Guidance for Improving Weight of Evidence 

Through Identification of Additional Emission Reductions, 

Not Modeled

by

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Emissions, Monitoring, and Analysis Division

Air Quality Modeling Group

Research Triangle Park, NC 27711

November 1999

Introduction

This paper provides guidance for using information from photochemical grid modeling

and ambient air quality monitoring to estimate additional levels of emission reductions needed to

support the 1-hour NAAQS for ozone beyond the reductions contained in the demonstrations

submitted by the States in 1998. Procedures for estimating improvements expected with the

implementation of the Tier 2 low sulfur program and benefits towards attainment are also

provided. Two techniques are described for estimating additional emission reductions, each with

its own strengths and weaknesses. Use is made of the fact that, since 1999 is more than half-way

from the model base year (1990) to the attainment year (2005 or 2007 in most areas), air quality

data from 1990 to 1999 allows modelers the opportunity to determine the representativeness of

the modeled predictions. These techniques identify the additional percentage reduction in NOx

and VOC from the 1990 emissions. 

General Procedures for Improving Weight of Evidence Through Identification of

Additional Emission Reductions, Not Modeled, Including Tier 2.

To strengthen the weight of evidence and account for high modeled peaks, estimate

additional measures that at a minimum bring the model estimated future design value to 124 ppb

or below. This is done by first estimating a future design value using the model predicted peaks.

Multiply the base design value by a ratio (average of model predicted peaks (across all days),

after controls divided by before controls). The base design value is an average of three years of

monitored design values that represent the modeled base case emissions. If the model estimated

future design value is at or below 124 ppb, additional emission reductions should not be needed.

If the model estimated future design value is greater than 124 ppb, estimate additional

measures by using two ratios 1) modeled change in design values to modeled change in

emissions and 2) air quality design value changes to NET/local emissions changes between two

reference years (e.g., 1990 and 1996). Do not include biogenic emissions. First, subtract 124
ppb from estimated future design value to identify additional ozone reduction needed. Then

multiply each ratio by the ozone reduction needed to estimate additional VOC and NOx emission

reductions needed to strengthen the weight of evidence argument for attainment. This results in

the additional percent reduction needed from the 1990 emissions.

To calculate the level of emission reductions needed (in tons per day) multiple the 1990

base emissions by the percent reductions. This results in the total tons per day reduction which

are additional reductions needed in the attainment year. To incorporate the impact of Tier 2

subtract the emission reduction estimates being applied towards attainment for Tier 2 from the

additional reductions. The remaining reductions may be adjusted to reflect other unmodeled

control measures which have been quantified. The following are more details of the procedures

with examples.

Estimating Additional Emission Reductions

Each of the methods described in the remainder of this paper begins with a monitored

ozone concentration which can be extrapolated to the attainment year and compared with the

standard. If the attainment year concentration is over 124 ppb, the methods described in this

paper can be used to estimate what would constitute additional emission reductions needed to

support a weight of evidence argument for attainment. The differences among the methods lie in

the factors used for this extrapolation. These are summarized in Table 1.

Both methods are based on the assumption that we can estimate the relationship between

ozone and its precursors (VOC and NOx). We can estimate this relationship by either (1)

comparing changes in model predicted ozone to changes in modeled emissions or (2) comparing

changes in observed air quality to changes in actual emissions. Both methods for estimating a

relationship are equally valid. Both have inherent uncertainty in estimates of emissions

inventories and estimates of the change in ozone air quality. Utility of either method is

dependent on the availability of data which shows a response in ozone due to a decrease in VOC

and NOx emissions. For example, if an area wants to apply method 2 using the NET inventories

for the 1990 and 1996 reference years, the VOC and NOx totals for the nonattainment area must

show a decrease in VOC and NOx between 1990 and 1996. If this is not the case then use of the

NET data for those two reference years in not appropriate.
Table 1. Summary of Methods for Estimating Additional Emission Reductions 

Method Ozone Concentration Being

Extrapolated

Extrapolation Ratio

(normalized reduction factor)

1 Future Air Quality Design Value Change in modeled emissions

 From base to attainment year 

 Change in modeled concentration

2 Future Air Quality Design Value Change in actual emissions

 Between two reference years 

Change in monitored concentration

Estimate a Future Air Quality Design Value 

Both methods make use of the results of past modeling to derive a modeled response of

ozone design values to VOC and NOx controls to estimate a future air quality design value.

Relative reduction factors are derived and used similarly to what is described in U.S. EPA,

(1999), Draft Guidance on the Use of Models and Other Analyses In Attainment Demonstrations

for the 8-Hour Ozone NAAQS, EPA-454/R-99-004. If the estimated future design value is < 124

ppb, no additional emission reductions are needed to strengthen the weight of evidence argument

for attainment. 

(1) Calculate an average (over all modeled days) predicted daily maximum (within the

nonattainment area and the down wind plume, or domain wide) 1-hour ozone concentration, first

with the base emissions (e.g., 1990) and then with the future emissions (e.g., 2007). 

(2) Using results from step 1, calculate the relative reduction factor, RRF, by taking the ratio of

the average daily maximum 1-hour ozone concentration obtained with future emissions to that

obtained with the base emissions.

RRF = AVGf / AVGc (1)

where 

AVGf = average (across all days) predicted daily maximum 1-hour ozone concentration

for future emissions, ppb.

AVGc = average (across all days) predicted daily maximum 1-hour ozone concentration

for base emissions, ppb.

(3) Calculate the base design value, DVB, as the average of 3 nonattainment area ozone design

values that represent the period used to predict ozone for base emissions (e.g., if 1990 emissions

are used, average design values for 1990, 1991 and 1992)1. The nonattainment area ozone

1Note, 1990, 1991 and 1992 design values reflect observations for 1988-90, 1989-91, and
design value is the maximum monitored design value from all sites in the nonattainment area. 

(4) Estimate the future design value, DVF, for the nonattainment area as the product of the

relative reductions factor (step 2) and the base design value (step 3). If the future design value is

< 124 ppb additional emission reductions should not be needed, no additional steps are required.

If the future design value is > 124 ppb proceed to apply method 1 and method 2 (below) to

identify additional emission reductions. 

Example 1: Estimate Future Air Quality Design Value

Given: Past results from modeling indicate predicted peaks (for three days) before controls in

1990 are 195, 180, and 165 ppb and after controls in 2007 are 155, 150 and 145 ppb. There are

two monitor sites in the nonattainment area. The monitored air quality design values for each

site are 185 and 176 in 1990, 145 and 152 in 1991, and 155 and 140 in 1992. 

Find: Estimate the future air quality design value in 2007. 

Solution:

(1) Compute the base and future average 1-hour daily maximum concentration. The average of

the model predicted peaks (in and downwind of the nonattainment area) for the base before

controls is: (195 + 180 + 165) / 3 = 180 ppb and for the future after controls is: (155 + 150 +

145) / 3 = 150 ppb. 

(2) Using the results in step 1 the relative reduction factor is: 150/180 = 0.83.

(3) Determine the nonattainment area design values representative of the episode used in the base

emissions and calculate the base design value. The nonattainment area design value for 1990 is

MAX(185, 176) = 185, for 1991 is MAX(145, 152) = 152, and for 1992 is MAX (155, 140) =

155 ppb. The base ozone design value is (185 + 152 + 155) / 3 = 164 ppb. 

(4) The estimated future design value is (0.83) (164) = 136 ppb

This is > 124 ppb, so we need to apply the following methods to determine additional emission

reductions. 

1990-92, respectively. All of these periods include “1990", the year of the base emissions.
Method 1: Estimate Additional Emission Reductions Using Modeled Responses

Method 1 uses the change in nonattainment area monitored base ozone design value and

estimated future ozone design value along with changes in modeled emissions before controls

(base emissions) and after controls (future emissions) to estimate additional emission reductions.

(1) Calculate the change in air quality design value by subtracting the estimated future design

value (e.g., 2007) from the base air quality design value (e.g., 1990). Estimate the percent

reduction in NOx emissions and VOC emissions which occurred within the nonattainment area

before and after controls. Do not include biogenic emissions. Divide the percent reduction in

NOx emissions by the change in the air quality design value and divide the percent reduction in

VOC emissions by the change in the air quality design value. This step results in two reduction

factors, one for changes in NOx emissions and one for changes in VOC emissions. 

(2) Estimate the amount of additional ozone reduction needed by taking the difference between

the future design value and 124 ppb, the maximum ozone design value consistent with meeting

the NAAQS.

(3) Calculate additional necessary emission reductions by taking the product of each of the

reduction factors (step 1) and the amount of ozone reduction needed (step 2). 

Example 2: Calculate reduction factor using model predictions and apply to model estimated

future design value

Given: Results from modeling used in Example 1 indicate an estimated future design value is

136 ppb and the monitored air quality ozone base design value representative of the

nonattainment area is 164 ppb. The control strategy reflects a 30% reduction in VOC and a 35%

reduction in NOx emissions. These reductions were obtained by comparing the modeled 1990

base emissions to the modeled 2007 attainment year emissions for the nonattainment area.

Find: The amount of additional VOC and NOx reduction needed to reduce the model estimated

future design value to 124 ppb, so that a convincing weight of evidence argument can be made

for attainment. 

Solution:

(1) Calculate the change in air quality design value as 164 - 136 = 28 ppb. The estimated percent

reduction in VOC and NOx are given 30% VOC and 35% NOx. The reduction factor for VOC is

30% / 28 ppb = 1%/ ppb and for NOx is 35% / 28 ppb = 1.2%/ ppb. 

(2) The amount of additional ozone reduction needed is (136 - 124) = 12 ppb.

(3) Therefore, the additional reduction needed in VOC is (1%) (12) = 12% of the VOC

emissions. And, the additional reduction needed in NOx emissions is (1.2%) (12) = 14% of the

NOx emissions. 
Method 2: Estimate Additional Emission Reductions Using Observed Air Quality Changes

This method uses monitored ozone air quality design values and emissions estimates for

the nonattainment area to calculate the reduction factors for VOC and NOx. These reduction

factors are then applied to the model estimated future design value as calculated in Example 1 to

estimate additional emission reductions. 

(1) Calculate the percent reduction in NOx emissions and VOC emissions which occurred within

the nonattainment area from an earlier year (e.g., 1990) to a more recent year (e.g., 1996). The

National Emissions Trends (NET) inventory provides an example of these data. Do not include

biogenic emissions. 

(2) Calculate the change in the nonattainment area’s ozone design value using the same reference

years. To account for fluctuations in meteorology average three years of design values to

estimate the design value for each of the reference years. The nonattainment area average design

values are used to assess the observed change in air quality from the “early” time period to a

“recent” time period. Monitors that were only online during one of these periods may not be

representative of the actual change in air quality. Rationale for excluding a monitor should be

documented.

(3) Divide the percent reduction in NOx emissions by the change in the area’s ozone design

value. Divide the percent reduction in VOC emissions by the change in the area’s ozone design

value. This step gives two reduction factors, one for changes in NOx emissions and one for

changes in VOC emissions. 

(4) Calculate the additional amount of ozone reduction needed by subtracting 124 ppb from the

model estimated future design value (see Example 1). 

(5) Calculate additional necessary emission reductions by taking the product of each of the

reduction factors (step 1) and the amount of ozone reduction needed (step 2). 

Example 3: Calculate reduction factor using change in ozone air quality design values and

nonattainment area emissions, and apply to model estimated future design value

Given: There are two monitors in the nonattainment area. The monitored air quality design

values for each site for reference years 1990 and 1996 are presented in Table 2. Emission

reductions between 1990 and 1996 are 30% reduction in VOC and a 35% reduction in NOx

emissions. These reductions were obtained by comparing the 1990 NET inventory to the 1996

NET inventory for the nonattainment area. The model estimated future design value in 2007 is

136 ppb. 

Table 2. Air Quality Design Values (ppb)

Monitor

1990 Reference Year 1996 Reference Year

1990 1991 1992 1996 1997 1998
1 185 145 155 140 146 139

2 176 152 140 135 145 130

Find: The amount of additional VOC and NOx reduction needed to reduce the future design

value to 124 ppb, so that a convincing weight of evidence argument can be made for attainment. 

Solution:

(1) The estimated percent reduction in VOC and NOx are given 30% VOC and 35% NOx.

(2) Calculate the change in the nonattainment area’s ozone design value. Determine the design

value for each reference year by first taking the maximum design from the two sites for each of

three years and then averaging the three years design values. The nonattainment area’s ozone

design value for 1990 is (185 + 152 + 155) / 3 = 164 and for 1996 is (140 + 146 + 139) / 3 = 142

ppb. The change in air quality design value as 164 - 142 = 22 ppb.

(3) The reduction factor for VOC is 30% / 22 ppb = 1.36%/ ppb and for NOx is 35% / 22 ppb =

1.59%/ ppb. 

(4) The amount of additional ozone reduction needed is (136 - 124) = 12 ppb.

(5) Therefore, the additional reduction needed in VOC is (1.36%) (12) = 16% of the VOC

emissions. And, the additional reduction needed in NOx emissions is (1.59%) (12) = 19% of the

NOx emissions. 

Incorporate Tier 2 and other unmodeled control measures

Once the percent reductions for VOC and NOx have been determined they can be

converted into tons per day reductions. Control measures used to address these additional

reductions must be quantified as estimates in tons per day reductions and compared to the level

of additional reductions needed. Sufficient additional measures have been identified when the

total from all unmodeled controls are equal to or greater than the estimated additional reductions.

(1) Convert the estimated percent reduction in VOC and NOx to tons per day by taking the

product of the percent reduction and the total emissions in the base case inventory for each

category of emissions, VOC and NOx. This results in tons per day for VOC and tons per day for

NOx. These are the additional level of controls needed.

(2) Subtract the Tier 2 emission reduction estimates being applied towards attainment from the

additional level of controls for each category of emissions, VOC and NOx. All other unmodeled

controls should be subtracted as well. Repeat this step until no additional reductions remain.

Example 4: Adjust additional emission reductions to account for Tier 2

Given: The nonattainment area total emissions in 1990 for VOC and NOx are 1197 tpd and 927
tpd, respectively. Also, as shown in Table 3 the estimated Tier 2 reductions in VOC and NOx are

10 tpd and 25 tpd, respectively. The estimated additional emission reductions are 16% VOC and

19% NOx, as calculated in example 3.

Table 3: Nonattainment Area Emissions Summary (tpd) without Tier 2

Year

VOC NOx

Point Area Mobile Total Point Area Mobile Total

1990 400 447 350 1197 300 377 250 927

2007 241 282 200 723 150 312 125 587

 Estimated Tier 2 Reduction = 10 25

Find: What are the additional emission reductions in tons per day still needed after incorporating

Tier 2?

Solution:

(1) The additional reductions are (.16 * 1197 tpd) = 192 tpd for VOC and (.19 * 927) = 176 tpd

for NOx.

(2) After subtracting Tier 2 reductions the remaining reductions are (192 - 10) = 182 tpd for VOC

and (176 - 25) = 151 tpd for NOx.
Use of Results 

The results from both methods should be considered along with other weight of evidence

presented in the technical analyses for the attainment demonstration. For example, where model

predicted peaks show greater improvement when low level NOx emissions are reduced verses

VOC or elevated NOx, substituting an equal amount of low level NOx reductions for the VOC

reductions is acceptable. Also, where modeling demonstrates substantial improvements in model

predicted peaks when emission reductions are applied to adjacent counties, the area of controls

may be extended to include adjacent counties. However, if emissions from adjacent counties are

used they must be included in the total emissions for the base and future. Modeling the

additional emission reductions would normally address these two example as well as the

following: change in boundary conditions due to transport, location of emissions (such as point,

area or mobile), elevated vs low level emission reductions, chemistry and wind flow patterns.

Model sensitivity runs may be used to help identify the appropriate controls measures to fill the

additional emission reductions needed to provide for attainment in the weight of evidence

analyses.

For guidance on VOC and NOx substitution use the, “NOx Substitution Guidance”, EPA

1993; “Transmittal of NOx Substitution Guidance”, memorandum from John Seitz, 1993;

“Clarification of Policy for Nitrogen Oxides (NOx) Substitution”, memorandum from John Seitz,

1994; and “Guidance for Implementing the 1-Hour Ozone and Pre-Existing PM10 NAAQS”,

memorandum from Richard D. Wilson, 1997. The 1993 and 1994 guidance was primarily

designed for the post-1996 rate of progress (3%/year VOC reduction) requirement and allowed

NOx reductions to be substituted for the otherwise mandatory VOC reductions as long as the

NOx reductions were shown to be consistent with the attainment demonstration (in other words,

if the attainment demo relied only on VOC reductions, the area could not substitute NOx

reductions for the 3%/year requirement, and if the attainment demo relied on both VOC & NOx

reductions, NOx could be substituted in part ). The 1994 guidance document (Guidance on the

Post-1996 Rate-of-Progress Plan and the Attainment Demonstration, EPA-452/R-93-015, Jan.

1994) provided equations & procedures for calculating the amount of NOx reductions that could

be substituted for VOC for the rate of progress requirements. Also, the 1997 guidance

establishes the 100 & 200 km distances for substitution of emission reductions outside the

nonattainment area. These documents are located on the EPA website:

“www.epa.gov/ttn/oarpg/t1pgm.html”.