~~_0 - - C) k ~~ P. (3, L5TL) 

May 5, 2011 

Office of Environmental Information Docket (OEI) 

Docket ID: EPA-HQ-ORD-2011-0050 

Mailcode: 2822T 

U .S . Environmental Protection Agency 

1200 Pennsylvania Avenue NW 

Washington, DC 20460 

Karen St John 

Senior Director 

Regulatory Affairs 

MAY 0 g 2011 

SUBJECT: BP America Comments on the "Integrated Science Assessment of Ozone and 

Related Photochemical Oxidants', EPA Document EPA/600/R-10/076A, Released March 

2011 

BP America (BP) appreciates the opportunity to provide written comments to the U.S . 

Environmental Protection Agency (EPA) on the Integrated Science Assessment (ISA) of 

Ozone and Related Photochemical Oxidants . 

BP's comments are focused on Policy Relevant Background; new modeling, studies, and 

information which should be considered in assessing Policy Relevant Background; and the 

capabilities of available models in relation to the form of a standard and Policy Relevant 

Background . We believe that EPA needs to reevaluate the PRB in the next draft of the ISA 

to address a number of issues to more properly characterize the PRB and the standard, 

notably that : 

1) The level of the standard being contemplated is approaching non-controllable 

background levels . 

2) Currently available modeling systems do not have the capability to 

simulate/capture the extreme events which is the form of the standard. 

3) EPA's use of the models and the form of the standard needs to be bounded by the 

capabilities of the models and robust model performance evaluation is needed . 

4) EPA's current construct of Policy Relevant Background is not valid at the current or 

contemplated level of the standard and needs to be extensively revisited with 

seasonality, geographical location, and statistical form taken into account. 

5) Emissions from Canada and Mexico, or any other country, should not be 

considered controllable for US air quality regulations 

MAY p 5 2011 
6) Anthropogenic emissions in the US necessary to maintain basic human and health 

services should be included in reevaluation of Policy Relevant Background instead 

of a zero emissions scenario . 

7) There is a disconnect between the extreme form of the standard and the 

"average" form of the PRB that must be reconciled. 

8) There is no mechanism available for States to deal with the complex and 

overlapping causes for high ozone from the standpoint of "exceptional events" 

exclusion . This needs extensive work. 

9) Hourly estimates of PRB should be paired in space and time with observations for 

health risk assessments, and the effect of models underpredicting the frequency 

and magnitude of high ozone events on risk assessments must be quantified ; 

As our detailed comments show, the current EPA-established PRB of 15-35 ppb is too low. 

It is our view that EPA must allow open scientific consideration of the issues presented by 

the PRB before any proposal for a new ozone NAAQS standard becomes final . BP provides 

a number of recommendations to inform a PRB reevaluation . 

These comments were prepared by Nicole Downey, Ph .D . (Earth System Sciences) ; Doug 

Blewitt, CCM (Air Quality Resource Management) ; Dana Wood, PE (BP) . 

BP's detailed comments follow this cover letter . Thank you for your consideration of BP 

America's comments. If you need any further information, you can contact me at 

sttohnk@bp .com (202-457-6594), or alternatively Gordon Smith at ordonreid .smith@bp .com (281-366-2683) . 

Sincerely, 

. 

Karen St. John 

Sr. Director, Regulatory Affairs 

BP America, Inc. 

202-457-6594 (W) 

202-351-1399 (cell) 
Comments on the `Integrated Science Assessment of Ozone and 

Related Photochemical Oxidants' 

EPA Document EPA/600/R-10/076A 

Released March 2011 

Docket # EPA-HQ-ORD 2011-0187 

May 5, 2011 

Prepared by: 

Nicole Downey, Ph.D . 

Doug Blewitt, CCM 

Dana Wood, PE 

For 

BP America 

bp 
1 .0 Introduction 

EPA is charged with protecting human health, and the primary ozone standard is 

meant to reduce the risk of negative health effects due to tropospheric ozone 

pollution. A key component of the standard-setting process, is calculating the 

benefit of tropospheric ozone reductions on human health relative to background 

ozone concentrations . EPA has defined Policy Relevant Background (PRB) 

concentrations as `those concentrations that would occur in the United States in 

the absence of anthropogenic emissions in continental North America (defined 

here as the United States, Canada, and Mexico). Policy Relevant Background 

concentrations include contributions from natural sources everywhere in the 

world and from anthropogenic sources outside these three countries' (EPA 2006, 

2011). In their 2006 criteria document, EPA defines PRB as 15-35 ppb (EPA 

2006) based on modeling work by Fiore et al. (2002, 2003). In the current ISA 

document, PRB is defined as 15-35 PPB based on the work of Fiore et al. (2002, 

2003) along with an updated model run by Wang et al. (2009). Here we argue 

that 

1) The model runs reported by EPA are not the most recent simulations 

of PRB. Newer model runs using the GEOS-Chem model have been 

completed for 2006-2008 and the results have been provided to 

EPA. These model runs were run at a higher spatial resolution, and 

model accuracy has been evaluated against CASTNET ozone 

observations across the country. Model simulations have shown an 

increase in predicted ozone across the entire US, but particularly in the 

Western US, compared to previous simulations of PRB. 

Additionally, PRB has been calculated using the CMAQ and CAMx 

nested grid models at 12 km resolution for 2006 with GEOS-Chem 

boundary conditions . The newer PRB modeling evaluations indicate 

that model grid size is critical to simulate the highest ozone events in 

non-urban areas. This is especially true in elevated terrain in the 

intermountain West where smaller grid sizes are necessary to capture 

the processes responsible for PRB impacts in elevated terrain . 

Preliminary results from the CAMx simulation show that the 4th 

highest PRB value exceeds 60 ppb for -1/3 of the Western United 

States in 2006. The simulation shows that nearly '/Z of the United 

States (by area) has a 4`h highest PRB value greater than 50 ppb. 

2) While modeling is an important tool in estimating PRB, recent 

analyses of air quality monitoring data have been conducted that are 

relevant to issues related to PRB. The following analyses should be 

cited and discussed in the ISA 

a. Jaffe (2010) - Relationship between Surface and Free 

Tropospheric Ozone in the Western US. Environmental 

Science and Technology. 
b. Lefohn et al. (submitted) - The importance of stratospherictropospheric transport in affecting surface ozone 

concentrations in the western and northern tier of the United 

States. Atmospheric Environment. 

c. Cooper et al. (2010) - Increasing springtime ozone mixing 

ratios in the free troposphere over western North America. 

Nature. 

These papers show that there is significant correlation between ozone 

in the free troposphere/lower stratosphere and the surface, and that 

ozone in the free troposphere is increasing due to increases in Asian 

emissions. This national effect is most pronounced in the Western 

United States (including California) where downwelling air masses 

from the free troposphere transport ozone and ozone precursors to the 

surface for a large portion of the year. The recent Ca1Nex field 

campaign also found significant impacts of stratospheric intrusion and 

long-range transport of emissions on both free tropospheric and 

surface ozone in California . Any literature recently published 

describing these results should be included in the ISA. 

3) The results of the model runs EPA includes in the ISA are not 

represented accurately when compared to the peer-reviewed 

publications they are derived from. EPA applies arguments related to 

the contribution of PRB ozone during events exceeding 85 ppb to all 

ozone levels . Fiore et al. (2002, 2003) and Wang et al. (2009) both 

argue that the contribution of PRB is highest between 50 and 70 ppb, 

particularly in the intermountain West. EPA assumes that PRB 

decreases with high ozone, but this is not valid for ozone less than 75 

ppb (from Fiore et al.) in the Eastern United States, and is not valid at 

any concentration range over the remainder of the US . In the most 

recent GEOS-Chem runs, decreasing ozone during high ozone events, 

is not observed at any ozone level (Zhang and Jacob, in prep). 

4) EPA has failed to expand its conceptual models regarding causes of 

`high' ozone as it has adjusted the proposed standard downward over 

time from 120 ppb (1-hour) to 85 ppb (8-hour) to 75 ppb (8-hour) to a 

stated level of 70 to 60 ppb (8-hour). The paradigms that explain 

`high' ozone formation at 85 ppb (8-hour) in low elevation urban areas 

are still valid; however if the standard is lowered, rural regions will 

exceed the standard. In both urban and rural areas, very different 

mechanisms explain `high' ozone at concentrations below 75 ppb. 

EPA must thoroughly re-evaluate its conceptual models regarding the 

causes of high ozone, when high ozone is defined below 75 ppb. 

5) Modeled PRB values used in health risk assessments must be 

paired in time and space. Restricting PRB to mean values tends to 

underestimate the influence of PRB on high-ozone days, and therefore 

overestimate the health risk benefit of lowered ambient ozone. Using 

mean values for PRB masks potential model performance issues, 

creating more uncertainty with the model results . PRB varies 
seasonally, by elevation and on an event basis (Fiore et al. 2002), and 

these variations must be taken into account in health risk assessments. 

Risk assessments should use PRB estimates at an hourly time scale. 

Monthly average hourly values are not sufficient. 

6) EPA has defined PRB as a model derived value, but it has applied 

these values in ways that are not scientifically valid or technologically 

feasible. For discussions regarding attainment, PRB must be 

defined in a statistical form that is consistent with the standard. 

Restricting PRB to its current definition precludes using observational 

data to define PRB, but the models used to define PRB cannot capture 

high-ozone events in a manner consistent with the definition of the 

standard. Both the statistical form of PRB and the statistical form 

of the standard must be defined in a manner that is consistent with 

model performance limitations . Realistic analysis of the effects of 

model bias at high ozone must be conducted and included in the 

decision-making process. Models should be rigorously tested against 

observational data, and model performance evaluation should be 

focused on reproducing high-ozone events . Additionally, it should be 

recognized that the influence of background ozone on ambient 

ozone concentrations has significant interannual variability that 

may not be captured by a models run for a limited time period. 

Observational data, such as Jaffe et al. 2010 should be evaluated to 

estimate the magnitude of this uncertainty . 

7) EPA justified setting a relatively low PRB in the 2006 Criteria 

Document based on the idea that PRB decreases during high ozone 

events. EPA continues to argue that PRB decreases during high ozone 

events in the current ISA. This appears to be true for ozone events 

above -85 ppb in Eastern US urban areas . This is not, however, 

generally valid for ozone below 75 ppb in the East, or ever valid over 

the remainder of the United States . Wang et al. (2009) clearly 

demonstrates that for ozone levels below 75 ppb, there is no significant 

decrease with high ozone events . More recent work by Zhang and 

Jacob (in prep) shows that PRB increases with higher ozone values 

in the Western United States during spring and summer, and in the 

Great Lakes region during spring. At no concentration range does 

the most recent GEOS-Chem modeling work show a significant 

decrease in PRB with and increase in ozone. 

8) Defining PRB as the concentration of ozone that would exist in the 

absence of anthropogenic emissions in the United States, Canada and 

Mexico implies that the United States can influence control of both 

Canadian and Mexican emissions. To date, no treaties exist between 

the United States and Canada or Mexico that make this possible. 

9) EPA's focus on ozone levels in the absence of anthropogenic 

emissions is outside the realm of possible control, and should not be 

considered possible targets for compliance. Instead, realistic 

emission targets should be evaluated for optimized health benefits 
and emission reductions . These targets should be achievable in the 

3-20 year time frame, and should include accounting for emissions 

required for maintaining basic public health services and a reasonable 

standard of living . A zero emission scenario would have negative 

public health impacts, which should be accounted for in the definition 

of PRB and the health benefits analysis. 

10) Policy relevant background varies on a dynamic event level basis due 

to multiple factors including : upper tropospheric/lower stratospheric 

downwelling (UT/LS), biomass burning, lightning and long range 

transport of pollution plumes. The current regulatory framework 

available for compliance does not allow for exclusion of high 

background events. No regulatory mechanism exists for excluding 

high ozone events caused by downwelling of upper tropospheric air, 

ozone production by lightning, or a combination of multiple 

background influences that cause exceedance of the standard. 

11) EPA fails to recognize that PRB varies significantly across the 

country, and that it is highly likely that significant portions of the 

United States cannot comply with a standard below 75 ppb due to 

background ozone. A system to identify and flag PRB events is 

essential for any standard below 75 ppb, and no system currently 

exists . For this reason, it is currently impossible to deal with PRB 

or exclusion of non-controllable high events in implementation. 

Observational data that could be used to identify background ozone is 

not available at most sites, and models cannot reliably simulate the 

highest ozone concentrations (and highest PRB impacts). The 

NAAQS ozone standard should be set within the valid range of 

models so that reliable and valid estimates of PRB events can be 

generated. 

2 .0 PRB simulations contained in ISA are outdated 

EPA asserts that `the ISA accurately reflects "the latest scientific 

knowledge useful in indicating the kind and extent of identifiable effects on 

public health which may be expected from the presence of [a] pollutant in 

ambient air"' (ISA p 1-1). In the case of policy relevant background (PRB) 

ozone, however, EPA has failed to consider the advancement of global chemical 

transport models, and has not included newer estimates of PRB derived from 

those advancements. Dr. Lin Zhang and Dr. Daniel Jacob (Harvard) have 

continued refining and testing the ozone simulation in GEOS-Chem, and have 

recently completed a 3-year PRB run spanning 2006 to 2008 using a nested-grid 

simulation at 0 .5 x 0.67 degrees over the contiguous United States . 
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0 20 40 60 80 1000 20 40 60 80 100 

MDAB ozone jppbv] MDAB ozone [ppbvj 

Figure 1. Distribution of MDA8 ozone concentrations for the standard (red) and 

improved (blue) GEOS-Chem ozone simulations for 10 elevated CASTNET sites (>1.5 km) 

in the Western US. Observational data are plotted in black. Notice that in Spring both 

models fail to capture the highest end of the probability distribution . Figure from Dr. Lin 

Zhang and Dr. Daniel Jacob (Harvard University) 

Figure 1 shows the probability of a given MDA8 ozone value occurring in 

observations (black), the standard GEOS-Chem model (red) and the latest GEOSChem model (blue) . A model bias of - 3 ppb in spring has been shifted to match 

observations . Notice, however, that the model still fails to capture the frequency 

and magnitude of enhanced ozone events in spring (> 70 ppb), and it remains 

unclear whether the mechanisms responsible for both mean and enhanced ozone 

events are accurately simulated in the model. 

This updated modeling data has been provided to EPA (Zhang and Jacob, 

personal communication) and represents the newer PRB estimation with the 

GEOS-Chem model than what is contained in the current ISA. Important changes 

relative to the simulations of Wang et al. (2009) and Fiore et al. (2002, 2003) are : 

a new parameterization of stratosphere to troposphere transport (STT), Asian 

emissions of NO,, that are more consistent with satellite based observations, 

higher spatial resolution, updated emissions inventories, and full-year model runs . 

The GEOS-Chem model continues to under-predict the frequency and magnitude 

of enhanced ozone events at CASTNET sites above 70 ppb (Figure 2), meaning 

that the model is biased low at enhanced ozone levels . For example, at several 

sites in the Western US, measurements regularly exceed 60 ppb, but the model 

generally fails to capture these events (Figure 3). This is likely due to 

underestimation of stratospheric tropospheric transport in the model. Further 

work needs to be done to improve parameterizations of the variability in STT and 

it's contribution to enhanced ozone events . 
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Figure 2. Model performance evaluation of the GEOS-Chem model. This figure shows the 

frequency of predicted vs . observed MDA8 greater than 60, 65 and 70 ppb for 12 CASTNET 

sites in the western United States. Notice that the model loses all predictive power at 70 ppb in 

spring . Figure from Dr. Lin Zhang and Dr. Daniel Jacob (Harvard University) . Figure 2. 

Model performance evaluation of the GEOS-Chem model. This figure shows the frequency of 

predicted vs . observed MDA8 greater than 60, 65 and 70 ppb for 12 CASTNET sites in the 

western United States . Notice that the model loses all predictive power at 70 ppb in Spring . 

Figure from Dr. Lin Zhang and Dr. Daniel Jacob (Harvard University) . 

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Figure 3 . Time series of observations, GEOS-Chem model predictions and predicted PRB 

at 4 CASTNET sites in the western United States . Dashed line is at 60 ppb, notice that the 

model fails to capture the highest events at all sites . Figure from Dr. Lin Zhang and Dr. 

Daniel Jacob (Harvard University) . 
p. 10 

The most recent GEOS-Chem simulations show that modeled PRB can be 

as high as 60 ppb on an event basis (Figure 4), and given that the model tends to 

under predict the frequency and magnitude of high ozone events, it is reasonable 

to assume that actual PRB exceeds 60 ppb on an event basis. Importantly, the 

more recent GEOS-Chem simulation was conducted for an entire year. The 

impact of PRB peaks in the spring, yet only summer was considered in Wang et 

al. (2009) . 

Contribution from the US background ozone concentration 

US background 

ozone concentration 

estimated by a 

sensitivity study 

without US 

anthropogenic 

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US background 

ozone for MDAa 

ozone exceeding 

60 ppbv 

SprinA Summer 

0 12 24 36 4e so ippbvl 

Comparing with the US anthropogenic contribution (previous slide), ozone 

exceedance in the western US are more likely due to increases in the US 

background ozone . ,7 

Figure 4. a) mean PRB ozone in Spring, b) mean PRB ozone in Summer, c) mean PRB 

ozone for days with MDA8 greater than 60 ppb in Spring, d) mean PRB ozone for days 

with MDA8 greater than 60 ppb in Spring. Slide from Dr. Lin Zhang and Dr. Daniel Jacob 

(Harvard) 

Additionally, Environ Corporation has completed a simulation of PRB 

using the CAMx and CMAQ models at 12 km resolution, and the modeling 

protocol has been shared with EPA. It is important to note that the methodology 

used in that analysis is consistent with typical CAMx or CMAQ applications . 

Boundary conditions were established using GEOS-Chem for the year 2006 and 

model predictions were then passed to 36 and 12 km CAMx and CMAQ runs. 

Environ's results are consistent with those of Zhang and Jacob (in prep), 

but they show a larger impact of biomass burning on PRB ozone (Figure 5). 

These differences are driven by the smaller spatial resolution of the model, and by 
p. 11 

the more-realistic temporal distribution of fire emissions. GEOS-Chem 

distributes a month's worth of fire emissions evenly over the month, whereas 

CAMx uses a daily biomass-burning inventory to drive emissions. The CAMx 

implementation is more realistic, and points not only to the important role fires 

play in PRB ozone, but also highlights the importance of using higher resolution 

(in time and space) emission inventories . Preliminary results indicate that CAMx 

exhibits better model skill than GEOS-Chem and one of the reasons could be due 

to the higher resolution. Additionally, the maximum PRB observed in various 

models increases with decreasing grid size (60 ppb at 0 .5 x 0 .67 degrees, 84 ppb 

at 36 km, and 116 ppb at 12 km). This illustrates that higher resolution models 

produce higher estimates of PRB, likely due to better representation of small-scale 

processes and higher resolution emission inventories . 

Simulated annual 4t" highest NA background MOA8 ozone 

GEOS-Chem 0.5 x 0.67 degree PRB (Harvard) CAMx 36 km PRB (Environ) 

90.0 

70.0 

80.0 

50.0 

40.0 

30.0 

20.0 

10 .0 

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Figure S . Comparison of 4th highest annual average PRB estimates from GEOS-Chem (Harvard) and 

CAMx (Environ) for the year 2006. Notice that the maximum of the scale for the GEOS-Chem figure 

is 60 ppb, whereas the maximum for the CAMx figures is 80 ppb. CAMx sees a much larger impact 

from biomass burning in the west, likely due to differences in the temporal and spatial resolution of 

emission inventories . 

3 .0 PRB values reported in ISA are not consistent with an ozone 

NAAQS below 85 ppb . 

EPA asserts that Fiore et al. (2002, 2003) and Wang et al. (2009) support 

a PRB of 15-35 ppb. These arguments were formed around a NAAQS standard 
of 85 ppb, and were largely based on the observations in urban areas in the eastern 

United States. At very high ozone values (i .e . > 85 ppb), PRB ozone decreased 

and local emissions dominated ozone formation. In the classical ozone model, 

meteorological stagnation events lead to an inversion cap. Emissions of ozone 

precursors are trapped in a shallow boundary layer. Chemical reactivity rapidly 

increases and background ozone is rapidly depleted. High ozone develops after 

several days, and the high ozone event persists until a weather system moves 

through and breaks up the stagnation event (Logan et al. 1985). This model 

works very well for describing the source of high ozone events > 85 ppb in the 

Eastern cities of the United States . It is not, however, an appropriate model to 

explain ozone in the 60-70 ppb range across the United States, especially in rural 

regions. Both Fiore et al. (2002, 2003) and Wang et al. (2009) found that PRB 

influence peaks between 50 and 70 ppb ozone. These values are within the range 

being considered for the 2011 ozone reconsideration . A multitude of factors 

contribute to high background ozone in the 50 to 70 ppb range (See Section 9 of 

this document), and most of these are not amenable to local or regional control 

strategies. 

Fiore et al. (2003) specifically argues that PRB `should be represented as 

a function of season, altitude and total ozone concentration', and they found that 

in their coarse 2x2 .5 degree model, PRB was as high as 50 ppb in the 

intermountain west in spring. Fiore et al. (2002) recognize that background 

ozone `represent[s] a major concern if the NAAQS were to be tightened' in the 

range of 50-70 ppb. Wang et al. (2009) show event level PRB as high as 50 ppb 

at Whiteface Base, NY and Tucson, AZ (Figure 4 of that paper). Wang et al. 

(2009) did not, however, calculate estimates for high-elevation sites or for spring, 

which leads to an underestimate of PRB in the intermountain west. Importantly, 

neither Fiore et al. (2002, 2003) or Wang et al. (2009) found that PRB 

decreases during high ozone events, when `high ozone' is defined in the 50-70 

ppb range. This was a critical assumption of the 2006 Criteria Document, and 

remains an integral assumption of the 2011 ISA. EPA used this observation to 

justify setting a relatively low PRB of 15-35 ppb, but this assumption is not valid 

in the 50-70 ppb range. Additionally, recent work by Zhang and Jacob (in 

prep) does not show a decrease in PRB at high ozone levels (Figure 6). 

Instead, they see an increase in PRB with increasing ozone concentrations for 

high-elevation sites, which is consistent with the observations of Fiore et al. 

(2003) (Figure 7). 
p. 13 

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Figure 6. GEOS-Chem model calculated ozone vs . observations at CASTNET sites in 4 regions of 

the United States . Whisker plots represent the contribution of North American (blue) and US 

only background ozone for 10 ppb bins. Notice that the contribution of background to high 

ozone does not decrease in any case . Also, notice that the contribution of background increases 

as ozone increases for the Intermountain West. Figure from Dr. Lin Zhang and Dr. Daniel Jacob 

(Harvard) . 11 
p. 14 

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Figure 7. Cumulative probability distributions for Elevated and Surface sites in the GEOS-Chem model 

developed by Fiore et aL (2003). Black symbols are observations, red triangles are GEOS-Chem predictions, 

and green diamond are GEOS-Chem calculated PRB. In this analysis, the contribution of PRB increased with 

increasing ozone in Spring at Elevated sites, but decreased at the highest levels (generally greater than 80 

ppb) . Notice that the model fails to reproduce the cumulative probability distribution of the measurements 

above - 60 ppb at elevated sites, and is biased low by -- 40 ppb at the highest end of the distribution for 

Summer at elevated sites . The model performs far better at surface sites. Figure taken from Fiore et al. 

2003. 

4 .0 For health effects studies, hourly estimates of PRB should be 

paired in space and time with observations 

EPA's current methodology for estimating the PRB contribution to the 

health effects of tropospheric ozone computes the difference between monthly 

mean diurnal cycles of PRB and observations . Because PRB varies significantly 

on an event basis, it is more appropriate to pair hourly PRB estimates with 

hourly observations in both space and time. EPA has been provided with 

hourly PRB estimates from the most recent 0 .5 x 0.67 degree GEOS-Chem PRB 

model runs. For each city where health effects studies are conducted, an hourly 

time series of PRB at that location should be used. Additionally, because it is 

well documented that GEOS-Chem under predicts the frequency of high 

12 
events, a detailed analysis of uncertainty must be conducted that quantifies 

the probable effect of this under prediction in health risk assessments. 

5 .0 For discussions regarding probable attainment issues, PRB 

should be defined in a statistical form consistent with the 

NAAQS standard 

Of primary importance in discussions regarding policy relevant 

background (PRB) is the statistical form chosen . NAAQS compliance is based on 

the 4th highest rolling 3-year average MDA8 ozone value. This is roughly 

equivalent to the 99`h percentile of the ozone distribution . PRB values, however, 

have generally been discussed as a mean value. An illustration of the difference 

between the mean and extreme values is shown in Figure 8. 

Gothic, Colorado 

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Figure 8. Mean, one sigma, two sigma and 4th highest MDA8 values for the Gothic, Colorado 

CASTNET site . Mean MDA8 for 2005-2010 was 50 ppb, mean + one sigma was 58 ppb, mean + two 

sigma was 67 ppb and the mean 4th highest MDA8 was 69 ppb. In this case, there is a 19 ppb 

difference between the mean and the 4th highest MDA8. 

Here, we have plotted the mean, la, 2a and 4`h highest MDA 8 for 

observations at the Gothic, Colorado CASTNET site for 2005 to 2010. The 

Gothic, Colorado CASTNET site is located at an elevation of 9,800 feet in central 

Colorado and there are minimal local sources in the region . The average 

concentration is 50 ppb, whereas the 4`h highest is approximately 69 ppb. The 

ISA presents modeling results for the mean and plus or minus one standard 

deviation from the mean. For this site, there is a 19 ppb difference between the 

mean and the fourth highest value. 

13 
Fiore et al. (2002, 2003), Wang et al. (2009) and the ISA discuss PRB in 

terms of means rather than extremes. It is, however, important to recognize that, 

with the current form of the standard, the 4`h highest PRB value should be used to 

gauge the effects of background relative to the standard . 

Due to the global nature of the atmosphere, characterization of PRB 

must include both global chemical transport models (CTMs). Because of the 

artificial definition of PRB, CTMs modeling must be used to quantify PRB, but 

only after the model accuracy has been compared against measurements at 

relatively remote sites to evaluate the accuracy their accuracy. CTM's, 

however, do not perform perfectly at all sites, and significant discrepancies 

still exist between modeled and observed ambient ozone. Observational data 

(both in-situ and satellite based) should be used to evaluate the large-scale 

processes such as STE, which appear to be significantly underestimated in 

models. 

CTM's, such as GEOS-Chem, tend to simulate mean conditions 

reasonably well, but fail at higher ozone values . As shown in Figure 7, the 

performance of GEOS-Chem degrades as ozone values increase, and the model 

estimates are up to 30 ppb low for high ozone values at elevated sites in summer. 

Rastigejev et al. (2010) show that GEOS-Chem, and other Eulerian CTM's, are 

not capable of simulating the ozone distribution above the 95`h percentile (Jacob 

2011), even if grid size is significantly decreased. Because models represent 

ensemble concentrations and because of the substantial uncertainties in model 

formulation and input data, models may never be able to accurately simulate 

extreme concentrations reliably . For example, numerical diffusion in Eulerian 

models (such as CMAQ, CAMx and GEOS-Chem) tends to disperse plumes far 

more rapidly than plumes actually disperse in the atmosphere, and a new 

generation of CTM's must be developed before these higher values can be 

properly accounted for in models. Fiore et al. (2002, 2003), Wang et ad. (2009) 

and Zhang and Jacob (in prep) all state PRB never exceeds 60 ppb in the model, 

but this may, in fact, be a model limitation. Model performance should be 

evaluated for both mean, and extreme observations, and a careful analysis 

detailing over what range each model is valid should be conducted. EPA should, 

therefore, formulate PRB and the NAAQS standard in a manner that is 

consistent with the model performance limitations of the models that are 

used. Furthermore, models should be thoroughly evaluated against 

observational data (including ozone precursor species) at relatively remote 

sites, and uncertainty in model estimates should be quantified and taken into 

account in health effects calculations . 

6.0 Canadian and Mexican emissions should not be considered 

controllable by the United States . 

Policy Relevant Background is defined by EPA as the background ozone 

concentration in the U.S . resulting from North American and transcontinental 

natural emissions as well as non-North American anthropogenic emissions . EPA 

14 
presumes that ozone precursor emissions from Canada and Mexico will or can be 

controllable to U.S . emission standards (either by Canadian and Mexican 

regulations or by U.S . sources paying for the installation of emission controls on 

foreign sources). Regulatory history has shown that this presumption is not true . 

EPA states that "For this document, PRB (policy relevant background) 

concentrations include contributions from natural sources everywhere in the 

world and from anthropogenic sources outside CNA (Continental North 

America). Background concentrations so defined, facilitate separation of 

pollution that can be controlled by U .S. regulations or through international 

agreements with neighboring countries from those that are judged to be generally 

uncontrollable by the U.S . Over time, consideration of potential broader ranging 

international agreements may lead to alternative determinations in which 03 

precursor source contributions should be considered by EPA as part of PRB ." To 

date there has been no international agreement entered in by the US that in any 

way requires or compels Mexico or Canada to control emissions or reduce 

emissions that are impacting the US. Under the La Paz Agreement of 1983 

(http_//www epa gov/Border2012/docs/LaPazAareement .odt) Annex V 

establishes the Joint Advisory Council but it only has the power to "develop 

recommendations for the Air Work Group" (pg. 21). The U.S.-Mexico 

Environmental Program, Border 2012, that resulted from the La Paz Agreement, 

has done nothing since its inception to improve air quality impacts from Mexico. 

The preamble for the 2008 Ozone NAAQS Reconsideration states "cross border 

03 contributions from within North America (Canada and Mexico) entering the 

U.S . are generally thought to be small. Section 179B of the Clean Air Act allows 

designated nonattainment areas to petition EPA to consider whether such a 

locality might have met a clean air standard "but for" cross border 

contributions." 

However, the study "Surface ozone background in the United States: Canadian 

and Mexican pollution influences" by Wang et al. (2009) shows that "simulations 

for summer 2001 indicate mean North American and US background 

concentrations of 26 _+ 8 ppb and 30 _+ 8 ppb, as obtained by eliminating 

anthropogenic emissions in North America vs. in the US only." "The Canadian 

and Mexican pollution enhancement averages 3 _+ 4 ppb in the US in summer but 

can be occasionally much higher in downwind regions of the northeast and 

southwest, peaking at 33 ppb in upstate New York (on a day with 75 ppb total 

ozone) and 18 ppb in southern California (on a day with 68 ppb total ozone)." 

Furthermore, the study found that "exceedances of the 75 ppb US air quality 

standard in eastern Michigan, western New York, New Jersey, and southern 

California are often associated with Canadian and Mexican pollution 

enhancements in excess of 10 ppb." In the 2009 analysis Wang concludes that 

"Unlike intercontinental transport that manifests itself mainly through 

enhancement of the hemispheric ozone background, pollution plumes from 

Canada and Mexico can be transported in the continental boundary layer to affect 

15 
p. 18 

US areas immediately downwind" (Wang, 2009). It should be noted that this 

study only considered impacts from emissions originating in Mexico and Canada 

for the July through August (summer) time period and does not address potential 

influences during the March through May time period (spring) when many sites 

exhibit high ozone levels . This study needs to be extended and the influence of 

Mexican and Canadian emissions understood prior to a re-evaluation of PRB and 

setting of lower standards. It is also important for EPA to quantify the ozone 

effects from emissions from Mexico and Canada to be considered consistent with 

the statistical form of the standard (3-year average of the 4`h highest daily 8-hour 

average) or on an event basis. Figure 9 (from Wang et al., 2009) shows the JuneAugust mean daily 8-hour maximum ozone concentrations for North America 

background (PRB), US background, and the Canadian and Mexican 

enhancements . 

a North American background (PRB) 

v 

Canadian and Mexican pollution enhancement 

Figure 9: June-Aug Mean Daily 8-hour Maximum Ozone Concentrations for a) PRB As defined in the 

ISA, b) if emissions from Canada and Mexico are included in PRB and c) the mean enhancement from 

Canadian and Mexican pollution (Figure taken from Wang, 2009) 

16 
As indicated in Figure 9, there is a mean enhancement of daily average 8-hr 

ozone concentrations during the summer of up to 15 ppb . However, compliance 

with the ozone standard is based on extreme concentrations (not mean 

concentrations) and to identify the actual ozone impact of emissions from Mexico 

and Canada of ozone compliance in the US a more event driven analysis is 

required . 

EPA needs to completely analyze the impact of Mexican and Canadian 

emissions on US ozone levels in a comprehensive analysis that delineates: 

emission inventories used, meteorology used, model accuracy and model 

sensitivity (grid size etc.) . Canadian and Mexican ozone contributions should 

be included in the quantification of PRB. In addition, EPA needs to identify 

what regulatory mechanism can be used to justify the assumption that 

Mexican and Canadian emissions will be controlled via. US emission 

standards. 

States are required to bring areas into NAAQS compliance via the SIP 

process, and states have no legal authority to forge international binding 

treaties, which would be necessary to control international emissions for US 

NAAQS. 

7 .0 International emissions inventories and long-range transport 

driving PRB are highly uncertain 

Policy relevant background (PRB) ozone includes anthropogenic emissions of 

ozone precursors outside of North America. In general, the quality of emissions 

inventories for countries outside of North America and Europe is low, and a 

significant amount of uncertainty exists regarding global NOX and VOC 

emissions. For example, Zhang et al. found that NOx emissions inventories for 

China, Japan and Korea were inconsistent with satellite-based observations, and 

that the emissions inventories required a factor of 2 increase to obtain agreement 

of modeling with observations. As the NAAQS ozone standard is decreased, 

understanding the contribution of international transport to ozone in the 50 to 70 

ppb range becomes extremely important. Cooper et al. (2010) found that ozone 

in the free troposphere has significantly increased over western North America, 

and they attribute this increase to rising Asian emissions. Currently, no-one has 

quantified the uncertainty associated with global emission inventories on PRB 

estimates, particularly at an event scale. Observations at Mt. Batchelor, Oregon, 

suggest that long range transport of Asian Pollution plumes impacts air coming 

into North America on a regular basis (Weiss-Penzias et al. 2006), and Obrist et 

al. (2008) saw evidence of long range transport of Asian pollution plumes at the 

Storm Peak Lab in Colorado . Oltmans et al. (2010) found that biomass burning in 

Eurasia can enhance ozone by as much as 20 ppb in western North America from 

the high Arctic to the mid-latitudes . As discussed in Section 5, global chemical 

transport models (CTMs) disperse plumes more rapidly than actually occurs in the 

atmosphere due to numerical diffusion, so they are not sufficient tools to identify 

the sources of high ozone events. One reliable method for parsing out the sources 

17 
of plumes has been developed by Weiss-Penzias et al. (2007) . This method 

utilizes multiple species of trace gases, which are simultaneously measured, and 

correlations between different species can be used to fingerprint sources of highozone events . For example, biomass burning plumes tend to be high in CO and 

aerosols, whereas stratospheric intrusions are low in CO and aerosols . These 

measurements are not available at AQMS across the United States, making it 

impossible to fingerprint sources of high ozone. A network of monitoring sites 

that simultaneously measure multiple trace gases (such as the NCORE 

network) must be deployed, and must include at least one site in the 

intermountain west (west of the continental divide) . At a minimum, the sites 

should measure 03, CO, aerosols and water vapor. These sites will help to 

characterize the sources of ozone as the standard approaches background levels, 

and will be extremely valuable for model performance evaluation . 

8 .0 Stratospheric intrusions and downwelling of upper 

tropospheric air masses remains a significant source of 

uncertainty in PRB simulations 

The current ISA asserts that "stratospheric intrusions that reach the surface are 

rare." Observational evidence, however suggests that stratospheric intrusions, 

folds, streamers and downwelling of ozone rich upper tropospheric air 

significantly contributes to surface ozone exceedance events . Models continue to 

underestimate the contribution of this effect to surface ozone concentrations . The 

recent Ca1Nex field campaign (2010) in California observed 8 stratospheric 

intrusions in 6 weeks (May 9 to June 26, 2010) in California (Oltmans et al. 

2010), and these events led to surface level MDA8 ozone > 75 ppb at at least one 

surface site (Langford et al. 2010). Additionally, the downward mixing of free 

tropospheric air enriched with Asian emissions of ozone precursors led to 

Paradise, CA exceeding an MDA8 of 75 ppb (Langford et al. 2010) . Hocking et 

al. (2007, 608032) concluded that stratospheric intrusions can be relatively 

frequent and that intrusions can significantly impact ambient ozone 

concentrations . Langford et al. (2009) also observes that stratospherictropospheric-transport (STT) leads to exceedances of the NAAQS, which has also 

been observed by Lefohn et al. (2010, submitted), and it is clear that global 

CTM's do not reproduce the frequency or magnitude of stratospheric intrusions 

and upper tropospheric downwelling events . A thorough analysis comparing 

observational evidence of STT, along with model predictions should be 

conducted. A technique to identify and flag UT/LS transport events based on 

widely available monitoring data must be developed and used for both 

implementation and model performance evaluation. 

9 .0 Regulatory framework available for compliance does not take 

background into account. 

Under the current regulatory framework, there are two significant issues regarding 

PRB. The first is that high background events in excess of the standard cannot be 

excluded. There is the Exceptional Events requirements of 40 CFR 50.14, but 

18 
p. 21 

that restricts flagging data to ozone exceedances that are from biomass burning, 

long range transport, or stratospheric intrusion . There is no mechanism to deal 

with downwelling of free tropospheric air or high ozone that is associated with 

lightning. Additionally, if a high background event is caused by multiple sources, 

including long range transport and natural ozone production, there is no legal 

mechanism to remove the data for attainment status . 40 CFR 50.14(c)(3)(iv) 

requires that "There would have been no exceedance or violations but for the 

event" which means there can only be one source . Further more, the definition of 

an "Exception Event" (40 CFR 50.1(j)) states that it "Is an event caused by human 

activity that is unlikely to recur at a particular location or a natural event," 

meaning that events that continue to occur from transport of anthropogenic 

emissions could not be excluded. 

The second significant issue is that ozone controls available at a state or county 

level are insufficient to demonstrate compliance with a standard that is very near 

background. Even with no local sources, many locations will exceed attainment 

due to transport of non-local, domestically produced ozone and ozone precursors. 

Multi-state and national coalitions of state governments and industry, such as the 

Four Corners Task Force, will be necessary to meet attainment goals. 

10 .0 The technological tools to deal with PRB in implementation 

do not exist. 

If the NAAQS ozone standard is tightened closer to background, it will become 

necessary to develop tools to filter out PRB events . The change in the frequency 

of grid cells with ozone exceedances predicted by the 0 .5 x 0.67 degree GEOSChem model increases nearly exponentially after 75 ppb (Figure 10). 

< 

i 

~.i 

U 

; 

tL"1 

Q 

2 

U 

L 

O_` 

i 

4000 

30'~'J 

, i)'. 7 

ts~ 75 7C- 65 '-"D 

()/ :~'IH ( :;Pt,~ 

=k< 

Figure 10. Number of grid boxes in the Harvard GEOS-Chem PRB full emissions simulation 

that exceed a 4`h highest MDA8 of 60 ppb. This figure was generated from model runs 

conducted at Harvard, which were provided to EPA. 

19