MEMORANDUM 

National Volatile Organic Compound Emission 

Standards for Aerosol Coatings 

Review of the Technical Basis for use of the One-dimensional MIR Scale in 

the National Volatile Organic Compound Emission Standards for Aerosol 

Coatings 

Prepared by: 

Deborah Luecken 

Office of Research and Development 

National Exposure Research Laboratory 

Atmospheric Modeling /Human Exposure and Atmospheric Sciences Division 

Prepared for: 

J. Kaye Whitfield 

Office of Air Quality Planning and Standards 

Sector Policies and Programs Division 

Research Triangle Park, North Carolina 27711 

March 15, 2007 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

NATIONAL EXPOSURE RESEARCH LABORATORY 

Research Triangle Park, NC 27711 

 Office of 

 Research and Development 

 March 15, 2007 

MEMORANDUM

SUBJECT: Review of the Technical Basis for use of the One-dimensional MIR Scale in 

the National Volatile Organic Compound Emission Standards for Aerosol 

Coatings

FROM: Deborah Luecken 

Atmospheric Modeling /Human Exposure and Atmospheric Sciences Division 

National Exposure Research Laboratory, ORD 

TO: J. Kaye Whitfield 

 Project Lead, 

 Office of Air Quality Planning and Standards 

 Sector Policies and Programs Division 

 Natural Resources and Commerce Group 

CC: J. David Mobley 

 Associate Director, Atmospheric Modeling Division 

 National Exposure Research Laboratory, ORD 

 Larry Cupitt 

 Associate Director, Human Exposure Research 

 National Exposure Research Laboratory, ORD 

 

I have reviewed the literature on reactivity to determine if it supports the use of a reactivitybased control for aerosol coatings, and in particular, the use of a single, one-dimensional, box 

model-based Maximum Incremental Reactivity (MIR) scale for national applicability. While I 

believe that we should continue to do more research work in this area to ensure that the best 

possible metric/scale is being employed, there’s evidence that, given what we already know, 

EPA can already make a compelling case for considering reactivity as a basis for VOC control. 

While no single scale will be a perfect predictor of ozone in every location at every time, a single 

one-dimensional, box model-based MIR scale, is remarkably consistent with the more complex, 

three-dimensional, Air Quality Model-based scales. The three-dimensional scales are attractive 

for their robustness and general applicability, but at the current time, the one-dimensional MIR 

scale has several advantages. We have a history of implementation and scientific review, as well 

as existing product formulations using the MIR, based on the 2001 CARB rule. We have peer-
reviewed values for over 800 compounds based on the box model MIR, but for only about 30 

explicit compounds from the three-dimensional models. Because it is much easier to 

implement, the box model-based MIR scale gives us a way to easily incorporate updates to the 

chemical mechanism and information on new chemical species that are not part of the existing 

800 compounds. 

In the following pages, I list four of the technical questions that we need to address in order to 

proceed with use of one-dimensional MIR values in a national regulation, and summarize how 

the scientific literature addresses these issues. Based on this review, I believe it is reasonable to 

proceed with the regulation, with the caveat that we allow the flexibility to modify the scale as 

the science improves. I include a more detailed discussion, plots, and references in an attached 

report to this memo. 

Question 1: Does the mechanism used to derive MIRs accurately characterize ozone formation 

from individual VOCs?

In general, the SAPRC99 chemical mechanism which has been used to predict the reactivity of 

ozone formation for VOCs, and derive the MIRs, has been shown to be a valid predictor of 

ozone formation under controlled evaluation studies. Ozone formation in the atmosphere is a 

complex process and there is still some uncertainty in the mechanism predictions. While the 

mechanisms may underpredict ozone at low VOC/NOx ratios, which are of most interest to this 

regulation, use of SAPRC99 minimizes this bias, and the bias errs on the side of being slightly 

conservative. There is some uncertainty in the detailed chemical mechanisms used to calculate 

reactivities of individual VOCs, but for the compounds used most often in aerosol coatings, this 

uncertainty is estimated to be largely limited to categories 1 or 2. I believe this is equal to 

(probably less than) the uncertainties involved in overall calculations of ozone changes due to 

mass reductions. 

Question 2: Is the MIR a reasonable way to define reactivity?

All of the different metrics that have been proposed, approximately 4 box model-based metrics 

and 8+ three-dimensional metrics, give generally consistent predictions in the relative ranking of 

reactivities. None of the detailed studies performed to date have shown serious deficiencies in 

the long-used MIR metric. The aldehydes are sometimes outliers, but these are not important 

components of aerosol coatings. 

The MIR describes ozone formation in areas where ozone is most sensitive to VOC emissions, in 

upwind areas with high emissions. It is complementary to NOx control programs which address 

ozone formation in NOx-limited areas. The box model-based MIR scale gives the largest 

difference between the high reacting compounds and the lower reacting compounds than any of 

the other scales. On the negative side, this allows a larger amount of overall mass to possibly be 

emitted in a reactivity-limited solvent, as compared with an equal mass-restricted solvent, which 

would benefit ozone in the immediate area, but may increase ozone slightly downwind (see 

Question 4 for further discussion). On the positive side, it provides more incentive for 

manufacturers to use low-reactivity chemicals in aerosol coatings – the major goal of this policy. 

In addition, the MIR has a long history of use and analysis, and is the basis of some existing 
paint formulations under California law. Given the uncertainties inherent in any air quality 

modeling, is a reasonable way to characterize reactivity. The three-dimensional AQM metrics 

are arguably more robust, but there are no detailed reactivity scales yet available for all of the 

possible chemicals involved in aerosol coatings using AQMs. 

Question 3: Does a single MIR scale account for potential spatial variability across the United 

States?

Several three-dimensional modeling studies have shown that spatial variations in reactivity are 

generally found to be small, and averaging time used in calculation of the scale (whether 1 hour 

or 8 hours) makes little difference in the values. When viewed on a relative basis, as is 

applicable for solvent mixtures, the one-dimensional, city-specific MIR values are approximately 

the same for the 39 different cities studied. This gives us confidence that one metric and scale 

can be used with equal applicability throughout the country. 

Question 4: Is the large effective range of the MIR scale a potential problem when replacing 

high reactivity compounds with lower reactivity chemicals?

A major benefit as well as a concern with the use of MIR is its large effective range. The box 

model-based MIR scale gives the largest difference between the high reacting compounds and 

the lower reacting compounds. This means that a substitution using MIR would result in a larger 

permissible amount of mass than using another metric, i.e. the MOIR. Compared with an equal 

mass-restricted solvent, would benefit ozone in the immediate area but may have some 

disbenefits downwind. 

Current studies indicate that some increases in ozone due to increased mass of low-reacting 

compounds are possible, but the studies have only looked at very extreme cases (i.e. substitution 

of all VOCs with equal-ozone amounts of ethane, tracking ozone formation downwind from 

substitutions only in an urban area). While ozone could increase due to these upwind 

substitutions, the increases tended to be much smaller (by a factor of 12-20) than the magnitude 

of concurrent ozone decreases. The substitutions had a larger effect on reducing the higher 

ozone concentrations than they did on increasing downwind concentrations. Even in these 

extreme cases, the benefits for ozone (reduction in ozone peak) were significant. 

Inherent in this rule is a reduction in ozone equivalent to a reduction in mass, therefore the 

substitutions will be done on a less-than-equal ozone basis, relative to the pre-rule mixture. 

Realistic changes in formulation, especially if limited to aerosol coatings are unlikely to result in 

a noticeable increase in ozone downwind, given that downwind areas are usually NOx-limited, 

so small amounts of additional VOCs won’t influence ozone formation much. 

More detailed and realistic studies are needed to determine whether any potential increase in 

mass due to reactivity-based VOC limits are truly a problem.