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February(18,(2011

Comments(on(the(statistical(form(of(Policy(Relevant(Background((PRB)(of(Ozone

EPA(Clean(Air(Scientific(Advisory(Committee

Ozone(Review(Panel(for(the(Reconsideration(of(2008(NAAQS

Nicole(Downey,(Ph.D.(L(Supported(by(BP(America(Production(Company

Slide(2:((Policy!relevant!background!(PRB)!of!ozone!is!defined!as!the!concentration!of!

ozone!that!would!be!present!in!the!United!States!with!zero!anthropogenic!emissions!in!

North!America!(including!Canada!and!Mexico).!!Currently,!EPA!defines!PRB!as!15J35!ppb!

based!on!modeling!results!from!Fiore&et&al.&[2002,!2003]!using!the!GEOSJChem!chemical!

transport!model.!!This!range!roughly!represents!an!average!across!the!entire!United!States!

over!a!year.!More!recent!GEOSJChem!modeling!work!suggests!that!PRB!is!far!higher!than!

that!estimated!by!Fiore&et&al.&[2002,!2003],!particularly!during!Spring!and!Summer!in!the!

Western!US.!!Here,!I!argue!that!the!current!EPA!definition!of!PRB!is!too!restrictive!to!

account!for!the!influence!of!background!ozone!on!ambient!ozone,!particularly!in!the!

Western!United!States.!!PRB!should!vary!by!season!and!by!elevation.!!Additionally,!an!

accurate!and!efficient!set!of!tools!to!evaluate!the!influence!of!background!on!highJozone!

events!must!be!developed!as!the!standard!approaches!background!over!large!areas!of!the!

United!States.!

Slide(3:((Recent!modeling!results!from!Dr.!Lin!Zhang!and!Dr.!Daniel!Jacob!(Harvard)!with!a!

higherJresolution!version!of!GEOSJChem,!show!that!in!spring!and!summer,!PRB!over!the!

entire!Intermountain!Western!United!States!exceeds!the!maximum!EPA!PRB!value!of!35!

ppb.!!As!shown!in!the!top!set!of!figures,!the!contribution!of!background!ozone!is!not!

constant!across!the!United!States,!nor!is!it!constant!between!seasons.!!PRB!is!generally!

higher!with!higher!elevation,!and!there!is!significant!seasonal!variability!depending!on!the!

location!of!interest.!!

The!second!row!of!figures!shows!the!contribution!of!PRB!to!ambient!ozone!on!days!where!

the!modeled!MDA8!exceeds!60!ppb.!!Consistent!with!other!studies,!along!the!heavily!

populated!eastern!seaboard!and!in!many!urban!areas,!background!is!low!during!high!ozone!

events.!!This!is!not!the!case,!however,!for!the!Western!United!States!and!Great!Lakes!area.!!

In!these!areas,!background!increases!with!higher!ozone,!highlighting!the!pivotal!role!PRB!

plays!in!ambient!ozone!above!60!ppb.!!

The!third!row!of!figures!shows!the!mean!ozone!enhancements!due!to!anthropogenic!

activity!in!the!United!States.!!Here!you!see!that!in!general,!in!the!rural!West!domestic!

anthropogenic!ozone!is!less!than!20!ppb.!!This!figure!highlights!the!distinct!differences!

between!urban!and!rural!ozone!environments.!!In!highly!urbanized!environments,!

domestic!anthropogenic!ozone!accounts!for!a!large!portion!of!the!observed!ozone!whereas!

in!rural!areas,!there!is!a!small!component!due!to!domestic!emissions.!!
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The!fourth!row!of!figures!shows!the!domestic!anthropogenic!ozone!signal!for!days!with!a!

modeled!MDA8!greater!than!60!ppb.!!Here!again,!the!difference!between!the!Western!and!

Eastern!United!States!ozone!regimes!is!clearly!visible.!!In!the!Intermountain!West,!the!

Denver!and!Salt!Lake!City!metro!areas!contribute!up!to!30!ppb!ozone,!but!in!rural!areas

domestic!anthropogenic!ozone!contributes!10J20!ppb!ozone!to!highJozone!events!(>!60!

ppb).!!

These!figures!clearly!show!that!the!proposed!levels!of!the!ozone!standard!(60!–!70!ppb)!are!

approaching!background!levels.!!Each!of!these!figures!represents!a!seasonal!average,!and!

on!an!event!basis,!PRB!can!be!higher!than!what!is!shown!here.!!Given!that!the!form!of!the!

standard!roughly!allows!for!exceeding!a!maximum!value!four!times!per!year,!accurate!

modeling!of!eventJlevel!highJozone!days!is!critical.!!

!!!!!!!!!!!!!!!!!!!!!!!!!

Slide(4:((Here!we!have!plotted!the!number!of!days!that!6!National!Park!sites!in!EPA!Region!

8!exceed!a!proposed!standard!of!60,!65,!70!and!75!ppb.!!As!the!standard!approaches!

background!(as!shown!in!Slide!3),!we!see!that!the!number!of!highJozone!days!increases!

dramatically.!!Also!notice!that!there!is!a!significant!amount!of!interannual!variability!in!

number!of!high!ozone!days.!!At!60!ppb,!some!sites!exceed!the!standard!25%!of!the!time!

(~80!days).!!All!of!these!highJozone!days!would!need!to!be!evaluated!to!determine!the!role!

of!background,!and!an!efficient!and!accurate!system!that!states!and!agencies!can!readily!

access!does!not!exist.!!Additionally,!EPA!has!not!adopted!a!formal!protocol!for!flagging!

highJozone!events!as!‘background’.!!

!!!!!!!!!!!!!!!!!!!!!!!

Slide(5:((In!addition!to!varying!by!season!and!elevation,!PRB!can!be!significantly!affected!by!

exceptional!events.!!These!include!stratospheric!intrusions,!streamers,!folds!etc...,!mixing!

down!of!free!tropospheric!air,!long!range!transport!of!Asian!anthropogenic!pollution!

plumes,!and!biomass!burning!emissions!(both!of!North!American!and!Asian!origin).!!The!

most!effective!means!for!identifying!exceptional!events!is!using!correlations!between!

multiple!coJlocated!trace!gas!measurements.!!Correlations!between!carbon!monoxide,!

methane,!humidity,!ozone,!and!mercury!can!be!used!to!fingerprint!the!sources!of!highJ

ozone!events.!!In!the!absence!of!experimental!data,!further!model!refinement!and!very!

detailed!emission!inventories!will!need!to!be!developed!to!identify!‘exceptional!events’.!

((((((((((((((((((((((((((
Comments(on(the(sta+s+cal(form(of(

Policy(Relevant(Background((PRB)(of(

Ozone(

Nicole(Downey,(Ph.D.(

Supported(by(BP(America(Produc+on(Company(

February(18,(2011(

EPA(Clean(Air(Scien+fic(Advisory(CommiNee(

Ozone(Review(Panel(for(the(Reconsidera+on(of(2008(NAAQS(
• PRB(is(defined(as(the(concentra+on(of(ozone(

that(would(be(present(in(the(United(States(

with(zero(anthropogenic(emissions(in(North(

America((including(Canada(and(Mexico)(

• Currently,(policy(relevant(background((PRB)(is(

defined(as(15T35(ppb(

• This(is(roughly(an(annual(average(of(PRB(

across(the(en+re(United(States(from(Fiore(et(

al.([2002,(2003](
Spring' Summer'

US'background'ozone'

for'MDA8'ozone'

exceeding'60'ppbv

US'background'ozone'

concentra=ons'es=mated'

by'a'sensi=vity'study'

without'US'

anthropogenic'emissions'

Ozone'enhancements'

from'US'anthropogenic'

emissions'for'MDA8'

ozone'exceeding'60'

ppbv

Mean'ozone'

enhancements'from'US'

anthropogenic'

emissions.'

Results(from(recent(GEOSTChem(modeling(at(Harvard((

(Zhang(and(Jacob,(11/8/2010)((

Current'EPA'PRB'Range'

Policy(Relevant(Background(

•(varies(by(season(

•(varies(by(eleva+on(

•(varies(by(ozone(concentra+on(

The'figures'at'right'show'that'

background'ozone'in'the'

en=re'intermountain'west'

exceeds'the'maximum'of'EPA’s'

defini=on'(35'ppb)'during'

Spring'and'Summer''

EPA’s(defini+on(of(PRB(should(

vary(seasonally(and(with(

eleva+on(

Addi+onally,(tools(must(be(

developed(that(can(quickly(and(

accurately(evaluate(the(

contribu+on(of(background(to(

highTozone(events(
Na+onal(Parks(Exceeding(Proposed(Standard(

• As(the(standard(

approaches(background,(

the(number(of(days(

exceeding(the(standard(at(

rural(western(Na+onal(Park(

sites(goes(up(drama+cally.(((

• In(some(years,(select(sites(

exceed(60(ppb(25%(of(the(

+me(

• The(current(form(of(the(

standard(allows(for(ozone(

to(exceed(the(standard(

roughly(4(+mes(per(year(
PRB(Varies(on(Event(Basis(

• PRB(can(be(strongly(influenced(by(extreme(events(

– Stratospheric(intrusions,(folds,(streamers(etc...(

• Recent(CalNex(field(campaign((May(9(to(June(26(2010)(observed(

8(stratospheric(intrusions(in(6(weeks(in(California((Langford(et(al.(

2010)(

– Mixing(down(of(free(tropospheric(air*(

– Asian(pollu+on(plumes((rising(Asian(emissions)*(

– Biomass(burning(plumes((Asian(and(North(American)(

• The'current'defini=on'of'PRB'is'too'restric=ve'to'

describe'important'variability'in'background'ozone'

contribu=on'to'high'ozone'events'