1992 to 2005 Meteorological Factors Conducive to Ozone 

Formation in the Portland-Vancouver Area 

(ODEQ, April 2006) 

Anthony Barnack 

Air Quality Division

Oregon Department of Environmental Quality 

(503) 229-5713 
Abstract 

Elevated ground level ozone concentrations in the Portland/Vancouver airshed have 

slightly decreased in the period between 1992 and 2005. In the same period, ozone 

precursor VOCs and NOx levels went down slightly while maximum summer time 

temperatures rose. Population and vehicle miles traveled also increased. When 

comparing similar temperature episodes between years the ozone levels declined 

slightly. Temperature episodes are defined as three or more days with maximum 

temperatures above 90oF, average wind speeds less than 10 mph, and solar radiation 

above 1.0 langley/hr. 

Introduction: 

Ozone is a secondary pollutant produced by a mixture of Nitrogen dioxide and Oxygen 

under strong ultraviolet radiation (hv). This reaction is driven by the creation of more 

Nitrogen dioxide from the reaction of volatile organic carbons under ultraviolet radiation 

and free radicals (See ozone chemistry following this report). The highest ozone levels 

occur during the summer months, during clear, hot days with poor ventilation 

Meteorological parameters which affect ozone formation: 

Temperature: 

In Portland/Vancouver the ozone levels start to approach Air Quality Index levels of 

“Unhealthy For Sensitive Groups” and the National Ambient Air Quality Standards 

(NAAQS) when temperatures are near or above 90oF for three consecutive days or more 

with poor ventilation. 

Ventilation: 

Ventilation refers to the amount of vertical mixing in the atmosphere. With good 

ventilation the dirty ground level air is transported up and replaced with cleaner air. In 

short, good ventilation reduces the ground level volatile organic carbon levels and the 

resulting ozone. The rule of thumb is for good ventilation to occur the average horizontal 

wind speed should be greater than 10 mph. 

Solar radiation (ultraviolet radiation): 

The time of year is important because the angle of the sun and duration of the day 

determine the ultraviolet radiation intensity and duration. 

Portland and Vancouver have many programs designed to lower the emissions of the 

ozone precursor pollutants of Volatile Organic Carbon and Nitrogen dioxide. One way to 

determine the efficacy of these programs is by analyzing the ozone trends while 

considering the trends for ozone precursors, vehicle miles traveled, and population 

growth. This analysis would not be complete, however, without accounting for variations 

in meteorological conditions such as temperature and ventilation. These parameters must 

be considered to place ozone levels in context. Once the meteorological conditions are 

identified a better comparison of ozone trends can be made using ozone episodes with 

similar weather conditions. 
Results 

1. Ozone 

The NAAQS for ozone is calculated by averaging three consecutive years of the fourth 

highest daily eight hour average ozone. This level must be below 0.8 ppm or it is a 

violation. The eight hour ozone standard replaced the one hour standard in 1997 and 

since that time the Portland/Vancouver Air Quality Maintenance Area has remained in 

compliance. However, the Portland/Vancouver area has had four exceedances (any eight 

hour daily max above 0.08 ppm)1. Figure 1 illustrates the exceedances both before and 

after the 1997 standard took effect. 

0

1

2

3

4

5

6

7

8

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Number of Exceedance

8hr Ozone Standand Begins

Figure 1. Portland/Vancouver number of exceedances of eight hour standard 1. 

In Figure 2, the three year average of the fourth highest maximum daily eight hour 

average trends down slightly with a slope of -0.001. 

y = -0.001x + 0.08

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Ozone (ppm)

Ozone NAAQS > 0.08 

8hr Ozone Standand Begins

Figure 2. Portland/Vancouver three year average of fourth highest daily maximum eight 

hour average. Eight hour standard begins in 1997. 
2. Horowitz D., 001_Portland-National-DMVT.xls, Portland Metro 

3. Population Research Center, College of Urban and Public Affairs, Portland State University. 

4 ODEQ measured VOCs at N. Roselawn, Portland and NOx measured at SE Lafayette, Portland. 

2. Vehicle Miles Traveled (VMT) and Population 

In Figure 3 the Portland/Vancouver VMT2 and Population3 have slopes of +0.08 and 

+0.04 respectively. The three year average of the fourth highest eight hour average 

Ozone value has a trend with a slope of -0.002 which is relatively flat. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Population (X Million) and VMT (X 10 Million)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Ozone (ppm)

VMT(X 10 million)

population (mil)

3yr Aver of 4th Highest Ozone

Linear (3yr Aver of 4th Highest Ozone)

Figure 3. 3yr average of 4th highest 8hr average Portland/Vancouver 1979-2004 2,3

3. Precursors: 

The Portland/Vancouver area ozone precursors are Nitrogen dioxide and volatile organic 

carbons (VOCs). Portland/Vancouver is VOC limited which means that reduction of 

VOCs reduces ozone levels more effectively than a reduction in NOx. TheVOCs that 

contribute most readily to ozone formation are aldehydes, alkanes, alkenes, and to a 

lesser extent aromatics. ODEQ has been measuring these compounds in Portland on a 

consistent basis from 2000 to 2005 (the time of this paper)4

. Figures 4 and 5 contain the 

May through September average concentrations measured by ODEQ in North Portland 

for Formaldehyde, Acetaldehyde, Propionaldehyde, and Acetone (some of the more 

common aldehydes). In all cases there is a negative slope as concentrations trend down 

over the six year period. Formaldehyde levels spiked in 2003 which also had three multiday ≥ 90oF episodes. Acetone levels dropped at a much steeper rate. Maximum Nitrogen 

dioxide, Nitric oxide, and Nitrogen oxides concentrations have also been declining as 

shown in Figure 6. Seasonal average Nitrogen dioxide, Nitric oxide, and Nitrogen oxides 

concentrations have been stable since 1995 with relatively flat slopes (Figure 7).
4 ODEQ measured VOCs at N. Roselawn, Portland and measured NOx at SE Lafayette, Portland. 

y = -0.2x + 2.6

y = -0.09x + 3.8

0

1

2

3

4

5

6

2000 2001 2002 2003 2004 2005

ug/m3

Acetaldehyde

Formaldehyde

Propionaldehyde

May - September Averages

y = -0.02x + 0.5

Figure 4. May through September 2000 to 2005 average Acetaldehyde, Formaldehyde, 

and Propionaldehyde concentrations in North Portland4

. 

y = -0.6x + 3.9

0

1

2

3

4

5

6

2000 2001 2002 2003 2004 2005

ug/m3

Acetone May - September Average

Figure 5. May through September 2000 to 2005 average Acetone concentrations in North 

Portland4

. 
4 ODEQ measured VOCs at N. Roselawn, Portland and measured NOx at SE Lafayette, Portland. 

y = -0.002x + 0.08

y = -0.01x + 0.3

y = -0.02x + 0.4

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

19921993199419951996199719981999200020012002200320042005

ppm

Max 1hr NO2

Max 1hr NO

Max 1hr NOx

Figure 6. SE Portland, May through September 1992 to 2005 maximum one hour 

average concentrations of Nitrogen dioxide, Nitric oxide, and Nitrogen oxides4. 

y = 0.0001x + 0.006

y = -0.0005x + 0.02

y = -0.0009x + 0.03

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

19921993199419951996199719981999200020012002200320042005

ppm

Aver 1hr NO2

Aver 1hr NO

Aver 1hr NOx

Figure 7. SE Portland, May through September 1992 to 2005 Seasonal average 

concentrations of Nitrogen dioxide, Nitric oxide, and Nitrogen oxides4.
5 ODEQ measured temperature at SE Lafayette, Portland. 

4. Temperature 

The Portland/Vancouver temperatures have been trending up from at least 1992 to 2005 

as measured by ODEQ at SE Lafayette. The number of days ≥ 90oF have gone up in the 

last 14 years as shown in Figure 8. There also have been numerous multi-day episodes of 

maximum temperatures ≥ 90oF. The years 1993-1996, 1998, 2000, and 2003-2005 all 

had three plus consecutive day episodes ≥ 90oF. Of these, 1996 and 2003-2005 had the 

greatest number of consecutive hot days. 

1 Day Trendline

y = 0.4x + 5.3

2 Day Trendline

y = 0.13x + 1.3

3 Day Trendline

y = 0.09x + 0.5 0

2

4

6

8

10

12

14

16

18

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

No. of 1, 2, & 3 Days +90 F

1 day >90oF

2 Consecutive days 

>90oF

3 Consecutive days 

>90oF

Figure 8. Number of consecutive days ≥ 90oF for one, two, and three day episodes as 

measured by Oregon DEQ at SE Lafayette5. 

A break down of the three plus day episodes (Figure 9) reveals the number of three, four, 

five, and six consecutive day events each year. Of days ≥ 90oF, 1993, 1994, 1996, 2003, 

and 2004 all had episodes of four days or longer. 1996 was the worst with the only six 

day episode and two three day episodes. More recently 2003 and 2004 had three and four 

day episodes. 

0

1

2

3

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

No. of 3, 4, 5, & 6 Days +90 F

3 Days in a Row +90o F

4 Days in a Row +90o F

5 Days in a Row +90o F

6 Days in a Row +90o F

Figure 9. Three, four, five, and six day episodes of consecutive days ≥ 90oF 5

. 
6. Ventilation indices 1977 to 1996 provided by ODEQ 1996 Meteorological Factors Conducive to Ozone 

Formation in the Portland/Vancouver Area. 

7. Ventilation indices 1998 to 2004 provided by Oregon Dept. of Ag. using Salem soundings. 

5. Ventilation 

The ventilation can be represented using the ventilation index which is calculated by 

multiplying the wind speed and the mixing height. The mixing height is the elevation 

below which all ground level mixing occurs. The lower the ventilation index the poorer 

the mixing. The ventilation index for days with the highest one hour ozone values from 

1977 to 1996 were presented in the 1996 Meteorological Factors Conducive to Ozone 

Formation in the Portland/Vancouver Area6. This graph is reproduced in Figure 10 

below. 

0

5

10

15

20

25

30

19771978197919801981198219831984198519861987198819891990199119921993199419951996

year

Ventilation Index

0

0.05

0.1

0.15

0.2

0.25

1 hr max Ozone (ppm)

Ventilation Index

max 1hr Ozone

Figure 10. Ventilation index on the maximum one hour ozone day 1979-1996 6

. 

The ventilation index for eight hour average ozone from 1998 to 20047 is shown in 

Figure 11. Not surprisingly, the ventilation index is poor for both one hour and eight 

hour average maximums ozone levels. 

For reference, the Washington Department of Ecology displays the ventilation index 

categories in Table 1 for air stagnation levels on their web site. 

http://www.ecy.wa.gov/programs/air/aginfo/MM5Instructions.htm . Jackson County, 

Oregon uses a ventilation index of level of 400 to ban woodstove burning in winter. 
6. Ventilation indices 1977 to 1996 provided by ODEQ 1996 Meteorological Factors Conducive to Ozone 

Formation in the Portland/Vancouver Area. 

7. Ventilation indices 1998 to 2004 provided by Oregon Dept. of Ag. using Salem soundings. 

0

5

10

15

20

25

30

35

40

45

50

1997 1998 1999 2000 2001 2002 2003 2004 2005

Year

Ventilation Index

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

8hr Ozone Average

Ventillation Index

Max 8hr

Figure 11. Ventilation index on the maximum 8hr ozone day 1998-2004 7. 

Table 1. Ventilation Index categories used for smoke management. 

Ventilation Index Dispersion 

0-50 Very poor 

50-2350 Poor 

2350-4700 Marginal-Fair 

> 4700 Good 
8. Temperature collected by ODEQ at SE Lafayette. 

6. Wind Speed 

The horizontal wind speed impacts the ventilation index as well as the location of the 

ozone plume. The wind speeds for the maximum eight hour ozone for each year were 

below six miles per hour as shown in Figure 12. A low wind speed is one of the elements 

for poor ventilation the other being low mixing heights. 

0

2

4

6

8

10

12

14

1990 1992 1994 1996 1998 2000 2002 2004 2006

Wind Speed mph

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

8hr ozone ppm

24 Average WS

Max 8hr Aver. Ozone

Figure 12. Wind speed on the maximum eight hour average ozone day 1992-20058. 

7. High Ozone Episodes 

A high ozone episode is defined in this report as a period of three or more consecutive 

days with maximum temperatures ≥ 90oF, daily average wind speeds < 10 mph, and 

maximum solar radiation > 0.1 Langleys/minutes. Episodes are grouped into weekday or 

weekend events. The weekday episodes occurs entirely within the week. The weekend 

events end on Saturday. 

Episode Identifying Method 

To find episodes, the maximum daily temperatures at ODEQ’s SE Lafayette, Portland 

site were identified and plotted from May through September. Figure 13 provides an 

example plot done for 1996 maximum temperatures. After these high temperature days 

were identified, the maximum daily eight hour ozone levels at all the Portland and 

Vancouver WA sites were retrieved. The wind speed, wind direction, solar radiation, and 

days of the week were also compiled. The dates with average wind speeds above 10 mph 

were removed from consideration. Days occurring in mid to late September were also 

not considered because their solar radiation levels were to low. The resulting three 

consecutive day episodes were compiled in Table 2. These days were grouped into 

weekday episodes and weekend episodes. One three episode per year was used for 

comparison between years. The episodes were selected based on similarities in the time 

of year, solar radiation, three day temperature trend, and low average wind speed. 
8. Temperature collected by ODEQ at SE Lafayette. 

1996 pdx max daily Temp

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

5/15/85/155/225/296/56/126/196/267/37/107/177/247/318/78/148/218/289/49/119/189/25

Date

Temp C

 Figure 13. Maximum daily temperatures for 1996 in Portland8

. 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

Table 2. Three day ozone episodes with maximum temperatures ≥ 90oF, average ws <10 

mph, average solar radiation > 1 Langley/hr 

Date Day 

SPR 

O3 

max 

daily 

8hr 

ave 

MHS/

MSJ 

O3 

max 

daily 

8hr 

ave 

SIS O3 

max 

daily 

8hr ave 

WA 

O3 

max 

daily 

8hr 

ave 

Max 

Tem

p 

max 

SR 

Av

er 

WS 

mp

s 

aver 

SR 

max 

WS 

MPS 

7/28/9

2 Tuesday 0.051 0.033 0.036 void 87 1.30 6.2 0.47 10.6 

7/29/9

2 

Wednesd

ay 0.064 0.043 0.051 0.048 91 1.30 7.0 0.46 10.8 

7/30/9

2 Thursday 0.082 0.078 0.066 0.097 100 1.30 4.4 0.46 8.1 

8/2/93 Monday 0.057 0.047 0.060 0.053 93 1.23 8.0 0.44 13.2 

8/3/93 Tuesday 0.047 0.045 0.065 0.041 98 1.23 8.1 0.39 13.9 

8/4/93 

Wednesd

ay 0.074 0.073 0.072 0.078 97 1.20 3.9 0.42 6.6 

9/6/93 Monday 0.058 0.053 0.043 0.043 90 1.03 5.3 0.30 9.7 

9/7/93 Tuesday 0.057 0.038 0.062 0.051 89 1.02 4.2 0.32 7.0 

9/8/93 

Wednesd

ay 0.058 0.051 0.054 0.048 90 1.05 4.2 0.33 6.6 

6/28/9

5 

Wednesd

ay 0.046 0.046 void 0.050 89 1.28 9.7 0.47 18.7 

6/29/9

5 Thursday 0.057 0.053 0.051 0.057 92 1.28 9.7 0.47 17.8 

6/30/9

5 Friday 0.083 0.063 0.062 0.069 97 1.29 7.1 0.46 12.1 

7/11/9

6 Thursday 0.067 0.054 0.059 0.057 88 1.33 8.2 0.49 14.3 

7/12/9

6 Friday 0.074 0.063 0.062 0.067 92 1.35 7.7 0.50 13.2 

7/13/9

6 Saturday 0.106 0.085 0.069 0.081 97 1.34 6.6 0.49 11.7 

8/7/96 

Wednesd

ay 0.051 0.038 0.053 0.048 89 1.27 6.9 0.45 12.1 

8/8/96 Thursday 0.073 0.057 0.066 0.067 92 1.26 6.9 0.44 12.3 

8/9/96 Friday 0.079 0.061 0.069 0.066 93 1.26 6.7 0.44 12.1 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

8/22/9

6 Thursday 0.048 0.036 0.040 0.046 89 1.21 6.9 0.40 10.3 

8/23/9

6 Friday 0.065 void 0.059 0.056 92 1.18 7.8 0.39 11.9 

8/24/9

6 Saturday 0.073 0.063 0.065 0.067 93 1.10 6.0 0.35 10.3 

7/26/9

8 Sunday 0.116 0.100 0.073 0.070 98 1.26 3.9 0.43 6.2 

7/27/9

8 Monday 0.088 void 0.077 0.078 97 1.27 4.0 0.38 7.7 

7/28/9

8 Tuesday 0.098 0.074 0.062 0.075 99 1.25 5.3 0.45 11.7 

6/26/0

0 Monday 0.056 0.049 0.054 0.054 91 1.28 7.3 0.48 11.2 

6/27/0

0 Tuesday 0.061 0.057 0.066 0.058 96 1.28 5.2 0.47 9.8 

6/28/0

0 

Wednesd

a 0.067 0.048 0.040 0.045 92 1.29 5.3 0.48 10.2 

8/8/01 

Wednesd

ay 0.049 0.040 0.047 0.046 86 1.24 8.0 0.43 14.4 

8/9/01 Thursday 0.081 0.067 0.055 0.063 96 1.25 4.7 0.43 6.4 

8/10/0

1 Friday 0.077 0.066 0.068 0.071 90 1.21 4.6 0.42 7.8 

7/22/0

4 Thursday 0.058 0.054 0.058 0.056 98 1.23 7.0 0.44 12.8 

7/23/0

4 Friday 0.051 0.051 0.061 0.049 102 1.24 6.5 0.45 9.5 

7/24/0

4 Saturday 0.084 0.077 0.058 0.065 100 1.22 6.1 0.44 11.9 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

Three Day Episode Results 

Figure 14 shows weekday ozone episodes from 1992 to 2005. Figure 15 displays 

weekend ozone episodes from 1996 and 2004. 

Ozone on the weekday episodes exhibit a slight negative trend with a slope of -0.004. 

The maximum temperatures for these episodes dropped more significantly -0.01. This 

temperature drop only refers to the episodes selected and is not reflective of the 

temperature trends in Portland/Vancouver. The maximum temperatures in the episodes 

selected were lower toward the end of the 14 year period and could explain in part why 

the ozone levels in this graph were also lower. The weekend episode data shown in 

Figure 15 closely track the meteorological parameters. 

Trendline y = -0.01x + 1.0

Trendline y = -0.002x + 1.3

 Trendline y = 0.004x + 0.5

Trendline y = -0.004x + 0.09

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

SR (Langley), Temp (F)/100, Aver. WS /10

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Ozone (ppm)

Max Temp/100

SR

Aver WS/10

Ozone max daily 8hr ave

Figure 14: Three day episodes with maximum temperatures ≥ 90oF, wind speed < 

10mph, Solar Radiation >1 Langleys/hr, all weekdays9

. 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

0.2

0.4

0.6

0.8

1.0

1.2

1.4

SR (Langley), Temp (F)/100, Aver. WS /10

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Ozone (ppm)

Max Temp/100

SR

Aver WS

Ozone max daily 8hr ave

July 

1996 Aug

1996

July 

2004

Figure 15: Three day episodes with maximum temperatures ≥ 90oF, wind speed < 

10mph, Solar Radiation >1 Langley/hr with third day on Saturday9

. 

1998 and 2003 episode comparison. 

In 2003 there were three multi-day episodes with temperatures ≥ 90oF but the maximum 

ozone level only reached 0.084 ppm. Conversely, in 1998 there was only one multi-day 

episode ≥ 90oF and the maximum level reached 0.116 ppm. If the maximum daily 

temperature criteria were lowered to ≥ 88oF, there are more episodes to consider. 

Three day episodes

In 1998 there is one three day episode and in 2003 there are two three day episodes ≥

88oF (Table 2). These episodes all include at least one weekend day, however, one starts 

on a Sunday, one ends on Saturday, and one includes both Saturday and Sunday. All 

three days in the 1998 episode have maximum temperatures in the high 90s and only one 

day in the 2003 episodes has a temperature in the high 90s. Solar radiation and wind 

speeds are comparable. Figure 16 compares these three episodes. There is a drop in 

ozone levels in 2003 from 1998, however, the temperature in the 2003 episodes is also 

lower and the wind speed are higher. The 1998 maximum ozone level occurs on the first 

day (Sunday) of the three day episode. The proceeding day had a maximum temperature 

of 85oF and a maximum 8 hour ozone of 0.067ppm. The eight hour ozone level had a 

jump of 0.05 ppm in one day. The first days of the 2003 episode were both only 

0.002ppm higher than the proceeding day. 

 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

Table 2. Three day ozone episodes with maximum temperatures ≥ 88oF, average ws <10 

mph, average solar radiation > 1 Langley/hr9 

Year Date Day 

Max 

O3 

Max Temp 

(X10) 

max 

SR 

aver 

SR 

Aver 

WS 

mph 

max WS 

MPS 

7/26/98 Sun 0.116 9.8 1.26 0.43 4 6.2 

7/27/98 Mon 0.088 9.7 1.27 0.38 4 7.7 1998 

7/28/98 Tue 0.098 9.9 1.25 0.45 5 11.7 

6/5/03 Thr 0.070 9.0 1.33 0.50 8 13.7 

6/6/03 Fri 0.071 9.6 1.31 0.49 6 10.4 2003 

6/7/03 Sat 0.071 9.0 1.29 0.47 5 9.3 

6/27/03 Fri 0.046 8.8 1.29 0.48 7 11.8 

6/28/03 Sat 0.066 9.2 1.32 0.47 7 13.1 2003 

6/29/03 Sun 0.055 8.9 1.29 0.47 9 14.0 

0.055

0.066

0.046

0.071

0.071

0.070

0.098

0.088

0.116

0

2

4

6

8

10

12

Temp F (X10), WS (mph)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Ozone (ppm)

Max Temp (X10)

Aver WS mph

max SR

Max O3

Sun - Tues, Jul 26 - Jul 28 Thr - Sat, Jun 5 - Jun 7 Fri - Sun, Jun 27 - Jun 29

1998 2003 2003

Figure 16. 1998 and 2003 three day episodes with temperatures ≥ 88oF, ws < 10mph, > 1 

Langley/hr9

. 
9. Ozone measured by ODEQ and Southwest Washington Clean Air Agency, Meteorological data 

measured at SE Lafayette, Portland. 

Four day episodes

Two four day episodes with similar meteorological patterns occurred in 1998 and 2003 in 

early September (Table 3). The maximum ozone levels during these two four day 

episodes were virtually identical and they both occurred in the second day of the four day 

stretch (Figure 17). 

Table 3. Four day ozone episodes with maximum temperatures ≥ 88oF, average ws <10 

mph, average solar radiation > 1 Langley/hr 

Year Date Day 

Max 

O3 

Max Temp 

(X10) 

max 

SR 

aver 

SR 

Aver WS 

mph 

max WS 

MPS 

8/31/98 Mon 0.079 9.6 1.1 0.36 6 9.2 

9/1/98 Tue 0.081 9.1 1.1 0.36 5 9.7 

9/2/98 Wed 0.041 8.8 1.1 0.36 7 11.2 

1998 

9/3/98 Thr 0.053 8.9 1.1 0.35 7 11.2 

9/2/03 Tue 0.055 9.2 1.0 0.33 5 6.8 

9/3/03 Wed 0.084 9.2 1.0 0.30 4 6.6 

9/4/03 Thr 0.075 9.2 1.0 0.33 4 6.6 

2003 

9/5/03 Fri 0.050 9.0 1.1 0.34 4 6.9 

1998 vs 2003 Four day episodes

0.081

0.041

0.055

0.084

0.075

0.050

0.079

0.053

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Temp F (X10), WS (mph)

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

Ozone (ppm)

Max Temp (X10)

Aver WS mph

max SR

Max O3

Tue - Thr , Aug 31 - Sept 3 Mon - Wed, Sep 2 -Sept 5

1998 2003

Figure 17. 1998 and 2003 four day early September episodes with temperatures ≥ 88oF, 

ws < 10mph, > 1 Langley/hr.
Discussion: 

In the last 14 years, ozone levels in Portland and Vancouver have been going down 

slightly while the population and the vehicle miles traveled continued to grow. The 

number of consecutive days with temperatures over 90oF went up during the same period. 

When similar high temperature episodes were compared over the years, ozone trends also 

were slightly negative. Portland also witnessed a drop in precursor pollutants 

concentrations which helps explain the lower than expected ozone levels. The wind 

speed and ventilation on maximum ozone days were typical for stagnant conditions and 

were not a factor in the ozone trending. 

The 2003 episodes were evidence of this. 2003 had more episodes than a similar hot 

year, 1998, but had a much lower maximum. The 1998 maximum occurred on a Sunday 

and jumped 0.05 ppm in one day. This is a very large rate of increase for the eight hour 

average and indicates that there was much more activity on Sunday than usual along with 

low wind speeds and high temperatures. The 1998 solar radiation levels were actually 

lower than the 2003 episodes however. The Monday following this event was a Clean 

Air Action Day and could have helped lower the precursor pollutants levels. The 2003 

episodes had lower ozone levels with similar meteorological factors which could mean 

lower precursor levels were present. The precursor trends shown in Figures 4 through 7 

above prove this to be true. 

Conclusion: 

The slightly negative ozone trend, the upward temperature trends, and the negative 

precursor trends would indicate that the ozone reduction programs are working. This 

argument can be made because the Portland/Vancouver population and total vehicle 

miles traveled continues to grow. However, many of the VOC controls are already in 

place and Portland/Vancouver could see more ozone with the expected population 

growth. The Oregon Low Emission Vehicle program is new in 2005-2006 and may 

result in further reduction in precursors per VMT. 
References 

1. Barnack, A. 2004 Oregon Air Quality Data Summaries, Oregon Department of 

Environmental Quality, Air Quality Division, May 2005. pg 44. 

2. Cai, Q. 2004 Oregon Population Report, Population Research Center, College of 

Urban and Public Affairs, Portland State University. Pg 7. 

3. Author unknown, 2002 Oregon Population Report, Population Research Center, 

College of Urban and Public Affairs, Portland State University. Pg 6. 

4. Turpel M., Principal Transportation Planner 

5. Horowitz D., 001_Portland-National-DMVT.xls, Portland Metro 

6. Finneran, B. 1996 Meteorological Factors Conducive to Ozone Formation in the 

Portland/Vancouver Area, Oregon DEQ. 

Simplified Ozone Formation 

Ozone (O3) is a secondary pollutant produced by the photolysis of Nitrogen Dioxide 

(NO2) to NO and a single oxygen (O). The single Oxygen (O) bonds to Oxygen (O2) in 

the presence of any solid material. In summary the reaction is: 

NO2 + O2 + m NO + O3 + m 

Ozone readily reacts with Nitrogen oxide (NO) to return to oxygen when the ultra violet 

radiation levels drop. 

NO + O3 NO2 + O2 

This is an equilibrium which can be driven to produce more ozone by the addition of 

more NO2. NO2 is formed indirectly from the degradation of volatile organic carbons 

(RHOC) in the presence of free radicals (OH•) and ultraviolet radiation. The RHOC and 

OH• produce another free radical HO2• which can react with NO to form NO2 and 

another free radical (OH•). 

 

OH• + RHOC NO + HO2• NO2 + OH• 

nohv

hv

hv