A mos. Chem. Phys., 24, 4305–4329, 2024
h ps://doi.o g/10.5194/acp-24-4305-2024
© Au ho (s) 2024. This wo k is dis ibu ed unde
he C ea i e Commons A ibu ion 4.0 License.
Resea ch a icle
Analysis o an in ense O3pollu ion episode on he
A lan ic coas o he Ibe ian Peninsula using
pho ochemical modeling: cha ac e iza ion o anspo
pa hways and accumula ion p ocesses
Edua do To e-Pascual1, Go zon Gangoi i1, Ana Rod íguez-Ga cía1, Es ibaliz Sáez de Cáma a1,
Joana Fe ei a2, Ca la Gama2, Ma ía Ca men Gómez1, Iñaki Zuazo1, Jose An onio Ga cía1, and
Mai e de Blas1
1Facul y o Enginee ing Bilbao, Uni e si y o he Basque Coun y (UPV/EHU), Bilbao, 48013, Spain
2CESAM, Uni e si y o A ei o, A ei o, 3810-193, Po ugal
Co espondence: Edua do To e-Pascual ([email p o ec ed])
Recei ed: 3 Ma ch 2023 – Discussion s a ed: 15 Sep embe 2023
Re ised: 2 Feb ua y 2024 – Accep ed: 13 Feb ua y 2024 – Published: 15 Ap il 2024
Abs ac . A oposphe ic O3pollu ion episode o e he A lan ic coas o he Ibe ian Peninsula du ing 2–6 Au-
gus 2018 has been analyzed. The episode was cha ac e ized by a pe manen wind shea h oughou he en i e
pe iod, making he obse ed ozone su ace dis ibu ion especially di icul o explain. A new me hodology is de-
sc ibed o analyze uppe -le el a mosphe ic pa ame e s, such as empe a u e, wind di ec ion, wind speed, and O3
concen a ions, added o he adi ional use o su ace pa ame e s, using he Wea he Resea ch and Fo ecas ing
(WRF) and Comp ehensi e Ai Quali y Model wi h Ex ensions (CAMx) models as well as a ailable su ace and
uppe -ai obse a ions. Resul s indica e ha he episode was cha ac e ized by a i s phase o a sudden inc ease
in O3concen a ions p oduced by umiga ion and in e - egional anspo p ocesses wi hin he Ibe ian Penin-
sula, ollowed by con inen al O3 anspo om Eu ope o he A lan ic coas . An A lan ic on p oduced he
dissipa ion o he episode, gene a ing an “ozone on ” accompanying he cold on passage ac oss he egion.
1 In oduc ion
Sou he n Eu opean coun ies a e hea ily exposed o high o-
posphe ic ozone (O3) concen a ions, pa icula ly hose su -
ounding he Medi e anean Basin (ETC/ACM, 2018; Eu-
opean En i onmen Agency e al., 2018). Accumula ion,
anspo , and eci cula ion p ocesses behind hese high con-
cen a ions ha e been ex ensi ely analyzed in he wes -
e n Medi e anean Basin and eas e n Ibe ia du ing he las
40 yea s (Millán e al., 1997; Gangoi i e al., 2001; Que ol e
al., 2016). The 2020 Eu opean ai quali y epo indica ed a
dec ease in O3le els compa ed o p e ious yea s. Howe e ,
le els emained no ably high, wi h maximum concen a ions
obse ed in cen al Eu ope, ce ain Medi e anean coun ies,
and Po ugal – loca ed on he A lan ic coas o he Ibe ian
Peninsula (IP) (EEA, 2022). These high O3concen a ions
a e due o a combina ion o he no he n midla i ude back-
g ound concen a ions (Cue as e al., 2013; Rod igues e al.,
2021) and he local and egional p oduc ion a o ed by he
egion’s ci cula ion wea he pa e ns (Russo e al., 2016) and
me eo ological condi ions, including empe a u e (Sá e al.,
2015) and sola adia ion (Sil a and Pi es, 2022). Howe e ,
despi e hei impo ance, O3episodes on he A lan ic coas o
IP, speci ically in no he n A lan ic Ibe ia (NAI) and wes e n
A lan ic Ibe ia (WAI), ha e no been examined in de ail. In
his egion, signi ican episodes o oposphe ic O3ha e oc-
cu ed, wi h alues exceeding he a ge alue o he p o ec-
ion o human heal h de ined by Di ec i e 2008/50/EC (Sil a
and Pi es, 2022). Mo eo e , conside ing he mo e s ingen
alues o he Wo ld Heal h O ganiza ion (WHO) ai quali y
guidelines (which a e planned o be inco po a ed in o he e-
Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union.
4306 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
ised ai quali y di ec i e by 2035), he si ua ion becomes
e en mo e conce ning.
In NAI, da a om he summe ligh campaigns o he
Eu opean MECAPIP p ojec (Millán e al., 1992, 1997)
e ealed long- ange anspo o pho ochemical pollu an s
om he English Channel in o he Basque Coun y (BC)
(Alonso e al., 2000). Pollu an s a e usually anspo ed unde
he ypical summe synop ic scena io, wi h he Azo es High
ex ending a idge o high p essu es o e he Bay o Biscay
and pushing no he ly winds o e NAI.
Gangoi i e al. (2002, 2006a) showed he impo ance o
e ical laye ing and anspo in he gene a ion o in ense O3
episodes in he BC unde a di e en synop ic si ua ion, wi h
pe sis en no heas e ly winds associa ed wi h blocking an i-
cyclones o e he B i ish Isles. Tha wo k also documen ed
he impo a ion o pollu an s in o he BC om se e al Eu o-
pean sou ce egions du ing he build-up o episodes, includ-
ing he Ibe ian Peninsula. Valdeneb o e al. (2011) showed
how O3 anspo e iciency inc eased a e he o ma ion
o accumula ion laye s o pollu ed ai masses alo , which
can a el la ge dis ances wi hin a s ably s a i ied ma i ime
bounda y laye (MBL) o o e he s able noc u nal su ace
bounda y laye o e land. The la e s udy demons a ed ha
anspo om and o he Eb o and Dou o alleys, bo h lo-
ca ed in he IP, plays a main ole in O3episodes in he BC.
Sáez de Cáma a e al. (2018) documen ed he ac ha O3
obse a ions in backg ound a eas o he BC may ha e p o-
duc ion and anspo o local o igin om su ounding a eas
du ing midday and a con ibu ion om he a i al o pollu ed
ai masses in he a e noon du ing he accumula ion and peak
phases.
Pas s udies o WAI in Po ugal showed ypical empo al
pa e ns wi h maximum mean mon hly concen a ions du -
ing sp ing and maximum hou ly concen a ions du ing sum-
me (Pi es e al., 2012). Concen a ions a e highe in inland
and u al a eas han in u ban egions. Howe e , O3episodes,
wi h concen a ions abo e he h esholds de ined o he p o-
ec ion o human heal h, also occu in u ban egions. Se e al
s udies (E yugina e al., 2007; Mon ei o e al., 2012, 2016)
showed he impo ance o sea b eeze ci cula ion in he build-
up o O3episodes h ough he Po uguese coas , poin ing o
he impo ance o p ecu so s emi ed in coas al a eas and O3
p oduc ion along he anspo owa ds inland a eas. He ig e
al. (2020) showed ha in Po ugal he occu ence o O3and
hea wa e e en s had he s onges ela ionship o eas e n
and no heas e n in low, highligh ing he impo ance o he
ad ec ion o O3pollu ion om he con inen al pa s o he
Ibe ian Peninsula. In addi ion o he egula an h opogenic
(e.g., a ic, indus y, ene gy p oduc ion) and biogenic (na -
u al) sou ces, ex ao dina y e en s such as o es i es play
an impo an ole in he O3episodes egis e ed in Po ugal
(Adame e al., 2012).
In his a icle, o he i s ime and as a as he au ho s
know, he oposphe ic O3p oblem is app oached in an in-
eg al way o he A lan ic coas o IP. We ha e selec ed an
O3pollu ion episode las ing 5 d occu ing om 2 o 6 Au-
gus 2018 (see Sec . 3.3), which a ec ed Spain and Po ugal.
This episode was cha ac e ized by a no able and simul ane-
ous inc ease in O3concen a ion le els ac oss bo h he NAI
and WAI egions du ing 2 Augus , high O3concen a ions
du ing consecu i e days un il 6 Augus , and inal dissipa ion
on 7 Augus . We u ilized he modeling sys em desc ibed in
Sec . 2 o examine he local o in e - egional anspo and
accumula ion o O3wi hin and be ween he wo coun ies,
as well as be ween hese coun ies and he es o Eu ope.
Valdeneb o e al. (2011) p oposed he exis ence o a po en-
ial anspo pa hway o O3and pollu an s along he A -
lan ic axis o he Ibe ian Peninsula in NAI. This hypo he-
sis, oge he wi h he ac ha Spain and Po ugal sha e h ee
main ai basins d aining om cen al Ibe ia in o he A lan ic,
implies ha he analysis should be ca ied ou as a whole o
he wo egions: no he n A lan ic Ibe ia (NAI) and wes e n
A lan ic Ibe ia (WAI), as shown in Fig. 1. We e eal how
in e - egional anspo o O3is a key elemen in explaining
he obse ed e olu ion o his episode.
1.1 A ea desc ip ion
The Ibe ian Peninsula, wi h Spain and Po ugal, has a com-
plex opog aphy and nume ous moun ain anges, wi h an a -
e age al i ude among he highes in Eu ope. IP is su ounded
by he Bay o Biscay o he no h, he Medi e anean Sea o
he eas , he Medi e anean and A lan ic Ocean o he sou h
whe e bo h mee h ough he S ai o Gib al a , and by he
A lan ic Ocean o he wes (Fig. 1). I is sepa a ed om he
Eu opean con inen by he Py enees Moun ain Range and a
high cen al pla eau la gely occupies i s su ace. The i e s
lowing in o he sea p oduce nume ous ai basins and alleys
ha a e decisi e o s udying a mosphe ic pollu ion due o
hei pa icula wind egimes.
To he no h and pa allel o he coas is he Can ab ian
Range, wi h ele a ions o mo e han 2500 m a.s.l. (me e s
abo e sea le el) in i s cen al zone. This moun ain ange ex-
ends om wes o eas o he wes e n end o he Py enees
and sepa a es he no he n coas o he IP om he no he n
peninsula pla eau. Basque Coun y is in he link be ween
he no he n coas o IP and he Py enees, wi h lowe -al i ude
moun ains, usually below 1500 m a.s.l. To he wes o IP, in
Po ugal, he e a e wo di e en a eas: no h o he Tagus
Ri e whe e moun ainous a eas also p edomina e and sou h
wi h la e landscapes and a ew low moun ains. Mos o
Po ugal is below 400 m a.s.l., and he highes al i udes a e in
he Se a da Es ela, which o ms a con inua ion o he Span-
ish Cen al Sys em. Bo h coun ies sha e he alleys o he
Dou o, Tagus, and Guadiana i e s.
O e he las yea s, he highes O3hou ly a e aged con-
cen a ions egis e ed in NAI and WAI ha e been con inu-
ously measu ed in u al moun ainous a eas such as Valde ejo
Na u al Pa k (VNP) s a ion in he Basque Coun y (Spain)
and Dou o No e (DN) s a ion in Al ão Na u al Pa k (Po -
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4307
Figu e 1. Topog aphic map o he Ibe ian Peninsula (c): he e i o y delimi ed o he le owa ds he A lan ic Ocean is Po ugal, o i s igh
Spain, and o he no h o he Py enees (F ance). (a) No he n A lan ic Ibe ia (NAI) and (b) wes e n A lan ic Ibe ia (WAI).
ugal). A bo h s a ions, O3exceedances a e nume ous, and
O3le els a e a ec ed by p ima y pollu an s ( ola ile o -
ganic compounds – VOCs; ni ogen oxides – NOx) emi ed
on hei co esponding coas line ha a e anspo ed inland
due o he sea b eeze ci cula ion. Besides he con ibu ion
o local sou ces, he concen a ion p o iles e lec he in lu-
ence o a mosphe ic anspo on a synop ic o egional scale
(E yugina e al., 2009; Ca alho e al., 2010; Mon ei o e
al., 2012; Bo ego e al., 2013, 2016; de Blas e al., 2019;
Gómez e al., 2020). All hese s udies ha e analyzed a ious
O3episodes o speci ic egions o he A lan ic coas o IP.
Howe e , he e is a lack o a common modeling and assess-
men me hodology o he whole egion. The mechanisms
p oducing O3episodes occu ing simul aneously in he wo
sub egions (NAI and WAI) o he A lan ic coas o IP a e s ill
in he p ocess o being u he documen ed.
1.2 Objec i es
The main objec i e o his pape is o analyze a oposphe ic
O3episode wi h a ema kable in ensi y o e a la ge e-
gion, co e ing no he n A lan ic Ibe ia (NAI) and wes e n
A lan ic Ibe ia (WAI), and o es ablish possible O3in e -
egional pa hways be ween hese egions and wi h he es
o he Ibe ian Peninsula and he Eu opean con inen . Fo
his pu pose, we ha e es ablished a me hodology based on
high- esolu ion me eo ological and pho ochemical modeling
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4308 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
o analyze he su ace concen a ion and e ical dis ibu ion
o O3. The p esence o a pe manen wind shea h oughou
he en i e episode added special complexi y and posed a chal-
lenge o he sea ch o he o igin o he obse ed O3impac
and he selec ion o he mos app op ia e educ ion policies.
This pape is o ganized as ollows: Sec . 2 desc ibes he
me hodology employed, con aining wo subsec ions. Sec-
ion 2.1. e e s o he modeling sys em used and Sec . 2.2.
he alida ion me hod. The esul s and discussion a e p e-
sen ed in Sec . 3, di ided in o h ee subsec ions, analyzing
he me eo ology o he episode in Sec . 3.1, s a is ical e alu-
a ion in Sec . 3.2, and O3concen a ions in Sec . 3.3. Finally,
in Sec . 4 we de ail he conclusions o his s udy.
2 Me hodology
In ecen yea s chemical anspo models (CTMs) ha e
been used o simula e and analyze sho -du a ion pollu ion
episodes in IP (Val e de e al., 2016; Escude o e al., 2019;
Pay e al., 2019). The use o ine g ids in models (wi h
high ho izon al spa ial esolu ions o 1–3 km) has gi en good
esul s in en i onmen s wi h complex opog aphy, whe e
mesoscale p ocesses become pa icula ly ele an o he in-
e p e a ion o he O3p oduc ion, accumula ion, anspo ,
and decay (Jiménez e al., 2006; Mon ei o e al., 2012). High
ho izon al spa ial esolu ion is also especially ecommended
when desc ibing O3 a iabili y in indus ial and u ban a eas
(Baldasano e al., 2011).
We ha e used a pho ochemical modeling sys em con igu-
a ion, combining me eo ological, emission, and pho ochem-
ical simula ions. Model execu ion ( o ini ial and bounda y
condi ions, among o he s) and alida ion equi e a a ie y o
expe imen al da a, all o hem desc ibed h oughou his sec-
ion. Model esul s ha e been p ocessed in o de o analyze
and ep esen e ical c oss-sec ions o he a mosphe e, and
we ha e calcula ed in eg a ed O3concen a ions om nea -
su ace a mosphe ic le els up o 2500 m a.g.l. (me e s abo e
g ound le el), acco ding o he a mosphe ic hickness abo e
he su ace a which O3accumula es (Que ol e al., 2018).
2.1 Simula ions
2.1.1 Me eo ology
The me eo ological pa ame e s equi ed o ai quali y simu-
la ions we e ob ained using he Wea he Resea ch and Fo e-
cas ing model (WRF) model e sion 3.9.1.1 (Skama ock e
al., 2008) wi h a modeling pe iod om 26 July o 9 Augus
2018. We de ined h ee domains wi h di e en esolu ions
(Table 1) and Lambe con o mal p ojec ion, as shown in
Fig. 2, wi h he cen e o he coa se domain a 45° N and
2.5° W and wi h 50 and 35° N as ue la i udes o he p o-
jec ion. The i s g id (d01) ex ension co e s a la ge pa o
he Eu opean con inen and no he n A ica wi h a 27 km
ho izon al esolu ion. This domain is in ended o include
Table 1. Spa ial cha ac e is ics o he domains used in WRF and
CAMx.
Domain Spa ial WRF CAMx
esolu ion numbe numbe
o cells o cells
d01 27 km ×27 km 162 ×162 160 ×160
d02 9 km ×9 km 195 ×150 193 ×148
d03 3 km ×3 km 393 ×186 389 ×182
la ge a mosphe ic ci cula ions be ween he No h A lan ic,
he Medi e anean Sea, and no he n A ica. I also includes
impo an sou ces o a mosphe ic emissions loca ed along
he English Channel (Uni ed Kingdom, no he n F ance, Bel-
gium, and he Ne he lands) and no he n A ica (Gangoi i e
al., 2006a). The second domain (d02), wi h a esolu ion o
9 km, inco po a es he en i e Ibe ian Peninsula, he sou h o
F ance, and he coas o no he n A ica. In his way, i can
documen he A lan ic on s o e he egion, he summe
an icyclones and associa ed mesoscale lows in he wes e n
Medi e anean Basin (Gangoi i e al., 2001, 2006b), and he
lows de eloping in he S ai o Gib al a (in’ Veld e al.,
2021; Massagué e al., 2021). We also included a hi d do-
main (d03), wi h a esolu ion o 3 km, co e ing he no h o
he Ibe ian Peninsula and he sou h o F ance, so ha a mo-
sphe ic lows de eloped o e he Eb o and he Dou o al-
leys could be ep esen ed wi h adequa e de ail. This hi d
domain includes a eas o special in e es wi h O3measu -
ing e e ence s a ions o he analysis o in e - egional O3
anspo (Na azo e al., 2008; de Blas e al., 2019; Gómez
e al., 2020). We used 31 ηlaye s co e ing up o app oxi-
ma ely 15 500 m a.g.l. The e ical esolu ion nea he bound-
a y laye , a he su ace, is g ea e han a highe le els, whe e
he dis ance be ween laye s inc eases (Table S1 in he Sup-
plemen ).
The physical pa ame e iza ions o he me eo ological
model a e de e minan s when simula ing ai quali y. In his
s udy, we p oposed a con igu a ion al eady used by o he
s udies in IP wi h sa is ac o y esul s (Bo ge e al., 2014;
Escude o e al., 2019). O he s udies, also ca ied ou in IP
(Pay e al., 2010; Bo ego e al., 2013; Banks and Baldasano,
2016), ha e used simila pa ame e iza ions wi h changes in
he con igu a ion o he plane a y bounda y laye (PBL). The
selec ed pa ame e iza ion is shown in Table S2. I is mainly
based on he con igu a ion o Bo ge e al. (2008), modi-
ying he longwa e adia ion scheme by he Rapid Radia-
i e T ans e Model (RRTM) (Mlawe e al., 1997) as ec-
ommended by he WRF de elope s. Addi ionally, sea su -
ace empe a u e (SST) supplied by NOAA has been used,
speci ically op imum in e pola ion SST (h ps://www.ncei.
noaa.go /p oduc s/op imum-in e pola ion-ss , las access:
28 Ma ch 2022), wi h a spa ial esolu ion o 0.25° ×0.25°
and a daily empo al esolu ion (Banzon e al., 2016).
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4309
Figu e 2. Domains used o he me eo ological and pho ochemical simula ions on he opog aphic map: d01 (27 km) – Eu ope and No h
A ica, d02 (9 km) – Ibe ian Peninsula, d03 (3 km) – no he n Ibe ian Peninsula. Red lines: loca ion and ex en o a mosphe ic e ical
c oss-sec ions analyzed in VNP and DN.
Ini ial and bounda y condi ions we e gene a ed using 6 h
eanalysis om he Eu opean Cen e o Medium-Range
Wea he Fo ecas s (ECMWF), speci ically ERA-In e im e-
analysis global da a (Be is o d e al., 2011) o 0.75° ×0.75°
ho izon al esolu ion. The e ical esolu ion is highe nea
he su ace (e e y 25 hPa om 1000 o 700 hPa), dec eas-
ing o highe le els. We also used he same da a o ou -
dimensional da a assimila ion (FDDA) abo e he PBL in he
coa se domain (d01).
2.1.2 Pho ochemis y and dispe sion
We used he Comp ehensi e Ai Quali y Model wi h Ex-
ensions (CAMx) e sion 6.50 (Ramboll En i onmen and
Heal h, 2018). The domain and ho izon al esolu ion selec ed
o his ai quali y model we e iden ical o hose used o
WRF model (see Table 1). We used 14 σlaye s going up
o app oxima ely 4800 m a.g.l. wi h he i s laye being ap-
p oxima ely 20 m hick. Concen a ions we e calcula ed a
he midpoin o each laye , so he modeled alues o he i s
laye co espond o a heigh o app oxima ely 10 m a.g.l., and
he same applies o o he laye s. The di e en hicknesses
o he laye s and hei co espondence wi h WRF laye s a e
shown in Table S3.
The gas-phase mechanism CB6 4 was used in his wo k
(Ramboll En i onmen and Heal h, 2018). Fo ino ganic
he modynamics and gas–ae osol pa i ioning CAMx uses
ISORROPIA (Nenes e al., 1998, 1999) and o d y depo-
si ion we chose he algo i hm o Zhang e al. (2001, 2003).
The O3column da a we e ob ained om he O3Moni o -
ing Ins umen (OMI) o NASA’s To al O3Mapping Spec-
ome e (TOMS) sa elli e, which has a daily empo al es-
olu ion and a ho izon al spa ial esolu ion o 1° ×1° (a ail-
able a h ps://acd-ex .gs c.nasa.go /anon p/ oms/omi/da a/
ozone/, las access: 28 Ma ch 2022). O3column da a we e
used in he oposphe ic ul a iole and isible (TUV) adia-
ion and pho olysis model used by CAMx: D . Mad onich’s
p ep ocesso o CAMx calcula es he pho olysis a es o
clea skies, and hen CAMx in e nally adjus s hese a es
in he case o clouds o ae osols (NCAR, 2011). The ini ial
and bounda y condi ions o he i s domain (d01) we e ob-
ained om he global ai quali y model CESM2.1: CAM-
Chem (Lama que e al., 2012). We i s an he i s domain
(d01), and o he o he wo (d02 and d03) we used he BN-
DEXT CAMx p ep ocessing p og am o gene a e ini ial and
bounda y condi ions ex ac ed om d01. In he i s simula-
ion o d01, we ex ac ed he ollowing simula ed pollu an s’
concen a ions o gene a e d02’s ini ial and bounda y condi-
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4310 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
ions: O3, NO, NO2, SO2, OH q, HO q
2, H2O2(hyd ogen pe -
oxide), CO, CH4, e hane, e hene, e hyne, p opane, o malde-
hyde, isop ene, mono e penes, benzene, oluene and o he
monoalkyl a oma ics, xylene and o he polyalkyl a oma ics,
HNO3, HONO (ni ous acid), PAN (pe oxyacil ni a e), and
NO q
3.
The CAMx domain con igu a ion was he same as ha
used in WRF (Fig. 2), as we e ype o p ojec ion, hei cen al
poin s, and e e ence la i udes. Howe e , a sligh educ ion o
dimensions was necessa y o i s co ec usabili y in CAMx
due o he way CAMx domains a e con igu ed and o he lim-
i a ion imposed by some emission models such as he Model
o Emissions o Gases and Ae osols om Na u e (MEGAN).
Also, o CAMx o p ope ly sol e he bounda y condi ions o
he nes ed domains, some cells, denomina ed as bu e cells,
we e added a he ou e edges a ound he pe ime e o each
domain.
CAMx inco po a es he WRFCAMX p ep ocesso e -
sion 4.6, which ans o ms he WRF me eo ological a iable
ields in o he speci ic me eo ological a iables equi ed by
CAMx. We chose o un his p og am wi h he YSU scheme
o he PBL (Hong e al., 2006) o be consis en wi h he PBL
con igu a ion used in WRF.
2.1.3 Emissions
We used he Model o Emissions o Gases and Ae osols om
Na u e (MEGAN) (Guen he e al., 2006, 2012), an empi ical
model o biogenic emissions mos widely used by he scien-
i ic communi y o he calcula ion o VOCs om ege a ion
(Sindela o a e al., 2014). The new e sion (MEGAN 3.0)
includes a p ocesso o calcula ing emission ac o s (EFs)
o di e en species o he i s ime, whe e he use may in-
co po a e cus om high-spa ial- esolu ion EF da abases om
speci ic ege a ion da a. This p ocesso con ains a wide se-
lec ion o EFs o mo e han 42 500 species ypes based on
he a ailable da abases (Guen he , 2017).
MEGAN equi es as inpu o he biogenic emissions
es ima ion o he di e en domains used in he CAMx
simula ion a me eo ological simula ion, a lea a ea index
(LAI) spa ial dis ibu ion, and EF spa ial dis ibu ions. We
inco po a ed me eo ological da a calcula ed by WRF in o
MEGAN 3.0 h ough he Me eo ology–Chemis y In e ace
P ocesso (MCIP) ool (O e and Pleim, 2010) using he p e-
p ocesso s included in his new e sion o MEGAN o cal-
cula e EF, bu we imp o ed i by upda ing Spanish land use
and ege a ion map da abases om he Na ional Fo es In-
en o y (To e-Pascual e al., 2021). O he exis ing global
LAI p oduc s, we chose he one gene a ed by he Mode a e
Resolu ion Imaging Spec o adiome e (o MODIS) ins u-
men o NASA’s Aqua and Te a sa elli es (Myneni e al.,
2002; Yang e al., 2006). The wide use o his p oduc is
due o i s high spa ial esolu ion (1 km ×1 km), high em-
po al esolu ion (e e y 8 d), and equen upda ing (Yuan e
al., 2011). Howe e , he ins umen shows unce ain ies due
o cloudiness and seasonal snow co e , and cu en MODIS
LAI p oduc s a e spa ially and empo ally discon inuous and
inconsis en (Zuazo e al., 2023). Thus, we used he 2010 e-
p ocessed MODIS LAI by Yuan e al. (2011) ins ead o he
one o 2018 because i was no a ailable a he ime o his
s udy.
We used he Emission Da abase o Global A mosphe ic
Resea ch (EDGAR) global an h opogenic emission in en-
o y (C ippa e al., 2018) e sion 4.3.2, published in Decem-
be 2017. EDGAR con ains an h opogenic emissions om
he Eu opean and A ican con inen s, which i he ex ension
o he main domain (d01) wi h a high spa ial (0.1° ×0.1°)
and empo al esolu ion (mon hly a e ages o 2010) o he
whole a ea. We selec ed he mos ele an compounds o
he analysis o oposphe ic O3pollu ion episodes, speci -
ically CO, NH3, non-me hane ola ile o ganic compounds
(NMVOCs), NOx, SO2, and CH4. Emissions due o a ia-
ion ha e been excluded, excep o landings and ake-o s,
as hey do no o igina e nea he su ace and can be expec ed
o ha e li le in luence on su ace and nea -su ace O3. We
used he SPECIATE ool (U.S. EPA, 2016) o specia e NOx
and NMVOC.
Emissions in EDGAR’s in en o y a e classi ied acco d-
ing o hei o igin ollowing he Con en ion on Long-
Range T ansbounda y Ai Pollu ion (CLRTAP) – Nomen-
cla u e o Repo ing (NFR) sec o s (Janssens-Maenhou
e al., 2019). We pe o med he daily and hou ly em-
po al dis ibu ion o emissions using he empo al dis-
ibu ion coe icien s in he LOTOS-EUROS CTM (De-
nie an de Gon e al., 2011). LOTOS-EUROS empo-
al p o iles we e de ined o SNAP (Selec ed Nomencla u e
o Sou ces o Ai Pollu ion) sec o s con empla ed in he
CORINAIR/EMEP me hodology (EEA, 2016). The e o e,
we eg ouped he la e sec o s based on he NFR-SNAP
mapping able (h ps://www.ceip.a / ileadmin/inhal e/ceip/
00_pd _o he /n 09_snap_gn .pd , las access: 20 Ma ch
2024). We used he Spa se Ma ix Ope a o Ke nel Emission
(SMOKE) model (h ps://www.cmascen e .o g/smoke/, las
access: 20 Ma ch 2024) o spa ial disagg ega ion and adap-
a ion o he domains, empo al disagg ega ion, and pollu-
an specia ion.
2.2 Valida ion
To alida e he WRF–CAMx simula ion, we combined he
analysis o he modeling esul s wi h he assessmen o me-
eo ological eanalysis, in pa icula ERA5 eanalysis, me-
eo ological obse a ions, and O3measu ed concen a ions.
This allowed us o e i y he model’s pe o mance wi h ex-
pe imen al da a, no only a he su ace le el bu also a up-
pe le els. Fo compa ison wi h expe imen al da a, we ha e
p e e en ially aken he highe -spa ial- esolu ion ou pu s o
he simula ions.
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4311
2.2.1 ERA5 hou ly eanalysis
We ha e aken as a e e ence o alida ing he me eo o-
logical simula ion he ERA5 eanalysis. ECMWF eleased
a new, imp o ed me eo ological eanalysis, namely ERA5
(He sbach e al., 2023, 2020), wi h a highe spa ial esolu-
ion (0.25° ×0.25°) and highe empo al esolu ion (hou ly)
han ERA-In e im. Due o he di icul y in collec ing me e-
o ological obse a ion da a and he un eliabili y o some o
he da a ound, we ha e decided o use ERA5 eanalysis as a
main e e ence, in addi ion o he selec ion o ou me eo o-
logical s a ions ha we men ion a e wa d, since i inco po-
a es mos o he o icial me eo ological measu emen s made
o his egion. We ha e compa ed ERA5 su ace empe a-
u e and winds (su ace and 750 hPa) wi h he WRF simula-
ion. Wi h all his in o ma ion, we ha e pe o med quali a-
i e compa isons o a p ope unde s anding o he episode.
In addi ion, we ha e examined he isible channel images
om he Me eosa sa elli e, a ailable on NOAA’s Global IS-
CCP B1 B owse Sys em (Knapp, 2008), shown in Fig. S1,
o e alua ing cloudiness and he synop ic e olu ion du ing
he episode.
2.2.2 Su ace and uppe -ai me eo ological
obse a ions
Among he me eo ological obse a ional da a, we ha e com-
pa ed adiosonde da a om Lisbon, San ande , and Bo -
deaux (loca ions shown in Fig. 1) wi h he WRF simu-
la ion, allowing us o e alua e he a mosphe ic e olu ion
a di e en al i udes h oughou he episode. We ga he ed
adiosonde da a om he da abase o he Uni e si y o
Wyoming (h p://wea he .uwyo.edu/uppe ai /bu aob.sh ml,
las access: 28 Ma ch 2022), as i has an ex ensi e compila-
ion o all adiosoundings conduc ed globally.
We compiled su ace obse a ion da a om wo s a-
ions o he Basque Me eo ological Ne wo k (EUSKAL-
MET), one o hem loca ed in Bilbao (Deus o) a sea
le el and ano he one in Oiz a 998 m a.s.l. (Fig. 1).
Fo Po ugal, we collec ed da a om wo s a ions
om he Global Hou ly–In eg a ed Su ace Da abase
(ISD) o he NCEI (h ps://www.ncei.noaa.go /p oduc s/
land-based-s a ion/in eg a ed-su ace-da abase, las access:
28 Ma ch 2022), one in Lisbon a sea le el and he o he in
Penhas Dou adas a 1398 m a.s.l.
2.2.3 Su ace O3measu emen s
To analyze ai quali y in he IP s udy a ea, we used
su ace obse a ion da a o hou ly O3concen a-
ions o Spain and Po ugal. The da abase om
Spain is a ailable a he MITECO websi e (h ps:
//www.mi eco.gob.es/es/calidad-y-e aluacion-ambien al/
emas/a mos e a-y-calidad-del-ai e/calidad-del-ai e/
e aluacion-da os/da os/Da os_o iciales_2018.aspx, las
access: 28 Ma ch 2022), which g oups all he da a om
he ai quali y ne wo ks o he au onomous commu-
ni ies, and o Po ugal om i s Ai Quali y Ne wo k
(h ps://quala .apambien e.p /, las access: 1 May 2021)
p o ided by he Agência Po uguesa do Ambien e. Fo he
s a is ical analysis o he wo egions analyzed in his pape ,
we ha e selec ed he s a ions in NAI om he Spanish
da abase shown in Fig. 1 and o WAI all he s a ions om
he Po uguese da abase.
2.2.4 S a is ical e alua ion o simula ed O3
concen a ions
The unce ain y associa ed wi h he models is de e mined by
compa ing he expe imen al da a (measu emen s) and he e-
sul s o hei simula ions. Se e al s udies ha e de eloped di -
e en me hodologies and he e is cu en ly no s anda dized
me hodology o his pu pose (Bo ego e al., 2008). In ecen
yea s, he use o a se ies o s a is ical indica o s has p e ailed
in he scien i ic e e ences (Bessagne e al., 2016; Oikono-
makis e al., 2018). In his wo k, we ha e chosen o use a se
o me ics commonly employed by he a o emen ioned s ud-
ies desc ibed in Table 2. This has allowed us o compa e he
me ics used he e wi h he wo k o o he au ho s. We ca ied
ou his e alua ion o he selec ed s a ions shown in Fig. 1
o NAI and WAI, also shown wi h hei coo dina es in Ta-
bles S4 and S5.
3 Resul s and discussion
Fi s , we ha e e alua ed he esul s o he me eo ological
model as hey de e mine he pe o mance o he pho ochem-
ical model. Secondly, we ha e assessed he pe o mance o
he simula ed O3concen a ions by CAMx wi h i s s a is i-
cal analysis, con as ing wi h obse a ions. A he end o his
sec ion, we ha e analyzed he e olu ion o he episode om
CAMx esul s and obse a ions o O3concen a ions.
3.1 Me eo ology
The 6-hou ly NCEP Clima e Fo ecas Sys em Reanalysis
(CFSR) his o ical a chi e in We e zen ale (h p://www.
we e zen ale.de/, las access: 20 Ma ch 2024) and he
ERA5 eanalysis a e used o desc ibe synop ic me eo ology.
Su ace and uppe -ai me eo ological obse a ions om a se
o s a ions (Sec . 2.2) a e also discussed in his sec ion in
he con ex o he di e en scales o he me eo ological p o-
cesses wo king oge he du ing he episode and he e en ual
adequacy o he esponse o he WRF model o he obse ed
me eo ology.
3.1.1 Synop ic analysis and uppe -le el winds
Following he NOAA NCEP Clima e Fo ecas Sys em Re-
analysis (CFSR) (Saha e al., 2014), he synop ic condi-
ions du ing he O3episode (2–6 Augus 2018) (Fig. 3)
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4312 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
Table 2. S a is ical me ics used o he pho ochemical simula ion.
S a is ical me ics Equa ion
Mean bias (MB) 1
N
N
P
i=1
(Modeli−Obsi)
Mean e o (ME) 1
N
N
P
i=1
|Modeli−Obsi|
No malized mean bias (NMB)
N
P
i=1
(Modeli−Obsi)
N
P
i=1
Obsi
Roo mean squa e e o (RMSE) s1
N
N
P
i=1
(Modeli−Obsi)2
Index o ag eemen (IOA) 1 −N·RMSE2
N
P
i=1Modeli−Obs+Obsi−Obs2
Pea son co ela ion coe icien ( )
N
P
i=1Modeli−Model·Obsi−Obs
sN
P
i=1Modeli−Model2·sN
P
i=1Obsi−Obs2
we e cha ac e ized by a la ge uppe -le el idge which ex-
ended om no he n A ica o wes e n Eu ope c ossing he
Ibe ian Peninsula and an associa ed la ge a ea o su ace
high p essu es. The su ace an icyclone co e ed he whole
Eu opean A lan ic coas om Scandina ia o Ibe ia. This
p essu e dis ibu ion is compa ible wi h eas o no heas e ly
winds a su ace le els ollowing he coas along he Eu o-
pean A lan ic and wa m sou he lies a uppe le els o e IP,
which b ing e ical s abili y and adequa e condi ions o O3
episodes (“accumula ion pe iods”) in cen al Ibe ia (Que ol
e al., 2018). The eas e ly winds in he ma ine bounda y laye
o he no he n coas o Spain and he sea b eeze inland con-
e gences a e behind mos o he O3episodes in he Basque
Coun y (Gangoi i e al., 2006a). These episodes, hough less
in ense, e en occu ed du ing he COVID-19 lockdown pe-
iod a e a signi ican educ ion o an h opogenic p ecu so s
(Gangoi i e al., 2021), and hey we e a ibu ed o O3impo -
a ion ac oss sou he n F ance and he Bay o Biscay.
The ERA5 eanalysis a he su ace and uppe le els shows
a mo e de ailed wind ield dis ibu ion and ime e olu ion
du ing he ini ia ion o he episode (Fig. S2). Eas e ly winds
in he Bay o Biscay and he no he lies a he coas o Po u-
gal (Fig. S2a.1 and b.1) in he ma ine bounda y laye we e
decoupled om he ela i ely wa m winds a uppe le els
(Fig. S2a.2 and b.2). Tha ai mass ci cula ed an icycloni-
cally a ound IP, a ac ha could ha dly be in e ed om he
CFS eanalysis in Fig. 3, o cing mode a e sou he lies o e
WAI, weak wes e lies o e NAI, and almos calm condi ions
o e he SW coas o F ance on 2 Augus (Fig. S2a.2). Tha
mean a maximum wind di ec ional shea o 180° in bo h
WAI and NAI egions. This ype o uppe -ai an icyclonic
ci cula ions seems o be a key componen o he no he n
A ican middle- oposphe e wind egime, which is behind
he dese dus anspo accumula ion and edis ibu ion in
he egion (Gangoi i e al., 2006b). F om 3 Augus onwa ds,
he e was a change in he wind ield a uppe le els, egis-
e ed by he ERA5 eanalysis: he wa m ai mass ci cula ing
a uppe le els mo ed o he wes , and he wind u ned o
he N o e he eas e n hal o IP. Tha comple ely changed
he a mosphe ic ci cula ion a hose le els, wi h he NAI be-
ing opened o he en y o ai masses o Eu opean o igin,
while he ci cula ion pa e n emained om he sou h o e a
la ge pa o he coas al WAI and om he NE o e sou h-
e n Po ugal (Fig. S2b.2). Howe e , hese changes we e no
obse ed a he su ace le el: a he A lan ic coas o IP, su -
ace winds did no show signi ican changes om he p e i-
ous day (Fig. S2b.1). The new wind con igu a ion a su ace
and uppe le els o e WAI, which las ed o he es o he
O3episode, was mo e simila o hose desc ibed in Gangoi i
e al. (2006a) and Valdeneb o e al. (2011) o he no he n
coas .
The end o he episode s a ed on 6 Augus wi h he de-
elopmen o an uppe -le el ough associa ed wi h a midla -
i ude dep ession o he sou h o Iceland (Fig. 3). The ough,
ma ked wi h a black a ow in he igu e, ex ended o NW
Ibe ia and o ced a su ace (and uppe le el) cold ai mass ad-
ec ion om he no hwes wi h a on al egion (dashed line
in Fig. 3), which c ossed NAI and WAI du ing he ollowing
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4313
Figu e 3. Synop ic condi ions du ing he O3episode wi h geopo en ial heigh a 500 hPa (geopo en ial dm, shaded) and su ace p essu e
(hPa, con ou s) du ing he pe iod o he high-ozone episode (2–7 Augus ). L and H mean “low-p essu e cen e ” and “high-p essu e cen e ”,
espec i ely. Con inuous black lines ep esen he wa me ai mass o e he IP and dashed black lines ep esen he A lan ic ad ec ion.
Sou ce: NCEP CFS eanalysis om h p://www.we e zen ale.de (las access: 20 Ma ch 2024).
24 h. The ERA5 wind and empe a u e da a in Fig. S3 show
he obse ed changes du ing he on al passage: he wes and
no hwes e ly wind ad ec ion s a ed on 6 Augus (Fig. S3a.1
and a.2) and mo ed o he wes du ing he ollowing day, wi h
wes e lies a he su ace and in ense sou hwes e lies a uppe
le els (Fig. S3b.1 and b.2). A simila wind ield dis ibu ion
has been es ima ed by he WRF simula ion du ing he whole
episode a bo h su ace and uppe le els, shown in Sec . 3.3,
in he con ex o he simula ed in e - egional O3 anspo and
dis ibu ion.
Radiosonde wind da a and WRF simula ed e ical p o-
iles a h ee sounding si es (Lisbon, San ande , and Bo -
deaux) o WAI and NAI a e ep esen ed in Fig. 4 o he
“ex ended” O3episode (1–8 Augus 2018). Obse a ions
(le ) and modeled e ical winds ( igh ) ag ee and co e-
spond o he su ace and uppe -ai wind ield eanalysis de-
sc ibed abo e. Du ing he ini ia ion o he episode (1–2 Au-
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4320 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
Figu e 9. Simula ed O3concen a ions (colo scale) and wind ields ( ec o s) by WRF–CAMx in d02 a 16:00 UTC on 1, 2, and 3 Augus
2018. Panels (a.1)–(c.1) show he ozone and wind concen a ion a he su ace and panels (a.2)–(c.2) he in eg a ed ozone concen a ion up
o 2500 m a.g.l. and wind a 1250 m a.g.l. Winds lowe han 2 m s−1ha e been omi ed.
end o 3 Augus caused he en y o new pollu ed ai masses
om F ance, bo h h ough he Bay o Biscay owa ds NAI
and h ough he Gul o Lion owa ds he Medi e anean Sea.
Tha anspo pa hway co obo a es one o he accumula ion
phase anspo pa hways p oposed by Gangoi i e al. (2006a)
and Valdeneb o e al. (2011). In his episode, howe e , he e
was no p e ious g adual accumula ion; i al eady s a ed wi h
an ab up ise in O3concen a ions on he p e ious 2 d du ing
he ini ia ion.
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4321
Figu e 10. Simula ed e ical O3concen a ions (colo scale) and p ojec ed wind ields (s eam composed by p ojec ed uwi h w(×10)) by
WRF–CAMx in d03 a 14:00 and 18:00 UTC on 1, 2, and 3 Augus 2018 o he VNP e ical c oss-sec ion. Concen a ions a e depic ed in
ppb as hey a e al i ude-independen (1 ppb ≈2 µg m−3a sea le el).
Pa icula ly in WAI, du ing 2 and 3 Augus he e is a no-
able inc ease in he O3simula ed concen a ions o e he sea
(Fig. 11c, d, e, and ). We ha e obse ed ha du ing 3 Augus
PM10 measu ed concen a ions in wes e n IP inc eased no-
ably, up o 60 µg m−3. These high concen a ions las ed un il
5 Augus (no shown in his pape ), and hey we e concu en
wi h lowe O3concen a ions in uppe laye s (Fig. 11c, d, e
and ), indica ing a anspo o mine al dus om he Saha a
o he WAI. Fumiga ion p ocesses on 2 Augus would ha e
anspo ed O3 om he uppe laye s o he su ace, whe eas
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4322 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
Figu e 11. Simula ed e ical O3concen a ions (colo scale) and p ojec ed wind ields (s eam composed by p ojec ed wi h w(×10)) by
WRF–CAMx in d03 a 14:00 and 18:00 UTC on 1, 2, and 3 Augus 2018 o he DN e ical c oss-sec ion. Concen a ions a e depic ed in
pa pe billion (ppb) as hey a e al i ude-independen (1 ppb ≈2 µg m−3a sea le el).
on 3 Augus i would be dus ins ead o O3(see he dus loca-
ion on he sa elli e map in Fig. S1). Those umiga ion p o-
cesses could in oduce O3and PM in o he sea–land eci -
cula ion cells, causing high measu ed concen a ions o bo h
pollu an s simul aneously.
In he VNP, o e NAI, an in usion o pollu ed ai h ough
he Bay o Biscay om he no h, o F ench o igin, was ob-
se ed, causing O3concen a ions o 100 µg m−3(50 ppb)
om he su ace le el o 1500 m a.s.l. (Fig. 10e and ). Ad-
di ionally, we obse ed how local pollu an s emi ed on he
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4323
coas line impac ed he inland alleys, p oducing mo e han
160 µg m−3o O3(Fig. S9). Thus, a signi ican local con-
ibu ion was added o he al eady exis ing egional ans-
po o pollu ed ai masses. In DN, o e WAI, he pa e n
o in low wi h he sea b eeze and impac on inland a eas
was epea ed as shown in Fig. S10, also documen ed in o he
s udies (E yugina e al., 2007; Mon ei o e al., 2012, 2016;
To e-Pascual e al., 2023).
Du ing 4 and 5 Augus he simula ed su ace O3con-
cen a ions again exceeded 130 µg m−3on NAI and WAI
(no shown). The anspo o pollu an s om F ance o
NAI h ough he Gul o Biscay and o he Medi e anean
Sea h ough he Gul o Lion con inued du ing hose days
and cycles o sea b eezes we e epea ed. In he case o he
Dou o No e s a ion, we obse ed he anspo o pollu ed
ai masses om he coas in o ha a ea due o he sea b eeze.
A sudden ise in O3hou ly concen a ions occu ing on 3, 5,
and 6 Augus was caused by he impac s o hese ai masses,
bu no wi h such in ensi y on 4 Augus (Fig. S10) because
o a mo e sou he ly ajec o y du ing ha day o he pollu ed
ai mass (no shown). The app oach o he cold on du ing
5 Augus caused p e on al winds o W-SW componen o e
IP ha ini ia ed he anspo o all pollu an s om he W o
he E o IP.
3.3.3 Dissipa ion
The A lan ic ai mass en e ed IP on 6 Augus , in oduc-
ing cloudiness (Fig. S1) and p oducing a wind shi o W
o e WAI. Tha change in oduced cleane ai o he wes
o he peninsula (Fig. 12). Howe e , in NAI, O3-pollu ed
ai masses coming om he W and SW o Ibe ia we e
anspo ed wi h he p e on al winds, adding O3 o he one
p e iously accumula ed days be o e (Fig. 12a.1 and a.2).
Tha si ua ion caused high simula ed O3su ace concen a-
ions (>130 µg m−3) and he highes obse a ions o e NAI
(Figs. 7 and 8). The passage o he on al sys em gene -
a ed a simula ed “ozone on ” also con i med by he mea-
su emen s. The inal en y o cleane ai om he A lan ic
Ocean du ing 7 Augus signi ican ly educed O3concen a-
ions bo h a he su ace and a al i ude in he NW egion
o IP (Fig. 12b.1 and b.2). In he E o IP and he wes e n
Medi e anean Basin, highe concen a ions we e s ill ound
in he simula ions, indica ing a possible episode du ing he
ollowing days in ha e i o y.
4 Conclusions
This pape analyzes a oposphe ic O3pollu ion episode ha
occu ed on he A lan ic coas o IP, speci ically o e WAI
and NAI, in Spain and Po ugal du ing 2–6 Augus 2018.
The episode was cha ac e ized by an almos simul aneous
ab up ise in O3concen a ions in bo h egions, which e-
mained high h oughou he en i e episode, exceeding he a -
ge alues and he in o ma ion h eshold o he EC/50/2008
EU Di ec i e. Using he me eo ological and pho ochemical
WRF–CAMx modeling sys em, we ha e iden i ied he ans-
po mechanisms behind his ype o episode, which is espe-
cially complex due o a me eo ology cha ac e ized by a pe -
manen wind shea h oughou he en i e pe iod. Addi ion-
ally, we ha e been able o cha ac e ize he possible sou ces
o pho ochemical pollu an s a ec ing hese wo a eas.
The episode began wi h an accumula ion o pollu an s in
he highe laye s abo e 2000 m a.g.l. o e IP due o a de-
coupling o high-al i ude and su ace ai masses. The o igin
o ha uppe -le el pollu ed ai mass was p obably he emis-
sion o pollu an s du ing p e ious days in IP i sel , which
we e hen apped below 2500 m a.s.l. due o he s abili y o
he uppe wa me ai . Subsequen ly, uppe ai masses umi-
ga ed on o he su ace h ough he di e en o og aphic chim-
neys along he A lan ic coas , p oducing he beginning o he
episode. Du ing he ini ia ion o he episode, he simula ion
poin ed ou ha he dominan p ocess was likely o be umi-
ga ion, wi h a con ibu ion o 30–40 µg m−3o he obse ed
O3inc ease. Measu ed O3maximum daily concen a ions in-
c eased mo e han 40 µg m−3 om he p e ious day’s highs
and simula ed O3mo e han 30 µg m−3.
F om 3 Augus onwa ds, he umiga ed ai masses we e
joined by o he pollu ed ai masses. Acco ding o he simu-
la ion, NAI ecei ed O3-pollu ed ai masses impo ed om
F ance, p o iding a minimum o 100 µg m−3backg ound O3
concen a ions, while WAI ecei ed O3 om he N and cen e
o IP, p obably sha ing he same minimum backg ound con-
ibu ion because o he con inui y o hose ai masses. The
mos in ense exceedances occu ed in he sea- acing slopes
o he main coas al anges a Valde ejo (Basque Coun y,
Spain) and Dou o No e (Po ugal) measu emen s a ions.
A hese si es, he e is an addi ional impac o local coas al
emissions o he al eady exis ing high backg ound concen-
a ions (100 µg m−3). Tha local con ibu ion, in oducing
“ esh” pollu an s inland wi h he sea b eezes, p oduced con-
cen a ions abo e 130 µg m−3o O3in he o m o a peak
in bo h measu ed and simula ed concen a ions, indica ing
a local con ibu ion o a leas 30 µg m−3o O3a bo h loca-
ions. Those concen a ions, as well as he anspo pa hways
obse ed h ough he simula ions, showed ha du ing he
episode he e we e di e en con ibu ions and in e - ela ed
anspo p ocesses: i s , an O3 umiga ion and in e - egional
anspo (wi hin IP) du ing 2 Augus and hen, om 3 Au-
gus onwa ds, a con inen al Eu opean O3 anspo and con-
cu en accumula ion wi hin coas al ci cula ions. The dissi-
pa ion o he episode occu ed g adually om W o E due o
an A lan ic ad ec ion, which in oduced colde and cleane
ai . A e he on passed h ough, pollu an s we e ca ied
om W o E, causing maximum hou ly concen a ions ha
we e signi ican p io o he episode dissipa ion.
CAMx simula ion con o med o he s a is ical pa ame e s
adi ionally used wi h hese models. We ha e in oduced o
he i s ime he analysis o winds a al i ude and he cal-
cula ion o in eg a ed O3concen a ions o a deepe unde -
h ps://doi.o g/10.5194/acp-24-4305-2024 A mos. Chem. Phys., 24, 4305–4329, 2024
4324 E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula
Figu e 12. Simula ed O3concen a ions (colo scale) and wind ields ( ec o s) by WRF–CAMx in d02 a 16:00 UTC on 6 and 7 Augus
2018. Panels (a.1)–(b.1) show he ozone and wind concen a ion a he su ace and panels (a.2)–(b.2) he in eg a ed ozone concen a ion up
o 2500 m a.g.l. and wind a 1250 m a.g.l. Winds lowe han 2 m s−1ha e been omi ed.
s anding o his episode. These newly p oposed analyses a e
necessa y o unde s and ai pollu ion episodes in a eas wi h
complex opog aphy whe e eci cula o y p ocesses can oc-
cu in bo h he uppe and lowe a mosphe e. They allowed
us o obse e he medium- and long- ange anspo s o pol-
lu ed ai masses, abs ac ing om local e ec s, and, in u n,
whe he he inc eases in O3concen a ions we e due o ai
mass ho izon al ad ec ions, umiga ions, o a combina ion
o bo h.
In iew o he di e si y o p ocesses in ol ed in his ype
o O3episode, he au ho s o his a icle ecommend ex end-
ing he analysis o modeling s udies o uppe le els o he
a mosphe e, pa icula ly in complex e ain applica ions and
wi h complex me eo ological si ua ions such as his case. In
o de o imp o e p edic ions as well as con ol s a egies,
da abases o obse a ions should be expanded a he su ace
and uppe le els o he a mosphe e. WAI su ace s a ion mea-
su emen s ha e p o en o be ep esen a i e o e alua ing he
episode and ag ee wi h he simula ions. Meanwhile, in NAI,
measu emen s a ions migh no be e y ep esen a i e o ad-
d ess his kind o episode due o hei p oximi y o indus-
ial si es. In uppe le els o he a mosphe e, O3soundings
and lida , among o he echniques o he cha ac e iza ion
o he e ical ozone dis ibu ion, should be used o u he
analyze he anspo pa hways and accumula ion p ocesses
add essed in his pape .
Code a ailabili y. The code we ha e u ilized is open-sou ce and
mainly consis s o he men ioned WRF and CAMx models. Fo he
g aphical ep esen a ions we ha e used he G id Analysis and Dis-
play Sys em (G ADS) a ailable a h p://cola.gmu.edu/g ads/ (las
access: 20 Ma ch 2024; Do y and Kin e , 1995). The WRF 3.9.1.1
sou ce code is a ailable a h ps://www2.mmm.uca .edu/w /use s/
download/ge _sou ce.h ml (las access: 28 Ma ch 2022; Skama ock
e al., 2008), and he CAMx 6.5 sou ce code is a ailable a h ps:
//www.camx.com/download/sou ce/ (las access: 28 Ma ch 2022;
Ramboll En i onmen and Heal h, 2018). Should he e be a need
o any o he in e media e p ocessing p og ams, please con ac he
co esponding au ho .
A mos. Chem. Phys., 24, 4305–4329, 2024 h ps://doi.o g/10.5194/acp-24-4305-2024
E. To e-Pascual e al.: O3episode on he A lan ic coas o he Ibe ian Peninsula 4325
Da a a ailabili y. The model g aphical hou ly ou pu is a chi ed
on he Zenodo da abase (h ps://doi.o g/10.5281/zenodo.10886332,
To e-Pascual, 2024).
Supplemen . The supplemen ela ed o his a icle is a ailable
online a : h ps://doi.o g/10.5194/acp-24-4305-2024-supplemen .
Au ho con ibu ions. ETP: concep ualiza ion, da a cu a ion,
o mal analysis, in es iga ion, so wa e, isualiza ion, o iginal d a
p epa a ion, e iew and edi ing. GG: concep ualiza ion, da a cu a-
ion, o mal analysis, in es iga ion, so wa e, isualiza ion, o igi-
nal d a p epa a ion, e iew and edi ing. ARG: concep ualiza ion,
da a cu a ion, o mal analysis, in es iga ion, so wa e, isualiza ion,
o iginal d a p epa a ion, e iew and edi ing. ESdC: concep ual-
iza ion, da a cu a ion, in es iga ion, so wa e, isualiza ion, o ig-
inal d a p epa a ion, e iew and edi ing. JF: concep ualiza ion,
da a cu a ion, in es iga ion, isualiza ion, o iginal d a p epa a ion,
e iew and edi ing. CG: concep ualiza ion, da a cu a ion, in es i-
ga ion, isualiza ion, o iginal d a p epa a ion, e iew and edi -
ing. MCG: concep ualiza ion, in es iga ion, isualiza ion, o iginal
d a p epa a ion, e iew and edi ing. IZ: concep ualiza ion, da a
cu a ion, in es iga ion, isualiza ion, o iginal d a p epa a ion, e-
iew and edi ing. JAG: concep ualiza ion, in es iga ion, isualiza-
ion, o iginal d a p epa a ion, e iew and edi ing. MdB: concep-
ualiza ion, in es iga ion, isualiza ion, o iginal d a p epa a ion,
e iew and edi ing.
Compe ing in e es s. The con ac au ho has decla ed ha none
o he au ho s has any compe ing in e es s.
Disclaime . Publishe ’s no e: Cope nicus Publica ions emains
neu al wi h ega d o ju isdic ional claims made in he ex , pub-
lished maps, ins i u ional a ilia ions, o any o he geog aphical ep-
esen a ion in his pape . While Cope nicus Publica ions makes e -
e y e o o include app op ia e place names, he inal esponsibili y
lies wi h he au ho s.
Acknowledgemen s. The au ho s wish o hank he Depa men
o Economic De elopmen , Sus ainabili y and he En i onmen o
he Basque Go e nmen (CONV 22/16) and he Uni e si y o he
Basque Coun y UPV/EHU as he sou ce o ou main inancial
suppo : GIA consolida ed Resea ch G oups (GIU21/050) (h ps:
//www.ehu.eus/es/web/gia, las access: 20 Ma ch 2024). This is in
addi ion o he Spanish Minis y o Uni e si ies and he Eu opean
Union o he Ma ga i a Salas g an (MARSA21/23) o Edua do
To e-Pascual, unded by he Eu opean Union – Nex Gene a ion
EU. These inancing bodies ha e played an exclusi ely economic
ole in he s udy.
Thanks a e also due o he inancial suppo o he con-
ac g an s o Joana Fe ei a (2020.00622.CEECIND) and Ca la
Gama (2021.00732.CEECIND), o CESAM (UIDP/50017/2020+,
UIDB/50017/2020+, and LA/P/0094/2020), and o FCT/MCTES
h ough na ional unds and he co- unding by he FEDER wi hin
he PT2020 Pa ne ship Ag eemen and Compe e 2020.
Financial suppo . This esea ch has ecei ed inancial sup-
po om he Uni e si y o he Basque Coun y (UPV/EHU)
h ough g an GIU21/050, he Depa men o Economic De-
elopmen , Sus ainabili y and he En i onmen o he Basque
Go e nmen h ough g an CONV22/16, and Edua do To e-
Pascual’s Ma ga i a Salas g an (g an no. MARSA21/23).
Funds we e also ecei ed om FCT/MCTES h ough na-
ional unds o suppo CESAM (g an nos. UIDP/50017/2020+,
UIDB/50017/2020+, and LA/P/0094/2020) and he con ac s o
Joana Fe ei a (g an no. 2020.00622.CEECIND) and Ca la Gama
(g an no. 2021.00732.CEECIN).
Re iew s a emen . This pape was edi ed by Rol Mülle and e-
iewed by h ee anonymous e e ees.
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