Weekday/Weekend Differences in Ambient Air Pollutant

Concentrations in Atlanta and the Southeastern United States

Charles L. Blanchard and Shelley Tanenbaum

Envair, Albany, CA

ABSTRACT

The authors quantified changes between mean weekday

and weekend ambient concentrations of ozone (O3) precursors (volatile organic compounds [VOC], carbon monoxide [CO], nitric oxide, and oxides of nitrogen [NOx]) in

Atlanta and surrounding areas to observe how weekend

precursor emission levels influenced ambient O3 levels.

The authors analyzed CO, nitric oxide (NO), and NOx

measurements from 1998 to 2002 and speciated VOC

from 1996 to 2003. They observed a strong weekend effect

in the Atlanta region, with median daytime (6:00 a.m. to

3:00 p.m. Eastern Standard Time) decreases of 62%, 57%,

and 31%, respectively, in the ambient levels of NO, NOx,

and CO from Wednesdays to Sundays, during the ozone

season (March to October). They also observed significant

decreases in ambient VOC levels between Wednesdays

and Sundays, with decreases of 28% for the sum of aromatic compounds and 19% for the sum of Photochemical

Assessment Monitoring Stations target compounds. Despite large reductions in O3 precursor levels on weekends,

day-of-week differences in O3 mixing ratios in and near

Atlanta were much smaller. Averaging overall O3-season

days, the 1-hr and 8-hr mean peak daily O3 maxima on

Sundays were 4.5% and 2.3% lower, respectively, than

their mean levels on Wednesdays (median of 14 site differences), with no sites showing statistically significant

Wednesday-to-Sunday differences. When restricted to

high-O3 days (highest 3 peak O3 days per day of week per

site per year), the 1-hr and 8-hr Sunday O3 mixing ratios

were 11% and 10% lower, respectively, than their mean

peak levels on Wednesdays (median of 14 site differences), with 6 of 14 sites showing statistically significant

Wednesday-to-Sunday differences. The analyses of weekday/weekend differences in O3 precursor concentrations

show that different emission reductions than normally

take place each weekend will be required to achieve major

reductions in ambient ozone levels in the Atlanta area.

INTRODUCTION

The occurrence of generally comparable, or even higher,

ambient concentrations of ozone (O3) on Saturdays and

Sundays than on other days of the week is commonly

known as the “weekend O3 effect.”1 Because emissions of

O3 precursors, including volatile organic compounds

(VOCs), oxides of nitrogen (NOx), and carbon monoxide

(CO), are lower on weekends than on weekdays, the weekend O3 effect is counterintuitive. A weekend effect has

been noted for many years and in a variety of locations.2– 6

It has been proposed that areas where O3 formation is

VOC limited tend to exhibit higher peak O3 on weekends,

because of a more pronounced weekend reduction of NOx

than VOC emissions.7 Other explanations for higher

weekend peak O3 values include possible higher precursor

emissions on Friday evenings with carryover to Saturday,

increased emissions of VOC in some areas on weekends

(including possibly higher ratios of VOC/NOx), changes

in the times of occurrence of emissions on weekends, or

increased reactivity of VOC emissions on weekends. The

weekend effect in Southern California has been studied at

length,8 –14 although differences of interpretation remain.

Fujita et al.10 and Lawson1 conclude that weekend reductions of NOx concentrations allow O3 to accumulate earlier in the day and to reach higher concentrations compared with weekdays and that proposed alternative

hypotheses are not supported by ambient data and do not

explain the weekend effect in Southern California. In

contrast, Croes et al.15 considered the available air quality

data and photochemical models inadequate to conclusively determine the causes of the weekend O3 effect in

Southern California.

Different day-of-week patterns of O3 concentrations

occur in Chicago, Philadelphia, and Atlanta.16 In Atlanta,

1995–1999 mean, median, and 90th percentile daily 1-hr

O3 maxima generally increased from Mondays or Tuesdays to higher levels on Fridays or Saturdays.16 Other

differences between Atlanta and Los Angeles that are of

potential significance to the weekend effect in each area

are the levels of NOx emissions from electric generating

units throughout the Southeastern United States and

Atlanta’s location relative to other major cities.

This paper provides additional analysis of the O3

weekend effect in Atlanta and its surroundings. The authors quantify the changes between weekday and weekend ambient concentrations of O3 and O3 precursors,

IMPLICATIONS

The analyses of weekday/weekend differences in O3 and

O3 precursor levels show that different emission reductions

of O3 precursors than normally take place each weekend

will be required before major reductions in ambient O3 can

be achieved in the Atlanta area. Although regional-scale

changes in O3 levels might result from future control programs, it is sobering that Atlanta-area sites exhibited only

minor weekend O3 reductions in spite of the large and

statistically significant weekend reductions of O3 precursor

concentrations.

TECHNICAL PAPER ISSN 1047-3289 J. Air & Waste Manage. Assoc. 56:271–284

Copyright 2006 Air & Waste Management Association

Volume 56 March 2006 Journal of the Air & Waste Management Association 271
describe the effects of weekend precursor changes on ambient O3 and particulate nitrate levels in Atlanta and

surrounding areas, and evaluate possible explanations for

the observed differences in the weekend behavior of O3

and O3 precursors.

For most states, photochemical grid modeling is the

primary planning tool for managing O3 levels. In some

cases, modeling will be supplemented by a “weight-ofevidence” demonstration, which might include analyses

of trends or application of other analyses of observations

to assess the effectiveness of possible emission control

programs. The day-of-week variation in ambient concentrations is one of type of analysis that could be performed,

along with others, as part of a complete weight-of-evidence demonstration.

EXPERIMENTAL WORK

Data Sources

Data were obtained for the years 1998 –2002 to provide a

multiyear record with focus on near-current conditions.

U.S. Environmental Protection Agency (EPA) Aerometric

Information Retrieval System (AIRS) (now Air Quality System) data were obtained for sites in Atlanta and in Fulton

County and for sites in the neighboring counties of Douglas, Cobb, Forsyth, DeKalb, Clayton, Fayette, Coweta,

Henry, Rockdale, Gwinnett, and Dawson (Figure 1). These

data included measurements of O3, nitric oxide (NO),

NOx, and CO. O3 data were also acquired for the nearest

Clean Air Status and Trends Network sites (Georgia Station, GA, Sand Mountain, AL, and Coweeta, NC). A variety of gas-phase and particle measurements were obtained

from four sites in the Southeastern Aerosol Research and

Characterization (SEARCH) network17: Jefferson Street

(Atlanta), Yorkville (GA), Centreville (AL), and Birmingham (AL). Particulate matter (PM) measurements were

obtained from EPA Speciated Trends Network for all of the

sites in Northern Georgia and Alabama. VOC data were

available from Photochemical Assessment Monitoring

Station (PAMS) measurements at four sites (Yorkville,

South DeKalb, Tucker, and Conyers) for the years 1996 –

2003. For both gas-phase and condensed-phase species,

the extension of the database to sites in Northern Alabama and Northern Georgia was intended to provide information on day-of-week concentration variations in areas potentially upwind of Atlanta on many days.

Statistical Comparisons

Day-of-week averages were determined by hour of the day

for hourly data (including CO, NO, and NOx) and for

24-hr measurements (including particulate nitrate and

sulfate). For O3, day-of-week averages were determined for

both peak 1-hr and peak 8-hr concentrations.

For the hourly data, statistical comparisons were

made for the hours of 6:00 a.m. Eastern Standard Time

(EST; indicative of fresh emissions, typically having maximum morning concentrations) and 12:00 p.m. EST (midday, higher photochemical activity), for 3-hr intervals

(6:00 –9:00 a.m., 9:00 a.m.–12:00 p.m., and 12:00 p.m.–

3:00 p.m. EST) and for daytime (6:00 a.m.–3:00 p.m. EST)

averages.

The O3 comparisons were made using all of the O3-

season days and for high-O3 days, the latter facilitating a

focus on conditions of greatest regulatory interest. As

defined by EPA, the O3 season in Georgia extends from

March to October.18 Twenty-one high-O3 days were selected from each year by determining the top three O3

days for each day of the week and each year. This procedure represents each day of the week and each year

equally and provides a set of 9% of the 245 days from

March to October. For each monitor, separate peak 1-hr

and peak 8-hr high-O3 data subsets were determined.

NOx, VOC, nitric acid (HNO3), and CO measurements were also restricted to the months of March to

October, although in the case of VOCs, measurements

were available only for the months of June to September.

Differences in weekday and weekend concentrations of

these compounds were computed using all of the days

within the selected months to provide as large a dataset as

possible for quantifying the changes in precursor levels

on weekends. For NO, NOx, and reactive odd nitrogen

(NOy), a second set of day-of-week averages was also computed for the high 8-hr O3 data subsets to determine

whether day-of-week precursor differences occurred on

high-O3 days, as well as on average days. PM measurements were analyzed for all days of the year, because

high-PM days often occur during winter months, and

because PM samples are collected less often (typically,

once every third or sixth day).

All of the statistical tests refer to t tests of the differences between means. The data points constituting each

mean were separated in time (e.g., Sunday noon compared with Wednesday noon) so that statistical assumptions of independence were not violated. Statistical methods exist for testing the significance of multiple

comparisons so that the joint significance probability of

all tests is controlled at a specified level (e.g., P 

0.05).19 –21 One such approach to simultaneous statistical

inference assigns each of “m” comparisons an error rate of

/m, which gives an experiment-wise error rate of .20,21

Depending on the compound, the authors tested differences at 4 –14 sites. For consistency, they used a fixed

comparison-wise significance P value (P 0.01) to evaluate statistical significance of the differences in site means.

Figure 1. Locations of monitoring sites.

Blanchard and Tanenbaum

272 Journal of the Air & Waste Management Association Volume 56 March 2006
Using P 0.01, 1 statistical test in 100 is expected to

yield an apparently significant result when no statistical

difference actually exists.

To facilitate comparisons, for each monitoring site

and each specified pollutant and time of day, the differences between mean Wednesday and mean Sunday concentrations were expressed as the percentage decrease

from the Wednesday mean: difference  100% 

(Wednesday mean  Sunday mean)/(Wednesday mean).

Statistical Characterization of

Weekday/Weekend Differences in Pollutant

Levels

Primary Pollutants. As determined from all of the O3-season days, the median daytime (6:00 a.m.–3:00 p.m.) declines of NO, NOx, and CO from Wednesday to Sunday

were 62%, 57%, and 31%, respectively (Figure 2). The

median decreases at 6:00 a.m. and 12:00 p.m. were similar

to the decreases for the 3-hr averaging times of 6:00 a.m.–

9:00 a.m. and 12:00 p.m.–3:00 p.m., respectively, and

bracketed the median daytime (6:00 a.m.–3:00 p.m.) average decreases. Declines exceeding 30 – 40% were statistically significant, and all of the urban monitors exhibited statistically significant weekend decreases of NO,

NOx, and CO during one or more time periods. Declining

ambient concentrations of NO and NOx also occurred on

high-O3 days.

Weekend declines of ambient NO, NOx, and CO concentrations could result from decreased emissions from

gasoline vehicles, on-road and off-road diesel vehicles,

stationary sources, or a combination of source types. The

emission contributions from different source types in the

Atlanta area constrain the possible emission reductions

from the major source categories. According to the EPA

2001 national emission inventory, CO emissions derive

predominantly from gasoline vehicles (79%), whereas

gasoline and diesel motor vehicles contributed 18% and

25% of U.S. NOx emissions, respectively, or 43% combined (including both on-road and nonroad vehicles).22

Within Fulton County (metropolitan Atlanta), on-road

and nonroad motor vehicles accounted for 79% of NOx

emissions (split as 38% diesel, 35% gasoline, 6% aircraft,

railroads, and other) and 93% of CO emissions in 1999

(90% from gasoline vehicles).23

Stationary sources in Fulton County accounted for

the remaining 21% of countywide NOx emissions, so the

60% weekend reduction of ambient NOx levels could

not have derived solely from changes in stationary-source

emissions. However, SO2 measurements made at the

SEARCH site in Atlanta showed lower mean levels during

morning and early afternoon hours on Sundays compared

with Wednesdays (Figure 3), consistent with the occurrence of weekend decreases in operations of some commercial or industrial facilities.

The 31% median reduction in ambient CO mixing

ratios on Sundays could not have occurred unless gasoline

vehicle CO emissions (90% of total CO emissions) were

lower on Sundays than on Wednesdays, which also implies that NOx emissions from gasoline vehicles were

lower on Sundays than on Wednesdays. However, gasoline vehicles were not the only contributor to weekend

NOx reductions, because the ambient NOx reductions

(60%) on Sundays exceeded the gasoline-vehicle portion (35%) of NOx emissions in Fulton County.

Because mobile sources accounted for 79% of NOx

emissions in Fulton County, changes in the ambient NOx

levels are expected to correlate with changes in CO and

black carbon (BC), both of which derive primarily from

mobile sources.11 As noted, 90% of the Fulton County CO

emissions are attributable to gasoline vehicles. In contrast, BC is found in both diesel and gasoline exhaust,

although the BC emission rates are greater for diesel than

for gasoline engines: depending on temperature, age, and

engine condition, diesel vehicles may emit BC at 3 to

30 times the rate (grams per mile basis) as gasoline

vehicles.24 In the Denver area, mean BC emission rates

Figure 2. Median percent decreases in NO, NOx, and CO from

Wednesdays to Sundays for three 3-hr periods (6:00 a.m.–9:00 a.m.,

9:00 a.m.–12:00 p.m. and 12:00 p.m.–3:00 p.m.), hourly (at 6:00

a.m., 9:00 a.m., and 12:00 p.m.), and averaged over the period 6:00

a.m.–3:00 p.m. The data are from March to October, 1998–2002.

The medians were determined from all Atlanta-area AIRS monitoring

sites with available data (Confederate Avenue, Conyers, South DeKalb, and Tucker for NO and NOx; DeKalb Technical and Roswell for

CO). The SEARCH site at Jefferson Street, Atlanta, was not included

in the computation of the medians, but during the period 1999–2002

it exhibited Wednesday to Sunday declines comparable to those

shown here (NO, NOy, and CO declines at 12:00 p.m. of 64%, 51%,

and 35%, respectively).

Blanchard and Tanenbaum

Volume 56 March 2006 Journal of the Air & Waste Management Association 273
from diesel vehicles averaged 12 times the BC emission

rates from high-emitting gasoline vehicles and 490 times

the BC emission rates from low-emitting gasoline vehicles.11,25

The overall contributions of diesel and gasoline vehicles to ambient BC concentrations depend not only on

emissions rates, but also on vehicle miles traveled. In the

Denver area, enhanced chemical mass balance modeling

using organic compounds as marker species yielded estimates of diesel to gasoline vehicle contributions to BC

concentrations in the approximate ratios of 1:1 at one

location and 3:1 at a second site.25 In Los Angeles, BC was

found to be a good indicator of diesel exhaust (albeit not

a unique source marker) through comparison with vehicle activity data and highway sampling.11

The median Wednesday to Sunday decline in 24-hr

BC concentrations at urban sites in Atlanta and Birmingham was 31%, with statistically significant differences at three of the five urban sites (Table 1). The 24-hr

BC declines on Sundays were smaller at the two nonurban sites of Centreville (16%) and Yorkville (13%).

Hourly measurements of BC at SEARCH sites indicate

that the reductions of BC on Sundays were larger during

some time periods than others at both urban and rural

sites. The median reductions of BC concentrations at

the four SEARCH sites were 44% during the midday

hours (6:00 a.m. to 3:00 p.m.) and 47% during the

period 9:00 a.m. to 12:00 p.m. (Table 1). Again noting

that gasoline vehicles generated 25–50% of ambient

BC in Denver, whereas the observed median weekend

reduction of CO levels was 31%, one would expect that

weekend reductions of gasoline vehicle traffic in Atlanta might lower ambient BC concentrations by 5–

15% on weekends. The observed 31% reductions of

ambient concentrations of BC at urban sites indicate

that diesel exhaust emissions were also lower on weekends than on weekdays.

For comparison, weight-in-motion measurements

and survey results in Los Angeles show that counts of

heavy-duty truck traffic on freeways declined by 60 – 80%

on weekends compared with midweek, whereas surface

street traffic volumes (primarily light-duty vehicles) decreased by 15–30% on weekends.9,13 Mean CO and NOx

levels declined by 26% and 44%, respectively, on Sundays compared with midweek.12

The authors have not attempted a precise quantification of the relative source-type reductions, but the observed weekend decreases in the ambient concentrations

of BC and CO imply that both diesel and gasoline vehicle

emission reductions contributed to the observed Sunday

Figure 3. Diurnally averaged mixing ratios of O3, CO, SO2, and NO

at Jefferson Street (Atlanta). All hourly measurements from the years

1999–2002 during the months March to October were used in determining the averages. Each point is the average of 130 samples.

Table 1. Differences between mean BC concentrations on Wednesdays and Sundays computed for various times of day and temporal resolutions.

Site Location

No. Days Wednesday to Sunday Decrease (%)

24-hr

Data

1-hr

Data 6:00 a.m. 12:00 p.m.

6:00 a.m.–

9:00 a.m.

9:00 a.m.–

12:00 p.m.

12:00 p.m.–

3:00 p.m.

6:00 a.m.–

3:00 p.m.

12:00 a.m.–

12:00 a.m.

010730023 N. Birmingham 32 0 NA NA NA NA NA NA 35

010732003 Wylam 11 0 NA NA NA NA NA NA 23

130890002 S. DeKalb 30 0 NA NA NA NA NA NA 37

BHM N. Birmingham 105 87 42 47 44 50. 46 43 31

CTR Centreville 122 39 37 50. 31 41 55 42 16

JST Jefferson St 219 98 55 56 56 62 56 58 31

YRK Yorkville 126 42 34 48 39 44 48 44 13

Notes: Positive numbers represent higher Wednesday concentrations. Statistically significant (P 0.01) differences are shown in italic typeface; The averages

were determined from all available monitoring days during 1998 –2002; Sampling periods were: 2000 –2002 for AIRS/PM station sites (S. DeKalb/Atlanta GA, N.

Birmingham, and Wylam/Birmingham AL), 1998 –2002 for 24-hr resolution SEARCH data (Birmingham AL, Centreville AL, Jefferson St./Atlanta GA, and Yorkville

GA), and 2001–2002 for 1-hr resolution carbon measurements at the four SEARCH sites.

Blanchard and Tanenbaum

274 Journal of the Air & Waste Management Association Volume 56 March 2006
reductions of ambient NO and NOx levels. More specifically, comparison of the ambient CO reductions on Sundays (median: 31%) with the gasoline-vehicle portion

(35%) of NOx emissions in Fulton County suggests that

the larger portion of the 60% Sunday reduction of NOx

levels was because of weekend declines in diesel exhaust

emissions. This conclusion is consistent with the results

from Southern California, as well as with Atlanta-area

declines in BC levels and the Denver results showing that

diesel exhaust contributes as much or more to ambient

BC levels as does gasoline engine exhaust. As a caveat, it is

noted that both CO and BC may be transported over

substantial distances, so urban CO and BC concentrations

likely include regional background levels. The reductions

of CO and BC emissions within Fulton County may have

been greater than the corresponding declines in ambient

concentrations.

The ambient hydrocarbon measurements were more

limited than the NOx and CO data but indicated that

VOC declines occurred on Sundays, especially at urban

locations. For the four Atlanta-area PAMS sites, the median midday (6:00 a.m.–3:00 p.m.) Wednesday to Sunday

decreases in ambient hydrocarbon concentrations computed from all of the O3-season days were 16% for alkanes, 13% for alkenes, 28% for aromatics, 21% for benzene, 5% for isoprene, and 19% for the sum of PAMS

target compounds (Table 2). The largest declines were

observed for aromatics, including benzene (Figure 4), for

which gasoline and gasoline vehicles are a known major

source. The aromatics declines are of comparable magnitude to the previously discussed changes in CO levels.

VOC/NOx Ratios. An important, although coarse, indicator of O3 sensitivity to precursors is the ratio of VOC/NOx,

with higher ratios indicating greater sensitivity to

changes in NOx.26,27 The differential changes in ambient

NOx and VOC levels in Atlanta imply that weekend ratios

of VOC/NOx differ from weekday ratios and suggest that

local O3 formation on weekends may be more NOx-sensitive than on weekdays. Considering the two urban locations, the mean VOC/NOx ratios increased from 1.3:1

to 2.3:1 at South DeKalb and from 2.7:1 to 4.4:1 at Tucker

when computed from all of the O3-season days (Table 3),

all of which are within the ranges historically considered

to be VOC limited.26,27 The later discussion of local O3

formation will show that ratios of O3/HNO3 and HNO3/

NOy also increased on weekends.

HNO3, Nitrate, and Sulfate. The median Wednesday-toSunday daytime (6:00 a.m.–3:00 p.m.) decline in HNO3

mixing ratios at the four SEARCH sites was 21% (Table 4),

whereas the median 24-hr nitrate decrease at the same

four SEARCH sites plus two Speciated Trends Network

Table 2. Differences between the mean Wednesday and mean Sunday ambient concentrations (6:00 a.m.–3:00 p.m.) of alkanes, alkenes, aromatics,

benzene, isoprene, and the sum of PAMS target species.

Site AIRS No. Wednesdays

Wednesday to Sunday Decrease (%)

Alkanes Alkenes Aromatics Benzene Isoprene Sum of Species

South DeKalb 130890002 90 24 15 32 22 2 25

Tucker 130893001 64 33 26 58 39 17 40

Yorkville 132230003 37 8 13 0.1 24 9 8

Conyers 132470001 46 9 12 25 20 8 13

Mean 14 10 29 14 4 17

Median 16 13 28 21 5 19

Notes: Positive differences represent higher Wednesday concentrations; Statistically significant differences (P 0.01) are shown in italics; Data are from 1996

to 2003.

Figure 4. Diurnal profiles of mean ambient benzene levels at

Yorkville, South DeKalb, Tucker, and Conyers during the period

1996–2003, shown for each day of the week.

Blanchard and Tanenbaum

Volume 56 March 2006 Journal of the Air & Waste Management Association 275
sites was 19% (Table 5). Although the 6:00 a.m., 12:00

p.m., 6:00 a.m.–9:00 a.m., 9:00 a.m.–12:00 p.m., 12:00

p.m.–3:00 p.m., and 6:00 a.m.–3:00 p.m. HNO3 levels

declined on Sundays at all four of the SEARCH sites, only

the 9:00 a.m.–12:00 p.m. decrease at Yorkville was statistically significant (P 0.01). None of the 24-hr Wednesday/Sunday PM nitrate concentration differences was statistically significant, although the differences were

consistent, ranging from 15% to 25% at five of the six

sites. Mean Wednesday/Sunday sulfate concentration differences were not statistically significant; three sites

showed Sunday increases, and three showed decreases,

with the median change being a 1.4% decline on Sunday.

The results indicate that HNO3 and PM nitrate levels

decreased on Sundays but not in proportion to the magnitude of the NOx decline.

O3. Both peak 1-hr and peak 8-hr O3 concentrations were

lower on high-O3 Sundays than on high-O3 Wednesdays,

with one exception; these decreases were statistically significant at 6 of the 14 sites that were studied (Table 6). For

the high-O3 days, the median change among the sites was

a 9.5% decrease from the mean Wednesday peak 8-hr

concentrations to the mean Sunday peak 8-hr concentrations; the mean Sunday 1-hr concentrations were 11%

lower than the mean Wednesday 1-hr concentrations (Table 6). In contrast, the differences between average

Wednesday and Sunday O3 levels nearly disappeared

when all of the days (March–October) were included in

the averages. For the all-days averages, the median change

among sites was a decrease of 2.3% from the mean

Wednesday peak 8-hr levels to the mean Sunday peak

8-hr values; the mean Sunday 1-hr mixing ratios were

4.5% lower than the mean Wednesday 1-hr mixing ratios

(Table 6). None of these differences was statistically significant. The modest changes in peak O3 levels on Sundays compared with Wednesdays contrast with the much

larger reductions in O3 precursor concentrations occurring on Sundays.

O3 Formation, Loss, and Accumulation

Ambient O3 levels are the net result of numerous atmospheric processes, any or all of which could exhibit

changes on weekends compared with weekdays. Different

processes may predominate during different periods of

the day, and all must be considered to understand the O3

response to precursor changes on weekends.

Overnight Surface Carry-Over. Because the nocturnal inversion confines pollutants within a shallow surface layer,

and fresh emissions tend to be lower at night, mixing

ratios from midnight to 4:00 a.m. typically reflect surface

carry-over of previous-day pollutants more than at other

times of day. All of the sites showed lower mixing ratios of

CO and NO and higher O3 levels on Sundays than on

Wednesdays from midnight to 4:00 a.m., as exemplified

by the Jefferson Street site (Figure 3). Thus, during this

time period, surface carry-over of O3 precursors was lower

on Sundays than on Wednesdays, whereas surface carryover of O3 was greater. The higher levels of surface O3 on

Sundays were associated with lower levels of NO during

the same hours (Figures 3 and 5), implying that reduced

titration of O3 by NO caused the higher Sunday O3 surface

carry-over. Ozone carry-over aloft need not have been

affected in the same way because of nocturnal isolation of

air aloft from surface-level NOx emissions.

Table 3. Mean daytime (6:00 a.m.–3:00 p.m. EST) concentrations of the sum of PAMS target compounds, NOx or NOy, and the ratios of VOC/NOx or VOC/

NOy at four locations, determined from all O3-season days having both VOC and NOx Data.

Site

Wednesday Sunday

No. Daysa VOC (ppbC) NOx (ppbv) VOC/NOx No. Daysa VOC (ppbC) NOx (ppbv) VOC/NOx

S. DeKalb 144 62  4 49  4 1.3 143 47  3 20.  2 2.3

Tucker 161 65  4 241 2.7 158 39  2 9  0.6 4.4

Conyers 168 18  1 8  0.4 2.2 167 20.  1 5  0.2 4.1

Yorkvilleb 134 36  3 11  0.7 3.2 132 33  3 6  0.7 5.8

Notes: The uncertainty limits are 1 SEM; a

No. days with NOx or NOy data; See Table 2 for No. days with VOC measurements; bNOy VOC/NOy.

Table 4. Differences between mean Wednesday and mean Sunday HNO3 concentrations at 6:00 a.m., 12:00 p.m., 6:00 a.m.–9:00 a.m., 9:00 a.m.–12:00

p.m., 12:00 p.m.–3:00 p.m., and 6:00 a.m.–3:00 p.m.

Site City

HNO3 Wednesday–Sunday Decrease (%)

6:00 a.m. 12:00 p.m. 6:00 a.m.–9:00 a.m. 9:00 a.m.–12:00 p.m. 12:00 p.m.–3:00 p.m. 6:00 a.m.–3:00 p.m.

BHM Birmingham, ALa 54 2.7 52 16 8.8 27

CTR Centreville, AL 20 20 22 21 19 20

JST Jefferson St/Atlanta, GA 30 17 22 12 22 19

YRK Yorkville, GA 27 19 26 23 19 22

Notes: Positive numbers represent higher Wednesday concentrations; Statistically significant (P 0.01) differences are shown in bold typeface; The averages

were determined from days during March to October of 1999 –2002; All locations are SEARCH sites; a

Data available from October 2000 to December 2002.

Blanchard and Tanenbaum

276 Journal of the Air & Waste Management Association Volume 56 March 2006
Fresh Emissions. Diurnal profiles indicate that fresh emissions of CO and NO were greater from 6:00 a.m. to 8:00

a.m. on Wednesdays than on Sundays (Figure 3). Because

O3 reacts with NO, O3 cannot accumulate until NO levels

fall below O3 mixing ratios.10 As a result of lower levels of

NO on Sunday mornings, less O3 was lost to NO titration

on Sundays. O3 accumulation began earlier on Sundays

than on Wednesdays, as indicated by the times when

morning O3 concentrations first exceeded the ambient

NO levels (Figure 5). At two sites, these times were 1 hr

earlier on Sundays compared with Wednesdays; at two

other sites, NO levels were always lower than O3 concentrations on Sundays but not on Wednesdays. O3 concentrations exceeded NO levels on all of the days at Yorkville,

but even there, NO levels during the hours of 6:00 a.m. to

8:00 a.m. were lower on Sundays than on Wednesdays.

Carry-Over Aloft. Ozone mixing ratios increased most rapidly during the hours from 9:00 a.m. to 12:00 p.m. (Figures

3 and 5). On Sundays, the mean temporal rate of O3 mixing

ratio increase from 9:00 a.m. to 12:00 p.m. was lower than

on Wednesdays, so that by 12:00 p.m. the mean Sunday and

Wednesday O3 levels were nearly identical at most sites,

although O3 levels at 9:00 a.m. on Sundays exceeded those

at 9:00 a.m. on Wednesdays (Figures 3 and 5). Concentration increases during morning hours could reflect either or

both O3 formation or increasing mixing depth and incorporation of higher O3 levels from aloft (as noted previously,

because aloft air masses are isolated from surface emissions

of NO overnight, higher O3 levels could have persisted

above the nocturnal inversion). Separation of the effects of

mixing from O3 formation is difficult. The authors did not

have vertical soundings, but they observed that average

surface wind speeds increased most rapidly between the

hours of 7:00 a.m. and 10:00 a.m., which suggests that

vertical mixing effects need to be considered especially during this time period. Additional analyses discussed next indicate that O3 transport from upwind was greater on

Wednesdays than on Sundays.

Regional Transport. The authors evaluated weekend

changes in O3 transport into the Atlanta area by comparing weekend to weekday peak 8-hr O3 levels at upwind

and downwind sites. The specific question to be addressed

was: are day-of-week differences in O3 mixing ratios

within and downwind of Atlanta attributable to day-ofweek differences in O3 production within the metropolitan area or to day-of-week differences in O3 transport

from upwind areas? Peak 8-hr O3 levels typically occurred

between the hours of 10:00 a.m. and 7:00 p.m., an interval during most of which substantial vertical mixing is

typical. They used surface wind directions to select subsets

Table 5. Differences between mean Wednesday and Sunday nitrate and

sulfate concentrations.

AIRS or SEARCH City

No.

Days

Wednesday to Sunday

Decrease (%)

Nitrate Sulfate

010730023 N. Birmingham, AL 32 8.7 1.2

130890002 S. DeKalb/Atlanta, GA 30 25 7.8

BHM Birmingham, AL 105 19 4.0

CTR Centreville,AL 122 21 1.2

JST Jefferson St/Atlanta, GA 219 19 5.1

YRK Yorkville, GA 126 16 5.4

Notes: Positive numbers represent higher Wednesday concentrations. None of

the differences are statistically significant (P 0.01). The averages were

determined from all days during 2000 –2002 (AIRS/PM station: S. DeKalb/

Atlanta GA and N. Birmingham) and 1998 –2002 (SEARCH: Birmingham AL,

Centreville AL, Jefferson St./Atlanta GA, and Yorkville GA).

Table 6. Differences between mean Wednesday and Sunday peak 1-hr and 8-hr concentrations.

AIRS/ SEARCH/ CASTnet City

No. Days Wednesday to Sunday Decrease (%)

Sunday Wednesday

Top 3 per Day of Week All Days

8-hr O3 1-hr O3 8-hr O3 1-hr O3

130890002 S. DeKalb/Decatur 158 157 5.3 9.0 1.4 4.0

130893001 Tucker 162 162 7.5 8.0 2.4 7.2

130970004 Douglasville 166 166 11 19 0.3 4.1

131130001 Fayetteville 165 166 15 18 2.9 6.8

131210055 Confederate/Atlanta 163 166 7.5 9.5 0.1 4.3

131270006 Brunswick 162 164 6.8 7.8 0.2 2.3

131350002 Lawrenceville 151 153 7.6 8.1 0.9 6.2

132470001 Conyers 166 165 15 17 6.2 8.6

CTR Centrevillea 134 137 15 15 5.0 4.6

JST Jefferson St/Atlantaa 137 137 11 12 4.4 1.1

YRK Yorkvillea 135 138 11 12 3.6 6.6

COW137 Coweeta 137 137 0.1 14 2.2 1.6

GAS153 Georgia Station 136 136 16 11 5.8 4.5

SND152 Sand Mountain 139 140 3.0 21 3.2 7.8

Mean 9.6 11 1.9 5.0

Median 9.5 11 2.3 4.5

Notes: CASTnet indicates Clean Air Status and Trends Network; Positive numbers represent higher Wednesday concentrations; Statistically significant (P 0.01)

differences are shown in bold typeface; The averages were determined from both all days and high-O3 days during 1998 –2002; All sites listed had 3 years

of data within the study period; a

1999 –2002.

Blanchard and Tanenbaum

Volume 56 March 2006 Journal of the Air & Waste Management Association 277
of days when it was reasonable to characterize one group of

sites as upwind from Atlanta and another as downwind and

confirmed the characterizations using HYSPLIT back trajectories.28 The transport analyses were carried out for high-O3

days (top three peak 8-hr daily maximum days per day of

week per year for each site). For each high-O3 day, hourly

surface wind speed and direction were obtained for the three

SEARCH sites (Jefferson Street, Yorkville, and Centreville)

and three Clean Air Status and Trends Network sites (Georgia Station, Sand Mountain, and Coweeta). The surface meteorological data from these six sites were also used for O3

sites lacking meteorological measurements (Yorkville for

Douglasville, Georgia Station for Fayetteville, and Jefferson

Street for Confederate Avenue, Tucker, DeKalb, Conyers,

and Gwinnett; Figure 1). The authors computed 24-hr vector average wind speed and direction for the high-O3 days

for each site (from the hourly averages with start times of

7:00 p.m. on a previous day to 6:00 p.m. on each high-O3

day). For each monitoring site, they then classified the 24-hr

surface wind direction according to quadrant of origin

(northeast, southeast, southwest, and northwest). The

HYSPLIT model was run using 3 trajectory levels (100, 500,

and 1000 m) for 10 days with vector-average surface winds

from the northwest quadrant and 10 with surface winds

from the southwest quadrant, randomly chosen from

high-O3 days (top three 8-hr days per day of week) during

June to August, 1999 –2002. Trajectories were carried back

from DeKalb County airport for 48 hr at 6-hr time intervals.

The three trajectory heights remained reasonably coherent

(within the classification of transport quadrants) and supported the classifications into quadrants using surface

winds.

Figure 5. Diurnally averaged mixing ratios of O3 and NO on Wednesdays and Sundays.

Blanchard and Tanenbaum

278 Journal of the Air & Waste Management Association Volume 56 March 2006
The urban/downwind sites exhibited mean mixing

ratios of 80 –100 ppbv, whereas the far-upwind sites

showed mean mixing ratios of 60 – 80 ppbv, thus indicating that transported O3 made a large (80%) contribution to peak O3 levels in and near Atlanta (Figure 6).

Note that the Atlanta urban plume could itself have contributed to the regional background depending on circulation patterns, as is suggested by the partially elevated

mean O3 levels at the nominally upwind sites of Yorkville,

Douglasville, and Fayetteville relative to Sand Mountain,

Georgia Station, and Centreville. All of the back trajectories for the cases with winds from the northwest quadrant

extended 48 hr back into areas to the northwest without

recirculation; 30% of the back trajectories arriving from

the southwest exhibited a trajectory rotation extending to

the southeast of Atlanta and suggestive of recirculation

around a high-pressure system. Other than Atlanta,

nearby urban areas, including, for example, Chattanooga,

could have contributed to the observed regional background when winds were from the northwest quadrant.

Figure 6. Mean daily 8-hr O3 maxima by day of week for high-O3 days at downwind sites (Conyers and Gwinnett), 1998–2002. The data have

been averaged across monitoring locations representing far upwind, near upwind, and urban/downwind locations. The error bars are 1 SEM. No

error bars are shown for averages based on one sampling day. (a) The days with winds from the northwest had 24-hr vector-average surface

wind directions of 270–360°. The far upwind site was Sand Mountain (Clean Air Status and Trends Network). The near upwind sites were

Douglasville (130970004) and Yorkville (132230003; PAMS Type 1). The urban sites were Jefferson Street (SEARCH), Georgia Tech, and

Confederate Avenue (131210048). The downwind sites were DeKalb (130891002; PAMS Type 2) and Conyers (132470001; PAMS type 3). (b)

The days with winds from the southwest had 24-hr vector-average surface wind directions of 180–270°. The far and near upwind sites were

Centreville (SEARCH) and Douglasville (130970004), respectively. The urban/downwind sites were Jefferson Street (SEARCH), Tucker

(130893001; PAMS type 2A), and Gwinnett (131350002).

Blanchard and Tanenbaum

Volume 56 March 2006 Journal of the Air & Waste Management Association 279
Previous studies have shown that transported O3 contributes substantially to the magnitude of peak O3 concentrations in the major metropolitan areas of the Southeastern United States.29

The differences between the mean 8-hr peak O3 levels

at the urban/downwind sites and the far upwind sites are

interpreted here as indicating the magnitude of the mean

contribution of the Atlanta urban plume on high-O3 days.

These differences were 10 –25 ppbv (Figure 6). The differences between the far upwind and urban/downwind

O3 mixing ratios varied somewhat among the days of the

week, but none of the differences was statistically significant (Figure 6). These analyses, therefore, show no statistically significant variation in the amounts of local O3

production in Atlanta by day of the week. However, other

comparisons of day-of-week differences in ratios of reactive species, discussed in the next section, indicate that

day-of-week differences occurred in the rates of local O3

formation at sites in Atlanta.

The mean far upwind concentrations were lower on

Sundays than on Wednesdays, Thursdays, or Fridays, although the differences were not statistically significant

(Figure 6), implying that regional carry-over of O3 was

lower on Sundays than on Wednesdays through Fridays.

The mean O3 mixing ratios at the far upwind sites were

marginally lower (1–3 ppbv) on Mondays and Tuesdays

than on Sundays, suggesting that some of the benefits of

weekend emission reductions in upwind areas appeared

on Mondays or Tuesdays, rather than on Sundays, that is,

some time lag occurred in the response of regional-scale

O3 levels to regional-scale weekend reductions of O3 precursor emissions. However, the differences between the

mean Sunday and the mean Monday or Tuesday O3 levels

were small (1–3 ppbv) and were not statistically significant. The differences between the minimum and maximum day-of-week means at the upwind sites indicate that

the regional contribution varied by 15–27 ppbv among

days of the week (Figure 6).

Under both northwesterly and southwesterly transport patterns, the mean day-of-week concentrations at far

upwind, upwind, and urban/downwind sites followed approximately parallel lines (Figure 6). Although we

grouped sites into far upwind, near upwind, and urban/

downwind locations for clarity, the results for individual

sites were also approximately parallel. The parallelism

(within the standard errors of the means) implies that the

day-of-week differences in mean O3 concentrations at

Atlanta sites were partially or wholly driven by day-ofweek differences in upwind transport concentrations.

That is, the magnitudes of the day-of-week variations at

downwind sites were approximately the same as those

occurring at upwind sites, because regional O3 levels substantially exceeded the amounts of local O3 formation

and because the amount of local O3 formation did not

show statistically significant day-of-week variations.

Local O3 Formation. The SEARCH sites provide measurements of one of the most important NOx reaction products, namely, HNO3, as well as NOy. For these sites, it is,

thus, possible to compare O3 levels to concentrations of

NOx reaction products and to compare product levels to

total oxidized nitrogen species concentrations. A key reaction product is HNO3, which is produced by reaction of

the HO radical with nitrogen dioxide (NO2). This reaction

removes a radical and an NO2 molecule. Because each

conversion of NO2 to NO produces an O3 molecule, with

some loss processes, removal of an NO2 molecule ends the

contribution of that molecule to O3 production (some

reaction products, e.g., peroxyacetyl nitrate, can serve as a

reservoir of NO2, however, because a reverse reaction may

occur). The ratio of O3 to HNO3 concentrations provides

a relative measure of the mean number of O3 molecules

produced per NOx molecule. The ratio of HNO3 to NOy

provides another relative measure of the amount of NOx

reaction product (the ratio of NOx to NOy is more indicative of the amount of reactant to the sum of reactants

and products, but the available NO2 record was much

shorter than the HNO3 record).

The mean Sunday HNO3 levels were lower than

Wednesday levels (Table 4). However, both central city

and rural SEARCH sites exhibited higher ratios of HNO3 to

NOy on Sundays compared with Wednesdays. Because the

atmospheric lifetime of HNO3 is of the order of hours to a

day, these day-of-week differences imply that a greater

fraction of the NO and NO2 was converted to HNO3 by a

specified hour on Sundays compared with the same hour

on Wednesdays. This result is consistent with the observed increase in ambient VOC/NOx on Sundays (Table 3),

which suggests the occurrence of a more reactive mix on

weekends.

For brevity, we refer to the mean number of O3 molecules produced per NOx molecule as O3 production efficiency.30 An increase in the ratio of O3 to HNO3 concentrations indicates that the efficiency of O3 production was

greater on Sundays than on Wednesdays (Figure 7). At

Jefferson Street, the O3/HNO3 ratios increased from 18:1

on Wednesdays to 24:1 on Sundays during the afternoon (sample start hours 12:00 p.m.– 4:00 p.m.), indicating an 35% increase in the number of O3 molecules

produced per NOx. The increase in O3 production efficiency acted in opposition to the decreases of NO emissions on Sundays. Modeling studies confirm the importance of the increase in O3 production efficiency as NOx

emissions decline.31

The ratios of O3/NOy or O3/NOx were also higher on

weekends than on Wednesdays at other monitoring sites.

Ratios of O3/CO were greater on Sundays compared with

Wednesdays at the three locations having CO data (Jefferson Street, Yorkville, and DeKalb), and on Saturdays relative

to Wednesdays at Jefferson Street and DeKalb.

Potential O3 Responses to Emission Controls

The analyses discussed previously indicate that local NOx

emission reductions on the order of 60% did not change

the amount of O3 formed locally, suggesting that local O3

formation was not typically limited by the availability of

NOx. Two other indicators support this conclusion. First,

the mean 6:00 a.m.–3:00 p.m. VOC/NOx ratios were low

(1.3:1 to 5.8:1) at all of the PAMS monitoring sites on

both Wednesdays and Sundays (Table 3; the 6:00 a.m.–

9:00 a.m. ratios ranged from 1:1 to 5.1:1). These ratios

underrepresent VOC, because they are based on the

sum of PAMS target compounds; the mean ratio of the

Blanchard and Tanenbaum

280 Journal of the Air & Waste Management Association Volume 56 March 2006
target compounds to the measured total VOC in this

dataset was 0.85. Historic EPA guidance has indicated

that 6:00 a.m.–9:00 a.m. ratios of ambient VOC/NOx

less than 8:1 are indicative of VOC-limited conditions.26,27 However, photochemical aging is as important as VOC/NOx ratios in determining the sensitivity

of O3 to changes in VOC or NOx emissions,32 so that

variations in O3 response to emission reductions should

be expected depending on the hour of the day, downwind distance, meteorological conditions and their relation to photochemical aging, and the influence of

highly reactive VOCs (e.g., isoprene).

A second indicator of where and when VOC or NOx

limits local O3 formation is the extent of reaction estimated from a smog production algorithm.33–36 The authors calculated this indicator on an hourly basis using all

of the available data, and they observed differences

among hours, days of the week, and types of days (i.e.,

high O3 or not). On high-O3 Wednesdays, the mean

hourly extent of reaction remained 60% (VOC-limited

conditions; refs 34 –36) before 10:00 a.m. at all of the sites

(Figure 8). By 12:00 p.m. and until 6:00 p.m., mean extent

exceeded 80% (NOx limited34 –36) at the nonurban sites of

Yorkville and Conyers. At the suburban sites of DeKalb

and Tucker, extent varied from 60% to 80% (transitional;

refs 34 –36) from 12:00 p.m. to 6:00 p.m., whereas at Jefferson Street the extent remained 60% during most hours

(Figure 8). In contrast to these high-O3 days, the mean

extent of reaction averaged 40% (VOC limited) from

12:00 p.m. to 6:00 p.m. at all of the sites when the averages were determined from all of the O3-season days.

According to this calculation, therefore, high-O3 days exhibited a different sensitivity to O3 precursors than did

other days, and the sensitivity of the peak hour was different than the sensitivity of the 8 hr comprising the 8-hr

Figure 7. Hourly ratios of the mean mixing ratios of O3 and HNO3 on Wednesdays and Sundays. Hourly averages were determined from

measurements made at the SEARCH sites at Jefferson Street (Atlanta; 1999–2002), Yorkville (GA; 1998–2002), Birmingham (AL; 2000–2002),

and Centreville (AL; 1998–2002) during the months of March through October, with each point computed as the average of 30 hourly

measurements per year.

Blanchard and Tanenbaum

Volume 56 March 2006 Journal of the Air & Waste Management Association 281
O3 peaks. In addition, the mean extent of reaction estimates between 12:00 p.m. and 6:00 p.m. on high-O3

Sundays were higher than during corresponding hours on

high-O3 Wednesdays by 10 –20% at Tucker and DeKalb, a

result that is directionally consistent with the observed

increase in VOC/NOx ratios at those sites on Sundays

compared with Wednesdays (Table 3). Increases in the

mean extent of reaction from Wednesdays to Sundays

were also observed for all of the sites when the means

were determined from all O3 season days.

Although the Atlanta weekend effect indicates that

local NOx emission reductions did not reduce the amount

of O3 formed locally, the response of locally formed O3 to

future local emission controls could differ from the

present weekend response, because the emission changes

occurring from weekdays to weekends might not exactly

reflect the emission reductions characteristic of control

plans. The authors observed median daytime (6:00 a.m.–

3:00 p.m. EST) decreases of 62%, 57%, and 31%, respectively, in the ambient concentrations of NO, NOx, and

CO from Wednesdays to Sundays during the O3 season

(March to October). They also observed decreases of 28%

for the sum of aromatic compounds and 19% for the sum

of PAMS target compounds. In comparison, the Georgia

Department of Natural Resources projects mobile-source

NOx emissions to decline from 304 tons per day in 2004

to 192 tons per day in 2010 (37%) in a 13-county area

around Atlanta and mobile-source VOC emissions to decline from 164 tons per day in 2004 to 112 tons per day in

2010 (32%).38 For the state of Georgia, EPA projects that

the recently adopted Clean Air Interstate Rule will yield

stationary-source NOx reductions of 23% by 2009 and

38% by 2015 relative to 2003.39 In the EPA air quality

modeling analyses of the effects of the Clean Air Interstate

Rule, Georgia VOC and NOx emissions from all sources

were projected to decline by 17% and 36%, respectively,

from 2001 to 2010.40 In overall magnitude, therefore, the

expected reductions of mobile-source and stationarysource O3 precursor emissions between 2001 and 2010 are

somewhat less than the weekend reductions in ambient

Figure 8. Hourly mean extent of reaction and O3, NO, NOx, and NOy mixing ratios on high-O3 Wednesdays (top three peak 8-hr

O3-concentration Wednesdays per year), 1998–2002. For Yorkville and Jefferson Street, two alternative formulations of the extent of reaction

were calculated using the measurements of NOy and NOx; the NOx data were available only for 2001 and 2002 at Jefferson Street and 2002

at Yorkville. For the remaining three sites, upper and lower bounds of the extent of reaction were calculated using the available NOx

measurements (which overestimate true NOx

37), and a best estimate is presented as the mean of the bounds.34,35

Blanchard and Tanenbaum

282 Journal of the Air & Waste Management Association Volume 56 March 2006
concentrations of NO, NOx, CO, and VOCs that are reported here. However, the locations and times of day

exhibiting future emission reductions might not correspond closely with the locations and times that now show

weekend reductions of ambient precursor concentrations.

For example, the emission projections include stationarysource NOx emission reductions from nonurban sources,

whereas the current weekend reductions of ambient NOx

may derive primarily from weekend decreases in motor

vehicle traffic.

In contrast to the small changes that were observed in

weekend relative to weekday O3 levels, EPA modeling

studies predict that the fourth-highest annual maximum

peak daily 8-hr O3 levels in Fulton County will decline

from 99 ppbv in 2000 –2002 to 85 ppbv in 2010.40 Ambient monitoring data now show that the corresponding

peak 8-hr O3 values in Fulton County dropped to 93 ppbv

in 2002–2004 and 91 ppbv in 2003–2005.41 It is possible

that the types of emission reductions that were modeled

by EPA might lower O3 levels more effectively than did

weekend reductions of urban VOC and NOx levels that

were observed; for example, geographically widespread

emission reductions might reduce regional O3 concentrations, whereas the weekend decreases of O3 precursors

that were observed in Atlanta did not change the amount

of O3 transported into the Atlanta area from upwind

regions. However, the observations could also imply that

projected future O3 decreases may be larger than what will

actually occur.

Potential PM Nitrate Responses to Emission

Controls

As demonstrated above, HNO3 and PM nitrate levels decreased in response to lower NOx levels on Sundays but

not in proportion to the magnitude of the NOx decline

(20% compared with 60%). Future changes in PM

nitrate levels will depend on changes in SO2 emissions

and sulfate concentrations, as well as on changes in NOx

emissions and HNO3 concentrations. In addition,

changes in ammonia levels may affect PM nitrate concentrations. Present cool-season concentrations of PM nitrate

in the Southeastern United States are low and tend to be

limited by the availability of ammonia.42,43 With declining sulfate concentrations, future PM nitrate levels could

become more responsive to changes in ambient HNO3

because PM nitrate formation would then be less ammonia limited. Model simulations show small predicted

changes in PM nitrate in response to future changes in

ambient levels of HNO3,43 and the actual changes in PM

nitrate levels will depend also on the responses of HNO3

to NOx reductions.

CONCLUSIONS

Future O3 concentrations in Atlanta will depend on the

responses of both transported and locally formed O3 to

emission controls. Regional O3 levels were 80% of the

ambient O3 concentrations observed in and downwind of

Atlanta, with the average O3 mixing ratios at sites in

Atlanta being 10 –25 ppbv higher than the levels measured at far upwind locations. Because of the substantial

influence of regional, upwind O3 on Atlanta’s observed

O3 levels, the day-of-week differences observed in Atlanta

were attributed in large measure to day-of-week differences in upwind transport conditions. Because transported O3 may have formed over a period of many days,

including both weekdays and weekends, the differences

between weekday and weekend concentrations of O3 do

not necessarily represent the changes that might occur in

response to regional-scale emission reductions. However,

our analyses indicate that local emission reductions on

the order of 30 – 60% did not change the amount of O3

formed locally, and, on weekends, O3 began forming 1

hr earlier, and more O3 formed per unit of NOx. The peak

O3 hours did not shift to an earlier mean time on weekends as might be expected if lower weekend NOx levels

were limiting O3 production. Because NO levels were

lower on Sundays, overnight surface carry-over of O3 was

greater, and morning titration of O3 by NO was reduced

so that local O3 accumulation began earlier on Sunday

mornings than on Wednesdays. During midday hours,

ratios of O3/NOx, O3/NOy, and O3/HNO3 were greater on

Sundays than on Wednesdays, indicating that, in addition to the titration effect, the mean number of O3 molecules formed per ambient NOx molecule increased on

Sundays. The available data did not allow for the evaluation of the effects of changing levels of NOx on hydroxyl

radical concentrations or whether O3 formation rates

changed with respect to specific VOC compounds.

In overall magnitude, the projected reductions of mobile-source and stationary-source O3 precursor emissions

in Georgia between 2003 and 2010 are less than, or comparable to, the weekend reductions in ambient concentrations of NO, NOx, CO, and VOCs that are reported

here. The locations and times of day exhibiting emission

reductions in the future may differ from the locations and

times that now show weekend reductions of ambient

precursor concentrations. Nonetheless, these results have

important implications for the potential efficacy of

planned emission reductions.

ACKNOWLEDGMENTS

The authors thank the Coordinating Research Council for

providing funding and the Council’s reviewers who offered suggestions that significantly improved this study.

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About the Authors

Charles L. Blanchard and Shelley Tanenbaum are Principals with Envair. Address correspondence to: Envair, 526

Cornell Ave, Albany, CA 94706; phone: 	1-510-525-6231;

fax: 	1-510-528-2834; e-mail: cbenvair@pacbell.net.

Blanchard and Tanenbaum

284 Journal of the Air & Waste Management Association Volume 56 March 2006