1 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!(includi ng!Canada!and!Mexico).!!Currently,!EPA!defines!PRB!as!15

J 35!ppb! based!on!modeling!results!from! Fiore&et&al.& [2002,!2003]!using!the!GEOS

J Chem!chemical! transport!model.!!This!range!roughly!represents!an!average!across!the!entire!United!States! over!a!year.!Mo re!recent!GEOS

J Chem!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!high

J ozone!
eve nts!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! higher

J resolution!version!of!GEOS

J Chem,!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!

cons tant!across!the!United!States,!nor!is!it!constant!between!seas ons.!!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,!back ground!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!t he!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.!!2 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!t he!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!10

J 20!ppb!ozone!to! high

J ozone!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!event

J leve l!high

J ozone!days!is!critical.!!!!!!!!!!!!!!!!!!!!!!!!!!! Slide(4:(( Here!we!have!plotted!the!number!of!days!that!6!National!Park!si tes!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!high

J ozone!days!increases! dramatically.!!Also!notice!that!there!is!a!significant!amount!of!inte rannual!variability!in! number!of!high!ozone!days.!!At!60!ppb,!some!sites!exceed!the!standard!25%!of!the!time! (~80!days).!!All!of!these!high

J ozone!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! high

J ozone!events!as!‘background’.!!!!!!!!!!!!!!!!!!!!!!!!! Slide(5:(( In!addition!to! varying!by!season!and!elevation,!PRB!can!be!significantly!affected!by!

e xceptional!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!co

J located!trace!gas!measurements.!!Correlations!between!carbon!monoxide,! methane,!humidity,!ozone,!and!mercury!can!be!used!to!fingerprint!the!sources!of!hi gh

J 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(GEOST Chem (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'