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 NO modeling and to recommend solutions to 2 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 (NO ) impacts from point sources. The second tier uses the Ambient Ratio 2 1 Method (ARM), where the ratio of modeled NO to oxides of nitrogen (NOx) is assumed to equal the 2 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 NO -to-NOx ratio or the non-guideline NO estimating method must be 2 2 approved by the reviewing authority.

BackgroundOLM/ARM Workgroup - Draft Recommendations May 27, 1998

2 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 NO is created as the exhaust mixes with the 2 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 NO formation, especially in rural areas. While the 2 reaction rate is essentially instantaneous, the total amount of NO conversion is limited by how quickly 2 the plume entrains surrounding air. Therefore, the amount of NO within the NOx plume increases as 2 the plume travels and disperses downwind of the stack. This increase will continue with time (plume travel), until the reactions that create and destroy NO reach quasi-equilibrium. An illustration of this 2 change in NOx composition with plume travel is provided in Figure 1.OLM/ARM Workgroup - Draft Recommendations May 27, 1998

3 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, thefinal plume NO -to-NOx ratio will equal the existing ambient NO -to-NOx ratio. Therefore, once the ambient 2 2 NO -to-NOx ratio is established, the predicted NO impact can be determined by multiplying the 2 2 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 [NO /NOx] is the observed (monitored) annual average NO concentration divided 2 observed 2

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, theOLM/ARM Workgroup - Draft Recommendations May 27, 1998

4 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 massemission rate of the source(s) is based on the molecular weight of NO , rather than an assumed mixture of NO and NO . This assumption is used in EPA’s

2 2 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 NO on a volume basis, it eliminates the need for including molecular weight differences in 2 calculating massemission rates. Therefore, observed volume ratios can be used in Equation 1 to adjust modeled massconcentrations, without adjusting for molecular weight differences.

The GAQM provides 0.75 as a national default of the observed NO -to-NOx ratio. This value is 2 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] maximumannual 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 NO -to-NOx ratio should also be 2 shown to be representative of the location where the maximumcollective impact from the new plus existing sources occurs.” (emphasis added)

The OLM/ARM Workgroup notes that EPA’s recommendation that the NO -to-NOx ratio 2 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 NO -to-NOx ratios that are inappropriate for 2 estimating the NO impact at more distant receptors

. 2OLM/ARM Workgroup - Draft Recommendations May 27, 1998

5 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 NO and 2 annual average NOx concentrations for purposes of determining site--specific NO -to-NOx 2 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 NO .” In addition, at least 75% of the days must have daily X 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” NO -to-NOx 2OLM/ARM Workgroup - Draft Recommendations May 27, 1998

6 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 NO -to-NOx ratios. However, the Workgroup has not been 2 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 NO concentrations at receptors within the immediate vicinity of the monitoring 2 site. In this situation, a different NO -to-NOx ratio would be required to estimate the NO 2 2 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 maximumconcentration increase from the proposed source or modification, 2) the location(s) of the maximumair pollutant concentration from existing sources, and 3) the location(s) of the maximumimpact 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 NO and NOx data should be collected for an entire year. The data should be validated using

2 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 NO and NOx values measured at concentrations below 20 ppb. In the meantime, the 2 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.

7 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 NO and NOx values below 20 ppb, or the 2 approved lower quantification limit, must be thrown out. Unlike compliance monitoring for the ambient NO air quality standard, data substitution can notbe used in the ARM method for concentrations 2 below the lower quantification limit. Compliance monitoring essentially deals with one compound, NO , and seeks to determine the maximum concentrations. Therefore, accuracy at low concentrations 2 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,” NO and “NOx.” In addition, ARM 2 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 NO and NOx concentrations without making an assumption regarding the NOx composition. In

2 effect, one would have to assume a NO -to-NOx ratio in order to substitute data, which circumvents 2 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 TechniqueOLM/ARM Workgroup - Draft Recommendations May 27, 1998

8 The next step in the ARM method is to determine the annual average NO and annual average NOx 2 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) NO -to-NOx ratio equals 2 the observed (monitored) NO -to-NOx ratio. This tenant is only true if the values used to establish 2 each ratio are based on the same averaging technique. Mathematically, the annual average NO to 2 annual average NOx ratio does not equal the average daily average NO to average daily average 2 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 NO -to-NOx ratios at night, and high 2 NO -to-NOx ratios during the day. The NO -to-NOx ratios based on daylight data are presumably 2 2 more representative of point source emissions. Therefore, the Workgroup concurs that the NO -to-NOx ratio for large urban areasshould 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 NO concentration is 2 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 NO concentrations using the methodology shown 2 in Equation 1.OLM/ARM Workgroup - Draft Recommendations May 27, 1998

9 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 NO and annual average NOx 2 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 NO concentration by the annual average NOx concentration to 2 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 NO Impacts. Air and Waste Management Association 2 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 NO Concentrations . Air and Waste Management Association 84 2 th 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.