United States
Environmental Protection
Agency
Environmental Monitoring Systems ••
Laboratory * r t \ \
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-81-074 Sept. 1981
Project Summary
Air Quality Data for
IMonmetallic Inorganic Ions:
Nitrate and Sulfate for
1977 and 1978 from the
National Air Surveillance
Networks
Gary F. Evans, John C. Puzak, John E. Frazer, and Allan Riley
The National Air Surveillance Net-
works provide information on air
quality for many urban and nonurban
Jocations 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 1977 and 1978. 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.
This Project Summary was devel-
oped by EPA's Environmental Moni-
toring 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's air.
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 concen-
trations remains the responsibility of
EPA's Environmental Monitoring Sys-
tems 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. Non-
urban-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 urban-
ization.
This report summarizes the 1977 and
1978 network data for nitrate and
sulfate ions.
Procedure
Laboratory Methods
Preweighed glass-fiber filters were
distributed to the cooperating local
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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 1977 and 1978
were performed by Northrop Services,
Incorporated - Environmental Sciences,
under Contract No. 68-02-2566.
An 8.3 percent section of each
particulate 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 dihydrochlor-
ide to form a reddish-purple azo dye
which was determined colorimetri-
cally at a wavelength of 520 to 540
nm.
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
difference 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,
monitored colorimetrically at wave-
lengths of 460 to 480 nm, was the
measure of the amount of sulfate in
the sample.
Quality Assurance Methods
Independent estimates of laboratory
precision and bias were determined for
these data to aid in interpreting analyt-
ical results. However, several potential
sources of measurement error associ-
ated with sample collection and handling
procedures (flow control, artifact forma-
tion, 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 deter-
mined 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 ana lysis. 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.
Estimates of bias and precision for the
1977 and 1978 analytical results are
given by concentration range in Table 1.
Results and Conclusions
Data are presented in Tables 2 and 3
in the form of annual cumulative
frequency distributions by ion 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 pg/m3.
The next entry is the maximum annual
value. Arithmetic means and standard
deviations, and geometric means and
geometric standard deviations are
presented in the lastfour columns in the
table. Comparison among years for a
given pollutant provides a relative
Table 1. Analytical Precision and Bias
indication of national trends. Nitrate ion
concentrations, for example, have
apparently increased over the 8-year
period in both urban and nonurban
areas, while sulfate ion concentrations
have remained relatively consistent.
Since typical levels of these pollutants
may be geographically dependent, the
national frequency distribution is not
helpful for judging the localized contri-
bution and relative severity of these
pollutants for an individual site. There-
fore, local site analyses should be
performed with data from surrounding
sites within the same general geo-
graphical area. Data for each site are
presented in the project report. Also,
some of the variability from year to year
in the national summary tables may be
attributed to different sets of sites being
used in the summarization.
A. 1977
B. 1978
Ion
Nitrate
Sulfate
Nitrate
Sulfate
Concentration
Range (ug/m3)
<3
3-6
>6
<6
6-20
>20
<3
3-6
>6
<6
6-20
>20
Bias (%l
+0.7
-5.6
-2.8
-6.8
-5.3
-5.9
+6.8
-4.5
-1.2
-7.3
-3.7
-5.5
Precision (%)
12
8
8
9
5
3
21
11
24
11
9
4
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Table 2. Urban National Cumulative Frequency Distributions
Number
Percent of time concentration (fjg/m3)
is equal to or less than
Arithmetic
statistics
Geometric
statistics
of
Ion Year Samples Min.
NO1 1971 3900 LD
1972 5519 LD
1973 4775 LD
1974 4562 LD
1975 4113 0.20
1976 3817 0.08
1977 4531 0.07
1978. 3610 LD
SOI 1971 3916 LD
1972 5519 LD
1973 4774 LD
1974 4564 LD
1975 4110 0.2
1976 3871 0.2
1977 4531 0.7
1978 3610 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
30
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
Table 3. Nonurban National Cumulative
50
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
70
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
90
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
95
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
99
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
Mean
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.O
8.8
9.1
9.4
Std.
Dev. Mean
2.30 2.13
2.61 2.32
2.87 2.44
3.40 2.36
3.31 2.79
3.83 2.58
3.97 2.87
4.10 3.07
6.8 7.84
7.7 9.15
7.0 8.06
6.7 8.34
7.0 8.22
5.8 7.33
6. 1 7.44
7.5 7.55
Std.
Dev.
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
Frequency Distributions
Percent of time concentration
Number
of
Ion Year Samples Min.
NOl 1971 671 LD
1972 928 LD
1973 831 LD
1974 706 LD
1975 630 0.20
1976 467 LD
1977 681 LD
1978 458 LD
SOI 1971 686 0.4
1972 929 LD
1973 831 LD
1974 706 0. 1
1975 630 0.2
1976 493 0.2
1977 681 LD
1978 458 LD
is equal to or
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
0.71
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
ffjg/m3)
Arithmetic Geometric
less than
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
statistics statistics
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. Mean
0.92 0.68
0.92 0.52
1.02 0.69
1.07 0.79
1.16 0.78
1.16 0.84
1.26 0.85
1.72 1.06
4.9 4.7
5.4 5. 1
5.9 4.2
6.2 4.4
5.7 3.8
4.9 3.8
5.4 4.0
5.3 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
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The EPA authors Gary F. Evans (also the EPA Project Officer, see below). John
C. Puzak, John E. Frazer, and Allan Riley, are 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 1977 and 1978 from the National Air Surveillance
Networks," (Order No. PB 81 -248 148; Cost: $ 12.50, 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 Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
US GOVERNMENT PRINTING OFFICE; 1981 —559-017/7432
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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