## Gerald Gentleman Station Sutherland, Nebraska Evaluation of Compliance with the 1-hour NAAQS for S0 2 

## September 17, 2015

Conducted by: 

### Steven Klajka, P.E. , BCEE

Wingra Engineering, S . C. Madiso n , Wiscon.'iillEm luati o n of Co mpliance with the / -hour i\'AA.QSfo r S0 1 Sep t e mb e r 1 7, 20 15 Page 2 l. 

### Introduction Win g ra E n g ine e rin g, S.C. was hir e d by S i erra Club t o conduct a n a ir m ode lin g im pact a nal ys i s t o he lp the U.S. Environment al Pro tec ti o n Age ncy (USE P A), s ta te a nd loca l air agenc ies i dentify facilities that a r e likel y cau s in g exceedences of the 1-ho ur s ulfur dioxid e (S02 ) N a ti o nal Ambient A ir Quality S tand ar d (NAAQS). T hi s document describes the re s ult s and pr oce dur es for a n eva lu a ti on co n ducted for the G e r a ld Ge ntl e man Stati o n l ocated in S uth e rl and , Ne bra s ka . The di s pers i o n modeling ana lys i s pr ed ic ted amb i ent a ir co ncentration s for co mp a ri so n w ith the 1- hour S0 2 NAAQS. The modeling was p e r fo rm ed u s ing the mos t rec e nt version of AERMO D , AERMET, and AERM INUTE, with data provided to Sierra Club by r eg ul a t o r

y a ir agencie s o r o btained through oth e r publicly-avai la bl e so urce s as docume nte d bel ow. The a na lys i s was con du c ted in adherence to a ll available US EPA gu id ance for ev a l uatin g sou rc e impac t s on atta inm ent o f the 1-h o ur S 0 2 NAAQS via aeri al di s pe r s io n mo delin g, in cl udin g the AERM OD Impl eme nt atio n Guide; USE P A 's Applicability of Appendix W M o d e li ng Gu id a nc e for th e !-h ou r S02 Nationa l A mbient Ai r Quality S tandard , A ug u st 23. 20 I0; modeling g uid a n ce promulgated by USE P A in A ppendix W to 40 C FR P a rt 5 1 ; USE P A's Mar c h 20 11 M o d e lin g G uidanc e for S0 2 NAAQS D es ig nati o n s; 1 and USE PA 's De ce mb e r 2013 S0 2 NAAQS Designations Te c hnic a l Assis tan ce Do c um ent. 2 

### 2. Co mpliance wit h the 1-b o ur 

### S02 NAAQS 2 .1 1
- hour 

### S0 2 NAAQS The 1-h o ur S0 2 NAAQS take s the fonn of a thr ee - yea r ave ra g e o f the 99th pe rcentil e of the annual di s tribution of da ily maximum !-hou r co n ce ntra ti o n s, wh i ch can n o t exceed 75 parts per billion (ppb). 3 Co mpliance with this s tandard was verifi e d us in g USE P A's AE RMOD air dispersion m ode l, whic h produces ai r co nc e ntrati o n s in units o f ll g/m3 • T he !-h our S02 NAAQS of 75 ppb eq ua ls 196.2 ll g/ m3 , and thi s i s th e value u se d fo r dete rmining whethe r m ode led impact s exceed the NAA Q S . 4 Th e 99th percent il e of the annua l di stributi on of d ai l y maximum !-h ou r co n ce n tra tio n s cor res po nd s t o the fourth-h ig h est va lu e at eac h rece pto r f or a give n year.

1 http: // www.epa. g ov / scramOO 1 / so2

_ modeling_guidance .htm 2 http: // www.epa . gov /oaqp sO O I / s ulfurdiox id e/pdf s/S 0 2Mo delingT AD.pdf 3 U S E PA, Ap pli cabi lity of A ppendix W Mode l ing Guid an ce for the 1
- h ou r S0 2 National Ambient A ir Qual ity Standard, A ugu s t 23, 20 10.

**~T he**  ppb to 1Jg/m3 conve rs io n is f o und in the source code to AERMO D v. 14134, s ubroutine Modules. The co n version calculatio n i s 75 /0.3823 

### = **96.2~Jg/m**

**•**£1'(1 /ualion of Co mpliance ll'ilh 1he /-h o ur : \:.JAQSfo r S01 Seple mber 1 7. 2015 Page 3 

### 2.2 Mode lin g Re s ults 

### Mo deling r es u lts for Ge r a ld Ge ntl e man S t ation a r e s umm ari ze d in Tab l e I . I t was determined that base d o n either current allowab le emissions o r mea s ured actu al em i ss ion s. the Gerald Gentleman S tation i s est imated t o c reate d ow nwind S0 2 

### conce ntrat i o ns which exceed the !-hour NAA QS. More spec ifi ca ll y, the modeling r es ults pr ese nt ed in Tab l e I , s ho w the pl a nt 's a llowable and actua l e mi ss io n s ca use exceedences of the NAAQS. "A llo wab le ' ' i s the peak e mi ss io n r ate from eac h unit as approved by the cu rre nt ai r qua li ty operation permit f o r th e facility. " Actua l " are th e meas ured emiss i ons f o r each hou r between Ja nuary I. 2 012 and Decembe r 3 1 , 20 14 a s taken fro m US EP A Air 

### Atfarkets Pr og ram Data. 5 

### A ir qua li ty impacts in Nebraska ar e base d o n a backg ro und conce ntrat i on of 88.9 Jl g/m 3

. 

### Thi s i s the 20 ll-13 de s ign va lu e f o r Dougla s Co un ty. Ne braska- the lowe s t m easu red background co ncentrati o n in the s tat e . Th i s i s the m ost r ece ntl y ava il ab l e de s i g n va lu e. See Section 5 fo r furth e r discuss i o n o f th e backg r o und concent r at i o ns used f o r this analy s i s . 

### Table I
- S0 1 

### Modeli ng R e sult sfor G e rald Gentlem an Station Modelin g Ana l ysis Averaging 99th Per ce ntile 1-h o ur Dai l y Maximum (J!g/m 3 ) Co mplie s with Emission Rates Pe ri od Imp ac t Background To tal NAAQS AAQS?

A ll owab l e 1-h o ur 809.7 88 .9 898.6 196.2 No Act ual 1-hour 128 .6 88 . 9 2 1 7.5 196.2 0 

### The e mi ss i o ns use d f o r the mo de lin g a na l ys i s are s ummarized in Table 2. 

### Ta bl e 2- Mo d eled S0 2 

### Emissio n s from G e rald G entleman Statio n 6 

### Stack U nit A I t owa ble Emi ss ion s ID lD 

### 2 -h o ur Ave rage (lbs / hr) SO l Unit I 18.4 22.5 S02 U nit 2 20,770.5 S tack Total A ll Un it s 39 , 19 3 .0 Based on th e modelin g re s ults , Tab l e 3 pr ov ides th e n ecessary e mi ss i o n redu ct i o ns from cur r ent a ll o wabl e r ates nece ssary to achi eve complia n ce with the 1- hou r NAAQS.

5 http: //ampd.epa.gov/ampdl 6 A llowable emiss io n s for Unit 1-1 are based on the 2-ho ur ave ra ge 2.5 lb s/ rnmbtu limitati o n in Pem1it t o Operate iss ued by the Nebras ka Departm e nt o f Envir o nm e ntal Qu a lit y. Aug us t 26 . 2002.Emluation of Complianc e with the !-h o ur N AAQSf or S 0 2 Se

pt

e mber 1 7
. 2015 · Pa

ge .J


### This assumes a one-hour averaging period fo r the emission rate , and that the e mis sio n rate is binding at a ll t im es. H oweve r, it is extremely lik ely th at eve n th i s limit is too hi g h t o protect the NAAQS, given the conservative aspects o f th is modeling protocol. For example, startup o r shutdown periods we re not evaluated. Du rin g these periods, decreased gas velocities and tempe r atures may lead to grea ter ambie nt impact s at g rou nd level. Further, the h ypothetica l em i ss i on limitati on in Table 3 wou ld allow Gerald Gentleman Stat ion to consume the enti re NAAQS, leaving little to no r oo m for any othe r source of S02 

### in the area. No consideration has been given to other so urce s and no margin of safety h as bee n in cluded in th e h ypothet ica l e mi ss i o n l im itati on. Table 3 - R eq uir ed Emission R ed u ction sfrom Gerald Gellfleman Sta tionf o r Compliance witlt tlt e 1-ltour NAA QS for S02 Acce ptabl e Impact Required Required Required (NAAQS- Background) T o tal Facility Total Fac i lity 99th Percentile Reduction Ba se d o n Total Facilit y 1-hour Average Emiss i on Rate 1-hour Da i

**~** 

### Max Al l owable Emissio ns (lbs/ hr) Emission Rate (J.lg/m ) (%) 

### (lbs/ mmbtu)

1 07.3 87% 5,193.8 0.33 

### Predi c ted exceedences of the !-hour NAAQ S for S02 based on allowable emissions extend throughout the region to a maximum distance o f 50 

### kilometers. Figure I s how s the extent ofNAAQS violations ba se d o n a ll owable emissions. Figure 2 shows the ex t ent ofNAAQS violations based o n actual hourly emissions. 2.3 Co n serva tiv e Modeling Assumptions 

### A dispersion modeling analysis requires the se l ection of numer o us parameter s which affect the pred icted concentrations. For the e nclosed analysis, seve ral paramete rs were sele cted which underpred ict facility impacts. Assumptions u se d in thi s modeling ana l ysis which likely under-e st imate concentrati ons include th e following:
• Allowable emissions are based on a limitation with an averaging period which is g reater than th e 1-hour average used f o r the S0 2 

### air quality s tandard. E mi ss ion s and impact s during any 1-h o ur period may be hi g her than assumed for the modeling analysis. • No conside rati o n of facility operation at le ss than I 00% 

### l oad. Stack param e t e r

s s uch as ex it fl ow rate and temperature are typica ll y lower at le ss than full load, reducing pollutant dispersion and increa sing pr edicted a ir quality impacts.Eva lua ti o n ofCompliance with th e 1-hour . NAAQSfo r S0 1 Sep le mb e r 17, 2015 Page5

• No co n s id e ra ti o n of buil din g o r stru ctu r e dow nwas h . T hese downwas h effects ty p ica ll y in c rease p r e di cte d co nce nt rat i o n s nea r t h e fac i lity.
• No co n side rat i o n of off

- s it e so urces . These o th e r so urces of S0 2 w ill in c r ease the predicte d impacts .Eva luatio n of Co mpliance with th e /-h o ur NAA QS fo r S0 1 Sep t e m be r 17, 20 15 Page 6

0 0 .;;·**~** 0 0 fl. "' 'f; 0 0 0**~** " 0**~-** "' 

# ..**~** "' 

# .. 0 0 0 ,,**~** " n 0

### ~ .,



# ..**·~** 

# .. "**~**

27!-000 281»JQ 285000 290000 29::.000 :100000 30f.QOO 3 1 0000 31:.000**3~0000** 325000 330000 UTM Eas t( mJ

### ·.**·**

I 335000 34000•) 3-1:.000 :.soooo**'~ ;.ooo**  ltlOOOO 1-hour average S02 concentrations ( ug per cubic meter) -All colored areas exceed the NAAQS. ---1-

------ ·11-**------ f~** **~~~~~-'-------'** 

### 1 9 6 

### 300 400 500 600 700 Fig ur e 1- Reg i onal View of Impa cts Based on Allowab l e EmissionsEw luatio n ofCompliance with th e /-hour NAAQSfo r S0 1 Septe mb e r 17. 20 15 Page 7

0 J 0.

### ~

§ s- -:;

317000 316000 Gerald Gentleman Station

)1WOQ noooo n•ooo j22000 UT M East(mj 325000 JZvOOO 1-hour average S02 concentrations (ug per cubic meter)- All colored areas exceed the NAAQS. 

### 196 210 

### Figure 2

- Regional View of lmpa ct s Based on Actual Emissions for th e 2012-14 Pe riodEm luatio n of Co mpliance with the /-h ou r NAAQSfo r S0 1 Sep tember 1 7 . 2015 Page8 

### 3. Modeling Methodology 3.1 Air Dispersion Model The modeling analysis used USEPA's AERMOD program, v. 14134. AERMOD, as avai l able from the Support Center for Regulatory At m os pheric Mode lin g (SCRAM) website, was used in conjunction with a third-party modeling softwar e program , 

### AERMOD View, so ld by Lakes Environmental Software. 

### 3.2 Control Options The AERMO D m ode l was run with the fo ll owing co ntrol op tion s: • 1-hour average air concentrations • Regulatory defau lts • F l agpole receptors To reflect a repre se ntative inhalation leve l , a flagpole height of 1.5 meters was u sed for all m odeled receptors. Thi s parameter was added to the receptor file when running AERMAP, as described in Sect io n 4.4. A n evaluation was conducted to determine if the modeled facility was located in a rural o r urban se tting u s ing USEPA's methodology out lin ed in Secti on 7.2.3 of the Guideline on Air Quality Mode ls. 7 For urban so urce s, the URBANOPT option is u se d in conjunction with the urban population from an approp ri ate nearby c ity and a d efa ult s urface rou g hne ss of 1.0 meter. Methods descr ibed in Section 4.1 we r e used to determine whether rural or urban di spe rsion coefficients were appropriate for the mode lin g analysis. · 

### 3.3 Output Options Th e AERMOD analysis was based on three years o f recent meteorological data. The modeling analy ses used o n e run w i th three yea rs of se quential meteo r o l ogica l data from 20 1 2 -2014. Consistent w ith USEPA 's Mode lin g Guidance for S0 2 NAAQS De s ignation s, AERMOD provided a tab l e of f o urth-high 1-hour S0 2 impacts concentrations cons is tent with t h e fonn ofthe l -hour S0 2 NAAQS . 8 Please r efe r to Tab l e I fo r the modeling results.

7 USEPA, Revision to the Guideline on Air Qua l ity Models: Adoption of a Pr eferred General Purpose (Flat and Complex Te rrain) Dispers ion Model and Other R ev i sio n s, Appendix W to 40 CFR Part 5 1, Nove mber 9, 20 05. 8 USE PA , Area Designations for the 20 10 Revised Prim ary Sulfur Dioxide Nat ional Amb ient Air Quality Stan dard s, Att achment 3, Ma rch 24, 20 II , pp. 24-26.Emluation ofCompliance ll'ith 1he /-hour NAAQSfor SO J Sept e mhe r 1 7. 2015 Pa

ge 9 4. Model Inp u ts 4.1 Geograph i ca l Inp u ts 

### The "ground floor" of all air dispersion modeling analyses is establishing a coordinate system for identifying the geog raphical locati on of em i ss ion sou rces and receptors . These geographical locations are used to determine local characteristics (such as l and u se and elevation) , and also to ascertain so urc e to r ece ptor distances and relati ons hip s . The Universal T ran sve r se Mercator (UTM) NAD83 coordinate syste m was used for identifying the easti ng (x) and northing (y) coordinates of the modele d so urce s and receptors. S tack l ocations were obtai n ed fr o m fac ilit y permits a nd pri or modeling file s provided by the s tate regulatory agency . The stack locations were then verified us in g aeria l photographs. Th e facility was evaluated to determine if it s hould be mod e led usi ng the rural o r urban dispersion coeffic ient opt i o n in AERMO D. A Geographic Inf ormati o n Syste m (GfS) was u se d to determine whether rural o r urban dispersion coefficients apply to a s ite. Land use within a three

- ki lometer radius circle su rroundin g the facility was considered. USEPA guidance s tates that urban di spe r s io n coefficients are use d if more than 50% of th e area within 3 kilometers has urban land uses. Otherwise, ru ral dispersion coeffic ient s are appropriate. 9 

### USEPA's AERSU RFACE v. 1 3016 was u se d to d evelop the meteo rol og ical data for th e mod e ling a nalysis . This m o d e l was also used to evaluate surro undin g land use within 3 kilometers . Ba se d on the outpu t from the AERSURF ACE, approximately 0.18% of s urro undin g land use around the modeled facility was of urb a n land use t ypes including Type 21 -Low In te nsity R esi d e nti a l, Type 22- High in te nsity Re side ntial and Type 23- Commercial I 

### Industrial/ Tra ns portati o n. This is less than the 50% value considered appropriate for the use of urban dispersion coefficients. Based on the AERSURF ACE analysis, it was conc lud ed that the rural option would be used for the modeling summari ze d in this report. Pl ease r efer to Sectio n 4.5.3 for a discussion of the AERSURF ACE a nal ysis .

9 U S E PA , Revision to the Guideline on Air Quality Mode ls : Adoption of a Preferred Genera l Purpose (F l at and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 5 1 , Novembe r 9 , 2005 , Sect i on 7.2.3.E valuatio n o f C ompliance wit h the !- ho ur N AAQSfor S 0 1 Sep 1 e mbe r 1 7
. 2015 Pa ge /0 

### 4.2 Emis s ion Rates and Source Para m e ters T he mode lin g a nalyses on l y co n s id e r e d S02 e mi ss io n s f ro m t he f ac ili ty. Of f- s it e so urces were no t co n side r ed. Co n ce ntra ti o n s we r e pred i cte d fo r th e s c e na ri os s how n in T a bl es l a nd 2 : I ) a ll owab le e mi ss io n s ba se d o n t he c urre nt pe r m it i ss ue d b y th e r eg ulat o ry age n cy , a nd 2) ac tu a l ho url y e mi ss io n s m eas ure d eac h ho ur b e t wee n J a nuary l , 2 01 2 a nd D ece m be r 3 1 , 20 14 as ta ken fr o m USE PA 

### A ir Ma r ke t s Pr ogra m Dat a. 10 Stack p a ra m eters a nd e mi ss i o n s u se d f o r th e m o d e lin g a nal ys is a r e s umm a ri ze d in T a b l e 4 . 

### Tabl e 4

- Facility Sta c k Param e t e rs and Emission s 11 Stac k S Ol S 0 2 D escr ipti o n B o il e r I B o il e r 2 X Coo rd. [m] 320 18 7 32 0 202 Y Coo rd. [m ] 455 0011 4549 89 3 Base E levat io n [ m ] 953 . 5 6 9 5 6 . 9 5 Re l ease H e i g ht [m] 1 6 7. 64 1 6 7.6 4 Gas Exit T e mp e ra ture [° K] 42 7.5 94 4 22. 0 39

Gas Ex it Ve l oc ity [ m / s] 24.42 2 4. 09 In s id e Di a m e t e r [m] 8.53 4 8 .5 34 A ll owabl e E mi ss i o n Ra t e [ g/s ] 2,32 1 2 , 6 17

Actua l E m i ssio n R at e [g/s ] 

### - - T h e above stack par a m e t e rs a nd e mi ss io n s we r e o bta i ne d fro m r eg ul a tory age n cy d oc um e n ts a nd d ata b ases i d ent ifi e d in Sec ti o n 2. 2 . Th e an a lys i s was co nd uc te d base d o n I 0 0% ope r atin g l oa d u s in g ma x imum ex h a u s t fl ow rates a nd te mp e ratu re s . Op e r ati o n a t less t ha n f ull c apaci ty l oa d s was no t co n s i dere d. T hi s ass um pt io n te nd s to un de r-pre di ct imp acts s in ce stac k p a r a me t e rs s uc h as ex it fl ow r ate a nd te mp e rat ure a r e t yp i ca ll y l o w e r at less t ha n f ull l oad , re du c in g p o lluta nt di spe rs io n a n d i nc r eas i ng pr e di cte d a ir qu a li ty imp ac t s . S tac k l ocat io n , he ig ht a nd d iam ete r were ve rifi e d u sing ae ri a l p h otog ra ph s, a nd flu e gas flow rate and t e m pe ra tu re we r e ve rifi e d u si ng co mbu s ti o n ca lc u lat i o n s.

10 http: //ampd.epa.gov/am p d/ 11 N ebraska Publ ic Power Di stri c t , Year20 10 Air Emi ss i o n

s In ve nt ory , Fac il ity ID 34385 , Ma r c h 28,20 1 1.Em!uati on ojCom p lianc e with th e /-hour NAAQSfo r S0 1 Sep t e m be r 17. 2015 Page II 

### 4 .3 Building Dimensions 

### No building dimensions o r prior downwash eva luati o n s we re ava il ab le . Th e r e f

o re this modeling a n alys i s did n o t a ddre ss the effects of d ownwa s h and thi s may und e r-pred ict impact s . 4.4 Receptors 

### F

o r Ge ra l d Gentl e m an Stat i o n, three receptor g rid s were e mp l oye d: I. A I 00-mete r Ca rt esian receptor g ri d centered o n Gerald Ge ntl e man S tation and extend in g out 5 kil o meter s . 2 . A 500-meter Cartesian recept o r g rid ce ntered o n Gera ld Gentlema n S tati o n and ex tendin g o ut I0 kilomete r s. 3 . A I ,000 -m eter Cartesian r eceptor grid centered o n Ge rald Gentle man Station and exte ndin g ou t 50 kil o mete rs . 50 kil o meters is th e

**~'< imum**  

### di s ta n ce accepted by USE PA for the use o f the A EIUAOD dispersion m o del. 12 

### A flagpo l e h e i

g ht of 1.5 meter s was used for all the se r ece pt o r s. Elevati o n s fro m s tac ks and receptors we r e ob t a ined f ro m National El evati o n Dataset (NE D) GeoTiff data. GeoTiff is a binary file that include s data de sc ript ors a nd gee- r efe rencing information ne cessary fo r ex trac tin g terrain e levations. The se elevat ions we re ex trac te d from I arc- second ( 3 0 m

ete r) r eso luti o n NED files. T he USE PA softwa re pro g ram AERMAP v. Ill 03 i s use d for the se task s . 4.5 M e teo ro logica l Data 

### To improve th e acc urac y of t he modeling analys is, r ecen t m e t

eo ro l og i ca l data for th e 2 01 2 -201 4 pe riod were pre par ed u s ing the US E PA 's progra m A ERM E T whi c h c r ea t es the model-ready su rf ace a nd profi l e data fi l es r equ ired by AER.MOD. R equired d ata input s t o AERMET included s urf ace meteorological m eas urement s, twice

-dail y so undin gs of upp e r a ir m eas u rement s, and the microrneteoro l og i c al param e t e r

s s urface r o ughne ss, albedo, a nd Bowe n rati o. One-minute ASOS data were avai labl e so US EPA m e th o d s we re u se d t o redu ce ca lm a nd mi ss ing ho urs . 13 

### The USE PA so ftware prog ra m A ERMI NUTE v . 14 23 7 i s used for th ese tasks. T hi s sec ti o n di scu ss es how the mete o r ologi cal data was prepared for use in the !-h o ur S0 2 

### NAAQS

12 USE PA , Rev i s io n t o the Gu id e line o n Air Quality Mode l s : Adop ti o n o f a Preferred Ge ne r a l Purpose (F lat and Co mpl ex Te rra in ) Di s pers io n Model and Other Rev i sio ns, App endix W t o 40 CF R Part 5 1, Sec ti on A.l.( I ), Nove mb e r 9, 2005. JJ US E PA , A rca De s ig nati o n s for the 20 I 0 Revi se d Primary S ui fur Di ox id e Na ti o nal Ambi cnt A ir Qua lity Standards. Atta c hm e nt 3, Ma r c h 24, 20 II , p. 19.E ra/uari o n of Co m pliance wirh rile /-hour XA.-IQSfor S0 1 Sepr e mber 1 7. 2015 Page 12

m ode l ing a nalyses. Th e U SE PA so ftware pro g ram A E RM E T v. 14 134 i s u se d f or th ese ta s ks. 

### 4.5.1 Surface Meteorolo gy Sur f ace meteorology was ob tained for Nort h Pl atte Reg i o na l A irp o rt located near th e Gerald Gentleman S t ation. Integrated S urf ace H ou rl y ( I SH) da ta for the 2012-20 14 pe ri o d were obtai ne d fr o m the Nat i o na l C limati c Data Ce nter (NC D C). Th e I S H s urface data was processed through AE RMET Stage I , w hich pe rf o rm s data extraction a nd qu a li ty co n t rol c hec ks . 

### 4.5.2 Upper Air Data U p pe r- a ir data are co ll ecte d b y a "w eather ba ll oo n ' ' that is released twic e per day at se lecte d l o ca ti o n s. As the balloon i s released, it ri ses through the atmos ph e re , and radios the data back to the s urface. The m eas urin g and transmitt in g device is known as e ither a radi osonde, o r r aw ind so nd e. Data co ll ected and rad i oed back in c lude : air pr essu r e, he i ght, temperatu r e , dew poi n t , w ind s pee d , a nd w ind direct i o n. T he upp e r air data we r e processe d throug h AERME T S ta ge I , w hi c h pe rfo rm s data extra c tion and qualit y co ntrol c hecks . Fo r Gerald Gentleman S tati o n , the co ncurrent 20 12-20 14 upp e r air data from twice-daily r adiosonde measu r e m e n ts o bt a in ed at th e m os t r e pre se n tat i ve l ocat i o n we re used. T hi s l oca ti o n was th e Nort h Platte , Neb r aska measurement s ta ti o n. Th ese data are in Fo r ecas t Sys tems Lab oratory (FS L ) f o rmat a nd we r e dow nl oaded in ASC ll text fo rmat f r o m NOAA's FS L website . 14 A ll r e p o rtin g le ve ls were downloaded and proces se d wit h AERM ET . 

### 4.5.3 AERSURFACE AE R SURFACE i s a program th a t ex tracts s urfac e roughness, a l bedo, and dayt im e Bowen rati o for an area s urro undin g a give n l ocati o n. AE R SURFACE uses land use and land cove r (LU L C) dat a in the U.S . Geological Survey's 1 992 Nat i o nal Land Cover D atase t to extr act the nece ssa r y m i crome teo rolog ica l data. LULC data was u se d for processing m eteo r o l ogica l d ata sets u sed as inpu t to AERM OD. AE R SU RFA CE v. 130 16 was u se d to deve l op s urf ace rou g hn ess, a lb edo, a nd daytime B owe n ra t i o va lu es in a re g i on s urroundin g the me teorol og ic a l data co ll ecti o n s it e. AE RS URFACE was u se d t o deve lop s urf ace roughne ss in a o ne k ilomete r radiu s s urroundin g the data co ll ec ti o n s i te . B owe n rati o and albe d o was developed fo r a I 0 kilometer by I 0 ki l omete r a r ea centered o n th e me t eorological data co ll ect i on site. Th ese micro m e teoro l ogica l data we r e p rocessed fo r se asonal periods using 30-degree sec t o r s . Seaso na l moi s t ure condi ti o n s we r e co n s id ere d ave r age with wi nt e r

14 Avai l able at: http: // es rl.n oaa.gov/ raobs/E valuation of Co mpliance with the !

- hour NA AQSfor S0 1 Sep tember 17. 2015 Page 13

m on th s h avi n g co nt inu o u s snow cove r . 

### 4 .5.4 D a ta Rev iew Miss in g meteoro l og ica l d ata were n ot fil l ed as t h e da ta fi l e me t USEPA ' s 90% d ata co mp le te n ess requ ireme nt. 1 5 Th e A ERMOD outpu t fi l e s hows th e re we re 0 .53% m iss in g d a t a . To confi rm th e re presentat i ve ness of t he ai r po rt meteo r o log i ca l d ata, t h e s urface cha rac te ri stics of the a irp ort data co ll ect io n site a nd th e mo d e l e d so urce l ocat io n we r e co m par e d. S in ce th e No rth P latte R eg io n a l Ai r port is l ocated c lose to G e ra ld Gentle m a n S ta ti o n , t h is m eteo ro l og ica l d ata se t was cons id e r ed app ro pr iate fo r t h is modeli n g a n a l ys i s. 16 Addi t io n ally, t hi s wea th e r s tat io n p rov i de d h i gh q u a li ty surface m eas ureme nt s fo r t he m ost rece nt 3-yea r tim e, a nd ha d s im il a r lan d u se, surface cha racter is ti cs, te rra in feat u res a nd c li mate . 

### 5 . Backg ro u nd S0 2 

### C o ncentrati o ns Backg r o und conce n t ratio n s were d eterm in e d co n siste n t wit h USE P A's Mode l i n g G u ida nce fo r S02 NAAQS Des i g n at ions. 17 To prese r ve t he fo rm of the ! -h o ur S 0 2 stan d a rd , b ased o n t he 99th perce n t il e of th e a nn ua l dist r ib ut io n of d a i ly m aximum 1 -hour co nce nt r a ti o n s ave r age d ac ross t he numbe r of yea r s mo d e l e d , t he back!! round fo u rth- hi g hest dai l y max i m um !- h o ur S 0 2 co nce ntrat io n was added to t he modele d fourt h

- h ig hest da il y max imum 1 -h o ur S0 2 co nce ntra ti o n. 18 Backgro un d conce n tratio n s we re based o n t he 201 1

- 13 desig n va lu e m eas ured by th e a m b ien t mo nito rs located i n Nebraska. 19 

### 6 . Re p orti n g Al l fi l es from the p rograms u sed fo r t hi s m o d e l ing a n alysis a r e avai l ab l e to reg u lato r y agenc ies . These in c lude a n a l yses p repa red w i t h AERSURFACE, AERME T , AERMAP, a nd AERMO D .

15 USE PA , Meteo r o logical Monito rin g Guidance for Regu latory Mode ling Applications, EPA-454/ R-99-05 , February 2000, Section 5. 3.2, pp . 5-4 to 5-5 . 16 USE PA, AERMOD Imp lementation Gu ide , Ma r c h 19.2009, pp . 3-4 . 17 USEPA, S02 NAA QS Des ignations Model ing Technica l Assista n ce Document. Dec . 2013 , section 8. 1, pp 27-28; USE PA. Area De s ig n ations for the 2010 Revised Primary Sulfur Diox ide Nationa l Ambien t A ir Quality Standards, A ttachmen t 3, March 24, 20 II , pp . 20

- 23 .. 18 USE PA , Applicability of Appendix W Mode lin g Guidance for the 1-hour S0 2 Nat ional Ambient Air Quality Standard, A ug us t 23.20 10, p. 3 . IQ http: // www .epa . gov/ ai rtren ds/ val ues .h m1l