters were
returned to the EPA for final weighing
and chemical analysis. Analyses for
inorganic ions during 1979 were
performed by Northrop Services, Incor-
porated - Environmental Sciences, under
Contract No. 68-02-2566.
An 8.3 percent section of each
paniculate filter was cut and extracted in
water for autoanalysis. The analytical
procedure for each ion was as follows:
1. The aqueous extract was analyzed for
nitrate ion by reduction of the nitrate
to nitrite by a copper-cadmium
reduction column. The nitrate was
reacted with sulfanilamide in acidic
solution to form a diazo compound.
This compound then coupled with N-
1 -naphthylene diamine dihydrochlo-
ride to form a reddish-purple azodye
which was determined colorimetric-
ally at a wavelength of 520to 540nm.
2. The extract was analyzed for sulfate
ion by the methylthymol blue (MTB)
method using a single channel
Technicon Autoanalyzer II system
equipped with a linearizer. The MTB
method is based on the spectral dif-
ference which exists in basic solution
(pH 12.5 to 13.0) between the barium
complex of MTB and free MTB. At this
pH, the barium complex is blue and
free MTB is brownish-red (absorbs
light at 460 nm). Thus, the color of
solutions containing both the free
MTB and the complex appears as
gray. The amount of free MTB, moni-
tored colorimetrically at wavelengths
of 460 to 480 nm, was the measure of
the amount of sulfate in the sample.
Independent estimates of laboratory
precision and bias were determined for
these data to aid in interpreting analytical
results. However, several potential
sources of measurement error associated
with sample collection and handling
procedures (flow control, artifact
formation, shipping and storage losses,
etc. that were not directly controlled by
EMSL/RTP) were not considered in the
estimates of data quality presented here.
Analytical precision estimates are
based upon percent differences
determined from analyses of two filter
strips taken from the same filter. Thus,
the measure of precision includes
variation caused by cutting, extracting,
and analytical processing as well as any
actual differences which may exist
between filter strips. Duplicate strips of
every 20th sample were analyzed for the
purpose of estimating precision.
Analytical bias estimates were ob-
tained through an audit program in which
"spiked" filter strips were introduced into
the ambient air sample group for routine
analysis. These quality assurance
samples contained known quantities of
each ion and thus provided a measure of
analytical recovery and its complement,
bias. Every 2 weeks a set of 10 audit
samples was provided to the analytical
laboratory for inclusion in the routine
sample processing.
Results
Estimates of the analytical precision
and bias associated with the 1979 results
are listed in concentration range in
Table 1.
Table 1. Analytical Precision and Bias
Concentration Bias
Ion Range ffjg/m3} (%)
Nitrate
Sulfate
<3
3-6
>6
<6
6-20
>20
+4.1
-2.5
+0.4
-6.7
•2.0
-2.9
Precision
±7
±3
±4
±6
±4
±1
Air quality data are presented in Tables
2 and 3 in the form of annual cumulative
frequency distributions by ion
summarized for urban and nonurban
locations, respectively. In the first line of
Table 2, the number 3900 is the number
of valid 24-hr samples analyzed for
nitrate in 1971. The next entry is the
minimum value detected that year, and
"LD" means below the minimum
detectable level of the instrument. The
next seven entries are the 10 through 99
percentile values. For example, the 90
percentile value of 5.02 indicates that 90
percent of the 3900 values, i.e.,0.90 x
3900 = 3510, were equal to or less than
5.02 //g/m3. The next entry is the
Table 2. Urban National Cumulative Frequency Distributions
Ion
/V03 '
S04 =
Year
1971
1972
1973
1974
1975
1976
1977
1978
1971
1972
1973
1974
1975
1976
1977
1978
Number of
Samples Min.
3900
5519
4775
4562
4113
3817
4531
3610
3916
5519
4774
4564
4110
3871
4531
3610
LD
LD
LD
LD
0.20
0.08
0.07
LD
LD
LD
LD
LD
0.2
0.2
0.7
LD
10
0.69
0.66
0.81
0.83
0.96
1.07
1.07
1.08
3.4
3.9
3.9
3.9
3.5
3.1
3.1
3.1
Percent of time concentration f/jg/m3)
is equal to or less than
30 50 70 90 95 99
1.55
1.58
1.71
1.74
2.00
1.99
2.09
2.21
5.7
6.7
6.3
6.3
6.1
5.4
5.7
5.7
2.29
2.47
2.58
2.54
2.91
2.80
2.99
3.24
8.0
9.4
8.1
8.3
8.3
7.4
7.6
7.8
3.22
3.55
3.65
3.63
4.12
3.83
4.15
4.56
10.9
12.7
10.9
11.3
11.5
9.9
10.3
10.6
5.02
5.90
6.24
6.46
7.18
6.48
6.76
7.78
18.0
20.3
17.5
17.9
18.1
16.5
17.1
16.6
6.48
7.88
8.31
8.62
9.90
9.53
10.40
11.18
22.0
25.4
22.7
22.8
23.8
20.0
20.9
20.4
11.85
13.50
14.61
17.50
17.07
20.65
20.63
22.91
33.5
37.9
35.4
34.8
35.8
29.2
29.9
34.5
Max.
26.17
24.99
37.36
54.89
33.71
43.16
97.67
56.96
69.2
75.9
162.0
69.1
72.6
66.7
76.4
228.4
Arithmetic
statistics
Std
Mean Dev.
2.77
3.05
3.25
3.36
3.73
3.71
3.89
4.22
9.6
11.1
9.9
10.0
10.0
8.8
9.1
9.4
2.30
2.61
2.87
3.40
3.31
3.83
3.97
4.10
6.8
7.7
7.0
6.7
7.0
5.8
6.1
7.5
Geometric
statistics
Std.
Mean Dev.
2.13
2.32
2.44
2.36
2.79
2.58
2.87
3.07
7.84
9.15
8.06
8.34
8.22
7.33
7.44
7.55
2.06
2.10
2.13
2.32
2.15
2.34
2.18
2.26
1.89
1.87
1.89
1.83
1.88
1.83
1.72
1.95
-------
Table 3.
Nonurban National Cumulative Frequency Distributions
Percent of time concentration f/jg/m3)
is equal to or less than
Arithmetic
statistics
Number of
Ion
NO3 -
5O4 =
Year
1971
1972
1973
1974
1975
1976
1977
1978
1971
1972
1973
1974
1975
1976
1977
1978
Samples
671
928
831
706
630
467
681
458
686
929
831
706
630
493
681
458
Min.
LD
LD
LD
LD
0.20
LD
LD
LD
0.4
LD
LD
0.1
0.2
0.2
LD
LD
10
0.06
LD
0.04
0.12
0.20
0.10
0.17
0.30
1.5
1.4
1.3
1.4
0.7
0.9
1.2
1.2
30
0.31
0.15
0.28
0.37
0.20
0.36
0.55
0.59
3.0
2.9
2.7
2.6
2.1
1.8
2.5
3.0
50
0.7 J
0.50
0.71
0.83
0.73
0.82
1.05
1.07
4.7
5.2
4.4
4.8
4.0
4.0
4.7
4.9
70
1.20
1.01
1.26
1.37
1.41
1.51
1.81
2.03
7.2
7.8
6.3
7.2
6.4
6.5
7.4
7.2
90
2.19
2.08
2.35
2.52
2.74
2.82
3.07
3.77
11.8
13.7
12.0
12.2
11.6
11.3
12.8
13.4
95
2.67
2.59
2.83
3.16
3.28
3.54
3.84
4.74
15.5
17.4
17.0
16.6
15.2
13.9
17.1
17.0
99
3.81
4.19
4.47
5.13
4.85
4.73
5.22
8.09
23.6
24.8
29.8
27.0
28.7
21.3
24.6
21.0
Max.
6.04
6.59
6.67
6.65
11.85
6.15
11.57
11.77
35.4
42.7
53.2
90.0
48.3
36.3
43.3
38.5
Mean
0.95
0.79
0.99
1.10
1.13
1.18
1.39
1.69
6.0
6.6
6.0
6.2
5.5
5.3
6.1
6.2
Std.
Dev.
0.92
0.92
1.02
1.07
1.16
1.16
1.26
1.72
4.9
5.4
5.9
6.2
5.7
4.9
5.4
5.3
Geometric
statistics
Mean
0.68
0.52
0.69
0.79
0.78
0.84
0.85
1.06
4.7
5.1
4.2
4.4
3.8
3.8
4.0
4.2
Std.
Dev.
2.26
2.52
2.34
2.25
2.35
2.27
3.21
2.78
2.03
2.05
2.29
2.30
2.36
2.22
2.72
2.71
maximum annual value. Arithmetic
means and standard deviations, and
geometric means and geometric standard
deviations are presented in the last four
columns in the table. Comparison among
years for a given pollutant provides a
relative indication of national trends.
Some of the variability from year to year,
however, may be attributed to different
sets of sites being used in the summari-
zation, depending upon the completeness
criteria for valid data and changes in state
and local participation in the network
over the years.
The annual arithmetic mean concen-
trations for nitrate and sulfate ions are
shown in Figures 1 and 2, respectively.
Conclusions
It is clear from Figure 1 that network
nitrate concentrations on a nationwide
basis increased substantially during the
1970's. In areas classified as urban, the
average annual rate of increase was 6.2%
while nitrate ion in nonurban areas
increased at an average annual rate of
9.6% during the nine-year period.
Daniel's Test for Trend snowed
significance at the 99% probability level
for the positive trends apparent in both
urban and nonurban nitrate ion
concentrations.
Average network sulfate concentra-
tions, shown in Figure 2, do not exhibit a
consistent trend through the entire
period, but do appear to have increased
fcsince 1976 in both urban and nonurban
'areas.
1
.o
i
o
-------
10.0
8.0
6.0
c
Q)
(J
C
O
o
O)
4.0
2.0
Urban
ft Nonurban
^«w
I
I
I
I
I
I
I
Year 71 72 73 74 75 76 77 78 79
Figure 2. Annual arithmetic average concentrations for NASN sulfate ion.
The EPA authors Gary F. Evans (also the EPA contact, see below), John C. Puzak,
and John E. Frazerare with the Environmental Monitoring Systems Laboratory,
Research Triangle Park, NC 27711.
The complete report, entitled "Air Quality Data for Nonmetallic Inorganic Ions:
Nitrate and Sulfate for 1979 from the National Air Surveillance Networks,"
(Order No. PB 83-172 502; Cost: $10.00, 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 author Gary F. Evans can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and . ^
------- |