United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-85/038 Aug. 1985
&ERA Project Summary
The National Air Pollution
Background Network Final
Project Report
Gary F. Evans
The U.S. Environmental Protection
Agency, in cooperation with the U.S.
Forest Service, operated a network of
ozone monitoring stations from 1976
through 1983 in selected National
Forests within the continental U.S. The
primary objective of this project was to
determine the level of ozone concentra-
tions occurring in remote areas, espe-
cially in relation to the National Ambi-
ent Air Quality Standard for ozone.
Secondary objectives included the eval-
uation of regional differences, the char-
acterization of seasonal and diurnal
patterns, and the assessment of long-
term trends for ozone concentrations in
remote areas. Annual mean ozone con-
centrations were found to vary from
site-to-site and year-to-year within a
range of 25 to 50 parts per billion (ppb).
Hourly ozone concentrations in excess
of 120 ppb, the current level of the Na-
tional Ambient Air Quality Standard,
were occasionally observed. Such
events, however, were rare and gener-
ally confined to the spring and summer
months at sites in the eastern half of
the U.S. and during the first half of the
study period. No such events were ob-
served after 1980. Seasonal mean
ozone concentrations were greatest
during the spring months (April
through June), and diurnal maximums
occurred most frequently during the
early afternoon (1-3 p.m.). While no sta-
tistically significant trends were ob-
served in mean ozone concentrations,
the frequency of exceedances of the Na-
tional Ambient Air Quality Standard de-
creased over the course of the study.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research Tri-
angle 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 or-
dering information at back).
Introduction
In 1971, the National Ambient Air
Quality Standard (NAAQS) for ozone
was established at an hourly average
concentration not to exceed 80 parts per
billion (ppb) more than once per year.
At that time, very few ozone measure-
ments were available from rural and re-
mote areas, and ozone concentrations
in such areas were assumed to be gen-
erally low and inconsequential relative
to the standard. Furthermore, ozone
from background areas was thought to
be largely removed by chemical scav-
engers upon entry into an urban envi-
ronment.
In the mid-1970's, the U.S. Environ-
mental Protection Agency (EPA) spon-
sored several field studies monitoring
ozone concentrations in rural areas.
This monitoring was conducted during
summer months in small cities and agri-
cultural areas in the eastern half of the
U.S. The results suggested that rural
areas experienced a greater range in
ozone concentration than had been pre-
viously supposed and that ozone trans-
ported from rural to urban areas should
not always be disregarded.
In 1976, in response to these and
other findings, EPA began to establish a
nationwide network of ozone monitor-
ing stations located in remote areas.
This project, originally called the Na-
tional Forest Ozone Study, was a joint
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undertaking of EPA's Environmental
Monitoring Systems Laboratory
(EMSL), and Office of Air Quality Plan-
ning and Standards (OAQPS), both lo-
cated in Research Triangle Park (RTP),
North Carolina. The Forest Service of
the U.S. Department of Agriculture,
working under an interagency agree-
ment, participated in the project by pro-
viding monitoring site locations within
National Forest (NF) areas and by per-
forming routine operations at the moni-
toring stations.
Eventually, the National Air Pollution
Background Network (NAPBN) con-
sisted of eight remote monitoring
stations, each collecting continuous
measurements of ozone by the chemilu-
minescence technique. Each site was lo-
cated as far as was practical from any
heavily used roadway and at least 100
miles from any major urban area. The
sites were located in open and relatively
elevated areas for good exposure to en-
sure representative sampling. An effort
was made to distribute the sites across
the continental U.S. to cover as many
regions of the country as possible. Site
locations are shown in Figure 1, and sta-
tion descriptions are provided in
Table 1.
The NAPBN was established to pro-
vide a reasonably long-term and contin-
uous record of ozone concentrations
and patterns in areas well removed
from anthropogenic sources of air pol-
lution and to make these data available
to EPA and other interested re-
searchers. The network was discontin-
ued at the end of 1983. All valid data are
on file and may be accessed through the
National Aerometric Data Bank (NADB),
U.S. EPA, Mail Drop 14, Research Trian-
gle Park, North Carolina 27711.
Procedure
The National Air Pollution Back-
ground Network (NAPBN) became fully
operational in late 1979 with the estab-
lishment of the eighth and final air mon-
itoring station which was located within
the Ochoco National Forest in Oregon.
At each site, U.S. Forest Service person-
nel visited the monitoring station once
per week to perform routine operation
and maintenance procedures and to cut
and label strip charts. These charts were
mailed to EMSL/RTP where they were
reduced to hourly average values which
were keypunched and entered into
EPA's mainframe computer. After vali-
dation, including statistical procedures
to test for outliers, the data were en-
Figure 1. Location of NAPBN Monitoring Sites.
Table 1. Description of NAPBN Monitoring Sites
National
Forest
Apache
Kisatchie
Mark Twain
Custer
Croatan
Ochoco
Green
Mountain
Chequamegon
State
AZ
LA
MO
MT
NC
OR
VT
Wl
Elevation
Above MSL
2500 M
65 M
350 M
1250 M
13 M
1350 M
390 M
440 M
Latitude/
Longitude
33°45'00"N/
109°00'00"W
31°30'00"N/
92°28'20"W
37°28'00"N/
90° 1 1 '00 "W
45°14'00"N/
106°15'00"W
34°59'05"N/
77°11'24"W
44°13'30"N/
1 19°42'25"W
43°56'00"N/
73°02'00"W
45°12'00"N/
90°37'00"W
Start
Date
9/76/79
5/26/76
12/18/78
6/23/76
3/13/78
70/04/79
10/24/76
8/10/78
End
Date
12/31/83
9/30/82
12/31/83
12/22/83
12/31/83
12/05/83
9/28/82
9/30/82
SAROAD
CODE
0300501 10A08
191490101A08
262950001 A08
2703W101A08
340945101A08
3804201 11A08
470265101A08
510490001A08
tered into EPA's SAROAD data storage
system. Site visits were made quarterly
by either EPA or contractor personnel to
audit and calibrate each ozone analyzer.
Calibration was performed using a certi-
fied UV photometer.
Results and Discussion
Annual summary statistics for the
ozone data collected at each NAPBN site
are presented in Table 2. Statistics tabu-
lated include the annual percent data
4
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Table 2. Annual Summary Statistics for NAPBN Ozone (ppb)
Year % Data Mean StdDev 50-%//e 95-%ile Max %>80 %>120
Arizona 1979 27.7 49.3 9.8
1980 95.6 47.4 13.0
Apache NF 1981 94.7 35.3 7.4
1982 91.1 41.2 9.2
1983 89.6 37.9 9.0
Louisiana 1976 39.4 31.5 21.4
1977 74.3 33.7 23.5
Kisatchie NF 1978 47.7 37.9 21.0
1979 79.8 26.8 14.7
1980 50.7 27.7 16.1
1981 30.7 30.1 16.7
1982 41.7 28.3 16.8
Missouri 1978 6.1 25.7 9.8
1979 95.6 39.3 18.2
Mark Twain 1980 53.9 45.4 20.8
NF 1981 89.6 31.7 14.3
1982 96.9 37.5 16.3
1983 92.7 38.5 18.3
Montana 1977 86.8 40.2 11.1
1978 51.8 41.6 8.9
Ouster NF 1979 71.6 36.2 9.9
1980 88.5 36.8 11.9
1981 72.6 30.1 9.0
1982 64.7 30.7 8.4
1983 90.9 35.2 9.0
North 1978 49.1 33.2 18.8
Carolina 1979 94.3 27.8 16.8
Croatan NF 1980 87.6 28.5 18.9
1981 84.2 27.4 15.4
1982 81.0 25.2 15.6
1983 89.6 25.2 16.1
Oregon 1979 23.9 29.2 6.7
1980 88.5 38.5 9.3
Ochoco NF 1981 88.7 31.2 7.6
1982 89.0 34.1 8.0
1983 83.2 34.4 7.5
Vermont 1976 12.1 29.3 11.5
1977 74.0 37.6 21.5
Green Mt. NF 1978 41.9 29.0 17.9
1979 73.3 31.6 16.6
1980 97.9 32.3 17.5
1981 83.7 28.5 14.4
1982 59.5 28.5 16.4
Wisconsin 1978 27.2 32.7 13.0
1979 87.7 35.2 14.8
Chequamegon 1980 72.2 38.8 19.3
NF 1981 92.6 33.1 12.3
1982 69.2 35.7 11.7
(the number of valid hourly ozone
values divided by the number of possi-
ble hours expressed as a percentage),
mean, standard deviation, 50th per-
centile (or median), 95th percentile.
maximum hourly value, and the per-
centage of valid hours with ozone con-
centrations greater than both 80 and
120 parts per billion (ppb).
50 65 80 0 0
45 70 90 0.3 0
35 50 65 0 0
40 55 75 0 0
35 55 70 0 0
30 70 125 2.6 0.03
30 80 135 4.4 0.08
35 75 125 2.8 0.07
25 55 700 0.1 0
25 60 105 0.3 0
30 60 95 0.3 0
25 60 90 0.2 0
25 40 50 0 0
35 75 125 2.4 0.01
45 80 155 4.5 0.08
30 55 115 0.4 0
35 65 95 0.5 0
35 70 110 1.8 0
40 60 80 0 0
40 55 75 0 0
35 50 70 0 0
35 55 70 0 0
30 45 70 0 0
30 45 55 0 0
35 50 65 0 0
30 65 105 0.3 0
25 60 85 0.1 0
25 65 150 0.9 0.07
25 55 90 0.1 0
25 55 95 0.2 0
25 55 85 0.1 0
30 40 50 0 0
40 55 80 0 0
30 45 75 0 0
35 50 65 0 0
35 50 60 0 0
30 45 60 0 0
35 75 145 4.8 0.23
25 65 105 1.4 0
30 65 105 1.0 0
30 65 115 1.5 0
30 55 105 0.2 0
30 55 100 0.5 0
30 60 100 0.1 0
35 60 110 0.7 0
35 75 115 2.7 0
30 55 80 0 0
35 55 90 0.1 0
EPA established in 1971 the first Na-
tional Ambient Air Quality Standard for
photochemical oxidants (primarily
ozone) at a 1-hour average of 80 ppb
which was not to be exceeded more
than once in any given year. In 1979,
EPA promulgated a revised standard
which stated that the expected number
of days per calendar year with daily
maximum ozone concentrations ex-
ceeding 120 ppb must be less than or
equal to one. This new standard differs
from the original in several important
ways, including the specific designation
of ozone, the emphasis on the daily
maximum concentration, and the statis-
tical interpretation of "expected ex-
ceedances." The most obvious differ-
ence, however, is the change in the level
of the standard from 80 to 120 ppb.
As may be seen from Table 2, ex-
ceedances of the 80 ppb level did occur
during most years at the five NAPBN
sites located in the eastern half of the
U.S., but in all cases the frequency of
such exceedances was less than 5 per-
cent of valid hours. At four of these sites
(Kisatchie, Mark Twain, Croatan, and
Green Mountain), exceedances of the
120 ppb level were observed. These in-
stances, however, were quite rare
(<0.25% of valid hours) and were con-
fined to the first half of the study period
(1976 through 1980). Technically, then,
the NAPBN sites have been in compli-
ance with the current ozone standard
since 1980.
Ozone data from each site were strat-
ified by quarter and averaged by hour-
of-day to evaluate seasonal and diurnal
cyclical behavior (examples are shown
for the sites at Custer NF, MT and
Croatan NF, NC in Figures 2 and 3, re-
spectively). Significant seasonality is
apparent in the data with the second
quarter (April through June) exhibiting
the maximum mean ozone concentra-
tion. Maximum hourly concentrations
generally occur in the early afternoon
(1-3 p.m.). Both seasonality and diurnal-
ity are more pronounced at the sites lo-
cated in the eastern half of the U.S.
Statistical trend analysis was applied
to the mean ozone values at each site,
and in no case was there evidence of
either an increasing or decreasing sys-
tematic pattern. However, it is clear
from Table 2 that, with the possible ex-
ception of the Mark Twain site, the fre-
quency of elevated ozone episodes
(hourly ozone concentrations >80 ppb)
decreased over the duration of the
study period. It should be noted that al-
though many urban sites were affected
by an ozone calibration change in 1979,
such was not the case for these data
where the UV calibration technique was
employed throughout the study.
Conclusions
• Although exceedances of the original
level (80 ppb) and the revised level
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80-
70-
60-
50-
40-
30-
20-
70-
0-
5 70 72 74 76 78 20 22 24
Hour Local Time
Legend: Quarter
Overall Average(PPB)
D-Q -O 2
47.7
34.9 47.7 38.8
Figure 2. Hourly mean ozone concentration by quarter—Ouster NF, MT.
tr-tr-tk 4
28.0
(120 ppb) of the NAAQS for ozone
were occasionally observed at the
NAPBN remote monitoring stations,
the latter were rare occurrences and
observed only during spring and early
summer months and at sites in the
eastern half of the U.S.
• The frequency of exceedances of the
80 ppb level was less than 5 percent of
valid hours for all site years, and no
exceedances of the 120 ppb level were
observed after 1980.
• Annual mean ozone concentrations
fell within a range of 25 to 50 ppb with
sites in the western U.S. recording
higher mean levels, but lower vari-
ances that those in the east.
• Diurnal maximum hourly ozone con-
centrations occurred most frequently
in the early afternoon (1 to 3 p.m.),
and the maximum quarterly mean oc-
curred in the spring (April through
June).
• Although statistical tests for trend re-
vealed no significant increases or de-
creases in mean ozone concentra-
tions, there were decreases in the
frequency of exceedances of the
NAAQS level(s) during the course of
the study.
80-
70-
60-
50-
40-
'•
30-
20-
70-
0-
a-..
0 2 4 6
Legend: Quarter
Overall Average (PPB)
10 12 74 76 78 20 22 24
Hour Local Time
1
&.-Q..Q
36.2
27.3 36.2 26.8
Figure 3. Hourly mean ozone concentration by Quarter—Croatan NF, NC.
A--A--A 4
79.9
S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20643
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The EPA author, Gary F. Evans (also the EPA Project Officer, see below), is with
the Environmental Monitoring Systems Laboratory, Research Triangle Park, NC
27711.
The complete report entitled "The National Air Pollution Background Network
Final Project Report," (Order No. PB 85-212 413/AS; Cost: $8.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The author can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-85/038
OOOD329 PS
U S CNVIR PROTECTION AGINCt
CHICAGO
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