DEVELOPMENT OF OZONE REACTIVITY SCALES

FOR VOLATILE ORGANIC COMPOUNDS

by

William P. L. Carter

Published in the

Journal of the Air and Waste Management Association

Vol 44, pages 881-899

January 20, 1994

Statewide Air Pollution Research Center, and

College of Engineering Center for Environmental Research and Technology

University of California

Riverside, California 92521-0312
Abstract

Methods for ranking photochemical ozone formation reactivities of volatile organic compounds

(VOCs) are discussed. Photochemical mechanisms for the atmospheric reactions of 118 VOCs were used

to calculate their effects on ozone formation under various NOx conditions in model scenarios representing

39 different urban areas. Their effects on ozone were used to derive 18 different ozone reactivity scales,

one of which is the Maximum Incremental Reactivity (MIR) scale used in the new California Low

Emission Vehicle and Clean Fuel Regulations. These scales are based on 3 different methods for

quantifying ozone impacts and on 6 different approaches for dealing with the dependencies of reactivity

on NOx. The predictions of the scales are compared, the reasons for their similarities and differences are

discussed, and the sensitivities of the scales to NOx and other scenario conditions are examined. Scales

based on peak ozone levels were highly dependent on NOx, but those based on integrated ozone were less

sensitive to NOx and tended to be similar to the MIR scale. It is concluded that the MIR scale or one

based on integrated ozone is appropriate for applications requiring use of a single reactivity scale.

Implications

Control strategies which encourage use of VOCs which form less ozone per gram emitted may provide

a less costly way to achieve ozone reductions. An example of this is to encourage use of alternative fuels

for motor vehicles. Practical implementation of such strategies requires some means to quantify ozone

formation potentials of VOCs. This paper discusses various methods to do this.

About the Author

William P. L. Carter is a Research Chemist with a joint appointment at the Statewide Air Pollution

Research Center and the College of Engineering Center for Environmental Research and Technology at

the University of California, Riverside CA 92521.

2
Introduction

The formation of ground-level ozone is a serious air pollution problem in many areas. Ozone is

not emitted directly, but is formed from the photochemical interactions of volatile organic compounds

(VOCs) and oxides of nitrogen (NOx). Many different types of VOCs are emitted into the atmosphere,

each reacting at different rates and with different reaction mechanisms.1 Because of this, VOCs can differ

significantly in their effects on ozone formation. These differences in effects on ozone formation are

referred to as the ozone "reactivities" of the VOCs. Although in the past such differences have often been

neglected and all non-exempt VOCs have been regulated equally, in recent years it has become recognized

that control strategies which encourage the use of less reactive VOCs could provide a cost-effective means

to achieve ozone reductions. An example of this is to encourage the use of alternative fuels for motor

vehicles. However, practical implementation of such strategies requires some means to quantify the

reactivities of VOCs.

There are a number of ways to quantify VOC reactivities, but the most relevant measure of the

effect of a VOC on ozone is the actual change in ozone formation in an airshed resulting from changing

the emissions of the VOC in that airshed. This depends not only on how rapidly the VOC reacts and the

nature of its atmospheric reaction mechanism, but also the nature of the airshed where it is emitted,

including the effects of the other pollutants which are present. Although the effect of VOCs on ozone

formation can be measured in environmental chamber experiments, the fact that these effects depend on

the environment where the VOCs react means that one cannot necessarily assume that quantitative ozone

impacts in the atmosphere will necessarily be the same as those measured in the laboratory. However,

the effect of a VOC on ozone in the atmosphere can be estimated using computer airshed models. While

the results of such calculations are no more reliable than the models of the chemical reactions and the air

pollution episode being considered, modeling provides the most realistic and flexible means to assess the

many factors which affect ozone formation from VOCs and for the development of VOC reactivity scales.

The effect of changing the emissions of a given VOC on ozone formation in a particular episode

will in general depend on the magnitude of the emission change and on whether the VOC is being added

to, subtracted from, or replacing a portion of the base case (i.e., present day) emissions. To remove the

dependence on this, it was proposed to use "incremental reactivity" to quantify ozone impacts of VOCs.2

This is defined as the change on ozone caused by adding an arbitrarily small amount of the test VOC to

the emissions in the episode, divided by the amount of test VOC added. This can also be thought of as

3
the partial derivative of ozone with respect to emissions of the VOC. Note that this does not necessarily

predict the effects of large changes in emissions, as might occur, for example, if all the motor vehicles

in an airshed were converted to another type of fuel. However, Chang and Rudy3 found that incremental

reactivities give good approximations to effects on ozone of alternative fuel substitution scenarios

involving changing up to 30% of the total VOC emissions. In any case, incremental reactivities will

predict the direction of an initial ozone trend which results when a control strategy is being phased in.

Incremental reactivities have been investigated in a number of computer modeling studies,3-9 and

the VOC’s reaction mechanism was found to be important in affecting its incremental reactivity. Some

compounds can cause the formation of 10 or more additional molecules of ozone per carbon atom reacted,

either directly or through its effects on reactions of other compounds, while others cause almost no ozone

formation when they react, or even cause ozone formation to be reduced9

. The predictions that VOCs

have variable effects on ozone formation, even after differences in how rapidly they react are taken into

account, and that some have negative effects on ozone formation under some conditions, have been

verified experimentally2,10.

The modeling studies also predict that incremental reactivities depend significantly on the

environmental conditions, particularly on the relative availability of NOx

5,9. NOx availability has

traditionally been measured by the ratio of total emissions of reactive organic gases (ROG) to NOx. In

general, VOCs have the largest incremental reactivities under relatively high NOx conditions (i.e., low

ROG/NOx ratios) and have much lower, in some cases even negative, reactivities under conditions where

NOx is limited (high ROG/NOx ratios). This is because under high NOx conditions the amount of ozone

formed is determined by the levels of radicals formed from the reactions of the VOCs, while under lower

NOx conditions it is the availability of NOx, which must be present in order for ozone to be formed, which

limits ozone formation. Other aspects of the environment in which the VOC is emitted, such as nature

of the other organics emitted into the airshed,8,11 the amount of dilution occurring,9 etc., can also be

important in affecting VOC reactivities, though investigations of these aspects are more limited.

The fact that incremental reactivities depend on environmental conditions means that no single

scale can predict incremental reactivities, or even ratios of incremental reactivities, under all conditions.

Thus the concept of a "reactivity scale" oversimplifies the complexities of the effects of VOC emissions

on ozone formation. Nevertheless, for some regulatory applications, the only practical choice is between

using some reactivity scale or ignoring reactivity altogether. The latter would be the appropriate choice

if reactivities were so variable that all VOCs could be considered to be the same to within this variability.

4
If this is not the case, and if the policy is adopted to use a reactivity scale, the issue becomes how one

would develop a scale whose use would result in the greatest overall air quality improvement for the range

of conditions where it will be applied.

An example of a case where the policy was adopted to use a reactivity scale is the "Low-Emission

Vehicles and Clean Fuels" regulations in California.12 In this regulation, non methane organic gas

(NMOG) exhaust standards for alternatively fueled vehicles are determined using reactivity adjustment

factors (RAFs), which are intended to relate the differences in ozone formation potential of the exhausts

compared to that of conventionally fueled vehicles.12 The regulation as presently adopted utilizes the

maximum incremental reactivity (MIR) scale developed by this author to calculate these RAFs.13

This paper gives the results of an investigation of alternative approaches for developing reactivity

scales, and describes the development of the MIR and other reactivity scales.

Chemical Basis of Reactivity

This section gives a summary of the fundamentals of the chemistry of O3 formation, which may

be useful for an understanding of this work. The only significant process forming O3 in the lower

atmosphere is the photolysis of NO2, which is reversed by the rapid reaction of O3 with NO.

NO2 + hν O(3P) + NO; O(3P) + O2 +M O3 + M; O3 + NO NO + O2

This results in O3 being in a photostationary state dictated by the NO2 photolysis rate and the [NO2]/[NO]

ratio. If reactive VOCs were not present, then significant amounts of O3 would not be formed. When

VOCs are present, they react to form radicals which either consume NO or convert NO to NO2, which,

because of the photostationary state relationship, causes O3 to increase. Although many types of reactions

are involved1

, the major processes can be summarized as follows:

VOC + OH RO2 + products

RO2 + NO NO2 + radicals

radicals OH + products

5
The rate of ozone increase caused by these processes is dependent on the amounts of VOCs present, the

rate constants for the VOC’s initial reactions, and the level of OH radicals and other species with which

the VOCs might react. Ozone production continues as long as sufficient NOx is present that reactions of

peroxy radicals (RO2) with NOx compete effectively with their reactions with other peroxy radicals.

Note that the OH radical levels are particularly important in affecting the O3 formation rate in the

presence of NOx because reaction with OH is a major (and in many cases the only) process causing most

VOCs to react. Thus if a VOC reacts in such a way that it initiates radical levels (or forms a product

which does), then it would enhance the rate of ozone formation from all VOCs present. This would give

it a high incremental reactivity compared to other VOCs. If the VOC has radical termination process

when it reacts in the presence of NOx, it will cause all VOCs to react slower and form less O3. In some

cases this reduced O3 formation from other VOCs may be more than enough to counter the ozone

formation formed from the VOC’s direct reactions. In such cases the VOC would have a negative

incremental reactivity in the presence of NOx.

Ozone formation stops once NOx is consumed to sufficiently low levels. Since NOx is removed

from the atmosphere more rapidly than VOCs (since most VOCs form product VOCs which also react),

it is NOx availability which ultimately limits O3 formation. If the NOx levels are sufficiently high that it

is not consumed before the end of the day, then it is mainly the rate of the VOC’s reactions, and their

effects on OH radicals, which affect incremental reactivity. Indeed, NOx inhibits O3 under high NOx

conditions because reaction of OH with NO2 is an important radical terminating process. If, however, NOx

is consumed before the end of the day, then O3 is NOx-limited, and increasing NOx would cause increased

O3 formation. Under such conditions, if a VOC’s reactions caused NOx to be removed more rapidly than

if the VOC were absent (such as, for example, by forming nitrogen-containing products such as PAN’s

from aldehydes and nitrophenols from aromatics), then this would have a negative effect on O3 yields, and

tend to reduce a VOCs incremental reactivity. Under highly NOx-limited scenarios, this becomes

sufficiently important to cause negative incremental reactivities for VOCs with significant NOx sinks in

their mechanisms — even for those which may have highly positive effects on O3 under conditions where

NOx is plentiful.

Thus NOx conditions are a major factor affecting reactivity. However, other scenario conditions

will also affect reactivity, by affecting how rapidly NOx is removed, by affecting overall radical levels and

thus how rapidly NOx and VOCs react, and by affecting other factors determining the efficiency of ozone

6
formation. This results in variations of incremental reactivities among the different airshed conditions,

even those with similar NOx levels. The relative importance of these factors are investigated in this work.

Methods

This paper uses a number of specialized terms and abbreviations. To assist the reader in following

this discussion, Table I gives a summary of these terms and abbreviations. These are discussed in more

detail below.

Scenarios Used for Reactivity Assessment

The assessment of ozone reactivities of VOCs under a variety of conditions requires calculating

their effects on ozone formation using a set of model scenarios which represent a realistic distribution of

environmental conditions. An extensive set of pollution scenarios has been developed for conducting

analyses of effects of ROG and NOx controls on ozone formation using the EKMA modeling approach14-18.

The EKMA approach involves use of single-cell box models to simulate how ozone formation in one day

episodes is affected by changes in ROG and NOx inputs. Although single-cell models cannot represent

realistic pollution episodes in great detail, they can represent dynamic injection of pollutants, time-varying

changes of inversion heights with entrainment of pollutants from aloft as the inversion height increases

throughout the day, and time-varying photolysis rates, temperatures, and humidities14,16,19. Thus, they can

be used to simulate a wide range of the chemical conditions which affect ozone formation from ROG and

NOx. These are the same as those affecting VOC reactivity. Therefore, at least to the extent they are

suitable for their intended purpose, an appropriate set of EKMA scenarios should also be suitable for

assessing methods to develop reactivity scales encompassing a wide range of conditions.

Base Case Scenarios. The set of EKMA scenarios used in this study were developed by the

United States EPA for assessing how various ROG and NOx control strategies would affect ozone

nonattainment in various areas of the country.18 The characteristics of these scenarios and the methods

used to derive their input data are described in more detail elsewhere.18,20 Briefly, 39 urban areas in the

United States were selected based on geographical representativeness of ozone nonattainment areas and

data availability, and a representative high ozone episode was selected for each. These were based on

7
Table I. Summary of terms and abbreviations.

Types of Scenarios and Scenario Characteristics

EKMA Scenario A model for an air pollution episode which can be represented in the EKMA model

formulation. This involves a single-cell box model with a fraction of ROG and NOx

pollutants present initially and the remainder emitted throughout the day, time-varying

changes in inversion heights and entrainment of pollutants from aloft as the height

raises, and time-varying humidities and temperatures.

Base ROG Mixture The mixture of reactive organic gases (ROGs) initially present or emitted in the

EKMA scenarios except for biogenic VOCs, VOCs present aloft, or VOCs added for

the purpose of calculating their incremental reactivities.

NOx (or ROG) inputs The sum of the initial NOx (or base ROG) and the total emitted NOx (or base ROG) in

the scenarios, in units of moles (or moles carbon) per unit area.

NOx Availability The condition of whether NOx is limiting O3 formation or whether NOx is in excess,

and the degree to which this is the case.

Base Case Scenario An EKMA scenario whose inputs are derived to represent a specific ozone exceedence

episode in an area of the United States.

MIR Scenario. Maximum Incremental Reactivity (MIR) Scenario. A scenario derived by adjusting the

NOx emissions in a base case scenario to yield the highest incremental reactivity of the

base ROG mixture.

MOR Scenario. Maximum Ozone Reactivity (MOR) Scenario. A scenario derived by adjusting the

NOx emissions in a base case scenario to yield the highest peak ozone concentration.

EBIR Scenario. Equal Benefit Incremental Reactivity (EBIR) Scenario. A scenario derived by

adjusting the NOx emissions in a base case scenario so VOC and NOx reductions are

equally effective in reducing O3.

Averaged Conditions A scenario whose inputs represent the average of those of the base case scenarios

Scenario

Measures of Reactivity

Incremental Reactivity Change in ozone formed caused by adding a VOC to the initial and emitted base ROG

in a scenario, divided by the amount of VOC added.

Relative Reactivity The Incremental reactivity of the VOC divided by the incremental reactivity of the

base ROG mixture.

Kinetic Reactivity Fraction of the VOC which reacts in the scenario.

Mechanistic Reactivity Change in ozone formed caused by adding a VOC to the initial and emitted base ROG

in a scenario, divided by the amount of VOC which reacted.

O3 Yield Reactivity Incremental or relative reactivity based on the effect of the VOC on the maximum

amount of ozone formed.

IntO3 Reactivity Incremental or relative reactivity based on the effect of the VOC on the O3 concentration integrated over time.

IntO3>90 Reactivity Reactivity based on the effect of the VOC on the sum of the O3 concentrations for

each hour when O3 ≥ 90 ppb.

8
Table I. (continued)

Reactivity Scales

Reactivity Scale A numerical ranking system where each VOC is assigned a number giving a measure

of how its emissions affect ozone formation.

Adjusted NOx Scales Reactivity scales derived from incremental reactivities in scenarios with a specified

condition of NOx availability.

MIR, MOIR, or EBIR The adjusted NOx scales consisting of the average of ozone yield reactivities in the

Scales MIR, MOR, or EBIR scenarios, respectively.

Base Case Scales Relative reactivity scales based on incremental reactivities in scenarios where NOx

inputs were not adjusted.

Base (AR) Scales Base case scales derived using the averaged ratio method. Averages of the relative

reactivities in the base case scenarios.

Base (L1) Scales Base case scales derived using the least squares error method which minimizes the

change in ozone caused by substituting the base ROG for the VOC using reactivity

weighting factors which the scale predicts has zero effect on ozone.

Base (L2) Scales Base case scales derived using the least squares error method which minimizes the

change in ozone caused by substituting the VOC for the Base ROG using reactivity

weighting factors which the scale predicts has zero effect on ozone.

Other

VOC Volatile Organic Compound. In this paper CO is also referred to as a VOC, but

strictly speaking it is not.

ROG Reactive Organic Gas. VOCs which react in the atmosphere to a significant extent,

i.e., VOCs other than CO, methane, chlorofluorocarbons, or other unreactive compounds.

NMOG Non Methane Organic Gases. VOCs excluding methane and CO.

EKMA Empirical Kinetic Modeling Approach. A method to estimate effects of ROG or NOx

controls on ozone based on box model calculations of one day episodes using a

particular computer program. In the context of this work, it refers to the box model

scenarios developed for this type of modeling analysis.

Null test A model simulation where one VOC or mixture of VOCs is replaced by another in a

proportion which a reactivity scale predicts would have no effect on ozone. The

resulting change in ozone is a way of measuring the error of a reactivity scale.

9
1986-88 data18. The initial NMOG and NOx concentrations, the aloft O3 concentrations, and the mixing

height inputs were based on measurement data for the various areas, the hourly emissions in the scenarios

were obtained from the National Acid Precipitation Assessment Program (NAPAP) emissions inventory,18

and biogenic emissions were also included. Table II gives a summary of the urban areas represented and

other selected characteristics of the scenarios.

Note that the initial NMOG and NOx concentrations are based on air quality data, so they are not

affected by uncertainties and possible errors in the emissions inventory. Errors in the inventory would

affect amounts of hourly emissions after the beginning of the simulation, which usually have less of an

effect on the ozone than the amounts of NMOG and NOx present initially. Thus if the NMOG inventory

were too low, then the base case ROG/NOx ratio would also be low, but to a lesser extent. However, this

would not significantly affect the ROG/NOx ratio in the adjusted NOx scenarios (discussed below.)

Several changes to the scenario inputs were made based on discussions with the California ARB

staff and others.13,21 Two percent of the initial NOx and 0.1% of the emitted NOx in all the scenarios was

assumed to be in the form of HONO. The photolysis rates were calculated using solar light intensities

and spectra calculated by Jeffries22 for 640 meters, the approximate mid-point of the mixed layer during

daylight hours. The composition of the NMOGs entrained from aloft was based on the analysis of Jeffries

et al. of aircraft data from a number of urban areas23. The composition of the initial and emitted ROGs

was derived as discussed below. Complete listings of the input data for the scenarios are given

elsewhere.20

This set of 39 EKMA scenarios are referred to as "base case" to distinguish them from the

scenarios derived from them by adjusting NOx inputs to yield standard conditions of NOx availability as

discussed below. No claim is made as to the accuracy of these scenarios in representing any real episode,

but they are a result of an effort to represent, as accurately as possible given the available data and the

limitations of the formulation of the EKMA model, the range of conditions occurring in urban areas

throughout the United States. When developing general reactivity scales it is more important that the

scenarios employed represent a realistic distribution of chemical conditions than any one accurately

representing the details of any particular episode.

Base ROG Mixture. The Base ROG mixture is the mixture of reactive organic gases used to

represent the chemical composition of the initial and emitted anthropogenic reactive organic gases from

all sources in the scenarios. It is referred to as the "base" mixture because it is used in the simulations

10
Table II. Summary of conditions of the EPA base case scenarios.

Calc. ROG NOx Final Init.+Emit Aloft

City, State Max O3 /NOx /NOx

MOR Height Base ROG O3

(ppb) (km) (mmol m-2) (ppb)

Atlanta, GA 163 7.3 0.8 2.1 12 63

Austin, TX 162 9.3 0.6 2.1 11 85

Baltimore, MD 275 5.2 1.2 1.2 17 84

Baton Rouge, LA 211 6.8 1.0 1.0 11 62

Birmingham, AL 223 6.9 0.6 1.8 13 81

Boston, MA 182 6.5 0.7 2.6 14 105

Charlotte, NC 137 7.8 0.4 3.0 7 92

Chicago, IL 251 11.6 0.6 1.4 25 40

Cincinnati, OH 183 6.4 0.8 2.8 17 70

Cleveland, OH 220 6.6 1.1 1.7 16 89

Dallas, TX 167 4.7 1.4 2.3 18 75

Denver, CO 172 6.3 1.3 3.4 29 57

Detroit, MI 217 6.8 0.9 1.8 17 68

El Paso, TX 162 6.6 1.1 2.0 12 65

Hartford, CT 160 8.4 0.5 2.3 11 78

Houston, TX 266 6.1 1.1 1.7 25 65

Indianapolis, IN 187 6.6 1.0 1.7 12 52

Jacksonville, FL 141 7.6 0.7 1.5 8 40

Kansas City, MO 146 7.1 0.7 2.2 9 65

Lake Charles, LA 257 7.4 0.7 0.5 7 40

Los Angeles, CA 483 7.6 1.1 0.5 23 100

Louisville, KY 191 5.5 0.9 2.5 14 75

Memphis, TN 205 6.8 0.8 1.8 15 58

Miami, FL 125 9.6 0.5 2.7 9 57

Nashville, TN 155 8.1 0.5 1.6 7 50

New York, NY 317 8.1 0.8 1.5 39 103

Philadelphia, PA 212 6.2 1.0 1.8 19 53

Phoenix, AZ 242 7.6 1.1 3.3 40 60

Portland, OR 152 6.5 0.8 1.6 6 66

Richmond, VA 212 6.2 0.9 1.9 16 64

Sacramento, CA 184 6.6 0.9 1.1 7 60

St Louis, MO 269 6.1 1.2 1.6 26 82

Salt Lake City, UT 173 8.5 0.7 2.2 11 85

San Antonio, TX 119 3.9 1.2 2.3 6 60

San Diego, CA 169 7.1 1.1 0.9 8 90

San Francisco, CA 167 4.8 2.0 0.7 25 70

Tampa, FL 192 4.4 1.3 1.0 8 68

Tulsa, OK 201 5.3 1.0 1.8 15 70

Washington, DC 250 5.3 0.9 1.4 13 99

Avg. Conditions 206 6.6 0.9 1.8 15 70

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without the added test VOC in the reactivity calculations. (When the reactivity of a VOC is assessed, that

VOC is added to this base ROG mixture in the simulation.) Consistent with the approach used in the

original EPA scenarios, the same mixture was used for all scenarios — except for the calculations where

the effects of changing this mixture are assessed. The speciation for this mixture was derived by Croes24

based on an analysis of the EPA database23 for the hydrocarbons and the 1987 Southern California Air

Quality Study (SCAQS) database for the oxygenates.25,26 This mixture consists of 52% (by carbon)

alkanes, 15% alkenes, 27% aromatics, 1% formaldehyde, 2% higher aldehydes, 1% ketones, and 2%

acetylene. The detailed composition of this mixture is given elsewhere.20

Adjusted NOx Scenarios. Since incremental reactivities are highly dependent on NOx

5,9,11, and

since the NOx conditions of the base case scenarios are variable (as indicated by the ROG/NOx ratios on

Table II), one would expect the incremental reactivities to also be highly variable. But if the NOx inputs

to these scenarios were adjusted to yield consistent NOx conditions, one might expect the incremental

reactivities, or at least the ratios of incremental reactivities, to be much less variable. In this case, the set

of incremental reactivities so obtained may provide a general reactivity scale which is at least applicable

to that particular condition of NOx availability. Comparing different reactivity scales for different NOx

conditions would provide a systematic means to assess how reactivity scales, and control strategies based

on them, would vary with NOx levels.

To develop a set of scenarios for this purpose, one needs a means to assess NOx availability, or

to establish equivalency of NOx conditions, which is applicable to a variety of scenarios. NOx availability

is determined both by the amount of NOx input and the rate at which it is removed. The latter is affected

not only by the reactivity and amount or ROGs which are present, but also by factors such as light

intensity, temperature, and dilution, which in general will vary from scenario to scenario. Therefore the

amount of NOx present or the ROG/NOx ratio are not necessarily reliable indicators of NOx availability.

However, if one examines how changes in ROG and NOx affect ozone formation as a function

of NOx inputs, essentially the same pattern is observed for all scenarios. This is shown on Figure 1, which

plots, against total NOx input, the changes in ozone caused by 1% increases in ROG or NOx inputs for

a typical scenario. The figure also includes a plot of the peak daily ozone concentration against NOx

input. Note that in all cases there is a NOx input level (designated "MIR" on the plot) where the ROG

input has the highest and most positive effect on ozone which is near or the same as the point where the

effect of NOx is the most negative; there is a lower NOx level ("MOR") which yields the maximum ozone

concentration and where the effect of NOx on ozone changes sign; and there is a yet lower NOx level

12
Figure 1. Qualitative dependencies on NOx inputs of maximum ozone and of relative changes in ozone

caused by 1% increases in total ROG or total NOx emissions for the "Averaged Conditions"

Scenarios. NOx inputs are shown relative to NOx inputs which give maximum ozone yields.

("EBIR") where the effects of fractional changes of ROG and NOx on ozone formation are equal.

Although these three points in general occur at different NOx inputs or ROG/NOx ratios for conditions of

different scenarios, they represent consistent NOx conditions in terms of how ozone formation is affected

by ROG and NOx changes. Thus, these can be used to define three conditions of consistent NOx

availability, which yield three sets of adjusted NOx scenarios. These are as follows:

Maximum Incremental Reactivity (MIR) Scenarios. In these scenarios the NOx inputs are

adjusted so that the base ROG mixture had the highest incremental reactivity. The NOx adjustment was

done by varying both the initial NOx and the emitted NOx by the same factor.20 These MIR scenarios

represent NOx conditions where emissions of ROGs have the greatest effect on ozone formation, and

where NOx has the strongest ozone inhibiting effect. Thus they represent conditions where ROG control

has the greatest effect on ozone. They can also be thought of representing approximately the highest NOx

levels which are relevant in considering control strategies for ozone, because ozone is suppressed to low

levels if NOx inputs are increased significantly above this level.

13
Maximum Ozone Reactivity (MOR) Scenarios. In these scenarios the NOx inputs are

adjusted to yield the highest peak ozone concentration. This represents the dividing line between

conditions where NOx is in excess and where ozone is NOx limited, or the "ridgeline" on ozone isopleth

plots.27 MOR scenarios represent NOx conditions which are optimum for ozone formation.

Equal Benefit Incremental Reactivity (EBIR) Scenarios. In these scenarios the NOx inputs

are adjusted so that the effect on ozone of a given percentage incremental change in ROG input is the

same as the effect of an equal percentage change in NOx. In other words, this is the point where the

incremental reactivity of the base ROG mixture, multiplied by the total amount of ROG input (excluding

aloft or biogenic ROGs), equals the incremental reactivity of NOx, multiplied by the amount of NOx input.

The EBIR scenarios represent the lowest NOx conditions where ROG control is of equal or greater

effectiveness for reducing ozone as NOx control. Thus they represent the lowest NOx conditions which

are of relevance to ROG control, since at lower NOx levels NOx control becomes much more effective in

reducing ozone.

Averaged Conditions Scenarios. It is useful for sensitivity studies and example calculations to

have a single scenario or set of scenario conditions which can be taken as being representative of the

larger set. For this purpose, we derived an "averaged conditions" scenario from the averages of the

relevant inputs of the 39 base case scenarios. This was then used as the basis for developing scenarios

with some input modified, such as the base ROG composition or the initial HONO. The MIR, MOR, or

EBIR versions of this scenario are determined as discussed above for the base case scenarios. Note that

when conducting sensitivity calculations on varied scenario conditions, the NOx adjustments to determine

MIR, MOR or EBIR conditions were done after the scenario condition was varied, so the effect of the

variation can be assessed on an equal NOx availability basis. Otherwise, the effect of the variation on NOx

availability may dominate the result.

Calculation of Reactivities in a Scenario

Incremental Reactivities. Incremental reactivities in a given scenario are calculated by

conducting model simulations of ozone formation in the scenario, and then repeating the calculations with

a small amount of the test VOC added. The amount of test VOC added depended on how rapidly it

reacted in the scenario, being determined such that the amount added is sufficiently large so that numerical

errors in the computer simulation do not significantly affect the results, yet is sufficiently small that the

14
effect of adding the VOC is within the linear range where the change in ozone is proportional to the

amount added.20 The incremental reactivities are then the change in ozone formed in the two calculations,

divided by the amount of test VOC added. The detailed methodology used for calculating incremental

reactivities in a scenario is given elsewhere.20

Incremental reactivities depend on how the amounts of VOC added and how the amounts of

ozone formed are quantified. In this work, incremental reactivities are based on VOCs quantified on a

mass basis, i.e., by the amount of ozone formed per gram of VOC added. This is most relevant for

control strategy applications because VOC emissions are quantified by mass. Alternative quantifications,

which have a closer correspondence to the chemical processes, are mole basis or carbon basis, where the

latter is more frequently used. Note that the VOC quantification affects relative reactivities of VOCs with

different molecular weights per carbon, such as oxygenates compared to hydrocarbons.

The way the amount of ozone formed is quantified will also affect incremental reactivities. The

following ozone quantifications are used:

Ozone Yield Reactivities. These are based on the maximum number of moles or grams of

ozone formed in the scenario, i.e., the moles or grams per unit area in the mixed layer at the time of the

maximum ozone concentration. This gives the same ratios of incremental reactivities as reactivities

calculated from peak ozone concentrations, but is preferred because it permits magnitudes of reactivities

in scenarios with differing dilutions to be compared on the same basis. Most previous recent studies of

incremental reactivity2,3,5,9,28 have been based on ozone yield or peak ozone concentration reactivities.

Integrated Ozone (IntO3) Reactivities. These are based on the ozone concentrations

integrated over time throughout the simulated day. Note that if two VOCs give the same maximum ozone

concentration when added in equal amounts in the scenarios, but one causes ozone to be formed earlier,

their IntO3 incremental reactivities would be different, even though the ozone yield reactivities would be

the same.

Integrated Ozone Over 90 ppb (IntO3>90) Reactivities. These are based on the extent to

which the ozone exceeds the California ambient air quality standard of 90 ppb, and the length of time of

the exceedence. In this work, this is quantified by the sum of the hourly ozone concentrations for the

hours when the ozone exceeds the standard in the calculations without the added VOC. (The hours when

15
the standard is exceeded in the calculations with the added VOC would be the same if the amount of VOC

added were sufficiently small.)

Relative Reactivities. For control strategy purposes, the ratios of incremental reactivities for one

VOC relative to others are of greater relevance than the incremental reactivities themselves. To define

a relative reactivity scale, one needs to select a VOC to use as the standard. For example, Chameides et

al.29 used propene, Russell and co-workers30 used carbon monoxide, and Derwent and Jenkins31 used

ethylene for this purpose. In this work, the standard we will use is the base case ROG mixture, i.e., the

mixture used in the model simulations to represent the initially present and emitted anthropogenic reactive

organic gases in the scenarios. Thus, the relative reactivity of a VOC is the ratio of the incremental

reactivity of the VOC to the incremental reactivity of the base ROG mixture. When defined in this way,

the VOC’s relative reactivity measures the effect on ozone of changing the emissions of this VOC

compared to the effect changing the emissions of all VOCs equally.

Kinetic and Mechanistic Reactivities. To provide a basis for examining in more detail how

differing aspects of VOC reaction mechanisms and scenario conditions affect reactivity, it is useful to

consider incremental reactivity as being the product of the "kinetic" and the "mechanistic" reactivities.9

The kinetic reactivity is the fraction of the emitted VOC which reacts in the scenario, and depends on the

VOC’s relevant rate constants and the levels of the radicals and species in the scenarios which react with

the VOCs. Mechanistic reactivity is the change in ozone formed caused by adding the VOC, divided by

the amount which reacts — or the incremental reactivity divided by the kinetic reactivity. The mechanistic

reactivities are independent (to a first approximation) on how rapidly the VOC reacts, but are affected by

factors such as the as number of conversions of NO to NO2 which occur when the VOC reacts, whether

the VOC’s reactions enhance or inhibit radical or NOx levels, the reactivities of the products they form,

and conditions of the scenario such as NOx availability and other factors which affect the overall efficiency

of ozone formation.9 These two components of incremental reactivity are affected by different aspects of

the VOC reaction mechanism and of the scenario conditions. Thus they are useful when explaining factors

which affect reactivity, and why reactivity can vary from scenario to scenario.

16
Derivation of Multi-Scenario Reactivity Scales

A total of 18 different general or multi-scenario reactivity scales were derived in this work,

depending on which type of ozone quantification, scenarios, or aggregation method was used. These are

discussed below.

Adjusted NOx Scales. Adjusted NOx reactivity scales are scales derived from incremental or

relative reactivities in the adjusted NOx scenarios. A total of 9 such scales were derived based on the

three conditions of NOx availability (MIR, MOR, and EBIR) and the three methods for quantifying ozone

(O3 yield, IntO3, and IntO3>90). Incremental reactivities in these scales were derived by averaging the

kinetic and mechanistic reactivities in the MIR, MOR or EBIR scenarios, and combining them to yield

aggregate incremental reactivities for these three NOx conditions. This gives essentially the same result

as simply averaging the incremental reactivities, but this approach was used because it also gives kinetic

and mechanistic reactivities characteristic of the adjusted NOx conditions. This is useful for analyzing

reactivity trends. Incremental reactivities were computed only for the ozone yield scales; for the IntO3

and IntO3>90 scales only relative reactivities were computed. Relative reactivities for the 9 adjusted NOx

scales were derived by averaging the relative reactivities in the adjusted NOx scenarios.

In the remainder of this paper the terms "MIR scale", "MOIR scale" or "EBIR scale" will be used

to refer to the ozone yield adjusted NOx scales. (MOIR stands for "maximum ozone incremental

reactivity.) The IntO3 or IntO3>90 adjusted NOx scales will be referenced explicitly as such when they

are discussed. This conforms to the terminology used elsewhere for the MIR and MOIR scales.12,13

Base Case Scales. Base case relative reactivity scales are derived from incremental reactivities

in the base case scenarios. Only relative reactivities are derived because the varying NOx conditions in

the base scenarios caused incremental reactivities to vary widely. For many VOCs relative reactivities also

varied widely in the base case scenarios, and different scales can be obtained depending on the methods

used to derive a single scale from the distribution of values among the scenarios. Three different methods,

discussed below, were employed. Combined with the three methods for quantifying ozone, these yielded

9 different base case relative reactivity scales.

The "Average Ratio" (AR) Method. This consists of simply averaging the relative

reactivities in the base case scenarios, with each scenario being weighed equally. This is the method used

to derive the relative reactivities in the adjusted NOx scales. However in this case, unlike the adjusted

17
NOx scales, the quantities being averaged are quite variable. The fact that this method weighs the relative

reactivities in all scenarios equally, despite the fact that ozone is much more sensitive to VOC changes

in some scenarios than in others, suggests that this may not give an optimum scale for control applications.

A more optimum scale should give greater weight to scenarios which are more sensitive to the quantities

being regulated.

The "Least Squares Error" Methods. These are based on minimizing the calculated sumof-squares change in ozone which would result if a substitution which the scale predicts would have zero

effect on ozone were applied throughout the set of scenarios. Model calculations of substitutions which

a reactivity scale predicts has no effect on ozone are referred to as "null tests" of the scale. For example,

if the relative reactivity of a compound in a scale were 0.5, then the scale predicts that substitution of 2

units of the compound for one unit of the base ROG would result in no net change in ozone. A null test

calculation would be a simulation of the effect of this substitution. Since in general relative reactivities

vary from scenario to scenario, a null test substitution would cause a change in ozone in at least some of

the scenarios no matter what relative reactivity were used. This change can then be thought of as a

measure of the "error" of the reactivity scale for the scenario. The least squares error relative reactivity

is the value which minimizes the sum of squares of this error, or change in ozone, resulting from this null

test. Note that this method gives greater weight to scenarios where ozone is more sensitive to VOCs.

Since relative reactivity is defined as reactivities relative to the base ROG, the relevant

substitution strategies for deriving these scales would involve either (1) reducing emissions of the VOC

and offsetting it by an increase in the emissions of all ROGs, or (2) reducing all ROGs and offsetting it

by an increase in the VOC. Least squares error method "L1" is based on minimizing the errors in null

tests of ROG for VOC substitutions, while least squares error "L2" is based on minimizing errors in null

tests of VOC for ROG substitutions. It can be shown20 that the Base (L1) relative reactivities are the same

as the weighted average of the relative reactivities in the individual base case scenario, where the

weighting factor is the square of the incremental reactivity of the base ROG mixture. On the other hand,

the Base (L2) relative reactivities are the reciprocals of the weighed averages of the reciprocals of the

relative reactivities in the scenarios, where the weighting factor is the square of the incremental reactivity

of the VOC.

Method L2 may seem preferable from a control strategy perspective because most substitutions

of interest involve replacing current emissions with some less reactive VOC, which is the basis of the null

test the L2 method is designed to optimize. However, method L1 is more tractable mathematically

18
because method L2 does not give well defined results for VOCs whose incremental reactivities vary

around zero, and because the relative reactivities of mixtures in the Base (L1) scales can be derived by

linear summations of relative reactivities of their components — which is not the case for the Base (L2)

scales.

Chemical Mechanism

The chemical mechanism used in this study is that of Carter,32 with updates for several VOCs as

indicated in footnotes to the reactivity results tabulations, below. A complete listing of the updated

mechanism is given elsewhere.20 This mechanism contains rate constant and product yield assignments

for almost 120 separate VOCs. It was evaluated by simulating results of a variety of environmental

chamber experiments, and was found to be able to simulate maximum ozone concentrations and rates of

NO oxidation and ozone formation to within ±30% for 63% of the experiments33. However, it had a slight

bias (~15%) towards overpredicting maximum ozone concentrations in the experiments designed to

represent ambient mixtures.33 This is comparable to or slightly better than the performance of the RADMII34,35 and Carbon Bond IV36 mechanisms in simulating the same35 or a similar36 database. This is as good

as can be reasonably expected given our current state of knowledge of atmospheric chemistry and

characterization of chamber artifacts.33,35

This mechanism is considered appropriate for reactivity calculations because it is at least as up

to date as the other available comprehensive mechanisms34,36,37 and it is the only one designed to represent

large numbers of VOCs which has been extensively tested against chamber data. However, its limitations

and uncertainties must be recognized. Most of the mechanism represents the state of knowledge as of

1989-1990 and is out of date in some respects. At the time it was developed, the available chamber data

were sufficient to test the representation of ~20 representative VOCs, and the reactions for most of the

others were derived by extrapolation or estimations.32 It has not been updated to take into account results

of recent experimental studies of incremental reactivities of a variety of VOCs.10 The uncertainties in the

reaction mechanism obviously must be taken into account when the results of model calculations of

reactivities are used to assess ozone control strategies. To aid in such assessments, the master listing of

reactivity results given in the following section includes footnotes indicating levels of uncertainty in the

mechanisms for the various VOCs, based on the amount of experimental data available to test the

mechanisms at the time they were developed.

19
Results and Discussion

Table III gives the incremental reactivities in the MIR scale and the relative reactivities in all 9

adjusted NOx scales for all the types of VOCs in the mechanism. Comparisons of relative reactivities in

the base case scales are shown for selected VOCs in figures given later in this paper. These data are

discussed below, first in terms of the variability and differences of incremental reactivities and its

components, and then in terms of the differences and variabilities of the relative reactivities. Finally,

reactivity adjustment factors for selected vehicle exhaust mixtures calculated using these various scales

are compared.

Distributions of NOx and Reactivity in the Scenarios

Figure 2 shows the distribution plots of the maximum ozone concentrations, two measures of the

NOx levels, and the base ROG incremental reactivities for the various scenarios. Note that the wide distributions of ROG/NOx ratios in the adjusted NOx scenarios indicate that the ROG/NOx ratios are, by

themselves, poor predictors of NOx availability. A much better predictor is the ratio of the NOx input to

the NOx yielding maximum ozone concentrations, or the NOx/NOx

MOR ratio. This ratio, which is 1 by

definition for the MOR scenarios, was found to be narrowly distributed around 1.5 for the MIR scenarios,

and 0.7 for the EBIR scenarios. On the other hand, as expected it varies widely among the base scenarios.

By this measure, most of the EPA base scenarios used in this work are between MOR and EBIR

conditions.

The distribution of maximum ozone levels is very similar in all the scenarios, being only slightly

lower in the MIR scenarios relative to the others. Thus, while MIR conditions are not optimum for ozone

formation, levels of ozone which exceed air quality standards are still formed. On the other hand, the

distribution of base ROG incremental reactivities are significantly higher in the MIR scenarios than in the

lower NOx MOR or EBIR scenarios. The wide distribution of base ROG reactivities in the base scenarios

is expected given their distribution of NOx conditions.

Note that in a previous derivation of general VOC reactivity scales,11 a different set of scenarios,

obtained from the studies of Gery et al.17 and Whitten38 were used. To show how the distribution of NOx

conditions from these scenarios compare with the EPA-derived scenarios used in this work, Figure 2 also

shows plots of NOx/NOx

MOR for the Gery et al.17 and Whitten38 scenarios. These are designated as "base

20
Table III. Tabulation of incremental reactivities in the MIR scale and relative reactivities in the

adjusted NOx scales, with notes concerning the uncertainty of the VOCs’ mechanisms.

MIR Relative Reactivity [b] Unc.

Compound Inc.Rct. Ozone Yield Integrated Ozone Int’d Ozone >90 ppb Notes

[a] MIR MOIR EBIR MIR MOIR EBIR MIR MOIR EBIR [c]

Carbon Monoxide 0.054 0.018 0.032 0.044 0.016 0.023 0.029 0.017 0.025 0.034 1

Alkanes

Methane 0.015 0.005 0.008 0.010 0.004 0.005 0.006 0.004 0.006 0.008 2

Ethane 0.25 0.079 0.140 0.18 0.061 0.078 0.088 0.069 0.091 0.109 2

Propane 0.48 0.16 0.27 0.33 0.128 0.17 0.19 0.139 0.19 0.23 2

n-Butane 1.02 0.33 0.57 0.70 0.26 0.33 0.36 0.29 0.38 0.44 1

n-Pentane 1.04 0.33 0.58 0.71 0.29 0.37 0.42 0.31 0.41 0.48 5

n-Hexane 0.98 0.31 0.55 0.65 0.28 0.35 0.38 0.29 0.39 0.44 5

n-Heptane 0.81 0.26 0.45 0.49 0.22 0.26 0.25 0.24 0.30 0.30 5

n-Octane 0.60 0.19 0.34 0.33 0.15 0.17 0.137 0.17 0.21 0.18 5

n-Nonane 0.54 0.17 0.30 0.27 0.132 0.125 0.063 0.150 0.16 0.113 5

n-Decane 0.46 0.146 0.26 0.22 0.109 0.091 0.021 0.128 0.130 0.070 7

n-Undecane 0.42 0.132 0.23 0.19 0.095 0.072 -0.002 0.114 0.110 0.046 8

n-Dodecane 0.38 0.118 0.21 0.16 0.082 0.058 -0.016 0.101 0.094 0.028 8

n-Tridecane 0.35 0.110 0.19 0.15 0.074 0.049 -0.025 0.093 0.082 0.018 8

n-Tetradecane 0.32 0.100 0.17 0.136 0.067 0.041 -0.031 0.085 0.073 0.011 8

Isobutane 1.21 0.39 0.63 0.80 0.33 0.43 0.51 0.36 0.48 0.60 7

Neopentane 0.37 0.117 0.18 0.21 0.092 0.105 0.104 0.104 0.123 0.126 7

Iso-Pentane 1.38 0.44 0.74 0.93 0.38 0.48 0.55 0.41 0.54 0.63 7

2,2-Dimethylbutane 0.82 0.26 0.43 0.51 0.21 0.26 0.27 0.23 0.30 0.33 7

2,3-Dimethylbutane 1.07 0.34 0.57 0.72 0.32 0.43 0.51 0.33 0.45 0.56 5

2-Methylpentane 1.5 0.48 0.76 0.90 0.42 0.49 0.52 0.45 0.55 0.60 7

3-Methylpentane 1.5 0.48 0.80 0.99 0.42 0.52 0.58 0.45 0.58 0.67 7

2,2,3-Trimetylbutane 1.32 0.42 0.68 0.84 0.38 0.49 0.57 0.40 0.53 0.65 7

2,3-Dimethylpentane 1.31 0.41 0.67 0.79 0.37 0.44 0.48 0.39 0.49 0.55 7

2,4-Dimethylpentane 1.5 0.48 0.73 0.86 0.42 0.48 0.50 0.44 0.53 0.58 7

3,3-Dimethylpentane 0.71 0.22 0.39 0.44 0.18 0.22 0.22 0.20 0.26 0.27 7

2-Methylhexane 1.08 0.34 0.57 0.69 0.30 0.37 0.40 0.32 0.41 0.46 7

3-Methylhexane 1.40 0.44 0.71 0.82 0.38 0.44 0.46 0.41 0.49 0.53 7

2,2,4-Trimethylpentane 0.93 0.29 0.46 0.49 0.24 0.27 0.24 0.27 0.31 0.29 7

2,3,4-Trimethylpentane 1.6 0.51 0.78 0.94 0.47 0.55 0.61 0.49 0.60 0.69 7

2,3-Dimethylhexane 1.31 0.41 0.67 0.79 0.37 0.44 0.48 0.39 0.49 0.55 7

2,4-Dimethylhexane 1.5 0.48 0.73 0.86 0.42 0.48 0.50 0.44 0.53 0.58 7

2,5-Dimethylhexane 1.6 0.52 0.79 0.96 0.49 0.59 0.67 0.51 0.63 0.74 7

2-Methylheptane 0.96 0.30 0.51 0.57 0.26 0.30 0.29 0.28 0.34 0.35 7

3-Methylheptane 0.99 0.31 0.53 0.60 0.27 0.32 0.32 0.29 0.36 0.38 7

4-Methylheptane 1.20 0.38 0.59 0.65 0.32 0.35 0.33 0.35 0.40 0.40 7

2,4-Dimethylheptane 1.33 0.43 0.63 0.72 0.37 0.41 0.40 0.40 0.45 0.47 7

2,2,5-Trimethylhexane 0.97 0.30 0.49 0.54 0.26 0.29 0.27 0.28 0.33 0.33 7

4-Ethylheptane 1.13 0.36 0.54 0.59 0.30 0.31 0.28 0.33 0.36 0.35 7

3,4-Propylheptane 1.01 0.31 0.47 0.50 0.26 0.26 0.22 0.29 0.31 0.28 8

3,5-Diethylheptane 1.33 0.43 0.63 0.72 0.37 0.41 0.40 0.40 0.45 0.47 8

2,6-Diethyloctane 1.23 0.39 0.58 0.68 0.35 0.37 0.38 0.37 0.42 0.45 8

Cyclopentane 2.4 0.76 1.19 1.46 0.67 0.80 0.89 0.71 0.89 1.01 7

Methylcyclopentane 2.8 0.90 1.32 1.5 0.82 0.96 1.05 0.86 1.03 1.17 7

Cyclohexane 1.28 0.41 0.63 0.69 0.35 0.38 0.37 0.38 0.43 0.43 7

1,3-Dimethylcyclohexane 2.5 0.81 1.18 1.43 0.76 0.89 1.00 0.78 0.95 1.10 8

Methylcyclohexane 1.8 0.59 0.84 0.96 0.51 0.56 0.55 0.55 0.62 0.64 5

Ethylcyclopentane 2.3 0.73 1.10 1.31 0.66 0.74 0.78 0.69 0.81 0.90 8

Ethylcyclohexane 1.9 0.62 0.86 0.97 0.55 0.57 0.56 0.59 0.64 0.65 8

1-Ethyl-4-Methylcyclohexane

2.3 0.73 1.00 1.15 0.66 0.70 0.71 0.70 0.76 0.81 8

1,3-Diethylcyclohexane 1.8 0.57 0.79 0.92 0.51 0.55 0.56 0.55 0.60 0.64 8

1,3-Diethyl-5-Methylcyclohexane

1.9 0.61 0.84 1.00 0.57 0.61 0.65 0.59 0.65 0.73 8

1,3,5-Triethylcyclohexane

1.7 0.54 0.74 0.86 0.49 0.53 0.55 0.51 0.57 0.62 8

21
Table III (continued)

MIR Relative Reactivity [b] Unc.

Compound Inc.Rct. Ozone Yield Integrated Ozone Int’d Ozone >90 ppb Notes

[a] MIR MOIR EBIR MIR MOIR EBIR MIR MOIR EBIR [c]

Alkenes

Ethene 7.4 2.4 2.8 3.2 2.2 2.4 2.7 2.3 2.6 2.9 1

Propene 9.4 3.0 3.2 3.7 3.0 3.0 3.2 3.0 3.1 3.4 4

1-Butene 8.9 2.9 3.0 3.4 2.8 2.7 2.8 2.8 2.8 2.9 4

1-Pentene 6.2 2.0 2.1 2.3 2.0 1.9 1.7 2.0 1.9 1.9 7

3-Methyl-1-Butene 6.2 2.0 2.1 2.3 2.0 1.9 1.7 2.0 1.9 1.9 7

1-Hexene 4.4 1.40 1.46 1.5 1.32 1.17 1.00 1.38 1.27 1.14 4

1-Heptene 3.5 1.10 1.15 1.18 1.02 0.88 0.69 1.08 0.97 0.82 8

1-Octene 2.7 0.85 0.90 0.89 0.76 0.62 0.41 0.82 0.71 0.55 8

1-Nonene 2.2 0.71 0.74 0.72 0.62 0.48 0.28 0.68 0.58 0.41 8

Isobutene 5.3 1.7 1.6 1.9 2.0 2.1 2.5 1.9 2.0 2.3 5

2-Methyl-1-Butene 4.9 1.5 1.6 1.9 1.7 1.9 2.2 1.6 1.7 2.0 7

trans-2-Butene 10.0 3.2 3.2 3.6 3.5 3.6 4.0 3.3 3.3 3.6 5

cis-2-Butene 10.0 3.2 3.2 3.6 3.5 3.6 4.0 3.3 3.3 3.6 5

2-Pentenes 8.8 2.8 2.8 3.1 3.0 3.1 3.2 2.9 2.8 2.9 7

2-Methyl-2-Butene 6.4 2.1 2.0 2.2 2.4 2.7 3.1 2.2 2.3 2.5 7

2-Hexenes 6.7 2.2 2.2 2.3 2.3 2.3 2.3 2.2 2.1 2.1 8

2-Heptenes 5.5 1.8 1.8 1.9 1.9 1.9 1.9 1.8 1.8 1.8 8

3-Octenes 5.3 1.6 1.6 1.7 1.7 1.6 1.5 1.6 1.5 1.5 8

3-Nonenes 4.6 1.46 1.45 1.5 1.48 1.40 1.32 1.46 1.37 1.29 8

13-Butadiene 10.9 3.5 3.5 4.1 3.6 3.5 3.6 3.5 3.4 3.6 8

Isoprene 9.1 2.9 2.9 3.3 3.2 3.1 3.2 3.0 2.9 3.1 6

Cyclopentene 7.7 2.4 2.3 2.5 2.9 3.1 3.5 2.5 2.6 2.8 8

Cyclohexene 5.7 1.8 1.9 2.1 2.0 2.0 2.1 1.8 1.8 1.9 8

α-Pinene 3.3 1.04 1.08 1.23 1.11 1.11 1.17 1.06 1.05 1.09 5

β-Pinene 4.4 1.40 1.42 1.6 1.5 1.47 1.5 1.44 1.41 1.46 8

Acetylenes

Acetylene 0.50 0.16 0.28 0.38 0.140 0.19 0.23 0.15 0.21 0.27 5

Methylacetylene 4.1 1.31 1.8 2.2 1.14 1.24 1.28 1.21 1.36 1.45 9

Aromatics

Benzene 0.42 0.135 0.114 0.051 0.112 0.113 0.097 0.123 0.122 0.101 4

Toluene 2.7 0.88 0.53 -0.023 0.77 0.71 0.54 0.82 0.72 0.48 4

Ethylbenzene 2.7 0.86 0.52 0.007 0.77 0.72 0.54 0.82 0.72 0.49 7

n-Propylbenzene 2.1 0.68 0.41 -0.016 0.59 0.55 0.41 0.64 0.56 0.37 7

Isopropylbenzene 2.2 0.71 0.43 -0.007 0.64 0.59 0.44 0.68 0.59 0.40 7

s-Butylbenzene 1.9 0.60 0.37 -0.014 0.54 0.49 0.38 0.57 0.51 0.33 7

o-Xylene 6.5 2.1 1.6 1.26 2.0 2.0 1.8 2.1 2.0 1.7 4

p-Xylene 6.6 2.1 1.7 1.29 2.1 2.0 1.8 2.1 2.0 1.8 7

m-Xylene 8.2 2.6 2.1 1.7 2.7 2.7 2.6 2.7 2.6 2.5 4

1,3,5-Trimethylbenzene 10.1 3.2 2.6 2.4 3.8 3.8 4.0 3.5 3.5 3.5 4

1,2,3-Trimethylbenzene 8.9 2.8 2.3 1.9 3.0 3.0 3.0 2.9 2.9 2.8 7

1,2,4-Trimethylbenzene 8.8 2.8 2.3 1.9 3.0 3.0 3.0 2.9 2.9 2.8 7

Tetralin 0.94 0.31 0.103 -0.23 0.33 0.26 0.147 0.31 0.23 0.076 5

Naphthalene 1.17 0.37 0.066 -0.43 0.36 0.26 0.064 0.36 0.24 -0.009 5

Methylnaphthalenes 3.3 1.05 0.65 0.21 1.11 0.96 0.75 1.07 0.90 0.65 8

2,3-Dimethylnaphthalene 5.1 1.6 1.16 0.79 1.8 1.7 1.48 1.8 1.5 1.31 5

Styrene 2.2 0.71 -0.26 -1.8 0.82 0.54 0.043 0.73 0.32 -0.40 8

Alcohols and Ethers

Methanol 0.56 0.18 0.23 0.28 0.149 0.18 0.19 0.17 0.20 0.23 1

Ethanol 1.34 0.43 0.61 0.72 0.33 0.36 0.37 0.38 0.43 0.45 1

n-Propyl Alcohol 2.3 0.72 0.95 1.07 0.60 0.63 0.61 0.66 0.71 0.70 7

Isopropyl Alcohol 0.54 0.17 0.27 0.35 0.17 0.22 0.27 0.17 0.23 0.30 7

22
Table III (continued)

MIR Relative Reactivity [b] Unc.

Compound Inc.Rct. Ozone Yield Integrated Ozone Int’d Ozone >90 ppb Notes

[a] MIR MOIR EBIR MIR MOIR EBIR MIR MOIR EBIR [c]

n-Butyl Alcohol 2.7 0.86 1.09 1.25 0.74 0.77 0.76 0.79 0.84 0.86 7

Isobutyl Alcohol 1.9 0.62 0.80 0.92 0.54 0.58 0.60 0.58 0.64 0.67 7

t-Butyl Alcohol 0.42 0.132 0.21 0.27 0.114 0.148 0.18 0.125 0.17 0.21 7

Dimethyl Ether 0.77 0.24 0.48 0.68 0.24 0.35 0.47 0.24 0.38 0.53 7

Meth. t-Butyl Ether 0.62 0.20 0.34 0.47 0.18 0.25 0.31 0.19 0.27 0.36 1

Ethyl t-Butyl Ether 2.0 0.64 0.88 1.11 0.62 0.77 0.92 0.63 0.80 0.98 7

Aldehydes

Formaldehyde 7.2 2.3 1.8 1.7 2.8 2.9 3.2 2.5 2.5 2.6 1

Acetaldehyde 5.5 1.8 1.8 2.2 1.7 1.5 1.31 1.8 1.6 1.6 4

C3 Aldehydes 6.5 2.1 2.1 2.4 2.1 1.8 1.49 2.1 1.9 1.7 5

Glyoxal 2.2 0.71 0.61 0.63 0.90 0.90 0.95 0.75 0.80 0.85 3

Methyl Glyoxal 14.8 4.7 4.0 3.9 6.5 7.1 8.2 5.3 5.5 6.1 3

Ketones

Acetone 0.56 0.18 0.17 0.18 0.16 0.142 0.134 0.16 0.16 0.149 5

C4 Ketones 1.18 0.38 0.46 0.53 0.32 0.30 0.29 0.34 0.34 0.34 5

Aromatic Oxygenates

Benzaldehyde -0.57 -0.18 -1.08 -2.7 -0.31 -0.74 -1.5 -0.25 -0.74 -1.7 5

Phenol 1.12 0.36 -0.45 -1.7 0.32 0.091 -0.42 0.33 0.0147-0.65 7

Alkyl Phenols 2.3 0.74 -0.53 -2.6 0.69 0.33 -0.53 0.70 0.20 -0.92 5

Others

Methyl Nitrite 9.5 3.1 3.5 5.2 5.2 6.9 10.2 3.5 4.7 6.9 3

Base ROG Mixture 3.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

[a] Incremental reactivities in units of grams ozone formed per gram VOC emitted for the ozone yield reactivity

scale for the MIR scale. Note that there are small differences in the last digit in the tabulated values

compared to those used in the California ARB Regulations12 for some VOCs. This is within the numerical

uncertainties of these calculations, and is because of minor changes in the software used20.

[b] Incremental reactivities of the VOCs (in units of ozone per gram of VOC) divided by incremental reactivities

of the base ROG mixture.

[c] Notes concerning the uncertainty of the mechanism are as follows:

1 Least uncertain mechanism, and tested against chamber data.

2 Mechanism probably not uncertain, but was not tested.

3 Laboratory data are available for the major reactions in the mechanism, but the mechanism was not

tested.

4 Uncertain portions of the mechanism are adjusted or parameterized to fit chamber data.

5 The mechanism is uncertain, and only limited or uncertain data were available to test it.

6 The mechanism was not optimized to fit existing chamber data.

7 The mechanism was estimated and was not tested.

8 The mechanism was estimated and was not tested, and must be considered to be highly uncertain.

9 The mechanism was estimated and was not tested, and is likely to be incorrect. Suitable only for

estimating reactivities of mixtures where this is a component.

23
Figure 2. Distribution plots of maximum ozone, the base ROG incremental reactivity, the ROG/NOx

ratio, and the ratio of NOx inputs to MOR NOx inputs for the MIR, MOR, EBIR and base

case scenarios. Base scenarios used previously11 are shown on the NOx/MOR NOx plot.

(1991)" on the figure. This shows that these scenarios have a much wider distribution of NOx conditions

than those used in this work. The implications of this on reactivity scales are discussed later.

Factors Affecting Reactivity

Dependence of Reactivity on Environmental Conditions. Table III shows that as expected the

incremental reactivities of a given VOC vary significantly with NOx conditions. However, it does not

show the extent to which the reactivities or their components vary among the individual adjusted NOx and

base case scenarios. An illustration of this is shown on Figure 3, which shows distribution plots of

kinetic, mechanistic, incremental, and relative ozone yield reactivities for carbon monoxide and toluene.

Although these are only two of the many types of VOCs, together they illustrate the trends which are

characteristic of most other VOCs to varying degrees.

24
Carbon Monoxide Toluene

Figure 3. Distribution plots of kinetic, mechanistic, incremental, and relative reactivities for carbon

monoxide and toluene in the MIR, MOR, EBIR and base case scenarios.

25
The sensitivity of kinetic reactivities to scenario conditions depends on how rapidly the VOC

reacts, with slowly reacting compounds being most sensitive, and having kinetic reactivities which are

essentially proportional to the integrated levels of species with which the VOC reacts. Rapidly reacting

compounds have kinetic reactivities approaching unity. Most slowly reacting compounds react only with

OH radicals, so variations in their kinetic reactivities reflect variations in integrated OH radical levels.

CO reacts so slowly that its kinetic reactivity is essentially proportional to the integrated OH, and thus the

distribution plot of its kinetic reactivities is the same as the distribution plot of integrated OH. The

distribution for kinetic reactivities of toluene, which reacts more rapidly, is qualitatively similar but varies

over a narrower range.

The distribution plots show that kinetic reactivities, and thus integrated OH radical levels, are

lower in the MIR scenarios compared to MOR and EBIR conditions. This is attributed to the fact that

NOx is involved in a number of radical termination reactions. However, the kinetic reactivities apparently

do not increase further to a significant extent as NOx is reduced from MOR to EBIR levels. Apparently

reduced termination caused by lower NOx when going from MOR to EBIR is offset by the increased

termination due to HO2 + HO2 and other peroxy + peroxy reactions which become more important once

NOx is consumed. On the other hand, the wide distributions of kinetic reactivities in the adjusted NOx

scenarios indicate that kinetic reactivities are significantly affected by other factors besides NOx. Factors

such as light intensity, temperature and dilution might be of equal or greater significance as NOx in

affecting radical levels and thus kinetic reactivities.

Since the dependence of kinetic reactivity on NOx is in the opposite direction as that for

incremental reactivities, the mechanistic reactivity must be the dominant factor affecting how incremental

reactivities vary with NOx. In contrast to the case with kinetic reactivities, for most VOCs there is also

almost no overlap in the distributions of mechanistic reactivities in the MIR and MOR scenarios; the data

on Figure 3 are typical in this regard. Thus, at least when NOx is above MOR levels, NOx availability

dominates over other scenario conditions in affecting mechanistic reactivity. While NOx is still important

in affecting mechanistic reactivities when NOx is below MOR levels, the other factors become relatively

more important, as indicated by the overlap in the distributions of the MOR and EBIR mechanistic

reactivities for both CO and toluene.

VOCs form ozone by producing HO2 and other peroxy radicals which react with NO to shift the

NO-NO2-O3 photostationary state towards ozone formation. The mechanistic reactivity of CO provides

a direct illustration how much ozone is formed by this process, since the reaction of CO involves only the

26
formation of a single HO2 radical. Thus, no more than one molecule of ozone can be formed from each

molecule of CO which reacts. This theoretical maximum is almost achieved under MIR conditions, but

the yield of O3 from HO2 decreases rapidly as NOx is reduced from MIR to MOR levels. This is because

the HO2+HO2 reaction, forming H2O2, begins to compete with the reaction of HO2 with NO in the

scenarios where all the NOx is consumed before the end of the simulation. Since essentially all VOCs

react to form peroxy radicals, this factor contributes to the NOx dependencies of mechanistic reactivities

of almost all cases.

For most other VOCs, this is not the only factor affecting how their mechanistic reactivities

depend on NOx. If a VOC is a significant radical source, it will have high mechanistic reactivities under

high NOx conditions where final ozone concentrations are determined how rapidly ozone is formed.

However, this enhancement would be less important under lower NOx conditions where ozone yields are

determined less by radical levels and more by NOx availability. On the other hand, if a VOC has NOx

sinks in its mechanism, it will cause low, and even negative, mechanistic reactivities under low NOx

conditions, but will not significantly affect high NOx (MIR) reactivities. Toluene is an example of a

compound where both factors are operative, and consequently its mechanistic reactivity, and thus its

incremental reactivity, decreases much more rapidly as NOx is reduced than is the case for CO and other

VOCs. Note that the NOx sink effect becomes dominant when NOx is sufficiently low, causing reactivities

to become negative. This is despite the fact that toluene is still calculated to form radical-initiating

products and radicals that react with NO. For the mechanism used in this work, the crossover for toluene

reactivity occurs at some NOx level around the equal benefit point, though the exact level appears to be

highly variable depending on other scenario conditions. (The crossover occurs at higher NOx levels when

the Carbon Bond IV mechanism is used.39)

One would expect relative reactivities to be much less dependent on scenario conditions than

incremental reactivities, at least for scenario conditions which affect reactivities of all VOCs in similar

qualitative ways. Thus, while decreasing NOx levels causes decreased incremental reactivities in all

compounds, this effect at least to some extent cancels out when considering relative reactivities. However,

if a VOC differs significantly in how its incremental reactivities vary with NOx than is the case for the

base ROG mixture, then its relative reactivity will also vary with NOx. For example, the mechanistic

reactivities of CO are less dependent on NOx than most VOCs so its relative reactivities increase with

decreasing NOx, while the opposite is true for toluene. For similar reasons, one would expect the

distribution of relative reactivities in the adjusted NOx scenarios to be much narrower than the

corresponding distribution of incremental reactivities. This is indeed the case for the MIR scenarios, but

27
the relative reactivities appear to be much more variable in the MOR and (especially) the EBIR scenarios.

Thus non-NOx scenario conditions appear to affect incremental reactivities of different VOCs similarly

under high NOx, MIR conditions, but this is apparently not the case under more NOx-limited conditions.

The variations in relative reactivity are discussed further in the following section.

Relative Reactivities

The relative reactivities of a variety of VOCs in the various scales are compared graphically on

Figures 4-6. (To aid in comparisons of the different scales, the dashed and dotted lines show the relative

reactivities in the MIR and MOIR scales, respectively.) Figures 7-9 show how well or poorly the relative

reactivities of a number of different VOCs compare in different scales by plotting the relative reactivities

of the VOCs in one scale against those in the other. The position of the points for the VOCs should be

compared to the 1:1 line where they would fall they had equal relative reactivities in the two scales. The

error bars on Figures 4-9 indicate the standard deviations of the averages or the derivations, and thus for

the adjusted NOx scales they indicate the importance of non-NOx scenario conditions in affecting relative

reactivities. These results are discussed in more detail below.

Ozone Yield Relative Reactivities — Adjusted NOx Scales. The results show that the ozone

yield relative reactivities can depend significantly on NOx for many VOCs, with the trend depending on

the type of VOC. Relative reactivities of the aromatics and other compounds with major NOx sinks

decrease significantly as NOx is reduced, with the effect being largest for the cresols and benzaldehyde,

the compounds with the strongest NOx sinks. The higher alkenes apparently have similar balances of

factors in affecting reactivity as the base ROG mixture, since their relative reactivities appear to be almost

independent of NOx, at least in the MIR to MOR regimes. The relative reactivities of compounds which

have weaker than average NOx sinks, such as CO, ethene and methanol, tend to increase with decreasing

NOx because their incremental reactivities are less sensitive to NOx than that for the base ROG mixture,

which includes a significant contribution from aromatics. In addition, relative reactivities of slowly

reacting compounds such as CO and ethane tend to increase with decreasing NOx because kinetic reactivities, which increase as NOx is reduced, are relatively more important in affecting reactivities of slowly

reacting compounds. Because CO is both slowly reacting and has essentially no NOx sinks, it provides

the most extreme case of a compound whose relative reactivity increases with decreasing NOx.

28
Figure 4. Comparison of relative reactivities of carbon monoxide, ethane, n-butane, n-octane, npentadecane, and ethene calculated using various methods. Points on right are ozone yield

relative reactivities for the varied averaged conditions scenarios.

29
Figure 5. Comparison of relative reactivities of propene, trans-2-butene, toluene, m-xylene, 1,3,5-

trimethylbenzene, and formaldehyde calculated using various methods. Points on right

are ozone yield relative reactivities for the varied averaged conditions scenarios.

30
Figure 6. Comparison of relative reactivities of acetaldehyde, acetone, methanol, ethanol, cresols,

and benzaldehyde calculated using various methods. Points on right are ozone yield

relative reactivities for the varied averaged conditions scenarios.

31
Figure 7. Plots of relative reactivities in the MOR scale against relative reactivities in the MIR scale

for selected VOCs. The left plot shows the full range of relative reactivities, while the

right plot has an expanded scale to show the less reactive VOCs more clearly.

The distribution plots on Figure 3 and the lengths of the error bars (the standard deviations) on

Figures 4-6 provide an indication of how other scenario conditions affect relative reactivities. For most

VOCs, the MIR relative reactivities are quite insensitive to scenario conditions, with the distributions

shown on Figures 3 being fairly typical. In general, the sensitivities to scenario conditions increase as the

NOx decreases, with the most extreme cases being the compounds, such as toluene, cresols, and benzaldehyde, with the large NOx sinks in their mechanisms.

Figure 7 shows the extent to which the relative reactivities of positively reactive compounds in

the MIR and MOIR scales correspond to each other. Although these relative reactivities are clearly highly

correlated, the MIR scale tends to underpredict the MOIR relative reactivities for CO, the alkanes and the

alcohols, and overpredict them for the aromatics, on a fairly consistent basis. On the other hand, Figure

8 shows that, except for toluene, the MOIR and EBIR relative reactivities correspond very well. These

32
Figure 8. Plots of relative reactivities in the EBIR scale against relative reactivities in the MOIR

scale for selected VOCs. The left plot shows the full range of relative reactivities, while

the right plot has an expanded scale to show the less reactive VOCs more clearly.

two scales are essentially equivalent to within the uncertainties caused by variabilities in non-NOx scenario

conditions if one considers only compounds which are positively reactive in both scales. These two scales

also agree in indicating that phenols and cresols are negatively reactive, while they are positively reactive

in the MIR scale. However, the discrepancy in the MOIR and EBIR scales in the relative reactivities of

toluene — and by extension the other monoalkylbenzenes, which the current mechanisms32,34,36 assume

have similar reactivities as toluene — is not insignificant in view of the relatively large emissions of these

compounds.

Ozone Yield Relative Reactivities — Base Case Scales. Figures 4-6 show the various base case

ozone yield relative reactivities for the representative VOCs, where they can be compared with those for

the adjusted NOx scales. As before, the "error bars" show the standard deviations of the averages or

derivations. The average ratio base case [Base (AR)] ozone yield reactivities have high standard

33
Figure 9. Plots of relative reactivities in the Base (L1), IntO3>90 scale against relative reactivities

in the MIR scale for selected VOCs. The left plot shows the full range of relative

reactivities, while the right plot has an expanded scale to show the less reactive VOCs

more clearly.

deviations because of the variation in relative reactivities in the scenarios due to the variation of NOx

conditions. In most cases the least squares error methods [Base (L1) and Base (L2)] give more welldefined values, having standard deviations which are comparable to or smaller than those for the adjusted

NOx scales. For most VOCs, the Base (L2) relative reactivities are essentially the same as the Base (L1)

values. Thus, as one might expect, a reactivity optimized for assessing substitutions involving replacing

current emissions with emissions of a less reactive VOC is essentially the same as one optimized for

assessing substitutions of highly reactive VOCs for increased emissions of all ROGs.

There are a few apparently anomalous Base (L2) values which can be seen from the data for the

cresols and n-pentadecane. These are cases when the incremental reactivities of the VOC are distributed

around zero, when the method used to compute the Base (L2) reactivities is most sensitive to the most

extreme values in the distribution. Because of the poor performance of this method in these cases, and

34
the fact that in most other cases it yields essentially the same result as the L1 method, it is concluded that

the L1 method is the better method to derive least squares error relative reactivities.

In most cases, the relative reactivities in the Base (AR) case scale tend to fall between those in

the MOIR and EBIR scale. This is as one would predict from the distribution of NOx/NOx

MOR ratios in

the base case scenarios. On the other hand, all of the Base (L1) and most of the Base (L2) relative

reactivities lie somewhere between the MIR and MOIR values. More MIR-like values for the least squares

error scales are expected because the method used to derive them puts more weight on reactivities in

scenarios where ozone is more sensitive to VOCs, i.e., which are closer to MIR conditions. However,

unlike the least squares error scale given in a previous study,11 where the Base (L1) scale corresponded

much better to the MIR scale than MOIR, in this case the Base (L1) reactivities are somewhat closer to

the MOIR scale.

The reason for the differences between this result and those given previously11 arises from the fact

that the scenarios employed in the previous study represented a more varied set of NOx conditions. This

is apparent from the distribution plots of NOx/NOx

MOR ratios on Figure 2, which include the distribution

for the base scenarios from Gery et al.17 and Whitten38 used in the previous11 study ("base (1991)"), where

they can be compared with distribution for the EPA scenarios used in this work. Although both sets have

average NOx/NOx

MOR ratios near the MOR range, the much wider distribution of NOx conditions in the

base (1991) set results in a larger fraction of scenarios which have near-MIR or higher-than-MIR NOx

conditions. Since reactivities in these high NOx scenarios are weighed the most heavily in computing the

least squares error scales, these scales are highly sensitive to the number of such scenarios in the

distribution. In general, the wider the distribution of NOx conditions in a set, the closer the least squares

error reactivity scale derived from it will correspond to an MIR scale.

The appropriateness of base case reactivity scales from this work obviously depends on how well

these EPA scenarios represent the distribution of conditions where ozone pollution episodes occur. It

should be recognized that MIR conditions probably occur in the atmosphere much more frequently than

represented by these EPA episodes. Each of these scenarios is based on the EPA’s assessment of the

conditions of a near-worst ozone episode in some area, and thus represents a meteorological condition

which is near to the most favorable for ozone formation in that area. Thus most other days would have

less favorable meteorological conditions for ozone, including many days when unacceptable ozone levels

may still be formed. These would include days with lower rates of NOx removal because lower

temperatures or light intensities cause lower rates of photochemical reactions. Slower NOx removal means

35
more NOx availability, and thus more MIR-like conditions. Since, as shown on Figure 2, ozone can still

exceed air quality standards under MIR conditions, such meteorological conditions, while not worst case,

are not irrelevant to the problem of urban ozone formation. If these conditions were represented in a more

comprehensive set of scenarios, the resulting least squares error scales would correspond much more

closely to the MIR scale than observed in this work.

Integrated Ozone and IntO3>90 Relative Reactivities. Figures 4-6 show relative reactivities

derived from the effects of the selected VOCs on integrated ozone concentrations (the IntO3 scales) and

from the effects of the VOCs on integrated ozone above 90 ppb (IntO3>90), where they can be compared

with the ozone yield reactivities discussed above. The IntO3, IntO3>90, and ozone yield relative

reactivities tend to agree well under MIR conditions, but then they tend to diverge as NOx as reduced, with

the IntO3 and IntO3>90 values changing less as NOx is reduced than is the case for ozone yield values.

In the cases of a number of the most reactive compounds, such as formaldehyde, m-xylene and trimethylbenzene, the IntO3 relative reactivities are essentially independent of NOx, despite the fact that the NOx

dependencies in the ozone yield relative reactivities are significant. The IntO3 relative reactivities of most

other compounds show the same trend with NOx as the O3 yield reactivities, except that the change with

NOx is less extreme. This lower sensitivity of IntO3 reactivities to NOx means that they are less variable

in the base case scenarios. Because of this, the base case relative IntO3 reactivities are less sensitive to

the method used to derive them, except for the few anomalous Base (L2) cases discussed above.

The reason this lower sensitivity of IntO3 relative reactivities to NOx — and their tendency to

correspond to MIR relative reactivities — is that integrated ozone levels are sensitive to the same

mechanistic factors which determine ozone yields under high NOx, MIR conditions. These are the factors

which affect how rapidly O3 is formed, as opposed to those which affect the ultimate O3 yield when NOx

is limited. In a high NOx scenario, both the ozone yield and the integrated ozone would be determined

by how rapidly O3 is formed. In a lower NOx scenario, the integrated ozone would still be sensitive to

the ozone formation rate, but if the O3 is NOx limited the maximum ozone yield is more sensitive to the

NOx availability than the ozone formation rate. While NOx availability has some influence on integrated

ozone under low NOx conditions, it tends to be less important a factor than the amount of time that the

highest levels of ozone were present in the scenario. Thus, since IntO3 reactivities and MIR reactivities

are both sensitive to the same aspects of the VOCs’ mechanisms, the relative reactivities tend to

correspond to each other. NOx sinks in the VOCs’ mechanisms, which become the dominant factor

affecting MOIR and EBIR reactivities, are only of secondary importance in affecting IntO3 reactivities.

36
One would expect the IntO3>90 reactivity scales to have characteristics somewhere between those

for the IntO3 and the ozone yield scales, and this is indeed what is observed. However, the IntO3>90

scales are closer to IntO3 scales than the ozone yield scales, and all the discussion above for the IntO3

scales are applicable to IntO3>90. There are a few cases, such as formaldehyde, trimethylbenzenes, and

(to a lesser extent) acetone, ethanol, and methanol, where there is a non-negligible difference between the

ozone yield and the integrated ozone reactivities under maximum reactivity conditions. In those cases,

the IntO3>90 reactivities tend to be closer to the MIR reactivities.

Because of this, MIR reactivities tend to give very good predictions of IntO3>90 reactivities in the

base case scenarios. This is shown on Figure 9, which gives plots of base case IntO3>90 relative

reactivities computed using the average ratio method against the values predicted by the MIR scale for

selected representative positively reactive VOCs. It can be seen that agreement to within the standard

deviations are attained for all but two VOCs, and for those the agreement is within 1.5 standard deviations.

This is much better than the correspondence of the base case IntO3>90 reactivities with the MOIR or EBIR

scales.

Effects of Variations of Other Scenario Conditions

The comparisons of reactivities in the adjusted NOx scenarios provide direct information on their

dependencies on NOx availabilities, and also, through the standard deviations of the averages, provide

indirect information on the importance of other conditions which were variable in the scenarios. However,

the composition of the base ROG mixture, the level and compositions of ROGs aloft, and the initial

nitrous acid (HONO) as a fraction of the NOx inputs were held fixed in all these scenarios, and thus these

data provide no information on the sensitivities of reactivities to these inputs. To assess this, modified

versions of the averaged conditions scenario were derived by varying these inputs as described below, then

the NOx inputs for each version were adjusted to derive corresponding MIR or MOR scenarios, and then

these were used to assess how these variations affect the MIR and MOIR reactivities. Sensitivities of

EBIR reactivities to scenario conditions are not discussed here, but they were generally found to be similar

to, though often greater than, those for MOIR reactivities.

Four different modifications of the composition of the base ROG were examined, all involving

relatively large changes to this mixture. These involved only changes to the ROGs associated with

anthropogenic emissions, a fraction of which (~60%) were present initially and the remainder emitted

37
throughout the day. The compositions and amounts of aloft and biogenic ROGs input were not varied.

The variations, and the code numbers used to designate them, are as follows: (1) "No Oxygenates": The

aldehydes and ketones were removed without modifying the levels of the other components. (2)

"Oxygenates x3": The aldehydes and ketones were increased by a factor of 3 without modifying the levels

of the other components. (3) "Aromatics x2": The aromatics were increased by a factor of 2, and the

alkanes and olefins reduced to keep the total carbon the same. (4) "Alkenes x2": The olefins were

increased by a factor of 2 and the alkanes and aromatics reduced to keep the total carbon the same.

One of the changes made to the EPA scenarios was assuming that ~2% of the initial and 0.1%

of the emitted NOx was in the form of nitrous acid (HONO), which is a powerful photoinitiator which

could help initiate the photochemical processes early in the day. The EPA scenarios as received from

Baugues,40 and the scenarios used previously,11 assumed no initial or emitted HONO. The (5) "No

HONO" modification, where the initial nitrous acid (HONO) was assumed to be zero and no HONO was

subsequently emitted, was used to assess the effect of this change.

The scenarios as received from Baugues all assumed a standard 30 ppb of VOCs aloft, and the

same chemical composition was used for this aloft mixture in all scenarios. The (6) "Aloft ROGs x5"

modification, where the concentrations of all ROGs aloft were increased by a factor of 5, was used to

assess how important aloft ROGs are in affecting reactivity calculations in these scenarios.

The MIR and MOIR relative reactivities calculated using these modified scenarios are shown for

the representative VOCs on the right hand side of the plots on Figures 4-6, where they can be compared

with the values for the corresponding averaged conditions scenario. The relative reactivities in the varied

scenarios are indicated by the code numbers on the plots, while the standard, or averaged conditions,

values are indicated by the dashed (for MIR) or dotted (for MOIR) lines.

The data on Figures 4-6 show that these relatively large variations in the base ROG mixture had

in most cases had only small effects on the relative reactivities. The variation which had the largest effect

was the increase in the aromatics ("3" on the plots), whose twofold increase, for example, caused a ~19%

decrease in the relative MIR reactivity of formaldehyde. Removing the oxygenates from the base ROG

("1") increased the relative MIR reactivity of formaldehyde by ~7%. The effects of these variations on

the other VOCs were generally smaller.

38
The removal of initial and emitted HONO from the scenarios ("5") had almost no effect on any

of the results except for formaldehyde, whose MIR and MOIR relative reactivities increased by ~15%, and

whose integrated ozone relative reactivities (not shown) increased by ~20%. This is a large sensitivity

in view of the almost complete insensitivities of the other results to initial HONO. Since both HONO and

formaldehyde provide early radical sources in the simulations, this shows that removing one such radical

source increases the sensitivity of the scenarios to the other.

The fact that these scenarios have initial HONO while those given previously11 do not might partly

explain why the formaldehyde reactivity in these scenarios is less sensitive to changes in aldehyde

emissions than calculated previously. In the absence of the initial HONO, we calculate that the relative

reactivity of formaldehyde increases by ~11% when the base ROG oxygenates are removed. (This is not

shown on the plots.) This is greater than the ~7% effect observed when the HONO is present, and

indicates that adding radical initiators such as HONO to the scenario reduces the sensitivity of

formaldehyde reactivities on initial aldehydes.

The fivefold increase in the aloft ROGs was found to have an insignificant effect on the relative

reactivity results. In view of this, the sensitivity to the composition of the aloft ROG mixture would also

be expected to be small.

Examples of Exhaust Reactivity Adjustment Factors

An example of a regulatory application of a reactivity scale is the utilization of reactivity

adjustment factors (RAFs) in the alternative fuel vehicle exhaust standards recently adopted in California.12

The mass emissions of exhausts from alternatively-fueled vehicles are multiplied by these RAFs to place

them on the same ozone impact basis as emissions from vehicles using conventional gasoline. The RAFs

are calculated from the ratios of incremental reactivities (as ozone per gram) for the exhaust mixtures from

alternative-fueled vehicles, relative to that for a mixture characteristic of exhaust from vehicles using

industry-average gasoline. The regulations as adopted utilize the MIR scale to calculate these RAFs,13 but

it is of interest to see how these would differ if other scales were used. This is shown on Figure 10,

which gives RAFs for selected vehicle exhaust mixtures calculated using the various reactivity scales. The

example mixtures, which are based on analysis provided by the CARB,13 include exhausts from vehicles

fueled with 85% methanol + 15% gasoline (M85), compressed natural gas (CNG), liquified petroleum gas

39
Figure 10. Comparisons of Reactivity Adjustment Factors for selected vehicle exhaust mixtures

calculated using various methods. Points on right are calculated from ozone yield RAFs

for the varied averaged conditions scenarios.

(LPG), and 85% ethanol + 15% gasoline (E85). The RAFs are calculated relative to the standard exhaust

mixture used by the CARB.13

The format for the data on Figure 10 is similar to that on Figures 4-6. The MIR, MOIR, EBIR

and Base (AR) RAF’s are the averages of the RAFs for the individual adjusted NOx or base case

scenarios, calculated from ozone yield, IntO3 and IntO3>90 incremental reactivities. The least squares error

(LSE) RAF’s shown are those which give the least squares error in ozone yield, integrated O3, or integrated O3>90 ppb in the null tests where the alternative fuel exhaust is substituted for the standard exhaust.

40
This is analogous to the Base (L2) method except that the reactivity of the standard exhaust is used in

place of that for the base ROG. The points under "Vary MIR" and "Vary MOIR" show the effects of

varying the various scenario conditions on the MIR or MOIR RAFs, as discussed in the previous section.

For all four exhausts shown, the RAFs tend to increase with decreasing NOx conditions, except

that the integrated O3 RAFs for E85 are almost independent of NOx. The M85 RAF is least sensitive to

the scale used, but is somewhat unique in that the integrated ozone and ozone yield RAFs have about the

same dependence on NOx. The RAFs for the other mixtures are more typical of relative reactivities in

general in that the integrated O3 RAFs are less sensitive to NOx than ozone yield values. The fact that

the MIR scale predicts the lowest RAF in all cases (except E85) may suggest to some that the MIR has

a bias towards giving undue credits to alternative fuels. This increase in RAF with decreasing NOx is due

to the fact that most alternative exhausts have more slowly reacting compounds whose reactivities are

affected by the lower radical levels in MIR scenarios, and by the fact that the alternative exhausts tend

to have less species with strong NOx sinks (e.g., aromatics) than the standard exhausts. However,

regardless of this variability, the range of RAFs do not overlap unity except for E85, which overlaps unity

only in some extremely NOx-limited scenarios.

The points on the right hand side of the plots show that the variations in the base ROG mixture,

the removal of initial HONO, and the increase in aloft ROGs will not significantly affect the RAFs in

these cases. Thus the main issue in affecting RAFs do not relate to these uncertainties, but to what type

of scale is most appropriate.

It should be noted that calculating reactivity adjustment factors for exhausts is not the only

regulatory application where a reactivity scale might be useful41. However, this is the only regulation

where such a scale has been applied to date. The results of this study should aid in assessing the

appropriateness of reactivity scales in other regulatory contexts.

Discussion of Issues and Research Needs

A quantitative reactivity scale which compares the effects of different types of VOCs on ozone

formation could be useful for a number of ozone control strategy applications. However, the development

of such a scale has a number of difficulties. These can be categorized into three major areas. The first

is that the gas-phase chemical mechanisms by which VOCs react in the atmosphere to form ozone are in

41
many cases highly uncertain. This results in uncertainties in the model predictions of the reactivity of a

VOC in any given scenario. The second is that the effects of VOCs on ozone formation — their

reactivities — depend on the environment in which they are emitted. This means that even if we are

capable of reliably predicting the reactivity of a set of VOCs in a set of scenarios, it is not obvious how

these results should be used in developing a single reactivity scale — or even whether a use of a single

reactivity scale has any validity. The third is that there are uncertainties in conditions of airsheds and

episodes where unacceptable levels of ozone are formed. The uncertainties in conditions of a specific

episode affect predictions of VOC reactivities for that episode, and uncertainties in distribution of

conditions affect the development of appropriate methods for aggregating scenario-specific reactivities into

a generalized reactivity scale.

The focus of this paper has been on the second of these problems, that of deriving a reactivity

scale given that reactivities depend on environmental conditions. This has been studied by deriving

reactivity scales using several different techniques, given a single chemical mechanism and a single set

of representative airshed scenarios. The chemical mechanism employed is uncertain for many VOCs, but

it incorporates our current best estimate of their atmospheric reactions, and represents most of the major

types of species which need to be incorporated in reactivity scales. The representative environmental

scenarios employed are even more uncertain, but they represent their developers’ best estimate of the

conditions of a wide variety of representative pollution episodes given the limitations in available data and

the constraints of the simplified physical formulation of the model used. This is sufficient at least for

evaluating methods for deriving reactivity scales.

Consistent with results of previous studies, it was found that the NOx conditions can significantly

affect relative as well as absolute reactivities. In addition, it was also found that relative reactivities can

depend on how ozone impacts are quantified, especially under low NOx conditions. Because of this,

different reactivity scales give different reactivity rankings for VOCs and in a few cases different orderings

of VOCs in these rankings. However, in most cases the qualitative rankings among the different scales

are very similar, and the quantitative differences between them are small compared to the full range of

reactivities of those VOCs which are now regulated as ozone precursors. The results of this study do not

support the conclusion that reactivities are so strongly dependent on scenario conditions that all VOCs can

be considered to be equal to within this variability. Therefore, use of some appropriate type of scale will

yield more a more efficient ozone control strategy than regulating all VOCs equally. The more difficult

issue is what is the optimum type of scale to use for this purpose.

42
Although a total of 18 different scales based on various NOx conditions and methods for

quantifying ozone were derived, essentially the choice is between scales which are sensitive to effects of

VOCs on how rapidly ozone is formed and scales which are sensitive to effects of VOCs on the maximum

amount of ozone which is formed when ozone is NOx-limited. Scales in the first category are the MIR

and the various integrated ozone scales, scales in the second category are the MOIR, EBIR and the

average ratio base case ozone yield scales. (Least squares error base case ozone yield scales are in the

first category if the base case scenarios represent a varied set of NOx conditions, but are intermediate

between the two if they represent only near-worst-case conditions.) Although there are arguments for each

type of scale, it is concluded that if only one scale can be used, a scale like MIR is more appropriate20.

While the MOIR and scales like it are most effective at addressing peak ozone levels under conditions

which are the most favorable for ozone formation, the scales like MIR it are more optimal when applied

to the wide variety of conditions where ozone is sensitive to VOCs, or when one is concerned with

reducing exposure to integrated ozone or ozone over the air quality standard.

Although these conclusions are based on reactivities calculated for highly simplified single day

scenarios whose accuracies are unknown, the scenarios employed are sufficiently varied that it is not

unreasonable to expect that similar results would be obtained if a more detailed and accurate scenarios

were employed. This is supported by the results of Russell and co-workers,30,41 who calculated integrated

ozone reactivities using a much more complex physical model,42,43 and obtained results which

corresponded very closely to the MIR and integrated ozone scales calculated in this work. The

calculations of Russell and co-workers30,41 also increased the level of confidence in the validity of the

reactivity scale derivation because they showed that a detailed physical model42,43 with condensed

chemistry44 can give essentially the same reactivity scale as a simplified physical model with detailed

chemistry. However, it is clear that further work is needed to develop and utilize a more comprehensive

and physically realistic set of scenarios for VOC reactivity assessment. All the scenarios used in this work

represented the reactions of the VOCs only over a single day, and scenarios involving multi-day episodes

and regional models are needed to assess the total impact of VOCs on ozone over their lifetimes. The

work of Russell and co-workers30,41 is an important start in this regard, but these results need to be further

evaluated using physically detailed models of other areas, and using regional models which can assess the

impacts of VOCs over longer time periods and in long range transport scenarios.

It should be recognized that regardless of which approach or set of airshed conditions is used for

developing a reactivity scale, model calculations of VOC reactivities are no more reliable than the

chemical mechanism used to calculate them. Modeling studies may give us an indication of the

43
magnitudes of the effects of these uncertainties, but will not reduce them. To reduce these uncertainties,

experimental data are needed to test the mechanisms used to derive the reactivity factors, or at a minimum

to test their predictions of maximum reactivity. Such experiments are underway in our laboratories.10

Conclusions

Practical implementation of ozone control strategies which take into account differences among

VOCs in their effects on ozone require use of some quantitative reactivity ranking scheme. Use of

incremental reactivity, or more particularly ratios of incremental reactivities or relative reactivities, is an

appropriate means to do this. However, relative reactivities can vary depending on environmental

conditions, how ozone impacts are quantified, and on what approaches are used to derive single scales

from reactivities under a variety of conditions. Although these variations can in some cases be significant,

in most cases they are smaller than the ranges of reactivity among non-exempt VOCs. Thus, despite the

variabilities, use of any appropriate reactivity ranking scheme would yield a more efficient ozone control

strategy than ignoring reactivity altogether.

The availability of NOx in the environment is the most important single factor affecting reactivity

rankings. This is often measured by the ROG/NOx ratio, though this is not always a good predictor of

reactivity characteristics because of variability of factors affecting rates of NOx removal. The ratio of NOx

to NOx levels giving maximum ozone concentrations is a better measure of this. Variations in the

composition of the base ROG mixture, the amount of initial HONO, and level of aloft VOCs are relatively

unimportant in affecting reactivity when compared to the NOx effect. However, the effect of NOx is less

when ozone impacts are quantified by integrated ozone concentrations, or by integrated ozone above air

quality standards, than is the case when ozone is quantified by peak ozone concentrations or ozone yields.

Under high NOx conditions where VOCs have their greatest effect on ozone, which is the basis for

deriving the Maximum Incremental Reactivity (MIR) scale, the relative reactivities are not strongly

affected by how ozone is quantified, and are also relatively insensitive to other scenario conditions. Under

lower NOx conditions, relative reactivities tend to become more sensitive to other scenario conditions, and

tend to differ depending on how ozone is quantified.

Thus the MIR scale is relatively well defined in the sense that it is fairly insensitive to the choices

of scenarios used to derive it, and in most cases it gives a reasonably good approximations to scales based

on integrated ozone under lower NOx conditions. The MOIR scale gives better predictions of effects of

44
VOCs on peak ozone yields in base case scenarios, but gave poor predictions of effects on integrated

ozone or integrated ozone above the standard. It is also more sensitive to the set of scenarios used to

derive it. Based on these considerations, and the fact that the MIR scale is based on environmental

conditions where VOC control is most important for affecting ozone, we conclude that the MIR scale (or

a scale similar to it, such as one based on integrated ozone over the standard) is appropriate for regulatory

applications which a reactivity scale is required. Airshed model calculations using a much more detailed

physical scenario (but a simpler chemical mechanism) lead to similar conclusions30,41.

Acknowledgements

The author acknowledges Mr. Bart Croes of the Research Division of the California Air Resources

Board (CARB) for his contributions to this work regarding the establishment of the scenario conditions

and for many helpful discussions. The members of the CARB’s Reactivity Advisory Panel also

contributed to the establishment of the scenario conditions and provided helpful input. The author thanks

Mr. Keith Baugues for supplying the EKMA scenarios used in this study; Dr. Harvey Jeffries for

supplying the actinic flux data used in these calculations and other useful data files and information; Dr.

Michael Gery for helpful discussions; Dr. Roger Atkinson for helpful discussions and for reviewing this

manuscript; and Mr. Dennis Fitz for reviewing this manuscript. Finally, the author acknowledges Mr.

Alvin Lowi for initiating the project which ultimately led to the adoption of an incremental reactivity scale

as a means to assess ozone impacts of vehicle emissions.

This work was supported by the California Air Resources Board through Contract No. A934-094,

and incorporates results of work carried previously out under EPA Cooperative Agreement CR814396-01-

0. The statements and conclusions in this report are entirely those of the author and not necessarily those

of the California Air Resources Board, the United States EPA, or any of the individuals listed above.

45
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