United States 

Environmental 

Protection 

Agency 

AIR 

Office of All Quality 

Planning and Standards 

Research Triangle Park, NC 2n11 

EPA-450/4-91-015 

May 1991 

~ • I ' J 

8~E PA CRITERIA FOR ASSESSING THE ROLE 

. . 

. --

OF TRANSPORTED OZONE/PRECURSORS 

IN OZONE NOt~ATTAINMENT AREAS 
.. 
aNVIHUI'\MiNlM 

PROTEC'T't8M 

AGiNCY 

8AU.AS, 1111&1 

liBRARY 

CRITERIA FOR ASSESSING THE ROLE OF TRANSPORTED 

OZONE/PRECURSORS IN OZONE NONATTAINMENT AREAS 

MAY 1991 

U. s. Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Technical Support Division 

Research Triangle Park, NC 27711 

TABLE OF CONTENTS 

LIST OF FIGURES . . . . . . . . . . . . . . iii 

LIST OF TABLES iv 

1.0 INTRODUCTION . 1 

2.0 ROLE OF TRANSPORT DETERMINATIONS IN IMPLEMENTING CLEAN 

AIR ACT REQUIREMENTS. . . . . . . . . . . • . . . . 3 

3.0 

2.1 

2.2 

Implications for Modeling and Monitoring ... 

CAA Modeling Requirements and Implications for 

Determining Transport . . . . . .. . . 

CONSIDERATION OF TRANSPORT IN THE DESIGN OF CONTROL 

STRATEGIES . . . . . . . . • . . . . . . . 

3.1 Determining Whether an Exceedance is Primarily Due 

3 

4 

9 

to Local Emissions . . . . . • . . • . . . . . 9 

3.2 Determining Likely Contributing Areas in cases of 

3.3 

overwhelming Transport . . . . . . . . . . 15 

3.2.1 A Predominant source Area is Identified 16 

3.2.2 No Predominant Source Area is Identified 17 

Consideration of Transport in Attainment 

Demonstrations . . . . . . . . . . . . . . 

3.3.1 

3.3.2 

3.3.3 

Use of Regional Scale Models 

Use of Monitored·Data to Estimate Model 

Boundary Conditions ..•..•..• 

Use of Modeling Procedures to Diminish 

Importance of Transport . . . . . . . 

18 

19 

23 

26 

4.0 MONITORING DATA FOR CHARACTERIZING TRANSPORT IN 

RECOMMENDED MODELS . . . . . . . . . . . . . 31 

4.1 Data Needed to Support Characterization of 

Transport in Models . . . . . . . . 

4.1.1 

4 .1. 2 

4.1.3 

4.1.4 

Trajectory Models . . . . . . . 

Urban Airshed Model . . . . . . . . . . . . 

Empirical Kinetics Modeling Approach . . . . 

Regional oxidant Model . . . . . . . . . . . 

i 

31 

31 

33 

36 

37 
4.2 Instrumentation/Deployment 

4.2.1 

4.2.2 

4.2.3 

4.2.4 

4.2.5 

Winds, Other Meteorological Data ..... . 

Ozone . . . . . . . . . . . . • . . . . 

NOX • • • • • 

co . NMOC . 

4.3 Quality Assurance of Monitored Data . 

5.0 SUMMARY 

REFERENCES CITED 

ii 

38 

38 

38 

38 

39 

39 

40 

43 

49 
1 

2 

3 

LIST OF FIGURES 

Multiperiod Back Trajectory With Area Most Likely 

Contributing to Observed ozone Identified 

Portion of United States Covered By Potential ROM 

Applications 

Future Ozone Transport as a Function of Present 

Transport 

12 

21 

27 

4 Minimal Ozone Monitoring Network . . . . . . . . . . . . 34 

.. 

iii 
1 

LIST OF TABLES 

Default Recommendations for Present Transported 

Boundary Conditions 

iv 

30 
1.0 INTRODUCTION 

The intent of this document is to present means (i.e. , 

criteria) for assessing the effect of transported ozone (03 ) and 

its precursors on 03 concentrations observed in locations not 

attaining the National Ambient Air Quality Standard (NAAQS)· for 03 • 

The primary purpose for making such an assessment is to foster 

design of control strategies which are most responsive to 

environmental conditions prevailing in a nonattainment area. 

Therefore, the criteria discussed herein address not only current 

conditions but future periods (after control strategies are 

implemented) as well. This is necessary in order to assess whether 

a proposed control strategy is likely to be successful in meeting 

prescribed deadlines for attaining the NAAQS for 0 3 • 

This document is prepared in response to Section 184(d) of the 

Clean Air Act (CAA) as amended in 1990. Section 184(d) states the 

following: 

"For purposes of this section, not later than 6 months after 

the date of the enactment of the Clean Air Act Amendments of 

1990, the Administrator shall promulgate criteria for purposes 

of determining contribution of sources in one area to 

concentrations of ozone in another area which is a nonattainment area for ozone. Such criteria shall require that 

the best available air quality monitoring and modeling 

techniques be used for purposes of making such 

determinations." 

The remainder of the document is organized in the following 

manner. Section 2 discusses why it is important to be able to 

characterize transport. This is done by identifying pertinent 

requirements in the amended Clean Air Act and by noting. how 

consideration of transport is a prerequisite for meeting these 

1 
requirements. Because Section 184(d), as well as other portions 

of the CAA, imply the need for quantitative estimates of transport, 

modeling is recommended as the prime means for making this 

determination. Moni taring is needed to support and estimate 

performance of the recommended modeling procedures. Section 3 

describes the appropriate sequence of modeling analyses for 

considering transport in the design of control strategies. This 

sequence proceeds by first noting the role of transport 

determinations in selecting locations in which to apply controls to 

most effectively reduce 03 during an observed episode. Next, 

procedures for considering transport in order to quantify the 

controls needed in identified ·areas to reduce 03 to the level 

specified in the NAAQS ( o .12 ppm) are described. Section 4 

presents monitoring recommendations (criteria) for estimating 

transported ozone and precursors needed in the modeling analyses 

recommended in Section 3. Section 5 summarizes identified modeling 

and monitoring criteria for determining contributions of transport 

in downwind nonattainment areas. 

2 
2. 0 ROLE OF TRANSPORT DETERMINATIONS IN IMPLEMENTING CLEAN AIR ACT 

REQUIREMENTS. 

2.1 Implications for Modeling and Monitoring 

The 1990 Clean Air Act amendments reflect a mix of 

ptescribed reductions of volatile organic compound (VOC) and 

nitrogen oxides precursors of (technology-based 

requirements). The Act also requires States to ensure that 

prescribed emission reductions are sufficient to meet the 0 3 NAAQS 

(air quality management requirements) in many nonattainment areas. 

It is in meeting the Act's air quality management requirements that 

proper consideration of transport is important. This consideration 

fulfills several roles: 

( 1) to help identify the primary reason (i.e. , transport 

or local emissions) for an observed "exceedance "* of the NAAQS in 

a nonattainment area, 

(2) to identify an upwind location(s) most likely to 

contribute to (and therefore the most appropriate locations to 

apply a set of strategies for reducing) an observed "exceedance", 

( 3) to allow quantitative estimates to be made regarding 

reductions in voc and/or NOx emissions needed in areas identified 

as likely to contribute to an observed exceedance in order to 

remedy the exceedance, 

( 4) to ensure and track effectiveness of promulgated 

control strategies, and 

*In thi~ document, the term "exceedance" is used to denote any 

observed daily maximum ozone concentration greater than the level 

specified in the NAAQS (0.12 ppm). 

3 
(5) to assess the extent of interstate transport 

throughout designated Transport Regions and assess strategies for 

mitigating interstate transport. 

Use of photochemical air quality dispersion models is the only 

practical way to meet the third and fifth roles identified above --

that of assessing whether contemplated emission control strategies 

will be sufficient to reduce 0 3 to ~0.12 ppm and whether interstate 

transport has been reduced sufficiently to make the goal a 

realizable one. 

The roles of meteorological and air quality monitoring are 

threefold: 

(1) to provide information concerning which areas and 

episodes to model; 

(2) to provide input information to models so that they 

might be used to make estimates of needed emission reductions in 

modeled areas; and 

(3) to aid in evaluating the performance of selected 

models. 

2.2 CAA Modeling Requirements 

Determining Transport 

and Implications for 

The CAA amendments categorize ozone nonattainment areas 

into five groups. These are, in descending order of severity, 

extreme, severe, serious, moderate, and marginal. Air quality 

management-related requirements are imposed in the first four of 

these categories. In addition, rural transport areas, as defined 

in ~ection 182(h), are subject to the same restrictions as a 

marginal nonattainment area, regardless of the severity of measured 

4 
ozone levels, if it can be shown that failure to attain in such an 

area is attributable to transport. Finally, certain small areas, 

as defined in Section 185(e), may be exempted from sanctions for 

failure to attain the NAAQS by a prescribed date, if attainment is 

prevented by transport-after all local State Implementation Plan 

(SIP) provisions are implemented. 

In extreme, severe, and serious areas, use of a photochemical 

grid model is required to demonstrate that proposed control 

measures will be sufficient to attain the NAAQS by specified dates. 

The Urban·Airshed Model (UAM) is the grid model recommended by the 

EPA for this purpose, and is expected to be widely used (Morris, et 

al. 1990 a,b; Douglas et al. 1990; causley 1990; Tang et al. 1990). 

In moderate nonattainment areas, use of grid modeling is 

required if the area includes territory in more than one state. 

Otherwise, grid modeling is preferred, but the Empirical Kinetics 

Modeling Approach (EKMA) is also an acceptable procedure (USEPA 

1989 a-c; Systems Applications, Inc., 1988; Meyer et al. 1989). 

Both the UAM and EKMA are regarded as "urban scale" models. That 

is, the modeling domain ordinarily addressed is sufficiently large 

to consider movement of pollutants over 12 daylight hours (e.g., 8 

a.m.- 6 p.m.). Generally, this has resulted in applications over 

domains on the order of a couple hundred kilometers on an upwind/ 

downwind axis at most. However, it is possible to consider larger 

domains for urban grid models. Domain size is usually limited by 

data base management considerations and computer limitations . 

.. 

5 
Criteria for considering transport must address the following 

issues relating to use of the UAM and EKMA models. 

( 1) Is use of one of these models appropriate for 

modeling the nonattainment area containing the ozone exceedance in 

question? If not, what should be done? 

( 2) What areajmodel ing domain should be modeled to 

adequately consider the effects of a proposed control strategy on 

an observed exceedance? 

(3) What inputs are needed to these models in order to 

reflect effects of transport? 

(4) How can the inputs needed for the urban scale 

models be derived? 

The EPA_has published guidance describing suitable ·procedures 

for using the UAM and EKMA in attainment demonstrations (USEPA 

1991a, USEPA 1989c). Much of the ensuing discussion regarding 

criteria for considering transport is drawn from these guidance 

documents and from Meyer, et al. 1989. Each model application is 

likely to have to respond to some area-specific characteristics 

which cannot be anticipated in general guidance. Thus, model 

application guidance has recommended formation of working groups 

consisting of representatives from each State affected in a 

particular modeling demonstration, appropriate US EPA. 

representatives and other interested parties (USEPA 1991a). It is 

expected that criteria for assessing the role of transported ozone 

and precursors identified in this document will be treated 

similarly. That is, a group of general procedures are identified 

6 
herein. It will be up to the working group responsible for 

developing and implementing a modeling protocol for a specific area 

to adapt this general guidance for use in specific areas. It is 

also conceivable that special circumstances prevailing in a 

particular area for which a modeling protocol is being developed 

may warrant use of procedures which differ from those described 

herein. In such cases, the rationale for a different procedure 

must be documented and submitted to the appropriate U.s. EPA 

Regional Office for consideration on a case-by-case basis. 

7 
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8 
3. 0 CONSIDERATION OF TRANSPORT IN THE DESIGN OF CONTROL STRATEGIES 

In order to quantify effects of transport and to ensure that 

transport is appropriately considered in the design of a SIP to 

demonstrate attainment of the NAAQS for ozone, it is necessary to 

use sophisticated, resource intensive modeling tools. To conserve 

resources, it is useful to focus modeling analysis on locations and 

incidents which, if remedied, would most likely result in 

attainment of the NAAQS. The discussion in Section 3.0 begins by 

describing several relatively simple tools (trajectory models) 

which may be useful in focusing analyses with complex models (e.g., 

UAM, Regional Oxidant Model (ROM)) needed to consider transport 

quantitatively. section 3.0 concludes with a discussion of urban 

and regional scale photochemical dispersion models needed to 

quantify effects of transport in SIP attainment demonstrations. 

3.1 Determining Whether an Exceedance is Primarily Due to 

Local Emissions 

The first step in assessing the role of transport is to 

determine whether transport is so overwhelming that the 

contribution of local emissions to an observed exceedance is 

relatively minor. such a case is referred to as "overwhelming 

transport". Procedures described in this step are useful in 

selecting episodes to test whether local control measures are 

sufficient to eliminate exceedances which are largely attributable 

to local emissions. In many cases, there may be an upwind/downwind 

gradient in observed 0 3 concentrations andjor man-made VOC and NOx 

emissJon density is much greater in the Consolidated Metropolitan 

Statistical Area (CMSA) or Metropolitan statistical Area (MSA) 

9 
under consideration than in surrounding locations. In such cases 

it is the prerogative of those responsible for the area's modeling 

protocol whether to perform an ·"overwhelming transport" 

determination using more rigorous procedures described below. If 

it is decided that a rigorous overwhelming transport determination 

is unnecessary, the analyst should proceed to Section 3. 3. Failure 

to perform an adequate "overwhelming transport" analysis when a 

detailed analysis is warranted could lead to very stringent 

prescriptions for control of local emissions. While such 

prescriptions might possibly work, they may not be a particularly 

efficient way to eliminate an exceedance which is largely a result 

of transport from external sources. 

If an assessment of overwhelming- transport is performed, the 

approach generally recommended is to perform trajectory analyses to 

assess the likelihood of overwhelming transport. The model 

ordinarily recommended for this purpose is the us EPA TRAJECTORY 

model (Meyer, et al. 1989). However, as described shortly, other 

trajectory models may be used if it can be shown that use of 

surface wind measurements alone inadequately describes transport. 

The EPA TRAJECTORY model uses surface wind data to construct a back 

trajectory from the site of the observed exceedance, beginning at 

the time of the daily maximum 03 concentration observed on the day 

of the exceedance. 

There is usually considerable uncertainty regarding the exact 

position of an air parcel nominally following a computed 

trajectory. The TRAJECTORY model incorporates this by considering 

10 
variability in wind data noted at nearby stations. The further one 

progresses from the back trajectory's starting point (i.e., the 

ozone monitoring site recording the exceedance) the greater this 

uncertainty becomes. As a result, one obtains a cone-like figure, 

emanating fro~ the monitoring site, which d~scribes the_ probable 

position of pollutant-laden air during periods preceding the 

observed exceedance. This is shown conceptually in Figure 1. 

Figure 1 depicts a back trajectory from a monitoring site with an 

observed daily maximum 0 3 concentration at 2:00 p.m. The "v1 t" 

represent distance traveled during each hour. The 61 are measures 

of uncertainty whose derivation is described in Meyer et al. 

( 1989). Significance of the shaded area will be described shortly. 

The trajectory is computed using surface wind data from all 

National Weather Service and other suitably sited wind monitors 

(see USEPA, 1987) within 160 km of the most likely computed 

position of an air parcel following the trajectory at any hour. 

Because the nighttime surface layer may be very shallow and winds 

aloft may differ significantly from surface observations at night, 

TRAJECTORY may only be applied for daytime hours (e.g., 8 a.m.- 8 

p.m.). 

An underlying assumption in the USEPA TRAJECTORY model 

application for identifying overwhelming transport is that if an 

air parcel corresponding with an observed exceedance is within the 

nonattainment area during 8 a.m. - 12 noon on the day of the 

exceedance, there is a good chance that local emissions play a 

significant role in causing the exceedance. This assumption is 

11 
t-' 

[\) 

V·t 5 

~t 

Figure ~ - Multlperlod Back Trajectory With 

Likely Contributing to Observed Ozone 
supported by diurnal meteorological conditions as well as by 

typical diurnal emission patterns for many important source 

categories of voc and NOx precursors for 0 3 • Thus, if all or part 

of the local CMSA/MSA containing or in close proximity to the site 

with the observed exceedance falls within ~he shaded area in Figure 

1, it can be assumed that its emissions play a significant role in 

leading to the observed exceedance. As noted in Meyer et al. 

1989, if the average resultant wind velocity is low, it should be 

assumed the local area contributes to the exceedance. If such a 

finding is made, the analyst can skip Step 2 in the procedure for 

considering transport (i.e., Section 3.2) and proceed to Step 3 

(Section 3.3). If a finding of overwhelming transport is made, the 

episode in question can be ignored in the local area's attainment 

demonstration unless it is not possible to identify an upwind 

CMSA/MSA(s) likely contributing to the observed exceedance. In 

this case, see the discussion in Section 3.2.2. 

It should be noted that it is possible for the TRAJECTORY 

model to yield misleading results under some circumstances. For 

example, if an exceedance occurs as the result of mid-morning 

fumigation of ozone which remains aloft overnight, it is 

conceivable that this is a result of transport rather than 

recirculated locally generated pollution. For exceedances 

occurring at 11 a.m., local civil time (LCT), or earlier, a 

trajectory model such as the Atmospheric Transport and Dispersion 

Model (ATAD) or the Branching Atmospheric Trajectory model (BAT) 

(Heffter 1980, 1983) is recommended to assess the likelihood of 

13 
overwhelming transport. These models utilize upper air data. 

Prior to applying such a trajectory model, the modeling protocol 

working group should consider whether the exceedance is due to 

fumigation. Review of available diurnal mixing height information 

and time series analysis of ai~ quality data may be helpful ~n 

distinguishing a fumigation-induced problem from one resulting from 

photochemistry relatively early in the day. If a mul tiday 

trajectory model is used to assess likely sources of a morning 

exceedance, attention should be given to MSA/CMSA's traversed by a 

swath surrounding the trajectory from 8 a.m. - 6 p.m. on the 

preceding day. Width of the swath is a function of uncertainty 

associated with the trajectory estimates. Uncertainty can be 

determined by a climatological review of trajectories computed 

under similar synoptic conditions and wind patterns. 

The TRAJECTORY model generally remains the model of choice for 

addressing exceedances occurring after 11 a.m., because it utilizes 

more dense surface monitoring networks and its application is 

confined to periods of the day when wind shear is likely to be 

minimized. Thus, surface winds would be more representative of 

flow in the mixed layer in most locations. Choice of the 

trajectory model and its underlying rationale must be documented by 

the working group preparing a modeling protocol and is subject to 

review by the u.s. EPA. 

It is next appropriate to consider rural transport areas, as 

described in CAA Section 182(h), more explicitly. Use of 

trajectory models, as described in the previous paragraphs, is 

14 
generally the preferred approach for confirming that an area, which 

otherwise qualifies, may be treated as a rural transport area. 

However, it may happen that the wind data available near such 

locations are too sparse to be reliable, or available trajectory 

models are otherwise.unsuitable. If the workipg group responsible 

for a modeling protocol determines that available trajectory models 

are inappropriate for the purpose of identifying overwhelming 

transport into a proposed rural transport area, the reasons for 

this determination should be documented and are subject to review 

by the U.S. EPA. If the EPA agrees with this assessment, other 

methods may be used to justify treatment of a candidate 

geographical location as a rural transport area. An area may be 

treated as a rural transport area if justified by the weight of 

supporting evidence of two or more of the following types (1) NOx 

and VOC inventories are much less than those in locations where it 

is plausible to believe pollution may be originating, (2) a past 

photochemical grid modeling analysis supports the hypothesis of 

overwhelming transport, (3) a field study supports the likelihood 

of overwhelming transport, or (4) other pertinent gu~dance can be 

presented. If an area is determined to be a rural transport area, 

exceedances occurring therein should be considered in selecting 

episodes to model and in the choice of modeling domains for nearby 

CMSA's/MSA's. 

3.2 Determining Likely Contributing Areas in Cases of 

Overwhelming Transport 

Once it is determined that "overwhelming transport" is 

likely, the next step is to try to establish the most likely 

15 
predominant source(s) of an observed "exceedance". If the 

TRAJECTORY model is used, this is done by noting any CMSA/MSA's 

located completely or partially within the shaded area in Figure 1. 

If a model using upper air data is used to estimate the cause of a 

morning exceedance, CMSA's/MSA's loca~ed within a swath surrounding 

the trajectory during 8 a.m. - 6 p.m. of the preceding day should 

be identified. As noted in Section 3.1, the width of the swath is 

a function of uncertainty in the trajectory path as estimated from 

climatology of the area with the observed exceedance. Results of 

applying any of the trajectory models may yield any of several 

outcomes: 

(a) one of the identified CMSA/MSA's has many more NOx 

or voc emissions than any of the other identified CMSA/MSA's. 

(b) Several CMSA/MSA's are identified, but it is not 

obvious which one(s) exerts the predominant influence on 

transported ozone/precursors; 

(c) No CMSA/MSA's are identified. 

3.2.1 A Predominant Source Area is Identified 

If the emissions of NOx or VOC in 9ne of the identified 

CMSA/MSA's greatly exceed those in the others, it may be assumed 

to be the predominant cause of the observed exceedance. In this 

case, the episode in question should be among those considered for 

modeling for this upwind CMSA/MSA. Procedures for selecting 

episodes to model with UAM or EKMA are described in USEPA 1991a and 

in USEPA 1989c, respectively. If the UAM is used and the episode 

in question is one of those selected for modeling, the downwind 

16 
boundary of the modeling domain for this upwind CMSA/MSA should 

extend beyond the monitoring site, as described in USEPA 1991a. 

3.2.2 No Predominant Source Area is Identified 

Outcomes (b) or (c) above are possible if an exceedance 

is the.composite .effect of- emissions in many areas or is a result 

of multiday transport. Several approaches are possible if one of 

these outcomes occurs. First, if there are a number of such 

incidents, this might serve as one consideration in deciding 

whether to petition the EPA Administrator to establish a Transport 

Region in accordance with Section 176A of the CAA. Cumulative 

effects of SIP's in several CMSA's/MSA's or States are best 

simulated with regional scale models, such as the EPA ROM, (Lamb, 

1983) used in concert with urban scale models. Regional models are 

also the recommended approach for quantifying interstate transport, 

as required in Transport Regions. Application of ROM and its role 

in assessing effects of transport will be described more fully in 

Section 3.3. 

If there is a limited number of "unattributed" exceedances, it 

may be possible to address the problem without establishing a 

Transport Region. A second approach could be use of an urban scale 

photochemical grid model, like the UAM, and a large modeling 

domain. The domain would encompass monitoring site(s) observing 

unattributed exceedances as well as the several CMSA/MSA's 

identified through use of trajectory methods as potential 

contributors to those exceedances. The episode(s) including the 

unattributed exceedances would be among those considered for 

17 
simulating over the large modeling domain (USEPA, 1991a). In such 

cases, the effect of emissions in one or several CMSA/MSA's on air 

quality downwind can be assessed through model sensitivity tests. 

If it is not feasible to exercise either of the preceding two 

approaches, it would be necessary to focus once again on the local 

area's attainment demonstration. As noted previously, it is not 

necessary to simulate effects of local controls on an exceedance 

which is a product of overwhelming transport. Instead, it should 

be treated as an "irreducible exceedance". 

If EKMA is used for the local area's attainment demonstration, 

guidance with respect to irreducible exceedances presented in Meyer 

et al. 1989 should be followed. In essence, this requires reducing 

the number of incidents in which an exceedance is allowed after 

simulation of proposed controls. A similar procedure is 

recommended if a photochemical grid model (e.g. , UAM) is used. For 

example, suppose guidance in USEPA 1991a permitted one post-control 

modeled exceedance. In this case, presence of one or more 

irreducible exceedances would mean that a proposed control strategy 

would no longer be acceptable if it resulted in any modeled 

exceedances. 

3.3 Consideration of Transport in Attainment Demonstrations 

The preceding two sections addressed qualitative 

procedures for identifying location ( s) most likely to be the 

primary cause of an observed exceedance. Once these are 

identified, an urban scale modeling analysis should be focused on 

the identified area ( s) using guidance in USEPA 1991a or USEPA 

18 
1989c. An identified area is, by definition, one whose control 

strategies may significantly affect whether an observed exceedance 

is eliminated. Effectiveness of controls in such an area may, 

nevertheless, be influenced by transport. The purpose of this 

·section is_to present recommendations for considering transported 

ozone and precursors in urban scale modeling used for attainment 

demonstrations. In attainment demonstrations, transport is 

considered by specifying boundary conditions for urban scale 

models. Approaches for doing this are enumerated below. 

3.3.1 Use of Regional Scale Models 

The preferred approach for generating boundary conditions 

for urban scale modeling applications is to "nest" the urban scale 

modeling domain within a regional modeling domain. Typical 

regional modeling applications consider domains on the order of 

1000 km or more on a side. Regional scale modeling simulations are 

begun two or more days prior to the period to be simulated by using 

a gridded photochemical model such as the US EPA's ROM. The 

regional model is used to simulate base case boundary conditions as 

well as regional i~pact of control strategies likely to be applied 

in and between the various CMSA's/MSA's within the regional 

modeling domain. The procedure followed in the regional (and 

urban) modeling analysis is to assume past meteorological episodes 

corresponding with high ozone are characteristic of future episodes 

with ozone-conducive conditions, and apply a control strategy for 

some future year. Generally, the year chosen would be the year in 

which the CAA specifies attainment of the NAAQS should be reached. 

19 
As noted in CAA Section 181 (a) , this varies according to how 

serious the present ozone problem is for a CMSA/MSA. 

Use of a gridded regional model requires a large amount of 

computer capacity. Further, the domain is often likely to 

encompass many political· jurisdictions, making it difficult to have 

access to the requisite emission information over the entire 

domain. This latter problem can only be overcome if states submit 

point, area and mobile source information to some centralized data 

system which can be accessed by the regional model. Because of the 

difficulties listed in this paragraph, it is recommended that the 

us EPA ROM model be used as the model of choice whenever feasible. 

States should submit emissions data to the us EPA's Aerometric 

Information Retrieval System (AIRS) data base (USEPA, 1989d) by 

November 1992, so that regional modeling efforts might make use of 

these estimates within the limited time allotted by the Clean Air 

Act. The us EPA will run ROM for as many episodes and locations as 

resources permit. Figure 2 depicts that portion of the United 

States for which it will be possible to run ROM within the next 3-4 

years. Clearly, the task of ensuring consistent data bases and 

strategy assumptions in regional and urban scale modeling analyses 

requires extensive coordination. For this reason, whenever 

regional models are used, the EPA should be represented on the 

technical or policy work group responsible for conducting urban 

scale attainment demonstrations. 

An infrastructure exists to enable States to readily access 

and use the results of ROM simulations as input to their urban 

20 
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, , ..... . '~ . . ·;1 . ../~· :·' lj.J 

\ ( ./_. r-·---.. ~··~, -::. .. 

46 

45 

44 

4.3 

42 

41 

40~· ..... 

39 

38 

37 

36 

35 

.) 

\ 

._ ----·.... l· ~-·r~· I 

I 

34+1 ··., ··i • .. 1 

... , ' . ,. .\ ;· 

t ' •. •. '',.··· .. 

{, I I • ·-~ ~ ' / 

~ 1... t 

.... t ' ~ t 33 

) /' \' \ . ' I ~ ............ ~ ,. ............... \ .\ 

( '•, ' . . ..... -.. ·-----.. ·-/'""' .. /':'""'"'· jl. }/,, .... ,,-. I i t.. .. ~:ri · · .. - ·· ~.: J""· ... 

I ' ~. .. . ' ' \ ~ , ... J t., ·: . .-. \\ 

32 

31 

30 

\:··· .......... ' .~,'\ .. . ... \ 2 9 -H I'. · .. " ·"':'~j· '•iJ' L 

2T .. r 

- souTHEAsT t \ , 

~I ~ 27+ /i \ 

26 + '~ + + I I I I I I I I I I I I ·,\ I ) I I I I I I 

99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 '84 83 82 81 80 79 78 77 76 75 74 

Figure 2. Portion Of The United States Covere

Potential ROM Applications 
scale modeling analyses. States may access ROM-generated data 

using the Gridded Model Information Support System (GMISS) 

(Computer Sciences Corporation, 1991). The us EPA archives air 

quality predictions as well as certain input information 

corresponding with · each application of ROM into GMISS. Groups 

using the UAM can readily make use of data retrieved from GMISS 

using the ROM/UAM Interface Program System (Tang et al. 1990). 

Similar means exist for using data retrieved from GMISS if EKMA is 

used (USEPA, 1991b). 

As implied by Figure 2, there are locations within the United 

states where it will not be feasible to use ROM in the foreseeable 

future. Further, ROM results may not be available for every 

episode which is of interest in every nonattainment area. Use of 

alternative regional scale models to generate bounda~y conditions 

for urban scale models is acceptable. Such use should be 

specifically addressed in modeling protocols developed for each 

urban modeling application, and coordinated with the appropriate 

u. s. EPA Regional Office(s) for approval on a case-by-case basis. 

Use of regional scale models is the preferred method for 

considering transport into a nonattainment area for several 

reasons. First, it is the most defensible means for estimating 

boundary conditions (i.e. , transport) after implementation of 

regional and urban strategies in upwind areas. As noted, 

attainment demonstrations must focus on future periods specified in 

the Clean Air Act. Therefore, credibility of estimated future 

transport is a key consideration. A second advantage of using 

22 
regional models is that they provide a far more comprehensive set 

of present boundary conditions than is feasible using any other 

method. This, in turn, replaces much of the subjectivity necessary 

in specifying present boundary conditions using other means. For 

example, the Urban Airshed Model generally considers five or more 

vertical layers in the atmosphere. It is necessary to specify 

boundary conditions for each grid cell in each of these layers 

along a domain's upwind boundary for each hour simulated. As noted 

in Section 3.3.2, it is impractical to collect monitored data for 

each of these grid squares. If monitoring data rather than 

regional models are used to specify boundary conditions, the 

specifications must be based on a limited number of direct 

observations and interpolation or some other, subjective procedure. 

3.3.2 Use of Monitored Data to Estimate Model Boundary 

Conditions 

If it is not possible to use a regional scale model to 

generate boundary conditions for urban scale models, an alternative 

approach is to derive estimates of boundary.conditions based on 

available air quality monitoring data. If the UAM is used, Section 

4.2.6 in Morris et al. 1990b shows grid cells in the UAM modeling 

domain for which boundary conditions must be specified for each 

hour of the simulation. As described in Morris et al. 1990b, it is 

possible to divide the modeling grid's boundaries into subsegments. 

This may be useful if monitored data suggest a distinct horizontal 

concentration gradient along the modeling grid's boundaries. The 

UAM software permits use of two procedures for estimating hourly 

boundary concentrations in surface level grid cells comprising each 

23 
segment of the domain's boundaries. The first of these assumes a 

constant concentration (of 0 3 , precursors) for each segment. The 

second approach performs a linear interpolation between 

concentrations specified at each segment's end points to estimate 

boundary conditions in each surface grid cell included in a 

boundary segment. Morris et al. 1990b note that the procedure 

generally followed is to assume a spatially invariant (i.e. , 

constant) concentration for cells in each segment. 

The UAM software also contains provisions for specifying 

hour-by-hour boundary conditions in grid cells aloft. These are 

described in Section 4.1.7 of Morris et al. 1990b. Two procedures 

have been most commonly used in past applications of UAM: 

( 1) Assume a concentration which is constant with 

height; 

(2) Specify a vertical concentration profile pattern 

which varies between a concentration estimated, as described 

previously, in a surface grid square and a concentration specified 

or calculated previously in a grid cell just above the mixing 

height (i.e., DIFFBREAK). 

The first method is the simpler of the two and may be 

justified, particularly during times of day with vigorous vertical 

mixing. The second method is seemingly more sophisticated but, if 

used, a rationale for the selected vertical profile should be 

included in the protocol describing the technical basis for the 

attainment demonstration. In the second method, concentrations 

24 
above DIFFBREAK are usually initially assumed to be at background 

levels (see Section 3.3.3). 

It may often happen that monitored data for ozone exist to 

enable specification of boundary conditions, but there are no data 

for the NO, N02 , CO; and voc species which must also be specified. 

In this case, information in Table 1 (see Section 3.3.3·) will have 

to suffice for specifying values for unmonitored species. 

Boundary conditions are considered somewhat differently if 

EKMA is used. In the OZIPM4 model underlying EKMA, one models 

concentrations within a moving column of air. As described in 

USEPA 1989c, the simulation begins at 8 a.m. on the day of an 

observed exceedance, with the column located over the central city 

in the modeled CMSA/MSA. The column follows a straightline 

trajectory which places it at the monitoring site recording the 

exceedance at the time of the exceedance. During the simulation, 

the column grows in height to reflect the typical diurnal rise in 

mixing height. As the column height grows, air is entrained from 

aloft. Boundary conditions are considered by specifying ozone and 

precursor qoncentrations in this layer aloft at the beginning of 

the simulation. These specified concentrations aloft are invariant 

with time. 

As noted previously, for attainment demonstrations, an 

estimate of future transported ozone and precursors is needed. In 

the absence of regional models, Figure 3 has been used to estimate 

future transported 0 3 , given an observed level of transported 03 

(USEPA 1989c). To illustrate use of the Figure, suppose a boundary 

25 
0 3 concentration for an isolated urban area is estimated to be 0.10 

ppm. The corresponding future boundary condition for 0 3 would be 

0.09 ppm. Figure 3 is based on application of OZIPM4/EKMA with a 

20 percent reduction in voc, no change in VOC speciation, and no 

change in NOx. Thus, for consistency, these latter assumptions 

should be used in conjunction with Figure 3.· In the absence of 

better information, this methodology may be applied for use with 

UAM as well as with EKMA. Use of alternative procedures for 

estimating future transport in the absence of regional modeling 

must be justified, and is subject to approval by the US EPA on a 

case-by-case basis. 

3.3.3 Use of Modeling Procedures to Diminish Importance of 

Transport 

Barring an intensive field study, it is clear from Section 

3.3.2 that specification of present boundary conditions based on 

monitoring data requires a number of arbitrary procedures andjor 

subjective judgment. Unfortunately, even when intensive field data 

are available, the period of an intensive study often will not 

coincide with 0 3 episodes of greatest interest. Further, the basis 

for estimating future boundary conditions is not strong. 

Therefore, it is appropriate to consider ways in ·which the 

sensitivity of model predictions to assumed boundary conditions can 

be diminished. 

sensi ti vi ty of UAM predictions to boundary conditions can 

likely be diminished by increasing the size of the modeling domain 

26 
N 

-...} 

0.12 

0. 1 1 

"""' 0.1 

/ 

/ 

/ 

/ 

/ 

/ 

,' 

0. 

a. 0.09 

.. /t 

1- 0.08 

0 

n. 

(/) 0.07 

z 

(2 0.06 rw 

z 0.05 0 

N 'r 0 0.04 

w 

:::> 0.0.3 

r-

:::> 

lL. 0.02 

0.01 

0 

0 

Source: EPA, 1989C 

/. 

/_, 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/J 

4 

0.02 0.04 0.06 0.08 0.1 0.12 0.14 

PRESENT OZONE TRANSPORT (PPM) 

Isolated /\rea ____ Mea :H th large 

FIGURE 3 

FUTURE OZOilE TnAIISPORT AS A FUIICTIOII OF PRESEIH TRAH
on the upwind side of the CMSA/MSA which is the focus of attention. 

There are, of course, practical limits to this in terms of data 

base management and computer constraints. Further, a tradeoff must 

be made against the desirability of extending the domain downwind 

beyond the location of monitors observing.exceedances attributable 

to the CMSA/MSA being modeled. 

Table 1 contains recommended default inputs for boundary 

conditions contained in USEPA 1989c. Defaults may be used if the 

UAM modeling domain is enlarged, as described above. The recommendations for nonmethane organic compounds* (NMOC), NMOC 

speciation, and 0 3 are based on a review of aircraft observations 

in an unpublished report by Baugues (1987). Subsequent to the 

Baugues (1987) review, doubt was cast on the observed NOx data. 

Hence the NOx defaults are based on earlier recommendations (USEPA 

1978). Recommendations for CO are based on a review of data 

produced from sampling aboard aircraft during the 1980 PEPE/NEROS 

study. The co recommendation differs from that in USEPA (1989c). 

The recommendations made herein are believed more representative of 

available data and should supersede the CO default value 

recommended in USEPA (1989c). In the absence of better, areaspecific information, these defaults may be assumed to be constant 

vertically and horizontally. In USEPA 1989c, it is noted that 

future levels of NMOC and co may· be reduced 20 percent, and NOx 

kept constant. NMOC composition is kept constant. 

*The term "nonmethane organic compounds" applies to ambient 

measurements of organic compounds. 

28 
Recommendations in Table 1 differ from some published 

elsewhere for use with the UAM (Morris et al. 1990c). The Morris 

et al. (1990c) recommendations are based on observations in more 

remote areas, whereas those in Table 1 are based on observations 

upwind and aloft of several cities. Unless the upwind bounds of 

the UAM modeling .domain are reflective of a remote area, 

information in Table 1 would appear more appropriate. If the 

Morris et al. (1990c) recommendations are considered more suitable 

in specific cases, future boundary conditions should be assumed 

identical to present boundary conditions. This is appropriate 

since the Morris et al. ( 1990c) suggestions are believed more 

reflective of natural background. 

As described in USEPA 1989c, the values in Table 1 may be used 

as defaults for the layer aloft considered in the OZIPM4/EKMA 

procedure. As with the UAM, the defaults for NMOC and co may be 

reduced by 20 percent for future boundary conditions. 

29 
Table 1 

DEFAULT RECOMMENDATIONS FOR PRESENT TRANSPORTED BOUNDARY CONDITIONS 

NMOC 

CBIV Speciation 

(Carbon Fractions) 

03 

NO" 

NO 

co 

*Source: PEPE/NEROS Study 

30 ppbc 

30 

40 ppb 

2 ppb 

0 ppb 

350 ppb* 

PAR = 0.498 

ETH = 0.034 

OLE = 0.020 

ALD2 = 0.037 

FORM = 0.070 

TOL = 0.042 

XYL = 0.026 

ISOP = 0 

NR = 0.273 
4.0 MONITORING DATA FOR CHARACTERIZING TRANSPORT IN RECOMMENDED 

MODELS 

An obvious question to ask upon reading Section 3.0 is, "What 

monitoring data are needed to drive the methods used to estimate 

boundary conditions for use with UAM and EKMA?" · This subject_ is 

addressed in Section 4. For serious, severe, and extreme 0 3 nonattainment areas, recommendations contained herein must be regarded 

as interim. Monitoring for these locations will be subject to 

regulations for enhanced monitoring networks in accordance with 

Section 182 (c)(1) of the Clean Air Act. These regulations are to 

be promulgated by June 1992. Recommendations in this Section which 

are inconsistent with those future regulations will be superseded. 

The discussion below proceeds by noting meteorological and air 

quality data needed to support each modeling procedure described in 

Section 3. Next, suggestions are made for sampling and analysis to 

obtain the data identified as necessary. Section 4 concludes with 

recommendations regarding quality assurance of coll~cted data. 

4.1 Data Needed to Support Characterization of Transport in 

Models 

4.1.1 Trajectory Models 

As described in Sections 3.1 and 3. 2, the trajectory 

models are used to establish which CMSA/MSA(s) is (are) most likely 

contributing to an observed exceedance. The key data required by 

the U.S. EPA TRAJECTORY model are hourly National Weather Service 

(NWS) surface wind data measured within 160 km of the most probable 

location of an air parcel during each hour from 8 a.m., LCT to the 

time of the observed daily maximum 0 3 concentration at the 0 3 

31 
monitor recording an exceedance. Because of concern over the 

representativeness of surface data at night, only 8 a.m.- 8 p.m., 

LCT, surface wind data recorded on the day( s) of an observed 

exceedance are needed. Table B-1 in Meyer et al. (1989) lists NWS 

sites for which data may be ordered. 

It is desirable to use as many wind observations as possible 

to construct trajectories with TRAJECTORY. However, prior to using 

additional observations, a note about NWS wind data is in order. 

These data reflect observations taken over a few minutes during 

each hour rather than continuous hourly averages. Allowances have 

been made in the TRAJECTORY model for this. It is recommended that 

data used to supplement the NWS observations be compatible with 

this characteristic. 

Two kinds of ancillary data are useful with the TRAJECTORY 

model. These are surface 03 measurements and upper air wind 

observations. Upper air wind data are useful as a guide to test 

whether adjustments made to surface data by TRAJECTORY are 

appropriate. These adjustments are made to make surface 

observations more representative of mean hourly winds in the mixed 

layer. Clearly, if a trajectory model, like BAT or ATAD, which 

requires use of upper air data is used, upper air wind information 

is not ancillary, but essential. In ATAD and BAT, upper air -data 

are used directly to estimate mean mixed layer winds or to estimate 

winds in discrete layers. 

ozone data are useful for testing the plausibility of 

conclusions drawn from trajectory models about whether or not 

32 
overwhelming transport is likely. Time sequences for ozone data 

observed along the track of an estimated trajectory may provide 

additional support for conclusions reached using the wind data. 

4.1.2 Urban Airshed Model 

The Urban Airshed Model makes use of a wide variety . 

of meteorological and air quality data. These are described in 

Morris et al. (1990b) and in USEPA (1991a). The discussion herein 

dwells solely on data useful to characterize boundary conditions. 

It is desirable to have air quality data collected at the surface 

and aloft (especially during periods of atmospheric stratification 

like nighttime). However, collection of data aloft is a resource 

intensive operation which needs to be designed on a case-by-case 

basis. The following discussion addresses minimum surface data 

needed to avoid having to rely entirely on the large domain/default 

assumption methodology described in Section 3.3.3. 

Figure 4 is a conceptual picture of a minimal ozone 

monitoring network suitable for use with UAM. For purposes of 

illustration, conditions conducive to high ozone most frequently 

occur with winds from U1 • Second most conducive conditions occur 

with winds from U2 • 

The following presents rough guidance for the siting of 

monitors in the network shown in Figure 4. 

Site 1: 15-50 km in the predominantly upwind direction from 

the city limits. Minimum distance upwind 

should be determined on a case-by-case basis, 

33 
U2 

® (1) 

/ 

U1 

® (3) 

@ (5) 

CENTRAL CITY 

LIMITS 

(§) (4) 

FIGURE 4. MINIMAL OZONE MONITORING NETWORK 

34 
Site 2: 

Site 3: 

site 4: 

Site 5: 

depending on city size and surroundings. Data from 

this monitor are to be used primarily to estimate 03 

transported to the model domain from upwind 

sources. 

Near the predominantly downwind edge of the city 

limits. 

15-35 km from the city limits in the predominantly 

downwind direction. 

30+ km from the city limits in the predominantly 

downwind direction. 

15-50 km in the second most prominent downwind 

direction. 

Under the first set of meteorological conditions (U1 ), Site 1 

is useful for estimating boundary conditions. Data from Site 5 may 

also be useful if, like Site 1, it measures ozone on a regional 

scale as defined in 40 CPR 58, Appendix D. Data from Site 4 may 

supplement those from·Site 1 in establishing boundary values under 

the set of conditions depicted by U2 , providing the data are 

regional in scale (40 CPR, Part 58, Appendix D). Data from sites 

not used for specifying boundary conditions may be used to assess 

model performance in predicting observed ozone concentrations. 

Care must be taken in interpreting surface ozone data for 

establishing transport during periods of high atmospheric stability 

(e.g., night). During such periods, surface data are unlikely to 

be representative of concentrations aloft. In the absence of more 

35 
direct measurements or indicators of ozone aloft, the following 

approximation of ozone aloft may be used: 

Use the higher value of the observed surface concentration for 

the hour in question, or the surface o~one concentration averaged 

over 9-11 a.m. LCT on the following morning. 

Precursors 

For purposes of characterizing boundary conditions for NMOC, 

NOx, and CO, sampling may be performed at sites {1), {5) and (4) in 

that order of preference. Care should be taken to ensure that the 

sampling sites are well-exposed. Sampling for NMOC within a 

vegetative canopy is unacceptable for purposes of characterizing 

regional boundary conditions. 

As with 0 3 , there is a concern that nighttime surface 

precursor data are unrepresentative of precursors aloft. Unlike 0 3 

however, the concern is that surface observations will overestimate 

concentrations aloft. In the absence of more direct information 

regarding concentrations aloft, during nighttime, the lower of the 

observed hourly surface measurement and the surface measurement 

averaged over 9-11 a.m., LCT, on the following morning may be used 

to approximate nighttime NMOC, NOx and CO aloft at regional sites 

intended to measure transport. During daylight hours, ozone and 

precursor data should be used as described in 

Section 3.3.2. 

4.1.3 Empirical Kinetics Modeling Approach 

Unlike UAM, EKMA only requires one assessment of 

boundary values for 0 3 , NMOC, NOx, and co per day. As described in 

36 
USEPA 1989c, the period chosen is the first full hour occurring 

after breakup of the nocturnal inversion. If this is not known, a 

10 a.m. - 12 noon LCT average surface concentration observed at 

upwind, regional sites may be used. For ozone, a minimal 

monitoring network similar to that in Figure 4 is suggested. The 

discussion in Section 4.1.2 also applies. Sampling of transported 

NMOC, analysis of speciated NMOC data and sampling/analysis of 

transported NOx and CO, though desirable, has a lower priority with 

EKMA than with UAM applications. 

4.1.4 Regional Oxidant Model 

Because of the spatial scales involved with this 

model, specification of its boundary conditions is likely to be 

less critical than is the case for urban models. However, 

measurements of regional ozone/precursors concentrations within the 

regional domain may be useful for evaluating ROM model performance. 

Monitored ozone data may be used to establish boundary values for 

ozone in ROM using applicable guidance described in Sections 3.3.2 

and 4.1.2 for UAM. Use of a natural background default value for 

ozone (Morris, et al. 1990c) is an acceptable alternative in the 

absence of sui table monitored data. Once boundary values are 

specified for ozone, a routine in the ROM establishes chemical 

equilibration with the specified ozone concentrations to derive 

concentrations for other chemical species. Hence, it is 

unnecessary for the user to specify these values. 

37 
4.2 Instrumentation/Deployment 

4.2.1 Winds, Other Meteorological Data 

Guidance provided in USEPA (1987) should be followed 

with respect to instrumentation and deployment at surface wind 

monitoring sites. Meteorological data aloft will be derived from 

sounding information collected twice daily at sites approximately 

500 km apart. These data are archived and may be obtained from 

the National Climatic Data Center in Asheville, NC. Data collected 

in special field studies such as the Lake Michigan Ozone study and 

the Southern Oxidant Study should also be used if available for 

periods of interest. 

4.2.2 Ozone 

ozone should be measured as described in Appendix D 

to 40 CFR, Part 50. For characterizing transport with UAM and 

EKMA, these measurements should be on a regional scale (40 CFR, 

Part 58, Appendix D). 

4. 2. 3 NOX 

UAM and EKMA require measurement of both nitric 

oxide (NO) and nitrogen dioxide (N02 ). Transported NO, NOx, and N02 

should be measured using alternative A (gas phase titration) 

described in Appendix F to 40 CFR, Part 50. Measurements should be 

collocated with the regional scale ozone measurements used to 

characterize ozone boundary conditions. 

38 
4.2.4 co 

Carbon monoxide should be measured as described in 

Appendix c to 40 CFR, Part 50. Measurements should be collocated 

with regional scale ozone measurements. 

4.2.5 NMOC 

Procedures for sampling and analyzing ambient 

organic compounds are rapidly evolving. It is no longer sufficient 

to use procedures outlined in Appendix E to 40 CFR, Part 50. 

Instead, it is recommended that sampling be done remotely using 

canisters. Contents of the canisters may be analyzed in a central 

laboratory. Sampling procedures and care of the canisters is 

described by McAllister et al. ( 1990). Collection of canister data 

to provide hourly boundary conditions for use in UAM would present 

very difficult logistical problems. Therefore, if it is decided to 

sample boundary conditions for NMOC using canisters, this will most 

likely have to be done on a discontinuous, sporadic basis. Threehour samples collected mid-morning (9-12), early afternoon (12-3), 

late afternoon (3-6), and at night (8-11) at 6-day intervals during 

the ozone season, may provide a sufficient basis for establishing 

representative boundary conditions and diurnal patterns needed for 

the UAM. Whether an NMOC sampling program for. UAM boundary 

.conditions should be undertaken is a decision most appropriately 

made by those responsible for deriving the modeling protocol for a 

CMSA/MSA's attainment demonstration (USEPA 1991a). If performed, 

sampling should be done at regional scale monitoring locations (40 

CFR, Part 58, Appendix D). Due to large concentration gradients 

39 
likely near vegetation, sampling should be out in the open, not 

within a canopy of vegetation. 

Sampling needs for estimating boundary NMOC for use with EKMA 

are.somewhat less demanding. Consistent with the discussion in 

Section 4.1.3, sampling needs to be performed over a single 3-hour 

period from 10-12 LCT as frequently as feasible. As with UAM, one 

of the issues addressed by those constructing the modeling protocol 

should be whether such sampling is necessary. 

Two approaches are possible for analyzing NMOC samples. The 

first approach is through use of gas chromatography and analysis of 

peaks on resulting chromatograms. Appropriate procedures for 

analyzing NMOC samples are described in Seila, et al. (1989). The 

principal advantage of this approach is that ·it enables retention 

of data which allow one to develop NMOC species profiles for 

boundary conditions. The major drawback is that it requires 

availability of a highly trained analyst and is, therefore, 

expensive. The second analytical approach is the cryogenic preconcentration approach (often abbreviated PDFID) (McElroy et al. 

1985). This approach is considerably less expensive than the 

first, but it does not allow one to examine speciated data. Choice 

of analytical approach is d.ictated by pri9ri ties in a particular 

study area and by available resources. 

4.3 Quality Assurance of Monitored Data 

The same scrutiny given all monitored data used to 

support UAM and EKMA analyses applies to data being considered to 

40 
derive boundary conditions. Appendix A to 40 CFR 1 Part 58 outlines 

a series of mandatory features for an acceptable quality assurance 

program. These are listed below. 

( 1) Documented selection ~ocedures for methods 1 

analyzers or samplers. 

(2) Provisions for training of personnel. 

(3) Installation of equipment. 

(4) Selection and control of calibration standards. 

(5) Adherence to calibration procedures. 

(6) Procedures for zerojspan checks and adjustments 

of automated analyzers. 

(7) Procedures for control checks and their frequency. 

( 8) Control limits for zero 1 span and other control 

checks and commitment to respective corrective 

actions when such limits are surpassed. 

(9) Calibration procedures and zero/span checks for 

any multiple range analyzers. 

(10) Provisions for preventive and remedial maintenance. 

(11) Quality control procedures for air pollution 

episode monitoring. 

(12) Recording and validating data. 

(13) Data quality.assessment (precision and accuracy). 

(14) Documentation of quality control information. 

41 
This page intentionally left blank 

42 
5.0 SUMMARY 

A series of modeling analyses has been identified for 

characterizing transport into nonattainment areas. These analyses 

enable one to: 

(a) determine the most likely principal cause of an 

observed exceedance (transport or local emissions); 

(b) identify upwind CMSA/MSA 1 s most likely to contribute 

to an exceedance if overwhelming transport is determined likely in 

(a); 

(c) quantify transport into CMSA/MSA 1 s identified in (b) 

so that such transport may be considered in estimating whether 

contemplated control strategies will be sufficient to eliminate an 

exceedance observed downwind. 

(d) estimate the cumulative effect of emissions in 

several CMSA/MSA 1 s on interstate transport. 

The discussion next focused on meteorological and air quality 

monitoring data needed to support the analyses noted above. Needed 

measurements, considerations 

instrumentation, operating 

provisions were identified. 

in monitoring network 

procedures and quality 

design, 

assurance 

To implement the procedure for characterizing transport into 

nonattainment areas, use of the following modeling and monitoring 

criteria are suggested. 

(1) Use of trajectory models is ordinarily recommended: 

(a) to establish whether an exceedance is primarily 

due to local emissions or overwhelming transport (Section 3.1); 

43 
(b) to identify contributing CMSA/MSA(s) if 

overwhelming transport is identified in (a) (Section 3.2); 

( 2) The US EPA TRAJECTORY model is appropriate for 

qualitatively estimating the prime cause (transport or local 

emissions) of most exceedances. 

To support use of the TRAJECTORY model, the 

following monitoring data are needed or desirable: 

' (a) Surface wind data at NWS sites within 160 km 

(Sections 3.1 and 4.1.1); 

(b) other surface wind data, as appropriate 

(Section 4.1.1); 

(c) surface ozone data (desirable) (Section 4.1.1); 

(d) upper air wind data.(desirable for TRAJECTORY, 

required for other trajectory models)(Section 4.1.1). 

(3) Trajectory models may also be used to identify rural 

transport areas. Several additional procedures are suggested for 

this purpose if it is determined that available data or models are 

inadequate for use in a particular application (Section 3.1). 

(4) For extreme, severe, serious, and some moderate 0 3 

nonattainment areas, the Urban Airshed Model (or other approved 

urban scale photochemical grid models) should be used to estimate 

controls needed in upwind CMSA/MSA' s to eliminate a downwind 

exceedance. Transport into a modeled CMSA/MSA ( i . e. , boundary 

conditions) may be considered using one of several approaches. 

(a) The preferred approach is use of the EPA ROM 

(Section 3.3.1). 

44 
(b) Use of other regional models is encouraged when 

it is not feasible to use ROM. Modeling procedures are subject to 

us EPA approval on a case-by-case basis (Section 3.3.1). 

(c) In the absence of regional modeling data, 

boundary conditions may be derived from monitored data (Section 

3.3.2). 

(d) In the absence of regional modeling results and 

sufficient monitoring data, expand the upwind dimension of the UAM 

modeling domain and use recommended default assumptions for 

boundary conditions (Section 3.3.3). 

(5) For some moderate 0 3 nonattainment areas, the 

Empirical Kinetic Modeling Approach (EKMA) may be used to estimate 

controls needed in a contributing CMSA/MSA to eliminate a downwind 

exceedance. Consider transport into a modeled CMSA/MSA (i.e. , 

boundary conditions) using one of several approaches. 

(a) The preferred approach is use of the EPA ROM 

(Section 3.3.1). 

(b) Use of other regional models is encouraged when 

it is not feasible to use ROM. Modeling procedures are subject to 

us EPA approval on a case-by-case basis (Section 3.3.1). 

(c) In the ~bsence of regional modeling data, 

boundary conditions may be derived from monitored data (Section 

3.3.2). 

(d) In the absence of regional modeling data and 

sufficient monitoring data, use recommended default values for 

boundary conditions (Section 3.3.3). 

45 
( 6) Regional ozone models, such as ROM, are recommended, 

when available, as the most appropriate procedures for estimating 

cumulative effects of emissions from many CMSA/MSA's on interstate 

transport of ozone. (Sections 3.2.2, 3.3.1) 

( 7) To estimate boundary conditions for UAM based on 

monitored data, the following measurements are minimal 

requirements: 

(a) surface ozone at regional site(s) (Sections 

4.1.2, 4.2.2); 

(b) surface NO, N02 , co, at regional sites 

(Sections 4.1.2, 4.2.3, 4.2.4); 

(c) NMOC may be sampled using canisters during 

several 3-hour periods at intervals ·of no more than six days 

(desirable) (Sections 4.1.2, 4.2.5); 

(d) NMOC may be analyzed from chromatographs 

(desirable) or using the PDFID approach. The former approach is 

necessary to obtain speciated data (Section 4.2.5). 

(8) To estimate boundary conditions for EKMA based on 

monitored data, the following measurements are minimal 

requirements: 

(a) use of late m~rning surface ozone data at 

regional sites (Sections 4.1.3, 4.2.2); 

(b) surface NO, N02 , and co observed late morning 

at regional sites (Sections 4.1.3, 4.2.3, 4.2.4). 

46 
(c) NMOC may be sampled during a 3-hour period in 

late morning using canisters at intervals of no more than six days 

(desirable) (Sections 4.1.3, 4.2.5); 

(d) NMOC may be analyzed from chromatographs. 

However, use of the PDFID approach and default recommendations for 

NMOC speciation is recommended (Sections 3.3.3, 4.2.5). 

(9) Monitored ozone data are ordinarily used to 

establish boundary conditions for regional scale models, but 

default natural background levels can also suffice. In ROM, 

boundary values for other chemical species are derived from 

specified ozone values. Data collected at regional scale 

monitoring sites for urban scale modeling analyses may be useful in 

evaluating performance of regional models (Section 4.1.4). 

(10) To be used as suggested herein, air quality data 

should be collected and analyzed consistently with mandatory 

quality assurance procedures (Section 4.3). 

47 
. ' 

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48 
. . REFERENCES CITED 

BAUGUES, K. A., (1987), Support Document for Selection of Default 

Upper Air Parameters for EKMA, Unpublished report. 

CAUSLEY, M. c., 1990, User's Guide for the Urban Airshed Model. 

Volume IV: User's Manual for the Emissions Preprocessor 

System, EPA-450/4-90-007D. 

COMPUTER SCIENCES CORPORATION, (1991), Gridded Model Information 

Support System (GMISS). UAM Subsystem, User's Guide, Volume 

II: (UAM Subsystem, EPA-450/4-91-009. 

DOUGLAS, S. G., R. C. KESSLER and E. L. CARR (1990), User's Guide 

for the Urban Airshed Model. Volume III: User's Manual for 

the Diagnostic Wind Model, EPA- 450/4-90-007C. 

HEFFTER, J. L., (1980), Air Resources Laboratories Atmospheric 

Transport and Dispersion Model ( ARL-ATAD) NOAA Technical 

Memorandum ERL ARL-81, Air Resources Laboratories, Silver 

Spring, MD. 

HEFFTER, J .. L., (1983), Branching Atmospheric Trajectory (BAT) 

Model, NOAA Technical Memorandum ERL ARL-121, Air Resources 

Laboratory, Rockville, MD. 

LAMB, R. G., (1983), A Regional Scale (1000km) Model of 

Photochemical Air Pollution, Part 1 - Theoretical Formulation, 

EPA -600/3-03-035. 

MEYER, E. L. and K. A. 

Transported ozone and 

EPA-450-4-89-010. 

BAUGUES, (1989), Consideration of 

Precursors and Their Use in EKMA, 

MORRIS, R. E. and T. C. MYERS, (1990), User's Guide for the Urban 

Airshed Model. Volume I: User's Manual for UAM (CB-IV), 

EPA-450/4-90-007A. 

MORRIS, R. E., T. C. MYERS, E. L. CARR, M. C. CAUSLEY and S. G. 

DOUGLAS, (1990b), User's Guide for the Urban Airshed Model. 

Volume II: User's Manual for the UAM (CB-IV) Modeling System, 

EPA-450/4-90-007B. 

MORRIS, R. E., T. C. MYERS, H. HOGO, L. R. CHINKIN, L.A. GARDNER 

and R. G. JOHNSON, (1990), Urban Airshed Model Study of Five 

cities, a Low-cost Application for the Urban Airshed Model to 

the New York Metropolitan Area and the City of St. Louis, EPA450/4-90-006E. 

49 
REFERENCES (CONTINUED) 

McELROY, F. F., V. L. THOMPSON and H. G. RICHTER, (1985), A 

Cryogenic Preconcentration - Direct FID CPDFIDl Method for 

Measurement of NMOC in Ambient Air, NTIS Publication Number 

PB-120631. 

McALLISTER, R. A., P. L. O'HARA, D. DAYTON, J. E. ROBBINS, 

R. F. JONGLEUX, R. G. MERRILL, JR., JO RICE and E. G. BOWLES, 

(1990), 1990 Nonmethane Organic Compound and Three-Hour Air 

Taxies Monitoring Program, Draft Final Report, N. F. Berg, 

USEPA Project Manager. 

SEILA, R. L., W. A. LONNEMAN and S. A. MEEKS, (1989), Determination 

of C2 to C, 2 Ambient Hydrocarbons in 39 U.S. Cities from 1984 

to 1986, EPA/600/3-89/058 (Appendix B). 

SYSTEMS APPLICATIONS, INC., (1988), A PC Based System for 

Generating EKMA Input Files, EPA-450/4-88-016. 

TANG, R. T., S.C. GERRY, J. S. NEWSOME, A. R. VAN METER, 

R. A. WAYLAND, J. M. GODOWITCH and K. L. SCHERE, (1990), 

User's Guide for the Urban Airshed Model. Volume V: 

Description and Operation of the ROM-UAM Interface Program 

System, EPA-450/4-90-007E. 

USEPA, (1978), ozone Isopleth Plotting Package COZIPP), 

EPA-600/8-78-014b. 

USEPA, (1987), on-site Meteorological Program Guidance for 

Regulatory Modeling Applications, EPA-450/4-87-013. 

USEPA, (1989a), User's Manual for OZIPM4 (Ozone Isopleth Plotting 

With Optional Mechanisms) Volume 1, EPA 450/4-89-009a. 

USEPA, (1989b), User's Manual for OZIPM4 (Ozone Isopleth Plotting 

With and Optional Mechanisms). Volume 2- Computer Code, EPA450/4-89-009b. 

USEPA, (1989c), Procedures for Applying City-Specific EKMA, 

EPA-450/4-89-012. 

USEPA, (1989d), Aerometric Information Retrieval System (AIRS), 

Volume I; Office of Air Quality Planning and standards, 

Research Triangle Park, NC 27711. 

50 

. ' 
TECHNICAL REPORT DATA ... {Please reDd Instructions on the reverse before completing) 

1. REPORT NO. ,2. 3. RECIPIE!'IT"S ACCESSION NO. 

EPA-450/4-91-015 ' 

4. TITLE ANO SUBTITLE 5. REPORT DATE 

Criteria for Assessing the Role of Transported Ozone/ May 1991 Date of Issue 

Precursors in Ozone Nonattainment Areas 6. PERFORMING ORGANIZATION CODE 

7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT NO. 

E. L. Meyer I i 

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT NO. 

U.S. Environmental Protection Agency (MD-14) A24A2F 

Office of Air Quality Planning and Standards 11. CONTRACT/GRANT NO. 

Technical Support Division 1 

Research Triangle Park, NC i 27711 None ! 

12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED 

Final 

14. SPONSORING AGENCY CODE 

Same I 

I 

15. SUPPLEMENTARY NOTES 

. 1 

16. ABSTRACT 

A series of modeling analyses appropriate for characterizing transport of ozone and 

its precursors into nonattainment areas is discussed. Air quality and meteorological measurements needed to characterize transport in identified modeling techniques are 

also identified. The report fulfills requirements in Section 184(d) of the Clean 

Air Act Amendments of 1990, in which the U.S. Environmental Protection Agency is 

directed to identify criteria for estimating transport of pollutants into ozone 

nonattainment areas. 

17. KEY WORDS AND DOCUMENT ANAL YSJS 

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSA Tl Field/Group 

ozone 

ozone models 

ozone/precursor monitoring pollutant transport 

• 

18. DISTRIBUTION STATEMENT 19. SECURITY CLASS (This Report) 21. NO. OF PAGES 

51 

Unlimited 20. SECURITY CLASS (TIJis page) 22. PRICE 

EPA form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE