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# OAR Box 1105 Prepped by Candice Davis



## Document

Number: 

# 110)11-1-26 5 Docket Number: 

# A-79-2 8

Printed 4/14/20083:11:59PM

Sheet 110of 131E_L

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# A-1?- ZS

STAF F ASSESSMEN TO F RECEN TCOMMENT S O N TH E SHORT-TER M SULFU R DIOXID E EXPOSUR E ANALYSI S

STRATEGIE S AN D AI R STANDARD S DIVISIO N OFFIC E O F AI R QUALIT YPLANNIN G AN D STANDARD S U.S . ENVIRONMENTA L PROTECTIO N AGENC Y. RESEARC H TRIANGL E PARK , NORT H CAROLIN A 2771 1

Jul y198 6P.3
_



### ' ' Disclaime r Thi s report ha s bee nreviewe db y th e Offic e o f Ai r Qualit y Plannin g and Standards , U.S . Environmenta l Protectio nAgency , an dapprove dfo r publica tion . Mentio nof trad e name s o r commercia l product s i s no t intende dt o constitut e endorsemen t o r recommendatio nfo r use .P. 4

Tabl e o f Content s 

# I Pa 

# 9 e

# |
I . Introductio n- 1 II . Finding s 3 1 ) NE M an dAi r Qualit y

Modelin g Error s 4

. 2 ) Definitio no f Exposure s o f Concer n5 3 ) Meteorolog y

andModelin g 9 4 ) Population s a t Ris kt o Shor t Term/Lo w Persistanc e Event s 1 2 5 ) Monitorin g Dat a Revie w 1 4 6 ) Siz e of.th e Sensitiv e Populatio n1 7 III . Implication s fo r th e Exposur e Analysi s 1 8 Reference sP. 5



### STAF F ASSESSMEN T O F RECEN T COMMENT S ON TH E SHORT-TER M

SULFURDIOXID E EXPOSUR E ANALYSI S 

# I; I . INTRODUCTIO N

" Analyse s o f populatio nexposure s t o short-term peak s o f SO 2 hav e bee n a key elemen t i nth e revie w o f th e sulfu r oxide s Nationa l Ambien t Ai r Qualit y Standards . Recen t healt hstudie s hav e show neffect s i nasthmatic s expose d t o short-ter m

peaks . Knowledg e o f thes e effect s ledt o question s regardin g th e degre e t o whic hth e curren t SO 2 NAAQ S protecte dagains t suc hpeaks . Pre liminar y analyse s o f short-ter m SO 2 ai r qualit y

wer e prepare dfo r th e Staf f Pape r i n198 2 (Strategie s andAi r Standard s Division , 1982) . Followin g closur e o

nth e Staf f Paper , a

nexpande deffor t examinin g short-ter m ambien t ai r qualit y

level s an

dth e potentia l fo r populatio nexposure s t o short-ter m

peak s wa s initiated . Thi s effor t wa s concerne dwit hdeterminin g populatio nexposur e t o pea klevel s o f SO 2 (generall y level s >

_0. 5 ppm) . T o d o this , th e NAAQ S Exposur e Mode l (NEM ) wa s modifie dt o calculat e th e probabilit y

o f populatio nexposure s aroundutilit y powe r plants . Th e decisio nt o focu s th e exposur e analysi s o npowe r plant s wa s guide db y th e earlie r ai r qualit y

studie s whic hshowe dtha t they wer e th e sourc e categor y mos t likel y

t o produc e hig hshort-term SO 2 level s i npopulate dareas . Analyse s wer e mad e fo r bot hcurren t condition s an dthos e tha t woul dobtai ni f th e curren t NAAQ S wer e jus t met . T o simplif y th e proble m

th e analysi s focuse d onwha t th e staf f define da s a nExposureo f Concer n(EOC) . Base donth e clinica l studie s availabl e i n198 2 th e EO C wa s define da s an exposur e t o >

_0. 5 pp m fo r 1 0 minute s whil e a t a ventilatio nrat e > 3 5 1/minute . Th e methodolog y

andresult s o f th e exposur e analysi s ar e describe di nsevera l paper s an

dreport s (U.S . EPA

198 4 andBille r e t al . 1984) .P. 6

2 Recently , however , concern s wer e raise d b ysevera l environmenta l an d publi cinteres t group s ove rth e relevanc e an d completenes s o f th e exposur e analysis
. Thi s pape rsummarize s recen t effort s t o asses s th e implication s o f th e scientifi can d technica l issue s whic h wer e raised
. I n some cases , thi s assessmen t provide s definitiv e answers , bu t i n man yinstance s thi s assessmen t ca n onl ydefin e th e bound s fo rthes e concerns . I n Novembe r198 5 representative s o f th e America n Lun g Associatio n (ALA ) met wit h th e Administrato ran d raise d a numbe ro fquestion s concerne d wit h th e exposur e analysi s (Godar , 1985)
. f) n behal fo fAL APr
. Dea n Sheppar d expresse d th e vie wtha t 0. 4 pp m fo r5 minute s woul d b e a mor e appropriat e definitio n o fa exposur e o f concer n tha n tha t use d i n th e exposur e analysis
. Dr
. Sheppar d suggeste d tha t th e sensitiv e populatio n wa s large rtha n jus t diagnose d asthmatic s an d shoul d includ e individual s wit h "twitch yairways. " AL Aals o expresse d judgment s regardin g th e seriousnes s o f response s o f asthmatic s t o suc h exposures . Th e Natura l Resource s Defens e Council , Inc
. (NRDC ) i n a Novembe r8 , 198 5 lette rt o th e Administrato rraise d stil l othe rissue s relate d t o th e exposur e analysi s (Hawkins , 1985)
. NRD C asserte d tha t sinc e th e exposur e analysi s relie d o n standar d dispersio n models , i t faile d t o accoun t fo r peak s du e t o certai n lo wper s istanc e meterologica l pvent s (e.g
. buildin g downwash , loppin g plume , an d fumigation)
. Theyals o pointe d ou t tha t peak s >^0. 5 ppm du e t o suc h event s coul d occu raroun d numerou s smal l source s no t addresse d i n ou ranalysis
. NRD C asserte d tha t EPA' s monito r sitin g criteri a an d guideline s o n spannin g o f instrument s have resulte d i n a systemati cunderestimat e o fhourl yexceedance s o f 0.5 ' ppm . I n a n August , 198 5 petitio n t o th e Administrato rth e Environmenta l Defens e Fun d (EDF ) presente d evidenc e o f populatio n exposure s t o ver yP. 7

3 hig h concentration s (>

_1 ppm ) i n th e vicinit yo f thre e smelter s non e o f whic h wer e i n complet e complianc e wit h th e curren t NAAQ S (Yuhnke , 1985)
. I n subsequen t submittal s ED F presente d evidenc e o findividual s requirin g hospitalizatio n as . a resul t o f exposure . Althoug h EDF' s immediat e concer n focuse d o n smelters , thei rinformatio n prompte d u s t o conside ragai n exposure s t o ver yhig h short-ter m level s o f SO 2 (>

_1 ppm ) aroun d powe r plants
. Apreliminar ystaf frevie wo fth e fact s foun d tha t ther e i s considerabl e scientifi cuncertaint ysurroundin g al l o fthes e issues , nonetheles s ther e i s some reaso n t o believ e tha t th e presen t exposur e analysi s migh t i n fac t underestimat e exposure . I n addition , i t i s als o possibl e tha t som e o f th e underlyin g analyti cassumption s ma yhave bee n incorrect
. Therefore , thi s staf f assessmen t wa s undertake n t o clarif yth e issue s a s much a

s possibl e an d t o determin e th e implication s fo rth e exposur e analysis
. I n th e proces s o f completin g thi s staf f assessment , a programmin g erro ri n th e SO 2 versio n o f NE

M wa s discovered
. Furthe rrevie wo fth e earlie r exposur e analyse s reveale d othe rproblem s wit h th e ai rqualit ymodeling
. Th e impac t o fthes e error s o n th e previousl yreporte d result s i s als o discusse d below
. I I
. FINDING S Thi s assessmen t focuse d o n si xarea s o finquiry : 1 ) NE M an d Ai r Qualit yModelin g Errors , 2 ) definitio n o f a n "exposur e o fconcern, " 3 ) meteorolog yan d modelin g o fver yshort-ter m peaks , 4 ) estimate s o f th e population s riskin g exposur e du e t o lo wpersistanc e events , 5 ) revie w o f monitorin g data , an d 6 ) siz e o fth e sensitiv e population
. Eac h o f theis e si xarea s i s addresse d below
.P. 8

4 1 ) NE M AN D AI R QUALIT

YMODELIN

G ERROR S A s a par t o fthi s assessment , th e result s o fth e earlie rexposur e analysi s wer e carefull yreviewed
. I n th e process , a n apparen t inconsistenc y wa s foun d i n th e reporte d outpu t fo ron e o f th e powe rplants
. I n trackin g down th e sourc e o f thi s inconsistency , a programmin g erro rwa s discovere d i n th e SO

2 varian t o f NEM
. Thi s erro rresulte d i n som e non-zer o probabilitie s o f exposur e bein g treate d i n th e mode l a s zer o probabilities , implyin g a systemati cdownward bia s i n th e previousl yreporte d results
. Preliminar y result s wit h th e correcte d versio n o fNE M indicat e tha t exposur e count s ma yincreas e bysom e 4 0 - 50%
. Havin g foun d th e on e error , a thoroug h examinatio n o f th e entir e SO 2 NE M cod e wa

s undertaken
. Th e examinatio n reveale d n o furthe rproblem s i n th e code
. Nevertheless , w e have decide d t o mak e th e mode l cod e availabl e fo rrevie wt o partie s outsid e EPA
. Finally , i t shoul d b e note d tha t th e codin g erro rdoe s no t impl ya f 1 a w i n th e underlyin g methodolog yo rlogic
. Ou rassessmen t indicate s tha t th e underlyin g methodolog yi s stil l sound
. Sinc e i t wa

s planne d t o examin e a numbe ro f ne wai rqualit yscenarios , a revie wo f th e previou s ai rqualit ymodelin g wa s made
. Aroun d severa l of th e powe rplant s studie d i n th e 198 4 report , i t wa s determine d tha t mor e representativ e estimate s o f backgroun d coul d b e made
. A t on e powe rplant , Gallagher , i t wa

s foun d tha t a

n erro rhad bee n made i n determinin g th e complian t fue l fo rth e curren t NAAQS
. Thi s erro rha s bee n correcte d i n analyse s no

wunderway
. Finally , change s wer e mad e t o th e meteorologica l dat a use d a t Portag e de s Siou xan d a t Gallagher
. I n summary , thi s staf f assessmen t resulte d i n th e discover yo fsevera l error s i n th e 198 4 exposur e analyses
. Th e correctio n o f thes e error s wil l lea d t o highe restimate d exposur e rates
. Despit e this , th e staf fP. 9

5
assessmen t indicate s th e error s wer e ver y technica l innatur e an dd o no t leadt o th e conclusio ntha t th e underlyin g methodolog y i s flawed . A

revise d 

# f 1 SO 2 exposur e analysi s repor t i s planne dfo r late r thi s year . 2 ) DEFINITIO N

OF EXPOSURE S O F CONCER N A ful l assessmen t o f ris kassociate dwit halternativ e SO 2 standard s woul dcombin e exposur e analysi s wit happropriat e dos e respons e information . Fo r th e bronchoconstrictiv e effect s o f SO2 , however , th e dose-respons e relationshi pi s a comple xfunctio no f SO 2 concentration , exercis e leve l
an dbreathin g patter no f subjects , sensitivit y o f subject , ai r temperature , humidity , an

dduratio no f exposure . T o simplif y th e analysis , w e develope d
a benchmar kterme da n"exposur e o f concern " (EOC) , whic hfixe dsome o f thes e variable s throug happropriat e assumption . Usin g studie s summarize d i nth e 198 2 criteri a documen t an dstaf f paper , th e EO C wa s define da s follows : SO 2 concentratio n- j

_0. 5 ppm Duratio n- 1 0 minute s Exercis e Leve l - Producin g > ^ 3 5 1/mi nventilatio n Sensitiv e - Exercisin g Asthmati c Populatio n Othe r Factor s - Assume dcomparabl e t o thos e studied , room temperature , humidit y Base do nth e limite dse t o f studie s availabl e i n1982 , i t appeare dtha t suc h exposure s coul dproduc e moderat e t o larg e change s i npulmonar y

functio n wit hperceptibl e symptom s i n2 0 t o 70 % o f mil dasthmatic s exposed . Th e dat a availabl e i

n198 2 di dno t provid e evidenc e fo r functio nchange s accompanie d b y symptom s a t lowe r concentrations . Th e EO C wa s not , however , intende da s a threshol do f response , bu t a leve l wher e a significan t fractio no f asthmatic s s o expose dmigh t experienc e potentiall y advers e effects .P.1 0

Leve l Th e AL A

concern s wit hth e origina l EO C wer e base do nresult s from studie s publishe dafte r closur e o nth e criteri a documen t an dstaf f p.aper . ALA suggeste dtha t a leve l o f 0. 4 pp m fo r 3 t o 5 minute s bette r reflect s th e curren t concensu s o f wher e advers e effect s woul dbegi nfo r exercisin g asthmatics . In,particular , dat a fro m Lin net . al . (1983 ) d o show moderat e grou pmea nfunctiona l change s wit hsymptom s tha t i na t leas t on e individua l
wer e clearl y

significan t wit h5 minut e exposures-t o 0. 4 pp m a t 4 8 t o 5 0 1/minute . Othe r studie s sugges t tha t a t highe r ventilatio nrate s bronchoconstrictio ncanoccu r at eve nlowe r levels , bu t th e physiologica l significanc e o f th e change s an dth e natur e o f an

y symptom s ar e unclear . Whil e thes e studie s addt o th e informatio nonth e effect s o f SO 2 belo w 0. 5 ppm , stil l othe r recen t studie s (Bethe l e t al. , 1983 ; Schacte r e t al. , 1984; - Roge r e t al. , 1985 ) expan donou r understandin g o f th e exten t o f effect s a t 0. 5 pp m an dabove . Thes e tendt o confirm ou r origina l
assessmen t tha t onl y a portio no f thes e asthmatic s respon dt o suc hexposure s wit hsignifican t functiona l change s andsymptoms , wit hexercis e rat e playin g a majo r role . At 3 5 t o 4 2 1/minut e an d5 t o 7 5 minut e exposure s t o 0. 5 pp

m SO- ; thes e studie s foundn o grou pmea nincreas e i nsymptom s an d i ntwo o f th e thre e studie s n o significan t chang e i nlun g function . Base dona reanalysi s o f Roge r e t al. , th e draf t criteri a documen t addendum (EPA 1986 ) foundapproximatel y 25 % o f th e asthmatic s exhibite dfunctiona l change s suggestiv e o f potentia l clinica l significanc e (100 % increas e i n airwa y resistance ) followin g suc hexposures . A t highe r exercis e rate s ( 6 0 1/minutes) , however , eve ngrou pmea nfunctiona l change s wer e large . A t th e lowe r concentratio n(0. 4 ppm ) wit hhig hexercis e ( 5 0 1/min) , onP.1 1

7 th e othe r hand , onl y 1 o f 2 3 asthmatic s selecte dfo r sensitivit y t o SU 2 experience da, clearl y significan t respons e (Linne t al . 1983) . Thu s 0. 5 pp m remain s a usefu l "benchmark " fo r assessin g exposure s whic hcoul dresul t i neffect s i na significan t fractio no f asthmatics . Clinicall y

significan t response s i nindividual s ar e importan t an d lea du s t o believ e tha t some exposur e analyse s o f 0. 4 pp m shoul db e conducted . However , whe nth e leve l o f th e EO C i s reduce dt o 0. 4 ppm , th e analysi s wil l nee dt o chang e th e minimum "high " exercis e rat e fro m 3 5 1/mi nt o 4 5 t o 5 0 1/mi nand/o r not e th e small , fractio no f potentia l responders . Unfortunately , th e structur e o f th e activit y file s use di n th e NE M analysi s mak e i t extremel y difficul t t o chang e th e definitio no f "high " exercise . Therefore , th e interpretatio no f result s fro m mode l . run s a t 0. 4 pp

m wil l nee dt o recogniz e th e smalle r percentag e o f asthmatic s potentiall y

responding . I nsummary , althoug hw e ar e currentl y plannin g t o addlimite danalyse s o f 0. 4 ppm.t o th e exposur e study , th e 0. 5 pp m leve l remain s a usefu l benchmar kandwil l b e employe dfo r mos t analyses . Duratio n Th e curren t exposur e analysi s already include s a sensitivit y analysi s fo r bminut e vs . 1 0 minut e exposures , whic hshow s a nincreas e i nth e percentag e expose dfo r th e shorte r averagin g period . However , th e America nLun g Associatio npointe dou t th e result s o f a mouthpiec e study whic hproduce d effect s afte r 3 minut e exposures . I t i.s alway s difficul t t o relat e result s from mouthpiec e studie s t o fre e breathin g subjects . Nevertheles s thi s and
a secon dsimila r study d o sugges t tha t ther e ma y b e reaso nfo r concer na t shorte r averagin g periods . A t thi s stag e ther e ar e many question s regardin g th e exten t o f respons e a t th e shorte r averagin g periods . .The review o f a recen t pape r b

y Hube r andPoole r (Hube r e t al . 1985a) . an drecen t analyse sP.1 2

8 o f monitorin g data , whic h sho wsubstantia l peak s lastin g u p t o 3 minute s increas e th e nee d t o resolv e thes e questions
. I n an effor t t o resolv e thes e uncertainties , th e proble m wa s discusse d wit h thre e differen t group s tha t conduc t SO 2 exposures . Th e EP Alaborator y i n Chape l Hil l has obtaine d approva l o fa stud yprotoco l addressin g thi s proble m an d has alread ybegu n work
. Thei rprotoco l include s exposur e period s fro m 3 0 second s u p t o 3 , minutes
. Dr
. Hackne yan d Dr
. Sheppar d als o expresse d interes t i n pursuin g th e question
. Thu s althoug h ther e i s currentl ysome uncertaint yove rth e exten t o frespons e fo rperiod s <_ 3 minutes , i t shoul d b e partiall yresolve d late rthi s year . I n th e meantime , w e wil l pla n t o shif t ou rfocu s fro m analysi s o f 1 0 minut e exposure s t o th e 5 minut e period
. Once th e clinica l studie s addressin g shorte rperiod s hav e bee n complete d an d reviewed , w e ma ywis h t o conside ranalyse s o fperiod s les s tha n 5 minutes
. Benchmarks fo rSever e Effect s Ou rexposur e wor k

ha s generall yfocuse d o n population s expose d t o concentration s j

_0. 5 pp m 10-minut e withou t regar d t o ho whig h th e concentration s migh t be
. Use o f th e 0. 5 ppm 10-minut e threshol d simplifie d th e analysi s ah d permitte d th e examinatio n o fa numbe ro fdifferen t powe rplant s a s wel l a

s th e effec t o falternativ e NAAQS
. Nonetheless , exposure s t o significantl y highe rconcentration s ar e o f concern . I n severa l case s populatio n exposure s aroun d utilit ypowe rplant s t o concentration s >

_ 1 ppm hav e bee n estimated
. Th e result s derive d fro m recen t run s o f th e revise d NE M mode l are

show n i n Tabl e 1
. A

s ca n b e seen , th e numbe ro f individual s expose d an d th e maximu m probabilit yo f exposur e a t thi s concentratio n leve l are significant
. A n analysi s o f measure d dat a aroun d 3 TV Apowe rplant s tend s t o suppor t th e modele d ai rqualit ydat a show n i n Tabl e 1
. Th e TV Adat a als o sho

wtha t th eP.1 3

. • 9 pea k concentration s ca n reac h 2. 0 ppm . Du e t o th e wa ythes e dat a wer e store d i t i s impossibl e t o sa yho wlon g thes e peaks lasted
. However , give n wha t i s known regardin g instrumen t respons e tim e i t i s likel yth e pea k persiste d fo r2- 4 minutes
. Alimite d analysi s o fth e 1. 0 ppm benchmar k has bee n prepare d (Thomas , 1986)
. 3 ) METEOROLOG

YAN D MODELIN

G Th e basi s fo rman

yo fth e concern s raise d b yNRD C i s a n analysi s complete d b

yAla n Hube ran d Fra n Poole ro f th e Atmospheri cScience s Researc h Laborator y(ASRL) , an d reference d i n th e Stac k Height s regulatio n (Hube re t al
. 1985a)
. Tha t analysi s indicate d tha t extremel yhig h concentration s coul d resul t fro m short-term/lo wpersistanc e event s no t explicitl ymodele d i n EPA' s dispersio n models
. Th e Huber/Poole rpape r. presente d it s conclusion s i n th e for m o f dilutio n ratio s an d expecte d annua l occurenc e rate s fo rtw o mode l facilities
. Thei rdat a indicate d peak s lastin g 3 0 second s t o 2 minute s coul d occu ro n th e orde ro f 100 0 time s peryea ra t anylocatio n withi n 0. 5 kilometer s o fa shor t stac k plan t (les s tha n GEP ) du e t o buildin g downwash
. Fo rloopin g plume s simila rpeak s coul d occu ro n th e orde r1 0 time s pe ryea ra t anygive n locatio n withi n 3 kilometer s o f an ysource . Hube ran d Poole rpoin t ou t i n thei rpape rtha t suc h calculation s ar e subjec t t o considerabl e uncertaint y an d are onl yaccurat e a

s orde ro fmagnitud e estimates
. T o plac e thes e number s i n perspectiv e w

e di d severa l things
. I n th e firs t place , Huber' s procedure s wer e use d t o calculat e th e dilutio n factor s du e t o buildin g downwas h aroun d a numbe ro factua l facilitie s (no t mode l facilitie s a s i n th e ASR

L work)
. Th e dilutio n factor s obtaine d wer e no t radicall ydifferen t fro m thos e i n Huber' s pape r(se e Tabl e 2)
. I n addition , w

e obtaine d stac k

gas concentration'estimate s fo rvariou sP. M


### 10
fuel s i n orde rt o calculat e groun d leve l concentrations
. Bot h stac k an d groun d leve l concentration s ar e show n i n Tabl e 2
. Give n th e assumption s o f thi s analysis * al l bu t th e ^ery cleanes t fuel s ar e capabl e o f producin g \ ver yshor t ter m level s >_ 0. 5 ppm a s a resul t o fbuildin g downwash
. Thus , concentration s HI. 5 ppm fo rver yshor t period s ma yoccu raroun d small , les s tha n GE

P stac k facilitie s burnin g relativel yclea n fuel
. Ou rearlie r screenin g studie s (whic h use d standar d dispersio n modelin g assumption s an d therefor e di d no t addres s buildin g downwash , loopin g plume , etc. ) ha d le d u s t o believ e tha t suc h facilitie s woul d no t b e a problem
. Althoug h i t i s clea rtha t th e exposur e analysi s doe s no t accoun t fo rexposure s du e t o buildin g downwash , ther e i s reaso n t o believ e tha t phenomen a suc h a

s loopin g plum e an d limite d mixin g are addressed
. I n th e exposur e analysi s th e probabilit yo fexceedin g 0. 5 pp m fo r1 0 minute s relie d o n th e hourl yaverage s predicte d b yCRSTER an d a n empiricall y derive d distributio n o f peak-to-mea n ratios
. Th e .distributio n o f ratio s Wa s obtaine d throug h an analysi s o f measure d dat a aroun d th e Kincai d powe rplant
. Th e Kincai d networ k wa s designe d t o pick-u p hig h concentration s du e t o phenomen a suc h a s loopin g plum e an d limite d mixing
. I n discussin g thi s wit h Dr.C.S
. Burton , a t System s Applications , Inc. , w e foun d tha t th e 2 0 highes t observation s wer e take n unde rstrongl yconvectiv e boundar y laye rconditions
. Thes e condition s woul d lea d t o what , observer s woul d identif ya

s loopin g plume
. Thu s i t i s reasonabl e t o conclud e that , excep t fo rbuildin g downwash , th e peak-to-mea n ratio s use d i s ou ranalysi s d o reflec t th e occurrenc e o f th e atmospheri cphenomen a cite d b yHube ran d Pooler
. [Give n th e stac k heigh t an d plum e temperatur e at Kincai d a

s wel l a s th e networ k

design , w

e ca n b^ e certai n tha t buildin g downwas h di d no t occu ran d woul d no t hav e bee n measure d i f i t had. ] I n fac t th e exceedanc eP.1 5


### 1 1

rate s o f 0. 5 ppm

estimate di

nou r modelin g studie s are consisten t wit h th e frequenc y

o f occurenc e number s calculate db y Hube r an

dPoole r fo r al l condition s excep t buildin g downwash . I nadditio nt o th e wor kdescribe dabove , a numbe r o f meeting s wer e hel dwit hASR L staf f t o determin e i f a mor e precis e characterizatio no f thes e phenomen a wer e possible . A s note dabove , ASR L staf f fel t thei r earlie r wor kgav e onl y a norde r o f magnitud e estimate . Afte r ou r meeting s an

da revie w

o f th e subject , ASR L determine dtha t mor e precis e estimate s wer e impossibl e withou t furthe r research. " Althoug hn o detaile d plan s wer e made , thei r initia l revie w an dplannin g indicate dtha t th e researc hcoul dtake.severa l year s to complet e an drequir e a budge t i n exces s o f $ 1 millio n(Hube r e t al . (1985b) . However , th e meeting s wit h \ ASR L a s wel l a s discussion s wit hothe r meteorologists , bot hinsid e an d outsid e EPA , reveale dstron g agreemen t tha t th e phenomen a describe di n th e pape r b

y Hube r an

dPoole r d o i

nfac t produc e hig hshort-ter m concentration s o f SO2 . However , ther e wa s som e disagreemen t a s t o th e frequenc y o f
occurrenc e an

ddilutio nfactors . I nsummary , th e initia l revie w o f th e meteorologica l an dmodelin g aspect s o f th e concern s raise db y NRD C indicate s tha t level s _ > 0. 5 ppm ma y wel l be occurrin g aroun dfacilitie s never addresse di

nou r earlie r exposur e analysis . Th e pea kconcentration s ar e likel y t o occu r withi n 0. 5 k m t o 3 k m an dreac hlevel s wel l ove r 0. 5 ppm . Althoug hi t i s certai nthes e phenomen a exist , ther e i s considerabl e uncertaint y ove r • thei r extent , frequency , an dmagnitude . Moreover , i t appear s tha t man y suc hpeak s ar e o f very shor t duratio n(3 0 second s t o 2 minutes) . A s note d above , th e effec t o f suc hshor t peak s onexercisin g asthmatic s i s th e subjec t o f a

nongoin g clinica l study . Additiona l researc hwoul db eP.1 6

1 2
neede dt o produc e bette r method s fo r calculatin g groun dleve l concentration s
an dfrequencie s resultin g fro m thes e lo

w persistanc e events . 4 ) POPULATIO N

A T RIS KT O SHOR T TERM/LOW PERSISTANC E PEAK S / I nth e shor t perio do f thi s initia l assessmen t i t wa s no t possibl e t o analyz e actua l populatio nexposure s associate dwit hphenomen a suc ha s buildin g downwas han dloopin g plume . However , a neffor t wa s mad e t o bette r defin e th e dimension s o f th e problem . Thi s initia l effor t focuse don tryin g t o determine - th e tota l populatio nlivin g aroun dsource s whic h migh t produc e thes e peaks . I t mus t b e stresse dtha t thi s effor t aime da t determinin g th e populatio na t ris ko f an exposur e andno t th e populatio n actuall y

exposed . Th e firs t difficult y inassessin g th e populatio na t ris ko f exposur e wa s simpl y determinin g whic hsource s migh t produc e short-ter m

peak s du e t o downwash . Sinc e dat a onbuildin g dimension s ar e no t availabl e o nNED S i t wa s impossibl e t o determin e whic hsource s hav e stack s les s tha nGEP . A s a surrogat e w e focuse do nsource s wit hstack s les s tha n6 0 m . I t shoul db e stresse dtha t fo r some source s stack s £ 6 0
m woul drepresen t GEP , whil e fo r other s > 6 0 m woul dno t b e GEP . Th e 6 0
m cutof f is , therefore , anextremel y roug hsurrogate . A program

wa s develope dt o coun t th e populatio ncentroid s lyin g withi nspecifie ddistance s o f facilitie s whic hha dat leas t on e stac k £ 6 0 m . I nour.firs t attempt s a t thi s w e foundtha t doubl e countin g o f centroid s wa s a significan t problem . Therefore , th e fina l versio no f th e program

calculate dth e population s withi na specifie ddistanc e o f 1,2,3,... ^ facilities . Th e result s o f thi s analysi s ar e show ni n Tabl e 3 . A s ca nb e seen , approximatel y 9 millio nindividual s liv e withi n D. 5 k m o f suc ha facilit y (wher e we ' presum e buildin g downwas hmigh t occur) . I f th e radi i ar e extende dt o 3 k m (are a withi nwhic hloopin g plume impact sP.1 7


### 1 4
I n summary , i t wa

s no t possibl e t o complet e a tru e exposur e analysi s i n thi s initia l study
. However , a n extremel yroug h estimat e o fth e populatio n a t rre k

wa s prepared
. Considerin g downwas h alone , th e estimat e wa s 9 million , increasin g t o 8 4 millio n i floopin g plum e wer e considered
. Th e ris k

o f exposur e i s very differen t fo rthes e two group s becaus e th e frequenc yo f occurrenc e o fth e tw o phenomen a ar e s o radicall ydifferent
. A t present , w

e hav e n o wa

yo f estimatin g th e percen t o fthos e livin g nea r suc h a facilit ywh o migh t actuall ysuffe ran exposure . 5 ) MONITORING DAT AREVIE W Th e revie wo f monitorin g dat a addresse d severa l differen t concerns . I n th e firs t place, - a n assessmen t wa s made o frawdat a submitte d byUAR G durin g th e Stac k

Heigh t rule-makin g whic h purportedl yshowe d evidenc e o f buildin g downwash
. Secondly , a n effor t wa s mad e t o locat e an yothe rdat a whic h migh t giv e evidenc e o f thes e lo wpersistanc e events
. Finally , a n effor t wa

s made t o determin e th e exten t t o whic h ou rSAROA D dat a wa

s biase d du e t o spannin g o fth e instruments
. UAR G Dat a Analysi s Th e revie wo f th e UAR G data*tend s t o confir m Ala n Huber' s calculations
. Tabl e 4 i s a portio n o f th e stri p char t take n fro m on e o fth e UAR G sites
. A s ca n b e seen , repeate d peaks 2
. 0. 5 ppm wer e recorde d wit h eac h pea k appearin g t o las t n o more tha n abou t a minute
. Dat a take n aroun d anothe r facilit ywher e th e instrumen t wa s locate d 1. 7 k

m away , sho

wpeaks lastin g 5-1 0 minutes
. Althoug h on e ca n no t b e sure , th e distan t locatio n o fth e instrumen t argue s tha t thes e latte rdat a sho wsom e conditio n othe rtha n buildin g downwash
. Th e revie wo f thes e stri p chart s b yOAQP S an d ASR

L staf fshowe d severa l othe rthings
. I n th e firs t place , th e instrument s wer e spanne dP.1 8



### 15

a t 1 ppm an d frequentl ypegge d out
. Thi s means tha t th e tru e pea k ca n no t b e readil yestimate d bu t wa s presumabl yi n exces s o f1 ppm . However , despit e thi s "pea k

lopping" , w

e conclud e tha t th e hourl yaverage s are unlikel yt o b e significantl yunderestimated
. Thi s conclusio n i s base d o n th e observatio n tha t th e are a unde rth e curv e a t th e pea k i s relativel y smal l an d thu s unlikel yt o significantl ychang e th e averag e fo rth e entir e hour
. Clearly , fo rpeaks o f longe rduratio n ( ^ 5-1 0 minutes) , "pea k lopping " woul d lea d t o a significan t underestimat e o f th e hourl yaverage . Peak s o f a t leas t 5-1 0 minute s wer e see n aroun d anothe rfacilit yi n th e UAR G dat a se t an d aroun d som e TV Afacilities
. I n suc h cases , i t i s possibl e tha t hourl yaverage s ma

yb e underestimate d du e t o spanning
. "Pea k lopping" , i f i t occurs , woul d als o bia s anyanalysi s o f pea k t o mea n ratios
. W

e determine d tha t thi s wa s no t a proble m wit h th e Kincai d dat a se t discusse d above
. However , any . futur e analysi s o fpea k t o mean ratio s woul d nee d t o addres s thi s problem
. Secondly , i t wa s determine d tha t eve n i f monitor s wer e se t ou t t o captur e peaks du e t o downwas h an d properl yspanned , instrumen t respons e tim e woul d lea d t o an underestimat e o fpea k concentrations
. Thi s woul d occu rbecaus e man yinstrument s no

wi n wid e us e requir e 4- 5 minute s t o reac h 95 % o fscale
. Thus , i fth e actua l pea k last s onl y3 0 second s t o 1 minute , mos t instrument s woul d no t respon d fas t enoug h t o registe rth e tru e peak
. Othe rDat a Set s Ou rsecon d area

o f inquir ywa

s t o determin e i f othe rshort-ter m dat a set s existe d whic h woul d hel p t o characteriz e th e frequenc yan d magnitud e o f peaks du e t o downwas h an d loopin g plume
. Th e Monitorin g an d Report s Branc h polle d th e Regiona l Office s an d foun d a t leas t 4 0 site s wher e instrument s ha

d bee n locate d nea rfacilitie s wit h les s tha n GE P stac k heigh t an d wher e very,shor t ter m dat a ( < 1 hour ) wa s als o available
. 

## IP.1 9

1 6

However , i nmany

instance s th e dat a wer e no t readil y availabl e and/o r wer e no t i na usabl e form . Therefore , unti l th e clinica l studie s discusse d abov e ar e complete dandw e have a bette r understandin g o f th e effec t o f ' < 3 minut e exposures , w e hav e hel dof f onrequestin g th e ra w data . I n additio nt o requestin g Regiona l Offic e data , w e als o contacte dORD . A s a result , w e foun dtha t 2 o r 3 o f th e RAP S site s (site s no t analyze di n Argonne' s earlie r study ) migh t show evidenc e o f loopin g plume . Ralp h Larsen , ASR L i s i nth e proces s o f analyzin g th e dat a fro m thos e andothe r RAP S sites . Preliminar y analyse s a t a singl e sit e indicat e very limite d exceedance s o f 0. 5 pp m 5 minut e average . A t ou r reques t TV A staf f provide d
a ful l yea r o f dat a recorde daroundeac ho f thre e powe r plants . Th e dat a includ e hourl y

averages an

dth e singl e highes t pea kwithi neac hhour . Th e TV A dat a wer e use di nanalyzin g peak s > ^ 1 pp m discusse dabov e an d als o show

significan t occurrence s o f pea kvalue s >

_0. 5 ppm . Effec t o f Spannin g onEarlie r Studie s Finally , w e examine dth e possibilit y tha t spannin g o f instrument s migh t hav e biase dearlie r studie s o f hourl y monitorin g dat a complete db y Nei l Fran kan

dTony Thral l (Franke t al . 1982) . Th e dat a se t use di nth e earlie r analysis wa s no longe r readil y available . Therefore , mor e recen t dat a fro

m the Nationa l Ai r Oat a Bankwer e examined . Th e objec t wa s t o deter min e i f a significan t numbe r o f instrument s wer e spanne dt o 0. 5 pp m whic h migh t the nresul t i na nunderestimat e o f hourl y average s >

^ 0. 5 ppm . A

tru e determinatio no f thi s woul drequir e a site-by-sit e review t o ascertai nhow eac hinstrumen t wa s spanne dan dth e frequenc y

wit hwhic hi t wen t t o ful l scale . Suc haneffor t wa s beyon dth e scop e o f thi s assessment , bu t a simplie r analysi s di dseem

possible . Base donou r earlie r studie s o f peak-to-mea n ratios , w e determine dtha t site s measurin g _>. 0.2 5 pp m 1-hour averag e coul dP.2 0

1 7 wel l hav e experience d
_ > 0. 5 pp

m a s a shorte r term

peak . Therefore , o f th e some 80 0 site s available , w e focuse donthos e tha t showe da t leas t 1 exceedanc e o f 0.2 5 pp

m l|-hou r I na 3 yea r period . I ntotal , ther e wer e 21 0 suc h\ sites , o f whic h10 0 showe da t leas t 1-hourl y averag e _>. 0. 5 pp

m an dtherefor e wer e almos t undoubtedl y spanne d> 0. 5 ppm . However , th e remainin g 11 0 site s recorde dhourl y peak s betwee n0.2 5 ppm , an d0. 5 ppm . I t is , therefore , possibl e tha t at thes e site s hourl y average s > 0. 5 pp

m may hav e occurre d bu t no t bee nrecorde ddu e t o spanning . Th e exten t t o whic hhourl y average s may hav e bee nunderestimate di s stil l unknown , sinc e a s note dabov e th e very shor t ter

m peak s woul dhav e t o persis t fo r th e hourl y averag e t o b e underestimated . Thes e 11 0 site s would , however , b e candidate s fo r a site by-sit e review

i f a mor e detaile dstud y wer e t o be undertaken . I nsummary , th e review o f th e monitorin g dat a di dno t conclusivel y answe r th e questio no f whethe r o r no t hourl y average s migh t be underestimate d du e t o spanning . Th e review did , however , convinc e u s tha t very short ter m peaks , (i.e. , >

_0. 5 pp m fo r 3 0 second s t o 2 minutes ) d o occu r an dhav e bee nmeasured . Sinc e th e peak s i nquestio nar e o f suc hshor t duration , hourl y average s may

no t be underestimated . Onth e othe r hand , ou r review clearl y demonstrate dtha t th e peak s themselve s may b e underestimate dbecaus e o f bot hspannin g an dinstrumen t respons e time . Moreover , wit hpeak s o f longe r duration , hourl y average s coul db e underestimate ddu e t o spannin g alone . A

searc hb

y th e Regiona l Office s an dother s turne du

pa numbe r o f othe r site s wher e thes e phenomen a may hav e bee nobserved . Apar t fro m th e limite dreview

o f th e RAP S data , w e hav e n o furthe r analyse s underwa y a t thi s time . 6 ) SIZ E O F TH E SENSITIV E POPULATIO N ALA i nthei r meetin g wit hthe.Administrato r stresse dtha t the y believe dtha t th e sensitiv e populatio nshoul dbe define dt o includ e mor eP.2 1

. . . 1 8 tha njus t diagnose dasthmatics . I nparticular , the y pointe dou t tha t ther e migh t b e u pt o 10 % o f th e nationa l populatio naffected , i f on e include din dividual s wit»

hsensitiv e airways . I t is , o f course , entirel y possibl e tha t
suc hindividual s migh t respon dt o anexposur e o f concern . Suc hindividual s ar e likel y

t o b e a subse t o f atopic s an di nth e staf f pape r wer e include d i nth e origina l sensitiv e population . Th e exposur e analysi s repor t tende d t o focu s o njus t asthmatic s a s th e sensitiv e population . However , th e siz e o f th e sensitiv e populatio ni s no t critica l t o th e exposur e analysis . Th e NE M analysi s use s th e entir e populatio no f anare a i nit s calculations . I n thi s work , th e measur e w e hav e tende dt o focu s oni s th e percen t o f th e popula tio nexpose dwhil e a t exercis e (define da s :

_3 5 1/min) . A

chang e i nth e siz e o f th e sensitiv e populatio nwil l no t chang e th e percen t exposed . Th e absolut e numbe r o f individual s i s als o presente dan dthi s woul dchang e i f th e siz e o f th e sensitiv e populatio nwer e altered . Thus , althoug hAL A ma y b e righ t o nthi s point , i t shoul dno t alte r an y o f th e genera l conclusion s w e hav e reached .
III . IMPLICATION S FO RTH E EXPOSUR E ANALYSI S - Clearl y th e codin g error s foundi nNE M indicat e tha t th e estimate s presente di nou r 198 4 report s (U.S . EPA , 1984 , Bi1le r e t al. , 1984 )
ar e no t accurate . Thes e error s hav e bee ncorrecte dandexposure s ar e bein g re-calculated . A

t thi s stag e i t seem s reasonabl e t o conclud e tha t th e revise dexposur e estimate s wil l b e reliabl e fo r situation s aroun dlarg e GE P sources . "Reliable " mus t b e understoo di nth e contex t o f th e caveat s outline di nth e exposur e report . I naddition , 1 t shoul db e emphasize dtha t th e revise destimate s appl y t o powe r plant s wit hGE P stac kheights . I t i s now clea r tha t numerou s smalle r source s o f SO 2 ar e als o likel y t o b e causin g very short-ter m

exceedance s o f an

dexposure s t o 0. 5 ppm . Thi s latte r poin t i s a t varianc e wit hou r earlie r conclusion s onthi s subject . Althoug hi tP.2 2

1 9 i s clea rtha t 5 minute s i s a mor e appropriat e perio d t o us e i n definin g a n EOC , i t i s no t clea rwhethe rperiod s _ < 3 minute s shoul d b e considered
. I f clinica l studies [ d o showeffect s a t thes e shorte rperiods , i t i s uncertai n t o wha t degre e th e additiona l exposure s aroun d smalle rsources,.wouI d ad d t o th e total s alread yestimated
. Th e firs t conclusio n regardin g th e reliabilit yo fth e revise d powe r plan t analyse s rest s i n par t o n th e result s o f dispersio n mode l evaluatio n studies
. Th e Kincai d an d Clift yCree k studie s indicat e tha t CRSTER predict s th e distributio n o f 1-hou rvalue s reasonabl ywell ; implyin g tha t th e 1-hou r value s i n th e exposur e analysi s ar e reliable
. Thi s despit e th e fac t tha t CRSTE R doe s no t explicitl ytrea t some lo wpersistanc e phenomena
. I n th e exposur e analysi s th e hourl yestimate s fro m CRSTER wer e use d a s on e inpu t t o th e calculatio n o f 10-minut e ai rquality
. Th e othe rinpu t wa s a distributio n 10-minut e pea k t o hourl ymea

n ratio s derive d fro m measure d dat a aroun d th e Kincai d powe rplant
. A s discusse d above , th e distributio n o f ratio s doe s reflec t th e occurrenc e o fphenomen a suc h a s loopin g plum e an d limite d mixing
. Therefore , i t seem s reasonabl e t o conclud e tha t th e exposur e analysi s aroun d GE P powe rplant s alread yaccount s fo rmos t o fth e phenomen a discusse d byHube ran d Pooler
. Th e genera l conclusio n regardin g th e soundnes s o fth e powe rplan t analysi s doe s no t exten d t o powe rplant s wit h significantl yles s tha n GE P stac k height
. A t suc h plant s downwas h probabl ylead s t o appreciabl y highe rexceedance s rate s o f 0. 5 ppm ; althoug h mos t o fth e peak s woul d likel yb e o f very

shor t duratio n (3 0 second s t o 2
. minutes)
. A s discusse d above , th e effec t o fsuc h short-ter m exposure s o n asthmatic s is , a t present , stil l uncertain
. Th e secon d conclusio n i s that , contrar yto-ou rearlie rassumption s regardin g smalle rsources , i t no wappear s tha t smal l source s wit h les s tha nf •••:' ' 2 0 GE P stac kheigh t ca ncaus e exceedance s o f an dexposure s t o >

^ 0. 5 ppm . Mos t
o f thes e exceedance s ar e du e t o buildin g downwash , occu r i nmos t case s withi n0. 5 k

m o f th e stack , andar e o f ver y shor t duratio n(les s tha n2 minutes) . ByvHuber' s calculation s an db y inspectio no f th e UAR Gdata , i t appear s tha t suc hexceedance s ma y occu r onth e orde r o f 100 0 pe r yea r a t a fixe dlocation . A t thi s poin t w e hav e no t bee nabl e t o accuratel y determin e how many source s ma y

b e subjec t t o downwas hno r th e population s wh o ma y b e a t ris ko f anexposure . Preliminary , bu t very rough , calculation s indicat e tha t th e number s ma y b e quit e significant . I naddition , smal l source s regardles s o f stac kheigh t ma y als o produc e very

short-ter m

( < 2 minute ) exceedance s o f 0. 5 pp m du e t o loopin g plume . Thes e exceedance s woul d likel y b e foun dwithi n3 k m o f th e stac kandoccu r o nth e orde r o f 1 0 time s pe r year . Thi s conclusio nrest s o nbot hHube r andPooler' s wor ka s wel l a s ou r ow nanalysi s o f peak-to-mea nratios . Smalle r source s wer e no t addresse d i nou r earlie r exposur e analysi s an dthei r inclusio nwoul donl y increas e th e estimate dexposures . Th e ver y

shor t averagin g perio do f th e peak s du e t o buildin g downwas h raise s th e questio no f th e appropriat e definitio nfo r a nexposur e o f concern . Base do ndat a no

w available , i t i s possibl e t o conclud e tha t 5 minute s i s a mor e appropriat e averagin g perio dtha n1 0 minutes . Whethe r o r no t shorte r period s shoul db e considere dwil l depen donth e result s o f clinica l studie s now

underway . W e als o conclud e tha t >

_0. 5 pp m i s stil l anappropriat e concentratio nbenchmark . Dat a ar e availabl e showin g functiona l change s wit hsymptom s a t 0. 4 ppm . However , whe nthi s lowe r concentratio ni s used , th e result s wil l hav e t o be interprete ddifferentl y reflectin g th e fac t a smalle r percen t o f asthmatic s migh t respondgive nsuc ha nexposure . Fo r thes e reasons , 0. 5 pp m stil l seem s a

nappropriat e benchmar kconcentratio n fo r mos t purposes , bu t 0. 4 pp m wil l be use dinsensitivit y analyses .2 1

. / I t i s importan t t o stres s tha t an analysi s o fsmalle rsource s woul d h e an almos t intractabl e analyti cproblem
. Sinc e buildin g downwas h woul d b e centra l t o th e problem , on e woul d nee d t o determin e th e numbe ran d typ e o f source s subjec t t o downwash
. However , downwas h depend s o h th e specifi c stack/building/nearb yobstacl e geometry ; wit h eac h sit e bein g different
. Thi s means i t woul d b e impossibl e t o drawa "representative " sampl e o f source s an d an yanalysi s woul d b e anecdotal
. Furthermore , wit h onl yon e exception , ou rpresen t dispersio n model s d o no t addres s eve n hourl y average s du e t o buildin g downwash
. ISCS Tdoe s hav e a downwas h algorithm ; however , i t produce s onl yhourl yestimate s and , simila rt o Huber , assume s th e plum e i s totall ydownwashe d fo rles s tha n GE P stacks
. Eve n i f hourl y estimate s wer e availabl e on e woul d stil l nee d t o estimat e th e 5-1 0 minut e peaks
. Roug h calculation s (simila rt o Huber's ) ar e possible , bu t onl yfo r th e simples t situations
. More typica l urba n situation s wit h numerou s sources , stree t canyons , an d terrai n woul d b e impossible
. Finally , th e geographi cscal e o fth e proble m doe s no t matc h th e scal e o f censu s data
. Ou rinitia l analyses , describe d above , indicat e tha t w e woul d haV

e seriou s difficultie s preparin g populatio n dat a fo rsuc h an analysis
. I f bette r method s t o calculat e peaks du e to.downwas h wer e developed , th e ai rqualit y dat a issu e woul d b e largel yresolved
. However , th e othe rissue s woul d remain
. I n summary , no

wtha t i t ha

s bee n revise d t o accoun t fo rcodin g error s th e analysi s o f exposure s aroun d powe rplant s shoul d b e reasonabl yaccurat e fo rcondition s aroun d GE P powe rplants
. However , th e analysi s doe s neglec t potentia l exposure s aroun d numerou s smalle rfacilitie s an d some powe r plant s wit h les s tha n GE P stac k heights
. A t presen t i t i s no t possibl e t o stat e conclusivel ywhethe rth e additiona l exposure s aroun d smalle rsource sP.2 5

2 2 woul d add significantl yt o th e total s alread yestimate d no rwhethe rsuc h exposure s woul d resul t i n a significan t respons e i n exercisin g asthmatics
. However , ou rpreliminar ywor k o n th e questio n indicate s tha t larg e number s o f individual s ma

yb e a t ris k

o fan exposure . Clinica l studie s are underway t o determin e i f th e respons e t o ver yshor t exposure s i s significan t o rnot
. Thus , althoug h tw o year s ag o w e conclude d tha t th e powe rplan t analysi s accounte d fo rmos t potentia l exposure s _ > 0. 5 ppm , th e recen t informatio n regardin g smalle rsources/lo wpersistanc e event s cal l thi s conclusio n int o doubt
. Unfortunately , wit h ou rpresen t dat a base s an d analyti ctools , a satisfactor yanalysi s o f smalle rsource s i s no t possible
.Tabl e 1
Modele dOccurrence s >

_ 1 pp m an dEstimate dExposures * (Curren t Emissions ) P.2 6

Plan t Expecte d Exceedance s 1 pp m 1-hour Expecte d

Exceedance s 1 pp m 5-minut e Asthmatic s

Asthmatic s Expose dExpose d>

_1 pp n > 1 pp m 5-minut e 5-minut e An y ^High " Exercis e (%) 2 Exercis e (% ) Gallaghe r 0.7 1 Portag e De s Siou x0.6 6 6. 4 5. 4 57 0 (2.8% ) 14 0 (0.85% ) 230 0 (12% ) 59 0 (3.5% )
1 ) Result s base do

nrevise d198 6 exposur e mode l 2 ) Define da s >

_3 5 1/mi nTabl e 2 Estimate d Pea

k Concentration s Du e t o Lo wPersistanc e Event s 

### V (ppm )

1 ) Win d Speed : 5 m/s ; Bld g Height : 2 5 m 2 ) Win d Speed : 1 0 m/s ; Bld g Height : 5 0 m 3 ) Win d Speed : 5 m/s ; Bld g Height : 45.7m , Stac k Height : 61 m 4 ) Win d Speed : 2 0 m/s ; Bld g Height : 57m , Stac k Height : 84m Coa l Sulfu r (lb/MMBtu ) 

### 0. 5 1. 0 1. 5 2. 0 2. 5 Stac k Concentratio n

(ppm ) 

### 25 8 51 6 77 4 103 2 129 0

Dilutio n Rati o Estimate d F r Distanc e fr o Perio d o f Ev equencyo f

Occurrenc e mStack (km ) en t (min. ) Rang e f o Hypothe t Plant s
38. 7 77. 4 115. 1 154. 8 193. 8

0.15 1

IO C < 0 O. E Downwa s ica l

0.6 5 1.2 9 1.9 3 2.6 0 3.2 0

.0025 2

) 0 5 >- 2 h

Rang e fo r Actua l Plant s
12.1 3 24.2 5 36.3 8 48.5 0 60.6 3

0.047 3 0.0 6

0.1 2 0.1 8 0.2 5 0.3 1

0.00024 4

100 0 <0. 5 0.5- 2 Looping

P 1 um e
0.8 . 

### 1. 5 2. 3 3. 1 3. 9

0.00 3

1 0 0.3- 3 0.5- 2P.2 8

Tabl e 3 Populatio n Residin

g Withi n Specifie d Distance s o f Shor t Stac k ( £ 6 0 m ) Source s o f SO? 1

(000s )

•s . Numbe r o f Sources 2 0. 5 k m1 k

m2 k

m3 k

m5 k

m 1 6,84 3 15,83 1 27,27 3 33,02 1 35,79 4 2 1,41 3 4,87 8 10,52 8 15,15 6 19,43 2 3 43 0 1,99 0 5,83 5 8,56 5 12,50 6 4 21 8 88 8 3,73 6 5,18 4 8,08 6 5 10 1 . 52 1 2,39 0 3,70 2 5,96 2 Any ' 9,18 6 25,76 1 57,78 3 84,11 4 120,95 6
1 ) Based o

n us e o f 198 0 U.S . Censu s data . Tota l considere d i s 226,546,000 . 2 ) Tota l numbe r o f source s i s 14,694 .

## :p—x|:tx3it|ix.i:r|x.iJxH:i' ! |-j-|-i-ri-i- | f t X j | ••••( •|- l i '•'-! •| i !• ! : | ; i i i | ! j i : j ! i | ' • •Reference s

Bethel , R . A. , Erie , D . J. , Epstein , J. , Sheppard , D. , Nadel , J.A. , an d Boushey , H.A.|(1983b) . Effec t o f Exercis e Rat e an dRout e o f Inhalation * o nSulfu r Dioxide-Induce dBronchoconstrictio ninAsthmati c Subjects . Am . Rev . Respir . Dis . 128 : 592-596 . Biller , W . F . e t al . (1984 ) Estimatio no f Short-Ter m Sulfu r Dioxid e Populatio nExposures . Pape r presente dat 77t hAnnua l Meetin g o f th e Ai r Pollutio nContro l Association , Sa nFrancisco . Frank , N.H . e t al . (1982 ) Relationshi pAmon g SO 2 Averagin g Time s an dAmbien t Standards . Monitorin g andDat a Analysi s Division , U.S . EPA , Researc hTriangl e Park , N.C. . Godar , Thomas , J . (1985 ) Lette r t o Le e M . Thomas , Administrator , U.S . Environmenta l Protectio nAgency , Novembe r 13 , 1985 . Hawkins , Davi dG . (1985 ) Lette r t o Le e M . Thoma s Re : Revie w o f Nationa l Ambien t Ai r Qualit y Standard s fo r Sulfu r Oxides , Novembe r 8 , 1985 . Horstman , D.H. , Roger , L . J. , Kehrl , H. , an dM . j . Hazuch a (unpublishe dmanuscript) . Airwa y Sensitivit y

t o Sulfu r Dioxide . HERL , U.S . EPA , RTP , NC . Huber , A.H . e t al . (1985 ) Comment s o nPea kGround-Leve l Concen tration s Du e t o Buildin g Downwas hRelativ e t o Pea kConcentration s Unde r Atmospheri c Dispersio nProcesses . (Unpublishe dpaper ) Atmospheri c Science s Researc hLaboratory , U.S . EPA , Researc hTriangl e Park,.N.C . Huber , A.H . e t al . (1985 ) Memorandum t o Josep hTikvart , Subject : Comment s o f Jun e 198 5 Analysi s o f Pea kConcentrations . Atmospheri c Science s Researc hLaboratory , U.S . EPA , Researc hTriangl e Park* , N.C . Linn , W . J. , Vent , T . G. , Shamoo , D . A. , Valencia , L . M. , Anzac , V . T. , Spier , C . E. , an dJ . D . Hackne y

(1983) . Respirator y Effect s o f Sulfu r Dioxid e inHeavil y Exercisin g Asthmatics . Am . Rev . Respir . Dis . 127:278-283 . Roger , L.J. , Kemrl , H.R. , Hazucha , M. , an dD.H . Horstman . Bronchoconstrictio ninAsthmatic s Expose dt o Sulfu r Dioxid e Durin g Repeate dExercise . 5 App . Phys . 59 : 784-791 . Schachter , E . N. , Witek , T . J. , Beck , G . J. , Hosein , H . R. , Colige , G. , Leaderer , B . P. , an dW . Cai n(1984 ) Airwa y Effect s o f Low Concentration s o f Sulfu r Dioxide : Dos e Respons e Characteristics . Arch . Environ . Healt h39 : 34-42 . Strategie s an

dAi r Standard s Divisio n(1982 ) Revie w o f th e Nationa l Ambien t Ai r Qualit y Standard s fo r Sulfu r Oxides : Assessmen t o f Scientifi c a.n

dTechnica l Informatio nOAOP S Staf f Paper . Strategie s an dAi r Standard s Division , U.S . Environmenta l Protectio nAgency , Researc hTriangl e Park , N.C , Novembe r 1982 .P.3 1



### U.S . Environmenta l Protectio nAgenc y (1984 ) Estimatio no f Short Term Sulfu r Dioxid e Populatio nExposures . Offic e o f Ai r Qualit y Plannin g an

dStandards , Researc hTriangl e Park , N.C . U.S . Environmenta l Protectio nAgenc y (1986 ) Secon dAddendu m to Ai r Qualit y Criteri a fo r Particulat e Matte r an dSulfu r Oxide s (1982) : Assessmen t o f New

Availabl e Healt hEffect s Information . Offic e o f Healt han

dEnvironmenta l Assessment , Researc hTriangl e Park , N. C Jul y 1986 .