Spatial Mapping of VOC and NOx Limitation of Ozone Formation in Six
Areas

**Paper no. 215 Session no. AB-2c**

**Charles L. Blanchard**

Envair, 526 Cornell Avenue, Albany CA 94706

**ABSTRACT**

Ambient aerometric data were used to qualitatively predict where ozone
formation at specific times and locations was limited by the
availability of volatile organic compounds (VOCs) and where it was
limited by oxides of nitrogen (NO~x~). The results are of interest for
corroborating the predictions of photochemical air quality simulation
models (PAQSMs), or for characterizing VOC- and NO~x~ limitation for
many different days and many different ozone seasons, potentially
corroborating the representativeness of the modeled episodes. The Smog
Production (SP) algorithm^1,2,3^ was applied to air quality monitoring
data from six areas: the northeastern and mid-Atlantic states (1994-99),
the southern Lake Michigan area (1991 and 1994-98), Atlanta (1990 and
1994-99), Texas (1993-1999), central California (1991-98), and southern
California (1994-97). Analyses were carried out for each day and each
site within each region. To summarize the results, maps of the results
were prepared using the five highest ozone days at each site during each
year. In addition, tables of summary results were prepared for all sites
and hours with ozone concentrations exceeding either 80 or 120 ppbv.
Comparisons with modeling studies and other types of data analyses were
also carried out, but are reported elsewhere.^3,4^

**INTRODUCTION**

Increased attention has been devoted in recent years to the use of
ambient aerometric data for qualitatively predicting whether ozone
formation at specific times and locations is limited by the availability
of volatile organic compounds (VOCs) or oxides of nitrogen
(NO~x~)^1,5,6,7,8,9^. Procedures for using ambient data are of interest
for corroborating the predictions of photochemical air quality
simulation models (PAQSMs) and for characterizing VOC- and NO~x~
limitation for many different days and many different ozone seasons,
potentially corroborating the representativeness of modeled episodes.

The Smog Production (SP) algorithm ^1,2,7,9^ has previously been applied
to data from the 1987 Southern California Air Quality Study^1^, the 1990
San Joaquin Valley Air Quality Study^10^, the 1993 Coastal Ozone
Assessment for Southeastern Texas (COAST) study and historical data from
the Houston area for the period 1988 through 1992^11^, the 1995
NARSTO-Northeast Study^12^, 1994-98 data from central California^4^, and
four metropolitan areas of Texas (Houston, Beaumont-Port Arthur,
Dallas-Fort Worth, El Paso) for the time period 1994 through
mid-1999^13^. These previous applications are extended here to cover the
time period from 1994 through 1998 or 1999 in each of those study areas,
as well as in Atlanta and the southern Lake Michigan area.

Besides the SP algorithm, other methods have been developed for using
ambient measurements to delineate between VOC and NO~x~ limitation.
These methods are the correlation of ozone with NO~y~ or NO~z~ (defined
as NO~y~ - NO~x~)^5^, indicator ratios^6,14,15,16^, radical budgets^17^,
and the Observation Based Model (OBM)^8,18^. Limited cross-comparisons
of the methods have been made. Previous work using data from six
research sites operated during the 1995 NARSTO-Northeast Study^12^
indicated that the results of the SP algorithm were generally consistent
with those derived from a ratio of ozone, corrected for estimated
background ozone, to the sum of all oxidized nitrogen species \[NO~y~,
where NO~y~ is defined as the sum of NO, NO~2~, and the products of the
oxidation of NO~2~, which include peroxyacetylnitrate (PAN), nitric acid
(HNO~3~), and other compounds\]. The NARSTO tropospheric ozone
assessment^19^ provides further discussion of the use of methods based
upon ambient measurements for corroborating modeling analyses.

**METHODS**

For the present study, applications of the SP algorithm were obtained
from previous studies and extended for six regions of the country. The
regions and time periods covered by the analyses are:

Central California 1991-98

Texas 1993-99 including 1-year intensive study (1993)

Southern California 1994-97

Atlanta 1994-99 and 1-year intensive study (1990)

Northeast and mid-Atlantic 1994-99 including 1-year intensive study
(1995)

Southern Lake Michigan area 1994-98 and 1-year intensive study (1991)

The findings are presented in terms of a parameter known as the extent
of reaction, which varies from 0 to 1^1,2^. Values of extent less than
0.6 indicate that ozone formation is locally limited by the availability
of VOCs. Values of extent exceeding 0.9 indicate that ozone formation is
locally limited by the availability of NO~x~. Intermediate values are
transitional or indeterminate.

Analyses were carried out for each day and each site within each region
using all daytime hours during time periods when the highest ozone
concentrations are typically reached. These periods ranged from June
through September for the northeastern U.S. and Lake Michigan area sites
to March through October for southern California. Statistical summaries
were prepared for hours with ozone exceeding 80 ppbv and for hours with
ozone exceeding 120 ppbv; map displays were prepared using averages over
the five highest ozone days at each site during each year.

Because data-analysis procedures use ambient monitoring data to predict
where and when peak ozone concentrations are limited by the availability
of VOCs or NO~x~, the accuracy of the ambient measurements strongly
affects the accuracy of the predictions. The SP algorithm requires
accurate measurements of ozone, NO, and either NO~x~ or NO~y~. However,
measurements of true NO~x~ (NO + NO~2~) or NO~y~ (the sum of NO~x~ and
NO~x~ reaction products) are not routinely available from compliance
monitors. The standard NO~x~ monitor measures NO~x~ plus unquantified
fractions of NO~x~ reaction products, such as peroxyacetylnitrate (PAN)
and nitric acid (HNO~3~)^20^. Such data are denoted as "NO~2~" and
"NO~x~" hereafter. These routine "NO~x~" data were used to provide upper
and lower bounds on the extent of reaction. The lower bound for extent
was computed by using the measurements of "NO~x~" as if they were true
NO~x~. The upper bound for extent was computed by using the measurements
of "NO~x~" as if they were true NO~y~. An example of the bounding is
shown in Figure 1. The two sets of extent values were plotted against
their means. Although the true extent is unknown, the mean of the bounds
is likely closer to the true value than are the bounds themselves. As
shown, for VOC-limited cases (less than \~0.6), the bounds are tight.
Thus, the predictions of VOC limitation are robust even when the
\"NO~x~" measurements are biased.

Figure 1. Extent of reaction computed using "NO~x~" as NO~x~ or as NO~y~
versus the mean of the two calculations at Houston area sites. The data
are from all peak hours having ozone mixing ratios greater than 80 ppbv
during the period 1994 through 1999. The time periods were March through
October of each year except 1999, for which data were available only
through July.

![](media/image1.wmf){width="6.5in" height="4.802083333333333in"}

The higher-extent values are subject to more uncertainty. When the mean
extent is approximately 1, the bounds are \~0.8 and 1.2. Values
exceeding one are overestimates, because "NO~x~" underestimates true
NO~y~ and uncertainties exist in the deposition rate of NO~y~.
Sensitivity analyses suggest that both bounds may depart from true
extent by roughly 0.1 to 0.2 units when the true extent is in the range
of \~0.8 to 1^1^, so that the mean of the bounds is a reasonable
estimate of the true extent. Here, the criterion for NO~x~ limitation is
assumed to be \~0.9 (mean of the bounds); the bounds themselves are
\~0.75 and \~1.05. This criterion appears reasonably conservative
because comparisons with modeling studies indicate that simulated ozone
concentrations generally respond to NO~x~ reductions when extent is
greater than \~0.8^3^.

Most current data bases, even for the principal field studies, include
measurements of \"NO~x~" rather than true NO~x~ or NO~y~. Therefore, the
results reported here use the mean of the bounds as a best estimate of
extent of reaction.

**RESULTS**

The majority of hours exceeding 80 ppbv were VOC limited at monitors
throughout the San Francisco, Los Angeles, San Diego, El Paso,
Washington DC, and New York metropolitan areas (Table 1). At monitors
located in other areas, approximately 40 to 60 percent of the hours
exceeding 80 ppbv were transitional, while about 10 to 40 percent were
NO~x~ limited. The Atlanta results illustrate the dependence of the
findings on the monitors used: the 1990 data were obtained from six
monitors, three of which were centrally located within the city, and
show a majority of VOC-limited hours. The 1994-99 data included only
three downwind monitors, as other locations did not report \"NO~x~"
measurements, and those downwind sites were either transitional or
NO~x~-limited (Table 1). As with Atlanta, the differences between the
1991 and 1994-98 data from the southern Lake Michigan area reflect
differences in the monitoring locations. In the 1991 study, more
nonurban monitoring sites were operated, and a larger proportion of the
hours were NO~x~ limited, whereas the more routine data from 1994-98 are
weighted toward more urban locations, which have more VOC-limited hours.

Table 1. Frequency of VOC-limited, transitional, and NO~x~-limited hours
for all monitors during hours when ozone concentrations were greater
than 80 ppbv.

+-------------+---------+------+-----------+-------------+-----------+
| Domain      | Years   | T    | Extent of |             |           |
|             |         | otal | Reaction  |             |           |
|             |         | H    | (percent  |             |           |
|             |         | ours | of hours) |             |           |
+=============+=========+======+===========+=============+===========+
|             |         |      | VO        | T           | NO~x      |
|             |         |      | C-limited | ransitional | ~-limited |
|             |         |      |           |             |           |
|             |         |      | Extent \< | 0.6        | Extent   |
|             |         |      | 0.6       | Extent\<0.9 | 0.9       |
+-------------+---------+------+-----------+-------------+-----------+
| **Central   |         |      |           |             |           |
| C           |         |      |           |             |           |
| alifornia** |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| San         | 1991-98 | 4269 | 81.3      | 18.1        | 0.6       |
| Francisco   |         |      |           |             |           |
|             |         |      |           |             |           |
| Bay Area    |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Sacramento  | 1994-97 | 4133 | 38.4      | 43.6        | 18.0      |
| Valley      |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| San Joaquin | 1994-97 | 3    | 32.4      | 60.2        | 7.5       |
| Valley      |         | 6293 |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| **Southern  |         |      |           |             |           |
| C           |         |      |           |             |           |
| alifornia** |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| South       | 1994-97 | 1    | 42.8      | 47.8        | 9.4       |
| Central     |         | 1826 |           |             |           |
| Coast       |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| South Coast | 1994-97 | 6    | 74.4      | 24.6        | 1.0       |
|             |         | 5416 |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Mojave      | 1994-97 | 1    | 32.9      | 56.0        | 11.1      |
|             |         | 5991 |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Salton Sea  | 1994-97 | 2844 | 32.6      | 54.6        | 12.8      |
+-------------+---------+------+-----------+-------------+-----------+
| San Diego   | 1994-97 | 6655 | 80.6      | 18.9        | 0.4       |
+-------------+---------+------+-----------+-------------+-----------+
| **Texas**   |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| # Houston   | 19      | 2979 | 41.9      | 43.5        | 14.6      |
|             | 94-99\* |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Be          | 19      | 509  | 21.6      | 56.4        | 22.0      |
| aumont-Port | 94-99\* |      |           |             |           |
| Arthur      |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Dallas-Fort | 19      | 1777 | 39.6      | 48.0        | 12.4      |
| Worth       | 94-99\* |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| El Paso     | 19      | 337  | 74.2      | 21.1        | 4.7       |
|             | 94-99\* |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| **Lake      |         |      |           |             |           |
| Michigan**  |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
|             | 1991    | 492  | 13.8      | 40.7        | 45.5      |
+-------------+---------+------+-----------+-------------+-----------+
|             | 1994-98 | 2271 | 38.9      | 42.3        | 18.8      |
+-------------+---------+------+-----------+-------------+-----------+
| **Northeast |         |      |           |             |           |
| and         |         |      |           |             |           |
| Mid         |         |      |           |             |           |
| -Atlantic** |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| #           | 1994-99 | 1482 | 50.1      | 40.1        | 9.7       |
| Connecticut |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Washington, | 1994-99 | 1692 | 51.7      | 38.4        | 10.0      |
| DC          |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Delaware    | 1995-99 | 416  | 53.8      | 40.9        | 5.3       |
+-------------+---------+------+-----------+-------------+-----------+
| Maine       | 1995-99 | 368  | 8.7       | 52.2        | 39.1      |
+-------------+---------+------+-----------+-------------+-----------+
| # Maryland  | 1994-99 | 610  | 37.5      | 53.3        | 9.2       |
+-------------+---------+------+-----------+-------------+-----------+
| Ma          | 1995-99 | 1783 | 30.0      | 56.1        | 13.9      |
| ssachusetts |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| New Jersey  | 1995-99 | 1551 | 16.7      | 56.8        | 26.5      |
+-------------+---------+------+-----------+-------------+-----------+
| # New York  | 1995-99 | 1071 | 72.5      | 26.3        | 1.0       |
+-------------+---------+------+-----------+-------------+-----------+
| North       | 1995-99 | 3567 | 39.4      | 56.0        | 4.6       |
| Carolina    |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| P           | 1995-99 | 1381 | 51.4      | 42.9        | 5.6       |
| ennsylvania |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Rhode       | 1995-99 | 162  | 25.3      | 45.1        | 29.6      |
| Island      |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| Virginia    | 1994-99 | 513  | 52.2      | 31.0        | 16.8      |
+-------------+---------+------+-----------+-------------+-----------+
| **Atlanta** |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| 1 upwind, 3 | 1990    | 736  | 54.1      | 39.7        | 6.3       |
| central, 2  |         |      |           |             |           |
| downwind    |         |      |           |             |           |
| sites       |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+
| 3 downwind  | 1994-99 | 3148 | 13.3      | 48.7        | 38.0      |
| site        |         |      |           |             |           |
+-------------+---------+------+-----------+-------------+-----------+

\* 1999 is a partial year

For hours with ozone exceeding 120 ppbv, only the San Francisco, Los
Angeles, and San Diego metropolitan areas showed a majority of
VOC-limited times and locations (Table 2). In other areas, 50 to 90
percent of the monitoring hours over 120 ppbv were transitional and 10
to 90 percent of the hours were NO~x~-limited. Through comparison of the
summary statistics in Tables 1 and 2, it is evident that important
differences occurred among days. In general, higher ozone concentrations
tended to be more associated with NO~x~-limited conditions. Thus, within
each region, the areas in which ozone formation was VOC or NO~x~ limited
varied among days, depending upon the intensity of the ozone episode.
More sites were NO~x~ limited on the highest ozone days than on moderate
ozone days.

Table 2. Frequency of VOC-limited, transitional, and NO~x~-limited hours
for all monitors during hours when ozone concentrations were greater
than 120 ppbv.

+-------------+--------+------+-----------+-------------+-----------+
| Domain      | Years  | T    | Extent of |             |           |
|             |        | otal | Reaction  |             |           |
|             |        | H    | (percent  |             |           |
|             |        | ours | of hours) |             |           |
+=============+========+======+===========+=============+===========+
|             |        |      | VO        | T           | NO~x      |
|             |        |      | C-limited | ransitional | ~-limited |
|             |        |      |           |             |           |
|             |        |      | Extent \< | 0.6        | Extent   |
|             |        |      | 0.6       | Extent\<0.9 | 0.9       |
+-------------+--------+------+-----------+-------------+-----------+
| **Central   |        |      |           |             |           |
| C           |        |      |           |             |           |
| alifornia** |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| San         | 1      | 184  | 48.9      | 45.1        | 6.0       |
| Francisco   | 991-98 |      |           |             |           |
|             |        |      |           |             |           |
| Bay Area    |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Sacramento  | 1      | 96   | 3.1       | 63.5        | 33.3      |
| Valley      | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| San Joaquin | 1      | 1347 | 1.2       | 53.2        | 45.6      |
| Valley      | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| **Southern  |        |      |           |             |           |
| C           |        |      |           |             |           |
| alifornia** |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| South       | 1      | 280  | 8.6       | 78.6        | 12.9      |
| Central     | 994-97 |      |           |             |           |
| Coast       |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| South Coast | 1      | 5878 | 51.6      | 46.7        | 1.7       |
|             | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| # Mojave    | 1      | 705  | 5.1       | 71.6        | 23.3      |
|             | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Salton Sea  | 1      | 104  | 3.8       | 84.6        | 11.5      |
|             | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| San Diego   | 1      | 64   | 14.1      | 78.1        | 7.8       |
|             | 994-97 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| **Texas**   |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| # Houston   | 199    | 364  | 7.7       | 58.0        | 34.3      |
|             | 4-99\* |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Be          | 199    | 16   | 0         | 31.3        | 68.8      |
| aumont-Port | 4-99\* |      |           |             |           |
| Arthur      |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Dallas-Fort | 199    | 55   | 5.5       | 56.4        | 38.2      |
| Worth       | 4-99\* |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| El Paso     | 199    | 10   | 60.0      | 20.0        | 20.0      |
|             | 4-99\* |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| **Lake      |        |      |           |             |           |
| Michigan**  |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
|             | 1991   | 55   | 0         | 40.0        | 60.0      |
+-------------+--------+------+-----------+-------------+-----------+
|             | 1      | 95   | 3.2       | 44.2        | 52.6      |
|             | 994-98 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| **Northeast |        |      |           |             |           |
| and         |        |      |           |             |           |
| Mid         |        |      |           |             |           |
| -Atlantic** |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| #           | 1      | 112  | 12.5      | 64.3        | 23.2      |
| Connecticut | 994-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Washington, | 1      | 38   | 2.6       | 50.0        | 47.4      |
| DC          | 994-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Delaware    | 1      | 32   | 0         | 75.0        | 25.0      |
|             | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Maine       | 1      | 9    | 0         | 11.1        | 88.9      |
|             | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Ma          | 1      | 36   | 2.8       | 77.8        | 19.4      |
| ssachusetts | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Maryland    | 1      | 54   | 1.9       | 64.8        | 33.3      |
|             | 994-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| New Jersey  | 1      | 93   | 0         | 61.3        | 38.7      |
|             | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| New York    | 1      | 38   | 23.7      | 76.3        | 0         |
|             | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| North       | 1      | 41   | 0         | 82.9        | 17.1      |
| Carolina    | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| P           | 1      | 39   | 23.1      | 69.2        | 7.7       |
| ennsylvania | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Rhode       | 1      | 16   | 0         | 12.5        | 87.5      |
| Island      | 995-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| Virginia    | 1      | 0    | 0         | 0           | 0         |
|             | 994-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| **Atlanta** |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| 1 upwind, 3 | 1990   | 39   | 5         | 74          | 21        |
| central, 2  |        |      |           |             |           |
| downwind    |        |      |           |             |           |
| sites       |        |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+
| 3 downwind  | 1      | 243  | 0.4       | 24.3        | 75.3      |
| site        | 994-99 |      |           |             |           |
+-------------+--------+------+-----------+-------------+-----------+

\* 1999 is a partial year

The map display for central California, based upon the top five ozone
days per site and year for each year from 1991 through 1998, indicated
that ozone formation at sites within the San Francisco Bay area was VOC
limited (Figure 2). On these high-ozone days, the transition between VOC
and NO~x~ limitation typically occurred in the southern Sacramento
Valley (northeast of San Francisco Bay) and the northern San Joaquin
Valley (southeast of San Francisco Bay). However, within inland cities
such as Sacramento, Stockton, and Modesto, fresh emissions provide
additional NO~x~, so that locally ozone formation again becomes VOC (or
radical) limited. Similarly, Fresno and Bakersfield, in the central and
southern San Joaquin Valley, respectively, are locally VOC limited but
appear to be surrounded by larger, rural areas where ozone formation is
NO~x~ limited. These analyses of the top five days were supported by the
statistical summaries (Tables 1 and 2), which also indicated that ozone
formation was VOC limited on many days throughout much of the San
Francisco Bay area but was more frequently NO~x~-limited in the
Sacramento and San Joaquin valleys.

Figure 2. Mean afternoon extent of reaction at sites in central
California. The data include the five highest ozone days at each site
during each year from 1991 through 1998. Symbol sizes indicate the mean
peak-hour ozone concentration and symbol types indicate the mean
afternoon extent of reaction. For each day, mean afternoon extent was
computed from the five hours between 10 a.m. and 7 p.m. having the
highest ozone values. Each site had at least two full years of data.

![](media/image2.wmf){width="6.5in" height="6.75in"}

The analyses of data from the top five days per site and year from 1994
through 1997 indicated that ozone formation at sites within the western
and central South Coast air basin (SoCAB) and the San Diego area was VOC
limited (Figure 3). The large number of monitoring sites, and the
consistency of the predictions, support the robustness of the
conclusion. Some sites in the eastern portion of the SoCAB appear to be
transitional between VOC and NO~x~ limitation on high-ozone days. A
transition from VOC to NO~x~ limitation also occurs in moving north from
the basin over the San Gabriel and San Bernardino mountains. Between
Atascadero and Ventura, considerable variability exists among monitoring
locations. Sites that are within the cities along the Highway 101
corridor are locally VOC-limited on high-ozone days. Such sites include
Atascadero, San Luis Obispo, Nipomo, Santa Maria, Gaviota, and Santa
Barbara. Sites located along the coast or inland were NO~x~ limited on
many or most high-ozone days.

Data from the 1993 COAST study and routine monitoring data during the
period 1994 through 1999 indicated that ozone formation on high-ozone
days at sites within central Houston and near the ship channel was VOC
limited (Figure 4). Areas to the north (northern and northwest Harris
County) and southeast (around the mouth of Galveston Bay) were NO~x~
limited. Ozone formation at sites near Beaumont and Port Arthur was
transitional between VOC- and NO~x~-limitation. The transitions between
VOC and NO~x~ limitation occurred over distances of \~15 to 75 km.
However, the density of monitoring was insufficient to precisely
delineate the transition zones.

The numbers of monitoring sites having measurements of both ozone and
"NO~x~" were more limited in Dallas-Fort Worth and El Paso than in
Houston. Ozone formation at the four sites located within the urban
Dallas-Fort Worth area was VOC-limited on most high-ozone days. A site
located about 50 km north of the urban center was NO~x~-limited on most
of the high ozone days. In El Paso, data from all four monitoring
locations strongly indicated that ozone formation was VOC limited on
high-ozone days.

Figure 3. Mean afternoon extent of reaction at sites in southern
California. The data include the five highest ozone days at each site
during each year from 1994 through 1997. Symbol sizes indicate the mean
peak-hour ozone concentration and symbol types indicate the mean
afternoon extent of reaction. For each day, mean afternoon extent was
computed from the five hours between 10 a.m. and 7 p.m. having the
highest ozone values. Each site had at least three full years of data.

![](media/image3.wmf){width="6.489583333333333in"
height="5.114583333333333in"}

Figure 4. Mean afternoon extent of reaction at sites in southeastern
Texas. For each site, the results are based on the five days having the
highest peak hourly ozone values each year from 1993 through 1999.
Symbol sizes indicate the mean peak-hour ozone concentration and symbol
types indicate the mean afternoon extent of reaction. For each day, mean
afternoon extent was computed from the five hours between 10 a.m. and 7
p.m. having the highest ozone values. Each site had at least two years
of data.

![](media/image4.wmf){width="6.5in" height="4.416666666666667in"}

Routinely collected data from 1994-1998 in the Lake Michigan area
indicated that ozone formation was locally VOC-limited within the
Chicago and Milwaukee metropolitan areas (Figure 5). Beyond the
metropolitan areas, sites near Lake Michigan were predominantly NO~x~
limited. More monitoring sites were operated during the 1991 Lake
Michigan Ozone Study (LMOS). The 1991 data followed a pattern similar to
the 1994-98 period, with ozone formation locally VOC-limited within the
Chicago metropolitan area. However, three additional sites in Michigan
and two of the three monitors located on boats in Lake Michigan showed
high (either \>0.8 or \>0.9) extent of reaction, suggesting a transition
between VOC and NO~x~ limitation occurring \~100 to 200 km from Chicago
over Lake Michigan. Thus, the urban, VOC-limited areas appear to be
surrounded by a larger region where ozone formation is predominantly
NO~x~ limited.

The data from northeastern and mid-Atlantic states indicated that urban
sites within the Boston, New York, Baltimore, Washington, and Pittsburgh
metropolitan areas were VOC-limited (Figure 6). Many sites outside the
major urban centers were NO~x~-limited, including sites located 20 to 30
km from downtown Washington or Baltimore. In general, the spatial
coverage of the sites was too limited to precisely delineate the
transition zones between VOC and NO~x~ limitation in any of the
metropolitan regions. However, the data are sufficient to indicate that
ozone formation was locally VOC-limited within the major metropolitan
areas, and that these VOC-limited areas are surrounded by a larger
region where ozone formation is predominantly NO~x~ limited.

During high-ozone days in 1994-99, one of the three downwind sites in
the Atlanta area was NO~x~-limited and two were transitional (Figure 7).
However, the 1990 data indicated that the central-city sites were
VOC-limited on many days. Other studies suggest that urban Atlanta sites
may be locally VOC-limited, but the adoption of anthropogenic emission
controls may be less effective than NO~x~ controls due to the
substantial influence of biogenic VOC emissions^8,18^.

Figure 5. Mean afternoon extent of reaction at sites in the southern
Lake Michigan area. For each site, the results are based on the five
days having the highest peak hourly ozone days each year from 1994
through 1999. Symbol sizes indicate the mean peak-hour ozone
concentration and symbol types indicate the mean afternoon extent of
reaction. For each day, mean afternoon extent was computed from the five
hours between 10 a.m. and 7 p.m. having the highest ozone values.

![](media/image5.wmf){width="6.052083333333333in" height="6.78125in"}

Figure 6. Mean afternoon extent of reaction at sites in the northeastern
and mid-Atlantic states. For each site, the results are based on the
five days having the highest peak hourly ozone days each year from 1994
through 1999. Some sites have data only during the 1995 NARSTO-Northeast
study. Symbol sizes indicate the mean peak-hour ozone concentration and
symbol types indicate the mean afternoon extent of reaction. For each
day, mean afternoon extent was computed from the five hours between 10
a.m. and 7 p.m. having the highest ozone values.

![](media/image6.wmf){width="4.875in" height="6.572916666666667in"}

Figure 7. Mean afternoon extent of reaction at sites in the Atlanta
area. For each site, the results are based on the five days having the
highest peak hourly ozone days each year from 1994 through 1999. Symbol
sizes indicate the mean peak-hour ozone concentration and symbol types
indicate the mean afternoon extent of reaction. For each day, mean
afternoon extent was computed from the five hours between 10 a.m. and 7
p.m. having the highest ozone values.

![](media/image7.wmf){width="6.291666666666667in"
height="5.760416666666667in"}

**CONCLUSION**

Overall, the findings indicate that substantial portions of most major
metropolitan areas were VOC-limited on most or all high-ozone days.
Differences among regions occurred in the geographical ranges over which
ozone formation was VOC-limited.

At most sites, important differences were observed among days of
moderate (\> 80 ppbv peak O~3~) and higher (\> 120 ppbv peak O~3~) ozone
concentrations. Higher ozone concentrations tended to be more associated
with NO~x~-limited conditions. The majority of hours exceeding 80 ppbv
were VOC limited at monitors throughout the San Francisco, Los Angeles,
San Diego, El Paso, Washington DC, and New York metropolitan areas, and
about 40 percent of hours with ozone concentrations over 80 ppbv were
VOC-limited in Houston and Dallas-Fort Worth. Approximately 40 to 60
percent of the hours exceeding 80 ppbv were transitional between VOC and
NO~x~ limitation in other areas. For hours with ozone exceeding 120
ppbv, only the San Francisco, Los Angeles, and San Diego metropolitan
areas showed a majority of VOC-limited times and locations; in other
areas, 50 to 90 percent of the monitoring hours over 120 ppbv were
transitional between VOC and NO~x~ limitation and 10 to 90 percent of
the hours were NO~x~ limited.

The San Francisco, Los Angeles, and San Diego metropolitan areas showed
the greatest geographical ranges of VOC limitation. In these cities,
ozone formation was VOC-limited at all locations within areas of
approximately 100 km on a side. The large number of monitoring sites,
together with the consistency of the results, provides strong evidence
that ozone formation was VOC limited. Ozone formation was NO~x~-limited
at distances exceeding 100 km from the urban cores on some or all days.
Because of their coastal locations, these California cities are unlikely
to experience significant ozone transport from areas where ozone
formation is NO~x~-limited. Thus, the findings support the application
of VOC controls in those cities. However, many sites within California's
Sacramento and San Joaquin valleys were NO~x~-limited on most or all
high-ozone days. In those areas, VOC controls alone may not reduce ozone
concentrations at all sites where exceedances now occur.

The central Houston and Dallas areas were also VOC-limited. However, the
areas in which ozone formation was VOC-limited were smaller than in the
California cites, being about 25 to 50 km on a side. Sites where ozone
formation was NO~x~-limited on some or all days were located \~25 to 50
km from the urban center. The number of monitoring locations was
insufficient to precisely delineate the transition from VOC to NO~x~
limitation. Thus, the findings support the potential utility of VOC
controls within portions of those cities. However, the results also
suggest that VOC controls alone may not reduce ozone concentrations at
all sites where exceedances now occur.

Urban sites within the Chicago, Boston, New York, Baltimore, and
Washington metropolitan areas were VOC limited. Sufficient monitors were
operating within the Chicago area to indicate that the area where ozone
formation was VOC limited extended along the Illinois and Indiana
shorelines and inland from \~10 to 40 km. In the other cities, the
number of monitoring locations was insufficient to delineate the
transition from VOC to NO~x~ limitation, though this study and
another^9^ both found that monitors located 20 to 40 km downwind of
downtown Baltimore and Washington were NO~x~ limited. Each of the
VOC-limited metropolitan areas is surrounded by larger regions where
ozone formation was predominantly NO~x~ limited. Thus, the findings
support the potential utility of VOC controls within portions of those
cities, while also suggesting that VOC controls alone may not reduce
ozone concentrations at all sites where exceedances now occur.

**ACKNOWLEDGMENTS**

Support for this project was provided by the American Chemistry Council.
I thank S. Tanenbaum for carrying out calculations and preparing
graphics and D. Baker for his review and oversight. Data were provided
by the California Air Resources Board, the U.S. EPA, the Houston
Regional Monitoring program, the NARSTO-Northeast Study, and the Texas
Natural Resource Conservation Commision.

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**KEY WORDS**

ozone, NO~x~ limitation, VOC limitation, control strategy assessment,
Smog Production algorithm