Sunday 

Morning Traffi c

AbstractWe evaluated day-of-week differences in mean concentrations of ozone precursors [nitric oxide (NO), nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon monoxide (CO) at monitoring sites in 23 states comprising seven geographic focus areas for the years 1998-2003. We used Wednesdays to represent weekdays and Sundays to represent weekends; we also analyzed Saturdays. Statistical t-tests applied to every monitor showed substantial and statistically signifi cant differences between Wednesday and weekend mean concentrations of ozone precursors in all study areas. At half the sites, ninehour (6 am through 3 pm) mean concentrations of NO, NOx, and CO declined by at least 65, 49, and 28 percent, respectively, from Wednesdays to Sundays. Large reductions in mean concentrations of ozone precursors on weekends did not reduce mean concentrations of peak ozone signifi cantly, and  in many areas, mean peak ozone increased on weekends. Ozone accumulation began earlier on Sundays than on Wednesdays, on average. We also evaluated day-of-week differences in mean concentrations of fi ne particle and nitrate at available monitoring sites for the same time period and same geographic focus areas. Based on the monitors’ differences in Wednesday and Sunday means, PM nitrate showed almost negligible weekend decline, despite large NO and NOx declines. Why We Did This Work

We need to understand the implications of EPA’s HD NOx emission standards on ambient ozone and particulate matter (PM2.5)For many years there has been a controversy regarding the least costly and most effective way to reduce ambient ozone levels- Debate has centered around NOx vs. VOC controls to 

reduce ambient ozone

The analysis of ambient air quality data is the best way to answer the question: “What is the most effective means to reduce ambient ozone levels?”- The weekend effect is a natural emissions control 

experiment 

What We Did

Obtained ambient air quality data from ozone problem areas in 23 states Analyzed data from 1998 through 2003 for NO, NOx, CO, ozone, and particulate nitrateAnalyzed weekday-weekend differences for pollutant species- For NO, NOx, VOC, CO, and ozone, evaluated March-Oct. 

ambient hourly data; for particulate nitrate evaluated annual 24-hour data

ImplicationsWeekends provide a natural experiment for understanding how urban ozone and PM nitrarespond to large reductions in precursor emissThe data suggest that VOC emission reductionreduce peak ozone, while NOx emission reductincrease ambient ozone levels at most urban Ulocations; there is little change in PM nitrate concentrations on weekends. Emission reductioprojected to take place by 2010 are similar in magnitude to current weekend reductions in oprecursor emissions. The fi ndings from this stumay require rethinking present control strategreduce urban ozone and PM nitrate exposure aozone transported downwind of urban locatioNote: Additional work investigating the weekend ozone effect currently is being conducted in the Southeast Michigan region.

Sunday to Wednesday Pollutant Ratios

6 a.m.–3 p.m. CO, NOx and daily one-hour and eight-hour ozone fractons, all days, March–October, 1998–200

Weekday Morning Traffi c

-80-70-60-50-40-30-20-10

0

10

NO (6 a.m.)

CO (6 a.m.)

NO (Noon)

CO (Noon)

O3 1-hr (All Days)

O3 8-hr (All Days)

O3 1-hr (High Days)

O3 8-hr (High Days)

PM2.5 Nitrate

Change, %

Wednesday to Sunday Pollutant Changes

Median Values, 23 States, 1998–2003

201 NO Monitoring sites249 CO Monitoring sites540 O3 Monitoring sites

69 PM Nitrate Monitoring sites

0.000.040.080.120.160.20

1 3 5 7 9 11 13 15 17 19 21 23

Hour (PDT)

NO

O3

Most important chemical reaction for ozone formation:NO + O3 NO2 + O2Solid Lines = WeekdaysDotted Lines = Weekends

O3 and NO (ppm)

Azusa (Los Angeles), Summer 1995

The Weekend Ozone Effect in U.S. Ozone Problem Areas Outside of California 

Lake M

Are

Northeast

U.S.

Offi ce of FreedomCAR and Vehicle Techn

Douglas R. Lawson - National Renewable Energy Laboratory • James J.