OLM/ARM Workgroup - Draft Recommendations May 27, 1998

EPA’s prescribed method for modeling NO emissions can be found in Section 6.2.3 of the 1

2

Guideline on Air Quality Models (GAQM). The GAQM is codified as Appendix W to 40 CFR Part

51.

The OLM/ARM Workgroup was established during the 1996 annual meeting of EPA/State/Local 2

modelers, to identify technical issues related to NO2 modeling and to recommend solutions to

EPA’s Office of Air Quality Planning and Standards. At the time of this writing, the

recommendations presented here reflect Workgroup views and do not necessarily reflect EPA

policy.

1

Use of the Ambient Ratio Method (ARM) for Estimating

Ambient Nitrogen Dioxide Concentrations

prepared by the OLM/ARM Workgroup

Draft for Comment

This is not official EPA policy guidance

Introduction

The U.S. Environmental Protection Agency (EPA) recommends a tiered approach for modeling

ambient nitrogen dioxide (NO2) impacts from point sources. The second tier uses the Ambient Ratio 1

Method (ARM), where the ratio of modeled NO2 to oxides of nitrogen (NOx) is assumed to equal the

existing NO -to-NOx ratio. EPA has established a national default ARM value (NO -to-NOx ratio) of 2 2

0.75, but allows site-specific values to be used if representative monitoring data is available.

The OLM/ARM workgroup felt additional guidance is needed regarding site-specific ARM values. 2

This document provides our recommendations, some of which differ from the ARM recommendations

originally presented at the 1991 Air & Waste Management Association annual meeting (Chu and

Meyer, 1991).

In summary, a site-specific NO -to-NOx ratio can usually be developed for areas where the ambient 2

NOx concentrations typically exceed 20 parts per billion (ppb). The resulting ratio can be used to

refine modeled estimates of long-term (annual average) NO concentrations. For areas with lower 2

NOx concentrations, a non-guideline method, such as the Ozone Limiting Method, may be required. In

all cases, the site-specific NO2-to-NOx ratio or the non-guideline NO2 estimating method must be

approved by the reviewing authority.

Background
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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The combustion process typically forms several types of NOx. For modeling purposes, the NOx 

emissions are typically assumed to be 90 percent (by volume) nitric oxide (NO), and 10 percent NO . 2

However, after the flue gas exits the stack, additional NO2 is created as the exhaust mixes with the

surrounding air. The typical atmospheric reactions that create and destroy NO are: 2

NO + O -> NO + O (oxidation of NO by ambient ozone) 3 2 2

NO + HC -> NO + HC (oxidation of NO by reactive HC) 2

C

NO + sunlight -> NO + O (photo-dissociation of NO ) 2 2

Oxidation by ozone is typically the main reaction for NO2 formation, especially in rural areas. While the

reaction rate is essentially instantaneous, the total amount of NO2 conversion is limited by how quickly

the plume entrains surrounding air. Therefore, the amount of NO2 within the NOx plume increases as

the plume travels and disperses downwind of the stack. This increase will continue with time (plume

travel), until the reactions that create and destroy NO2 reach quasi-equilibrium. An illustration of this

change in NOx composition with plume travel is provided in Figure 1.
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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Figure 1 - The Change in NOx Composition (Percent NO ) with Plume Travel 2

The basic tenant of the ARM theory is that on a long-term (annual average) basis, the final plume

NO -to-NOx ratio will equal the existing ambient NO -to-NOx ratio. Therefore, once the ambient 2 2

NO2-to-NOx ratio is established, the predicted NO2 impact can be determined by multiplying the

modeled NOx concentration by the ambient ratio, as shown in Equation 1.

Equation 1: [NO ] = [NO /NOx] x [NOx] 2 pred 2 observed pred

where

[NO ] is the predicted annual average NO concentration, 2 pred 2

[NO2/NOx]observed is the observed (monitored) annual average NO2 concentration divided

by the observed (monitored) annual average NOx concentration, and

 [NOx] is the predicted (modeled) annual average NOx concentration. pred

As previously stated, the ARM theory applies at distances where the typical NOx composition within

the plume has stabilized. The original description of the ARM method indicated this distance could be

greater than 10 kilometers (km) from the emission source (Chu and Meyer, 1991). However, the
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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existing ambient ratio (measured at a far-field location) can also be used to conservatively estimate

near-field NO impacts. 2

The ARM theory assumes that the NOx mass emission rate of the source(s) is based on the molecular

weight of NO2, rather than an assumed mixture of NO and NO2. This assumption is used in EPA’s

Compilation of Air Pollutant Emission Factors (AP-42) and is typically used in vendor NOx data

sheets. While this approach is inconsistent with the typical assumptions regarding the initial mixture of

NO and NO2 on a volume basis, it eliminates the need for including molecular weight differences in

calculating mass emission rates. Therefore, observed volume ratios can be used in Equation 1 to

adjust modeled mass concentrations, without adjusting for molecular weight differences.

The GAQM provides 0.75 as a national default of the observed NO2-to-NOx ratio. This value is

based on 106 NOx monitoring data sets from various parts of the country. The GAQM also allows the

use of observed NO -to-NOx ratios based on local data. 2

Issues

The Workgroup has identified the following issues regarding site-specific ratios that should be resolved

or noted.

1. Monitor Location

The GAQM states that a site-specific ratio may be used “if it can be shown that such a ratio is

based on data likely to be representative of the location(s) where [the] maximum annual impact

from the individual source under review occurs. In the case where several sources contribute to

consumption of a PSD increment, a locally derived annual NO2-to-NOx ratio should also be

shown to be representative of the location where the maximum collective impact from the new

plus existing sources occurs.” (emphasis added)

The OLM/ARM Workgroup notes that EPA’s recommendation that the NO2-to-NOx ratio

represent the maximum impact site(s) may differ from the original recommendation presented at

the Air & Waste Management Association (Chu and Meyer, 1991). Chu and Meyer

recommended that the NO -to-NOx ratio be based on data collected 15 to 80 km downwind of 2

the predominant emission source, to ensure the data represents a NOx concentration in

quasi-equilibrium. From the Workgroup’s experience, the maximum NOx impact site(s) typically

occur well within 1 km of the predominant NOx emission source. In addition, the maximum

impact site(s) appear to be in the region where the composition of the NOx plume is still changing. 

Therefore, locating a monitor at the maximum impact site may be contrary to the

equilibrium tenant of the ARM, and lead to NO2-to-NOx ratios that are inappropriate for

estimating the NO impact at more distant receptors. 2
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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In contrast, ratios based on more distance monitors may be used for all receptors located between

the source and monitor. However, the resulting ratio may be overly conservative at the maximum

impact site(s).

2. Averaging Technique

Chu and Meyer recommend a unique approach for determining the annual average NO2 and

annual average NOx concentrations for purposes of determining site--specific NO2-to-NOx

ratios. They recommend calculating the annual average as the average daily average

concentration. This approach is different than averaging the hourly concentrations, as used in

standard monitoring methods, including those used in NAAQS compliance.

In larger urban areas, Chu and Meyer also recommend limiting the annual concentrations to

daylight hours (7 a.m. to 6 p.m.). They stated, “urban ratios of annual averages tend to be biased

toward the low side due mainly to the influences of significant nighttime emissions of area sources,

particularly, the nighttime traffic.” Using only daylight data results in a more conservative

NO -to-NOx ratio. 2

3. Data Quality

Chu and Meyer stated that daily average concentrations below 20 parts per billion (ppb) should

be eliminated prior to determining the annual average concentration. The purpose is to “avoid

potentially large errors introduced by small signal to noise ratios typical of current monitoring

instruments at low ambient levels of NOX.” In addition, at least 75% of the days must have daily

average concentrations greater than 20 ppb. Annual average concentrations cannot be determined

if there are fewer days with acceptable data.

These data restrictions can limit the application of the ARM method in low impact areas. Chu and

Meyer summarized this limitation with the following statement:

The chemiluminescent instruments currently used in NOx monitoring are known to have

relatively large errors in detecting NOx less than about 20 ppb. In order to minimize the

estimation errors, it is suggested that, until better instruments become routinely available,

only data with daily averaged NOx concentrations greater than 20 ppb be used in the

calculations. This may eliminate or greatly curtail the use of rural NOx data to derive

NO /NOx ratios. 2

Workgroup Recommendations for Developing a Site-Specific Ratio

As previously discussed, the NOx composition within the plume varies with distance. Ideally, the

changing ratio could be characterized by an equation that would provide the “observed” NO2-to-NOx
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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ratio at a given distance between the source and receptor. Site-specific data could then be used to

adjust this equation to provide the local NO2-to-NOx ratios. However, the Workgroup has not been

able to develop a generalized equation that would be applicable for a wide variety of modeling

scenarios. Therefore, the Workgroup has developed recommendations that provide conservative

estimates of the local NO -to-NOx ratio for all receptors located within the source-monitor radius. 2

The following discussion represents the OLM/ARM Workgroup’s recommendations on how such a

site-specific NO -to-NOx ratio should be developed. 2

Monitoring Equipment, Siting and QA Procedures

The Workgroup recommends siting the NOx monitor at a location sufficiently downwind of the

predominate emission source to allow the NOx composition to stabilize. The original recommendation

of 15 to 80 km downwind and within the general direction (±22.5 ) of the maximum impact, is o

reasonable. A closer site may be used if it can be shown that the distance is adequate for the plume

NOx composition to stabilize. A closer site may also be selected if the resulting NO -to-NOx ratio will 2

only be used to estimate NO2 concentrations at receptors within the immediate vicinity of the monitoring

site. In this situation, a different NO2-to-NOx ratio would be required to estimate the NO2

concentrations at other receptors.

The Workgroup acknowledges that siting a monitor at quasi-equilibrium distances essentially precludes

the use of conventional pre-construction monitoring data collected under the Prevention of Significant

Deterioration (PSD) program. EPA’s Ambient Monitoring Guidelines for Prevention of

Significant Deterioration (PSD) states “[pre-construction] monitoring data should be representative

of three types of areas: 1) the location(s) of maximum concentration increase from the proposed

source or modification, 2) the location(s) of the maximum air pollutant concentration from existing

sources, and 3) the location(s) of the maximum impact area.” (emphasis added). Since NOx plumes

typically do not reach quasi-equilibrium at the maximum impact points, PSD applicants subject to NOx

review may have to obtain other site-specific NOx data for developing local ARM ratios, or site an

additional NOx monitor at a distant downwind location.

NOx monitoring should be conducted using a NOx analyzer and quality assurance (Q/A) procedures

approved by the reviewing authority. The EPA designated reference or equivalent method for

chemiluminescent NOx analyzers is acceptable in areas with relatively high ambient concentrations (i.e.,

greater than 20 ppb).

Data Quality and Sufficiency

Hourly NO2 and NOx data should be collected for an entire year. The data should be validated using

the procedures established in the approved Q/A plan.
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

The OLM/ARM Monitoring Subgroup is developing recommended Q/A procedures to allow the 3

use of NO2 and NOx values measured at concentrations below 20 ppb. In the meantime, the

reviewing authority may be able to justify a lower quantification limit below 20 ppb, depending on

the sensitivity of the monitor, the selected scale of operation, and the type of quality assurance

procedures used in the 0-30 ppb range during data collection.

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The current EPA reference method for chemiluminescent NOx analyzers does not include quality

assurance procedures for data collected below 30 ppb (the lower limit of EPA’s acceptable audit

range). In addition, most audit test points occur between 50 and 80 ppb. EPA has certified some

NOx analyzers that are capable of measuring concentrations well below 20 ppb, but once again, there

are no established Q/A procedures for data collected at this range. Therefore, data below the “lower

quantification limit” cannot be assigned a reliable value.3

The Workgroup concurs with Chu and Meyer that NO2 and NOx values below 20 ppb, or the

approved lower quantification limit, must be thrown out. Unlike compliance monitoring for the ambient

NO2 air quality standard, data substitution can not be used in the ARM method for concentrations

below the lower quantification limit. Compliance monitoring essentially deals with one compound,

NO2, and seeks to determine the maximum concentrations. Therefore, accuracy at low concentrations

is not required, and the lower quantification threshold value or some other conservative estimate can be

substituted for data below this level.

In contrast, ARM monitoring deals with two “compounds,” NO2 and “NOx.” In addition, ARM

monitoring seeks to determine the relative concentrations (NOx composition) across the entire range

of recorded values. Therefore, accuracy at all data ranges, low and high, is more critical. Since data

below the lower quantification threshold is questionable, there is no reliable method for substituting both

the NO2 and NOx concentrations without making an assumption regarding the NOx composition. In

effect, one would have to assume a NO2-to-NOx ratio in order to substitute data, which circumvents

the purpose for conducting the ARM analysis in the first place. In rural and low impact urban areas,

most or even all of the NOx data may be below the 20 ppb lower quantification limit, which makes the

concern with data substitution even more significant.

An ARM database must have acceptable data for at least 75 percent of the year. This equates to

6,570 NO and 6,570 NOx data points greater than the lower quantification threshold (e.g., 20 ppb). 2

The data points do not need to be matched. Databases with fewer values cannot be used to determine

annual average concentrations. In these cases, a different approach must be used to estimate annual

average NO concentrations. 2

Averaging Technique
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The next step in the ARM method is to determine the annual average NO2 and annual average NOx

concentrations. The annual average concentrations should be determined using the standard averaging

technique where the sum of the values is divided by the number of values.

The Workgroup notes that using the annual average concentrations is different than the average daily

average approach recommended by Chu and Meyer. The Workgroup believes the annual average

approach is consistent with current monitoring concepts. In addition, the resulting ratio is being used to

refine the annual average NOx concentration, not the average daily average NOx concentration. 

Further, the ARM theory is based on the tenant that the predicted (modeled) NO2-to-NOx ratio equals

the observed (monitored) NO2-to-NOx ratio. This tenant is only true if the values used to establish

each ratio are based on the same averaging technique. Mathematically, the annual average NO2 to

annual average NOx ratio does not equal the average daily average NO2 to average daily average

NOx ratio.

The Workgroup concurs with Chu and Meyer that large urban areas can have significant diurnal

variations in NOx concentrations. A review of South Coast Air Basin data shows a very noticeable

nighttime increase in NO levels, presumably from motor vehicles (Chico, et al., 1998). This diurnal

change in NO concentrations leads to relatively low NO2-to-NOx ratios at night, and high

NO2-to-NOx ratios during the day. The NO2-to-NOx ratios based on daylight data are presumably

more representative of point source emissions. Therefore, the Workgroup concurs that the

NO -to-NOx ratio for large urban areas should be based on the annual average concentrations for 2

daylight hours (7 a.m. to 6 p.m.).

ARM Value

Once the annual average concentrations are determined, the annual average NO2 concentration is

divided by the annual average NOx concentration to determine the site-specific NO -to-NOx ratio. 2

The resulting ratio can then be used to estimate the NO2 concentrations using the methodology shown

in Equation 1.
OLM/ARM Workgroup - Draft Recommendations May 27, 1998

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Method Summary

The recommended procedure is summarized below:

1. Site the NOx monitor at a location sufficiently downwind of the predominate emission source to

allow the NOx composition within the plume to stabilize.

2. Collect a year’s worth of NO and NOx data using a NOx analyzer and quality assurance (Q/A) 2

procedures approved by the reviewing authority.

3. Validate the NO and NOx data using the procedures established in the approved Q/A plan. 2

4. Eliminate all NO and NOx data below 20 ppb, or the approved lower quantification threshold. 2

5. Check if at least 75% of the data remains. If not, seek some other NO modeling method. 2

6. Use the remaining data to calculate the annual average NO2 and annual average NOx

concentrations. For large urban areas, use only daylight data (7 a.m. to 6 p.m.) to determine the

annual average concentrations.

7. Divide the annual average NO2 concentration by the annual average NOx concentration to

determine the observed NO -to-NOx ratio. 2

8. Use the resulting ratio and Equation 1 to estimate the ambient annual average NO concentrations. 2

References

Chico, Thomas, Herman Wong, and Alan Schuler. Successes and Failures of Using the Ambient

Ratio Method to Estimate Annual NO2 Impacts. Air and Waste Management Association

91 Annual Meeting, June 1998. (AWMA Document Number 98-TAB.10P. In Press). st

Chu, Shao-Hang, and Edwin L. Meyer. Use of Ambient Ratios to Estimate Impact of NOx

Sources on Annual NO2 Concentrations. Air and Waste Management Association 84th

Annual Meeting, June 1991. (AWMA Document Number 91-180.6).

EPA. Guideline on Air Quality Models (Revised). EPA Document Number EPA-450/2-78-027R,

40 CFR 51, Appendix W. Last revised August 1996.

EPA. Ambient Monitoring Guidelines for Prevention of Significant Deterioration (PSD). EPA

Document Number EPA-450/4-87-007. May 1987.