Exploring the influence of land use on the urban carbonyl sulfide budget: a case study of the metropolitan area of Barcelona

Abs ac Ca bonyl sul ide (OCS) is used o quan i y he ca bon cap u e po en ial o he biosphe e because o i s di ec co ela ion wi h CO2 up ake du ing pho osyn hesis. Howe e , o cons ain he u ban biosphe e signal, i is necessa y o e alua e po en ial an h opogenic sou ces. We conduc ed wo sampling campaigns in he Me opoli an A ea o Ba celona (AMB), Spain, du ing May ( ull COVID lockdown) and Oc obe 2020 o measu e he spa ial dis ibu ion and a iabili y o OCS in ou u ban land uses as ollows: buil , u ban o es , u ban pa k, and pe i-u ban ag icul u e. The OCS backg ound le els de e mined a Tibidabo (442m asl) we e app oxima ely 484±20 pp and 407±8 pp o May and Oc obe 2020, espec i ely, and ag eed wi h o he seasonal su eys conduc ed in Eu ope du ing ha same pe iod. A e aged emissions we e in he ange o +Δ12±40 pp o he ci y and we e +Δ9.4±40.9 pp o u ban +Δ22.1±48 pp o u ban g een +Δ20.7±42.9 pp o ag icul u al and −Δ4.8±19.6 pp o o es . The u ban alues anged om neu al o abo e backg ound, sugges ing nea by an h opogenic and ma ine emissions such as +Δ150 pp in Mon juic, which is downwind o Ba celona's ha bo . Du ing he c op-g owing season in May, he ag icul u al a eas consis en ly showed alues below he backg ound (up o −Δ76 pp in Ga à, up ake) a 7:00 UTC when he land b eezes we e dominan , while la e in he mo ning, when he sea b eeze a e de eloped, he plan sink is masked by he anspo o ma ine emissions. U ban o es s loca ed no h o Tibidabo showed OCS alues up o −Δ70 pp , sugges ing signi ican up ake by u ban o es s. We conclude ha de e mining he u ban biosphe e signal using OCS as a ace is mo e complex han expec ed because he ma ine and an h opogenic emissions om he po s ongly impac he spa ial- empo al dis ibu ion o OCS. Plain Language Summa y Ca bonyl sul ide (OCS) is a gas used o quan i y he ca bon cap u e po en ial o he biosphe e. Howe e , ci ies ha e po en ial an h opogenic OCS sou ces ha can impede a clea cons ain o he u ban biosphe e signal. We conduc ed wo sampling campaigns in he Me opoli an A ea o Ba celona (AMB), Spain, du ing May ( ull COVID lockdown) and Oc obe 2020 o measu e he a iabili y o OCS in ou u ban land uses: buil , u ban o es , u ban pa k, and pe i-u ban ag icul u e. The OCS backg ound le els de e mined a Tibidabo (we e app oxima ely 484±20 pp and 407±8 pp o May and Oc obe 2020, espec i ely). The u ban buil si es we e ypically abo e backg ound, sugges ing nea by an h opogenic and ma ine emissions. Du ing he c op-g owing season, he ag icul u al a eas consis en ly showed alues below he backg ound sugges ing plan OCS up ake when he land b eezes we e dominan , while la e in he mo ning, when he sea b eezes de eloped, he in luence o ma ine emission esul ed in alues abo e he backg ound. U ban o es s showed signi ican OCS up ake. We conclude ha de e mining he u ban biosphe e signal using OCS as a ace is mo e complex han expec ed because he ma ine and an h opogenic emissions om he po s ongly impac he spa ial- empo al dis ibu ion o OCS. ESTRUCH ETAL. © 2023. The Au ho s. This is an open access a icle unde he e ms o he C ea i e Commons A ibu ion-NonComme cial-NoDe i s License, which pe mi s use and dis ibu ion in any medium, p o ided he o iginal wo k is p ope ly ci ed, he use is non-comme cial and no modi ica ions o adap a ions a e made. Explo ing he In luence o Land Use on he U ban Ca bonyl Sul ide Budge : A Case S udy o he Me opoli an A ea o Ba celona Ca me Es uch1,2 , Sau eu Bel iso3 , Alba Badia1 , Ve onica Vidal1,4 , Roge Cu coll1,5, Mi eia Udina6 , Claudia G ossi5 , Josep-An on Mo guí1,7, Rica d Segu a1, Se gi Ven u a1, Yolanda Sola6 , and Ga a Villalba1,8 1Ins i u de Ciència i Tecnologia Ambien als (ICTA), Uni e si a Au ònoma de Ba celona, Ba celona, Spain, 2Eu eca , Cen e Tecnològic de Ca alunya, Clima e Change Resea ch Depa men , Ampos a, Spain, 3Labo a oi e des Sciences du Clima e de l’En i onnemen , LSCE, Pa is-Saclay, F ance, 4Depa men o Compu e A chi ec u e and Ope a i e Sys ems, Uni e si a Au ònoma de Ba celona (UAB), Ba celona, Spain, 5Ins i u e o Ene gy Techniques, Uni e si a Poli ècnica de Ca alunya, Ba celona, Spain, 6Depa amen de Física Aplicada–Me eo ologia, Uni e si a de Ba celona, Ba celona, Spain, 7Depa amen de Biologia E olu i a, Ecologia i Ciències Ambien als, Uni e si a de Ba celona, Ba celona, Spain, 8Depa men o Chemical, Biological and En i onmen al Enginee ing, Uni e si a Au ònoma de Ba celona (UAB), Ba celona, Spain Key Poin s: • The u ban buil si es we e ypically abo e backg ound, sugges ing nea by an h opogenic and ma ine emissions • Ag icul u al a eas consis en ly showed alues below he backg ound sugges ing plan Ca bonyl sul ide (OCS) up ake when he land b eezes we e dominan • De e mining u ban biosphe e signal using OCS is complex because ma ine and an h opogenic emissions impac i s spa ial- empo al dis ibu ion Suppo ing In o ma ion: Suppo ing In o ma ion may be ound in he online e sion o his a icle. Co espondence o: G. Villalba, [email p o ec ed] Ci a ion: Es uch, C., Bel iso, S., Badia, A., Vidal, V., Cu coll, R., Udina, M., e al. (2023). Explo ing he in luence o land use on he u ban ca bonyl sul ide budge : A case s udy o he me opoli an a ea o Ba celona. Jou nal o Geophysical Resea ch: A mosphe es, 128, e2023JD039497. h ps://doi. o g/10.1029/2023JD039497 Recei ed 23 JUN 2023 Accep ed 26 NOV 2023 Au ho Con ibu ions: Concep ualiza ion: Ca me Es uch, Sau eu Bel iso, Ga a Villalba Da a cu a ion: Josep-An on Mo guí, Ga a Villalba Funding acquisi ion: Ga a Villalba In es iga ion: Ca me Es uch, Sau eu Bel iso, Alba Badia, Ve onica Vidal, Roge Cu coll, Mi eia Udina, Claudia G ossi, Josep-An on Mo guí, Rica d Segu a, Se gi Ven u a, Yolanda Sola, Ga a Villalba 10.1029/2023JD039497 RESEARCH ARTICLE 1 o 21 Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 2 o 21 1. In oduc ion Ca bonyl sul ide (he ea e OCS) is an a mosphe ic ace gas used o s udy he CO2 seques a ion capaci y o ege a ion (Remaud e al.,2023; Whelan e al.,2018) because i is des oyed h ough pho osyn hesis in he same way as CO2 wi hou p oducing any emissions h ough espi a ion (P o oschill-K ebs & Kesselmeie ,1992). Acco ding o he la es es ima ions, e es ial ege a ion and soils akes up om 530 o 670GgSy −1 (Remaud e al.,2023) he la ges na u al sink o a mosphe ic OCS. Su ace oceans a e he mos impo an na u al sou ce o OCS con ibu ing di ec ly and indi ec ly, ha is, om he a mosphe ic oxida ion o ca bon disul ide and dime hylsul ide, be ween 270 and 320 (Lenna z e al.,2017). The global budge o OCS is pa ially balanced by an h opogenic emissions, which a e es ima ed o be 400GgSy −1 (Campbell e al.,2015). Te es ial sinks and na u al and an h opogenic sou ces a e gene ally sepa a ed in space and ime, and exhibi dis inc seasonal and spa ial a ia ions (Remaud e al.,2023). Hence, land use changes a e expec ed o ha e a signi ican in luence on he global budge o a mosphe ic OCS. Al hough many s udies ha e e alua ed OCS budge s a egional and global scales, li le is known abou OCS sou ces and sinks a he u ban scale. Unde s anding land use in luence on he budge o a mosphe ic OCS a he u ban scale is much mo e di icul o es ablish because sou ces and sinks can coexis o e small dis ances. Fo example, Commane e al.(2013) indi ec ly assessed OCS sou ces along he highly u banized coas line o Cali- o nia whe e hey de ec ed biogenic OCS emissions om we lands in Sac amen o Bay, an h opogenic emissions nea a s o age acili y and e ine y in he po o Los Angeles, and OCS loss du ing he day ime due o pho osyn- he ic up ake in ege a ed a eas. Besis e al.(2021) also conduc ed a s udy ha iden i ied po en ial sou ces o OCS a he ci y le el and obse ed he spa ial he e ogenei y in luencing OCS mixing a ios o Thessaloniki, G eece, and iden i ied h ee loca ions whe e OCS concen a ions in inne -ci y a eas we e h ee imes highe han hose a he sea on and whe e he was e managemen sys em was a sou ce o OCS. Acco ding o Zumkeh e al.(2018), an h opogenic emissions o OCS due o aluminum, esiden ial and indus ial coal, TiO2, sol en s and i es in he Me opoli an A ea o Ba celona con ibu e o 65% (178MgSy −1) o he o al emissions in he Ca alonia egion (276MgSy −1, see Figu e S1 in Suppo ing In o ma ionS1). Howe e , a s udy by Bel iso e al.(2023) demon- s a ed ha he Zumkeh e al.(2018) in en o y o F ance la gely o e es ima ed OCS an h opogenic emissions by app oxima ely an o de o magni ude. A p e ious s udy by he same g oup used di ec measu emen s om a sampling owe and compu ed back ajec o ies wi h he FLEXPART anspo model o ind ha he Pa is subu - ban a ea was ac ing mainly as an OCS sink (Bel iso e al.,2022). Ano he u ban s udy o Innsb uck, Aus ia, measu ed he hou ly exchange a es o OCS wi h an eddy co a iance owe and showed ha u ban OCS emissions a e signi ican ly highe a noon du ing weekdays han du ing weekends, which sugges s ha oad a ic could be a ne sou ce o OCS emissions. The use o OCS as a pho osyn hesis ace has ecen ly ecei ed a en ion as a po en ial me hod o de e mine he con ibu ion o he u ban biosphe e owa d educing he ca bon oo p in (Mallik e  al., 2016; Villalba e al.,2021). Mallik e al.(2016) epo ed he i s eco ds o annual OCS mixing a ios o e an Indian ci y, which we e subs an ially highe han oposphe ic alues, and hose alues changed seasonally wi h an obse ed 86% d op in OCS in No embe when ai masses changed om oceanic o con inen al. Villalba e al.(2021) also obse ed changes in OCS ela ed o ai masses coming om ma ine and con inen al sou ces. This wo k highligh ed he di icul y o cha ac e izing he biosphe e signal when he inpu masses a e con inen al due o he he e ogenei y, hey p esen in OCS con en compa ed o s able ma ine condi ions. This s udy aims o con ibu e o he unde s anding o he dynamics o OCS budge s in u ban a eas by explo ing o wha ex en di e en u ban land uses can ha e an impac on a mosphe ic OCS mixing a ios in e ms o he pa icula u ban clima e, geog aphy, and opog aphy a ec ing he anspo and mixing o OCS sou ces and sinks. Mo i a ed by he p esen lack o in si u OCS con inuous measu emen s o p ope ly in e su ace OCS (Remaud e al.,2022), we conduc ed an OCS measu emen campaign in he Me opoli an A ea o Ba celona (AMB). We selec ed ou di e en land uses as sampling si es as ollows: o es , ag icul u al land, u ban pa ks and impe ious a eas wi h he in en ion o unde s anding he ole o u ban land use on he OCS budge in a coas al ci y whe e we would expec a mixing a io g adien be ween he sea (whe e OCS is p oduced) and he land (OCS cap u e in ege a ed a eas). The selec ion o sampling a eas was based on hei po en ial as a sink o OCS based on pho osyn he ic a es (Campbell e al.,2017; Villalba e al.,2021; Yang e al.,2018) o a local sou ce based on an h opogenic in en o ies (Lee & B imblecombe,2016; Yan e al.,2019; Zumkeh e al.,2018). The OCS sampling campaigns we e unded and conduc ed wi hin he p ojec In eg a ed Sys em Analysis o U ban Me hodology: Ca me Es uch, Ga a Villalba P ojec Adminis a ion: Ga a Villalba Resou ces: Ga a Villalba Supe ision: Sau eu Bel iso, Ga a Villalba Visualiza ion: Ca me Es uch, Ve onica Vidal, Roge Cu coll, Ga a Villalba W i ing – o iginal d a : Ca me Es uch, Ga a Villalba W i ing – e iew & edi ing: Ca me Es uch, Sau eu Bel iso, Alba Badia, Roge Cu coll, Mi eia Udina, Claudia G ossi, Ga a Villalba 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 3 o 21 Vege a ion and Ag icul u e (URBAG), which was inanced by he Eu opean Resea ch Council wi h he o e all aim o unde s anding he in luence o g een in as uc u es on he u ban CO2 budge . As a coas al ci y wi h in ense sea b eezes du ing he day, he AMB is an ideal case s udy o de e mine he con ibu ion o he u ban biosphe e and o he an h opogenic sou ces o he o e all u ban OCS budge . The campaigns we e conduc ed du ing he sp ing and ea ly au umn o 2020 and almos coincided wi h he OCS sp ing maximum and au umn minimum (Mon zka e al.,2007). Fu he mo e, he May campaign ook place du ing he ull lockdown due o he COVID pandemic, which helped cons ain he biosphe e signal gi en he educ ion in an h opogenic ac i i ies ha a e po en ial sou ces o OCS such as ehicle exhaus (Zumkeh e al.,2017). We analyzed he obse ed OCS concen a ions in e ms o land-use in luence while unde s anding he ole o local wea he condi ions, synop ic e en s, and he plane a y bounda y laye by using a mosphe ic anspo models, clima e obse a ions and a mos- phe ic ace s. Mo e speci ically, we a emp ed o answe he ollowing ques ion: Wha is he land-use in luence on he u ban OCS budge ? 2. Me hods 2.1. S udy Si e The wo expe imen al campaigns ook place in he Me opoli an A ea o Ba celona (AMB), loca ed in he NE co ne o Spain, as shown in Figu e1. The AMB, wi h a o al su ace a ea o app oxima ely 640km 2 and o e 3.3 million inhabi an s (16,160 people/km 2), is he se en h la ges u ban a ea and eigh h mos densely Figu e 1. (a) A map o Spain showing he Me opoli an A ea o Ba celona (AMB, ed ec angle). O og aphy o he AMB (b) and land use (c), whe e iplica es o colo ed do s ep esen he upwind, a ge , and downwind sampling loca ions (d). Backg ound a mosphe ic OCS le els we e in es iga ed a he upwind sampling poin s a he si es o Tibidabo (highes al i ude in he PBL), Ga à and Poble Nou (closes o he sho e). These si es a e depic ed wi h colo ed do s wi h whi e ci cles in b and c. The ed diamond ep esen s he loca ion o ICTA and he blue one whe e he adiosondes we e launched and he adon measu emen s we e pe o med (c). 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 4 o 21 popula ed egion in he Eu opean Union. I is also one o he mos indus ialized a eas in he wes e n EU (Que ol e al.,2004), specializing in ood p oduc ion, chemicals, ehicle manu ac u ing, and gas p oduc ion and dis i- bu ion (Ins i u d’Es adís ica de Ca alunya,2023). The AMB is bounded by he Besós Ri e in he no heas , he Llob ega Ri e in he wes , he Collse ola moun ain ange o he no h, and he Medi e anean Sea o he sou h, as shown in Figu e1. The AMB has a Medi e anean d y subhumid clima e wi h an annual p ecipi a ion o app oxima ely 600mm, which is mainly concen a ed du ing sp ing and au umn, and a mean annual empe a u e o 16.5°C. The o og aphy o he ci y ex ends om sea le el o 516m abo e sea le el (a.s.l.) a he moun ain ange loca ed no hwes o he main u ban a eas. Mo e han hal o he AMB's su ace a ea is co e ed by ege a ion in a ious o ms, including u ban o es (43%), ag icul u al ields (8%) and u ban pa ks (4%), while 41% is co e ed by impe ious and o he in as uc u es (4%). Land uses a e shaped by he ollowing physical cons ain s: c ops a e loca ed in he coas al plains o med by he del a o he Llob ega Ri e , while u ban o es s composed o pine and holm oaks a e loca ed in he Collse ola moun ain ange (Figu e1). U ban g een spaces a e sca e ed h oughou he me opolis and popula ed wi h o na- men al species anging om acacias o plan ain ees and o namen al sh ubs and g asses. 2.2. Field Campaigns Ai samples we e aken a he eigh si es shown wi h colo ed do s in Figu e1c, which we e selec ed o ep e- sen he he e ogenei y o land use h oughou he AMB and a e summa ized in o ou land-use ca ego ies (see Table1): ag icul u al a ea (loca ed in he Ga à and P a a eas), u ban o es (Tibidabo and Collse ola), u ban g een (Mon juic and Guina dó) and u ban (Sag ada Familia and Poble Nou). We ook ai samples a each si e (he ea e e e ed o as he a ge si e) and upwind and downwind o each a ge si e in an a emp o ollow he ai mass low du ing i s ajec o y o e he a ge si e. The upwind and downwind sampling si es we e de e - mined based on his o ical wind pa e ns and ajec o ies ypical o each a ge loca ion unde simila a mosphe ic condi ions expec ed du ing he campaign and a e a ailable om he Me eo ological Se ice o Ca alonia (www. me eo.ca ) and shown in Tex S1 in Suppo ing In o ma ionS1. No e ha in May and Oc obe 2020, obse ed Campaign Si e (ele a ion) Land use ype Da e CWTs a SWC b PBLH ange (m) OCS 7:00 UTC (pp ) OCS 9:30 UTC (pp ) May 2020 Tibidabo (175–442m) Fo es 19/05 C N 19–441 478±20 494±9 Ga à (0–2m) Ag icul u al 21/05 U N 11–249 482.1±NA 495.5±10.7 Poble Nou (0–4m) U ban 21/05 U N 44–2,217 461±20 NA Mon juic (42–98m) U ban g een 20/05 C N 327–883 485±14 490±5 P a (0–2m) Ag icul u al 25/05 E E 737–1,182 502±NA 508±2 Sag ada (37–55m) U ban 26/05 E E 771–1,900 460±6 NA Collse ola (107–442m) Fo es 27/05 E E 395–1,079 441±39 497±3 Guina dó (115–169m) U ban g een 28/05 A SE 590–1,524 505±8 612±17 Oc obe 2020 Ga à (0–2m) Ag icul u al 23/10 U NW 21–5,019 401±9 403±3 Poble Nou (0–4m) U ban 15/10 CN N 287–571 405±23 426±45 Guina dó (115–169m) U ban g een 19/10 ASW SW 69–934 389±20 392±16 Sag ada (37–55m) U ban 20/10 SW SW 29–1,085 444±19 460±93 Mon juic (42–98m) U ban g een 21/10 CSW SW 92–734 425±NA 485±87 P a (0–2m) Ag icul u al 22/10 SW SW 103–495 468±24 490±37 No e. An unab idged e sion o his able wi h upwind and downwind OCS including a de ailed da a o backg ound measu emen s concen a ions (see Figu e4) is a ailable in Tex S1 in Suppo ing In o ma ionS1. aCWT: Ci cula ion wea he ype. The di e en ypes o CWTs a e cyclonic (C), an icyclonic (A), pu e ad ec i e (N, S, E, W, NE, SE, NW, SW), hyb id cyclone-ad ec i e (CN, CNE, CE, CSE, CS, CSW, CW, CNW), hyb id an icyclone-ad ec i e (AN, ANE, AE, ASE, AS, ASW, AW, ANW, and unde ined (U)). bSWC: Synop ic wind componen summa izing he p essu e pa e n, on s, wind di ec ion and speed. Table 1 Ci cula ion Wea he Type (CWT), Synop ic Wind Componen (SWC) and PBLH Ex ac ed F om WRF Simula ions Exp essed in Me e s Abo e G ound Le el (a.g.l), A e aged OCS Concen a ions o All Measu emen s Taken Each Day Du ing he May and Oc obe Campaigns, and Exp essed in pp (Mean Values F om he Th ee Si es, Mean Values±S anda d De ia ions) 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 5 o 21 wind pa e ns some imes di e ed om his o ical da a. The loca ions o he upwind, a ge , and downwind si es a e shown o each poin using he same colo in Figu e1c. The spa ial dis ance among he h ee sampling poin s o each si e anges om 500 o 4,000m. Sampling imes we e chosen o cap u e inland winds du ing he ea ly mo ning (i.e., 4:00 and 7:00 UTC am) and he ypical sea b eezes ha de elop du ing he mo ning and ea ly a e - noon (i.e., 9:30 and 11:00 UTC am). In each campaign, each place was measu ed one day, a he s ipula ed hou s. The i s campaign ook place be ween 18 May and 28 May 2020, which was du ing a pe iod o in ense anspo es ic ions due o he COVID-19 pandemic, and esul ed in highly signi ican ai pollu ion educ ions (Badia e al.,2021). An h opogenic emissions d as ically dec eased, especially o he a ia ion (95%), oad anspo (up o 85%), and mo e mode a ely acco ding o shipping (21%) and indus y (11%) sec o s (Gue a a e al.,2021). The second campaign ook place om he 13 o he 27 o Oc obe 2020, when he an h opogenic ac i i y e u ned o no mal. The empe a u e anged om 20°C o 24°C du ing May and 14°C o 20°C du ing Oc obe . Samples we e collec ed a 7:00 UTC and 9:30 UTC, and o some speci ic days chosen a he beginning and a he end o he campaign, we pe o med addi ional sampling a 4:00 UTC and 11:00 UTC o he Tibidabo, Ga à and Poble Nou upwind loca ions o de e mine he backg ound concen a ion o OCS. The wea he was s able du ing bo h campaigns wi h one day o sligh p ecipi a ion du ing each sampling campaign, which accumula ed 2.6 and 1.6mm du ing May 29 and Oc obe 26, espec i ely. The ai mass in luence was mos ly cyclonic and ad ec i e wi h some days cha ac e ized by an icyclonic ci cula ion. Synop ic winds we e mos ly no h and eas du ing he May campaign and p edominan ly sou hwes du ing Oc obe as summa ized in Table S2 in Suppo ing In o ma ionS1. We eco ded empe a u e, ai p essu e, wind di ec ion and wind speed a each sampling si e using a po able anemome e (Skywa ch Xplo e 4). The ela i e humidi y was ob ained om he Me eo ological Se ice o Ca a- lonia o he i s campaign (Se ei Me eo ològic de Ca alunya,2023), and addi ionally, i was egis e ed on si e using a digi al hyg ome e (TFA Dos mann®, p ecision ±4%) du ing he Oc obe campaign. 2.3. OCS Measu emen s and Analysis Samples we e collec ed in cylind ical bo osilica e glass lasks (1L olume, No mag Labo und P ozess echnik GmbH, Ge many) wi h Kel-F PCTFE al es i ed a bo h ends. This lask ma e ial showed he lowes pe mea- ion o gases compa ed o o he sealing ma e ials (S u m e al.,2004). To ensu e he quali y o he measu emen echnique and epea abili y o he expe imen , wo lasks connec ed o each o he in a po able de ice we e simul aneously illed in se ies (see Tex S2 in Suppo ing In o ma ionS1, U ban si es, o de ice pic u es). The inle was ins alled a 3.5m abo e g ound le el (m a.g.l.) and ai , a e passing h ough a magnesium pe chlo a e desiccan ube o emo e mois u e and a il e o emo e pa icles, was pumped o he lasks wi h he help o a diaph agm pump (KNF N84.4 ANDC). Be o e aking he sample, ai was lushed h ough he lasks o a leas 10min. A ha ime, we checked ha he low was s eady and be ween 2.0 and 3.5Lmin −1. The lask sampling ook abou o 10min o be comple ed. A e lushing, he exi al e was closed, and he lasks we e p essu ized wi h sample ai o app oxima ely 1.5ba . Samples om he a ge , upwind and downwind si es we e aken a he exac same momen o simula e an Eule ian app oxima ion in which he sampling poin emains ixed (Leelőssy e al.,2016). Flasks we e sen o he Labo a oi e des Sciences du Clima e de l’En i onnemen , LSCE, Pa is-Saclay, F ance, o OCS analyses. Flasks we e s o ed o a maximum o 2mon hs be o e analysis. OCS was analyzed ollowing he analy ical me hod desc ibed in Bel iso e al.(2016, 2022). In his p ocedu e, a mosphe ic OCS (500ml) is apped using an au oma ed sampling sys em (En ech P7100 p econcen a o ) and hen analyzed by gas ch oma og aphy wi h pulsed lame pho ome y de ec ion (PFPD, Va ian Model 3,800, Bel iso e al.,2016). Cali- b a ion was pe o med e e y week using a calib a ion gas supplied by Ai P oduc s (Bel iso e al.,2016). The di e ence be ween he assigned alue o an NOAA long- e m ai s anda d (NOAA-2004 scale) and he a e age o mul iple analyses o ha same s anda d was smalle han 1.5%. The d i be ween calib a ions was assessed on a daily basis using a sho - e m a ge gas (Bel iso e al.,2016). 2.4. De e mining he Plane a y Bounda y Laye Heigh I is necessa y o de e mine he plane a y bounda y laye heigh (PBLH) o unde s and i s in luence on he accumula ion and dilu ion o OCS concen a ion a each si e o e he u ban a ea. PBLH measu emen s we e 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 6 o 21 no possible a he eigh a ge si es, bu a leas he PBLH was calcula ed om measu emen s aken on he campus o he Uni e si y o Ba celona (Figu e1, label: Radiosonde/Radon) using wo di e en echniques as ollows: (a) based on obse ed measu emen s om a adiosonde (RSD) s a ion ins alled on he oo o he Physics Depa men o he Uni e si y o Ba celona (41°23′03″ N 2°07′01″E), which is pa o he Global Me eo ological Ne wo k (GCOS,2016); and (b) ia a lase ceilome e (CL-31, Vaisala Inc., Finland) loca ed a he same place wi h a measu emen ange up o 7.6km, 10-m e ical esolu ion and a empo al esolu ion o 16s. Fo he la e , he PBLH is es ima ed om he ceilome e da a using he Vaisala Bounda y-Laye View so wa e (BL-VIEW) Enhanced G adien me hod (VAISALA,2020)—see de ails in Ga cia-Dalmau e al.(2021)— ollowed by selec- ion algo i hms acco ding o he me hodology o Lo e ane e al.(2016). To de e mine he PBLH based on adiosonde measu emen s, we ollowed a obus nume ical p ocedu e p oposed by Liu and Liang(2010) using midday adiosonde (RSD) obse a ions om he selec ed pe iods o s udy and conside ing 1100 UTC as he launching ime. The PBLH was compu ed ollowing he ai pa cel me hod o a con ec i e domina ed bounda y laye (Holzwo h,1964; Seibe e al.,2000; S ull,1988) as he heigh whe e ai pa cel e olu ion ollowing a d y adiaba ic e olu ion om su ace in e sec s wi h he en i onmen empe a u e p o ile. The calcula ed PBLH om RSD was isually co ec ed o sol e he me hod e o s and mos ly ela ed o he p esence o se e al empe a u e in e sion laye s. A de ailed explana ion o he me hod and how we applied i o ou case s udy is p esen ed in Suppo ing In o ma ionS1 (Tex S3). In his s udy, we also analyzed adon, which is a adioac i e and noble gas known o be a good a mosphe ic ace (G ossi e al.,2012), o be e unde s and he e olu ion o he PBLH and o co obo a e he PBLH calcula ed om he ceilome e obse a ions using he me hod p oposed by G i i hs e al.(2013). Hou ly a mosphe ic concen a- ions o Rn we e measu ed du ing Oc obe 2020 using an A mosphe ic Radon MONi o (ARMON) a he Ba ce- lona School o Indus ial Enginee ing s a ion (41°23′03″ N 2°07′01″E, 10m a.g.l) a he same si e ha adiosonde and ceilome e da a we e eco ded and iden i ied wi h he label Radiosonde/Radon in Figu e1c. The moni o ing was designed and buil by he Uni e si a Poli ècnica de Ca alunya as desc ibed by G ossi e al.(2012). Due o COVID-19 es ic ions, no adon da a we e eco ded o May 2020 because he equipmen had o be shu down. The es ima ions o PBLH using adiosonde and ceilome e we e also compa ed wi h PBLH simula ed using he Wea he Resea ch and Fo ecas ing model (WRF). WRF simula ions we e pe o med o ob ain in o ma ion on he main anspo p ocesses and he e ical mixing condi ions h oughou he AMB as desc ibed in Sec ion2.6. 2.5. WRF Simula ions o he U ban A mosphe e The WRF 4.3 model (Skama ock e al.,2021) coupled wi h he mul ilaye u ban canopy scheme BEP-BEM (Salamanca e al.,2010) was used in his s udy o simula e he u ban a mosphe e o e he AMB o be e unde - s and he in luence o local wea he condi ions and PBL de elopmen on OCS concen a ions. The WRF con ig- u a ion used o his s udy consis s o 3 wo-way nes ed domains wi h ho izon al esolu ions o 9, 3, and 1km co e ing he Ibe ian Peninsula, he Ca alan e i o y, and he AMB, espec i ely, wi h 45 e ical laye s up o 100hPa (see Figu e S2 in Suppo ing In o ma ionS1). The physics pa ame e iza ion o he plane a y bounda y laye is he Boulac scheme, which has been shown o be he mos sui able o esol e he u bulence in ensi y in con ec i e si ua ions o he AMB (Segu a e al.,2021). We included 11 u ban local clima e zone (LCZ) clas- si ica ions (S ewa & Oke,2012) o he inne mos domain and each had a speci ic alue o he mal, adia i e and geome ic pa ame e s o he buildings and g ound, which a e used by he BEP-BEM o compu e he hea and momen um luxes in u ban a eas (Gilabe e al.,2021; Ribei o e al.,2021; Yu & S einbe ge ,2011). We conduc ed simula ions o May and Oc obe 2020 using me eo ological ini ial and bounda y condi ions om ERA5 (Cope nicus,2023). Table S3 in Suppo ing In o ma ionS1 u he desc ibes he model cha ac e is ics and expe imen al con igu a ions. The model simula ions we e alida ed using empe a u e, wind speed and wind di ec ion obse a ions om 14 me eo ological s a ions o he Me eoca ne wo k (h ps://www.me eo.ca / obse acions/xema). 2.6. FLEXPART Back T ajec o ies FLEXPART back ajec o ies we e calcula ed o de e mine he p ecedence o ai sampled a he a ious si es o es ablish whe he he ai mass in luence was local o egional. FLEXPART (FLEXible PARTicle dispe sion model) is a Lag angian anspo and dispe sion model ha is used o calcula ing he long- ange and mesoscale 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 7 o 21 dispe sion o ai pollu an s (S ohl e al.,2005). The FLEXPART e sion adap ed o he WRF mesoscale model (B ioude e al.,2013) was used o calcula e back ajec o ies o ai ace s om each o he sampling si es a he sampling imes. The back ajec o ies we e compu ed om he ou pu o he WRF model a he nes ed domain o 1km wi h 16 e ical le els om 0 o 2,000m a.g.l. Each back ajec o y was calcula ed o 24h a e he sampling ime. The numbe o pa icles eleased o he a mosphe e in he simula ion was 2,000. F om he 3D back ajec o ies, only he laye s om 0 o 100m abo e g ound le el we e used o he s udy o oo p in s, as i is whe e he exchange among he land/ocean - a mosphe e akes place. 3. Resul s 3.1. OCS Sample Repea abili y and Accu acy in U ban En i onmen s We pe o med a p ecision analysis o he GC measu emen s o lask-ai samples acco ding o he me hods de eloped by Bel iso e  al. (2022) o assess he epea abili y and accu acy o he OCS measu emen s in an u ban en i onmen . Eigh pai s o lasks in o al we e selec ed andomly om he May and Oc obe campaigns, and each lask was analyzed wice consecu- i ely o OCS. The a e age di e ence be ween duplica ed measu emen s, ha is, he in a lask di e ence, was 9.8±8.7 pp (median=8.2 pp , in e - qua ile ange (IQR)=3.6–12.7 pp , n=16, 90 h pe cen ile=16.5 pp , 6.3% o ou lie s), as shown in Figu e2a. I was sligh ly highe han ha epo ed by Bel iso o samples collec ed in a u al a ea o e whea and apeseed c ops (Bel iso e al.,2022), i.e., 9.3±7.2 pp (median=7.5 pp , in e qua ile ange (IQR)=4.5–12.5 pp , n=58, 5.2% o ou lie s, bu in ha case, he 90 h pe cen ile was 19.5 pp ). Figu e2c shows ha among he 54 lasks (each analyzed wice consecu i ely), ou displayed mixing a ios highe han he 90 h pe cen ile (i.e., 22.9 pp ), while he median and IQR o he in a lask di e ence (7.7 pp and 3.3–12.1 pp , espec i ely) appea o be highly consis en wi h he da a displayed in Figu e2a. Consis ency is also ound in e ms o he p opo ion o abe an de e mina ions o OCS mixing a ios (i.e., 6.3% (Figu es2a) and 7.4% (Figu e2c)). I was concluded ha he p ecision o GC measu emen s o lask-ai samples emained unchanged o e he wo campaigns and was consis en wi h ha epo ed by Bel iso e al.(2022). The a e age di e ence be ween lasks o he same pai (i.e., he in e lask di e ence) was also analyzed in a duplica e manne and was 10.1±14.0 pp (median=4.8 pp , IQR=0.05–18.6 pp , n=8, no ou lie , Figu e2b). Hence, he in e lask di e ence du ing he AMB s udy displayed highe a iabili y han ha o Bel iso e al.(2022), ha is, 5.5±4.5 pp (median=4.9 pp , IQR=2.2–8.0 pp , n=29, a 6.9% p opo ion o ou lie s was epo ed in ha s udy). The in e lask di e ence was also assessed om a single analysis o he con en o each lask. The median, IQR and 90 h pe cen ile we e 12.1 pp , 4.3–22.5 pp and 45.0 pp , espec i ely, wi h n=95 (Figu e2d). We epo ed he exis ence o six ou lie s (i.e., a p opo ion o 6.3%) exhibi ing in e lask di e ences highe han 45.0 pp and up o 360 pp . The in e lask di e ence calcula ed om pai s o lasks each analyzed only once o OCS (Figu e2d) displays highe a iabili y han ha om pai s o lasks analyzed wice consecu i ely (Figu e2b). Consequen ly, al hough he ou ou lie s displayed in Figu e2c should ha e been disca ded, we a bi a ily ein- co po a ed hose exhibi ing in a lask di e ences in he ange o 22.9 pp (i.e., he 90 h pe cen ile) and 36.8 pp . Acco ding o Bel iso e al.(2022), da a om pai s o lasks exhibi ing in e lask di e ences highe han 14 pp should be disca ded. No e ha his h eshold is app oxima ely wice ha se by Mon zka e al.(2007). Al hough he in e lask di e ence calcula ed om pai s o lasks each analyzed only once o OCS could o igina e om abe an da a (Figu e2d), nobody e e documen ed he high equency dis ibu ion o OCS mixing a ios nex o po en ial u ban sou ces o ha gas. In o he wo ds, al hough lasks a e supposed o be simul aneously illed in pai s, he p ocess o p essu iza ion o wo lasks connec ed in se ies could gene a e concen a ion g adien s Figu e 2. Analysis p ecision o OCS measu emen s om lask-ai samples collec ed in he AMB showing he 10 h, 25 h, median, 75 h and 90 h pe cen iles. Panels (a, c) show Δ in e lask di e ences in OCS concen a ion be ween wo consecu i e analyses o he con en o each lask (median o lask 1 minus ha o lask 2), whe eas (b, d) show he Δ in e lask di e ences in OCS concen a ion be ween lasks o he same pai . The diamonds co espond o he ou lie s (>90 h pe cen ile), which a e o e 23 pp in panel (c) and o e 45 pp in (d). Ou lie s a e labeled acco ding o he loca ion and ime o sample collec ion. The codes o each sampling si e a e si e, loca ion and ime UTC (si es: Tib: Tibidabo, P a: P a , Pob: Poble Nou, Mon: Mon juic, Gui: Guina dó; loca ion: U: Upwind, D: Donwind, T: Ta ge ). No e ha his o ical wind da a was used o ha pu pose. The label colo s ep esen he campaigns and co espond o ed o May and blue o Oc obe . 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 8 o 21 e lec ing he he e ogeneous dis ibu ion o OCS nex o po en ial u ban sou ces o ha gas, in ou case samples ook 10min o ill ou in se ies. In ligh o his possibili y, we decided o a bi a ily alida e he six ou lie s epo ed in Figu e2d and in es iga e whe he in e lask di e ences highe han 45 pp would be si e dependen o would esul om abe an da a. 3.2. E alua ion o WRF Simula ions We an WRF BEP-BEM simula ions in May 2020 and Oc obe 2020 o ob ain high- esolu ion me eo ological da a, a mosphe ic s abili y, and PBLH du ing he sampling pe iods. S a is ical analyses o he oo -mean-squa e e o (RMSE), mean bias (MB) and co ela ion ac o (R) we e used o e alua e pe o mances o he model compa ed wi h g ound-based me eo ological obse a ions om Me eoca (see Sec ion2.6). The nea -su ace ela i e humidi y (RH), empe a u e (T) and wind speed (WS) s a is ical analyses a e displayed in Figu es S3 and S4 in Suppo ing In o ma ionS1 o campaigns in May and Oc obe , espec i ely. The e is an o e all good pe o mance o he modeled RH, T and WS in compa ison wi h he obse a ions wi h an a e age e o o ±14% o RH, ±1.7 o 1.9°C o T and ±1.65ms −1 o WS. This s a is ical analysis is a ailable in Table S4 in Suppo ing In o ma ionS1. Du ing he May campaign, he e we e p edominan no he ly winds om he no h om he 18 h un il he 21s , and he ci cula ion wea he ypes we e hyb id-cyclonic o cyclonic egimes, while he second hal (25 h h ough he 28 h) was cha ac e ized by winds om he eas wi h a Medi e anean Sea ai mass in luence and ad en i e egimes, wi h he excep ion o he 28 h, which was an icyclonic wi h a sou heas synop ic wind componen and local ma i ime ai mass in luence. Du ing he Oc obe campaign, wes e ly and sou he ly winds cha ac e ized by ad en i e ci cula ion wea he ypes p e ailed o mos o he pe iod wi h a cyclonic no h in luence and egional con inen al ai mass in he i s 2days swi ching o a ma i ime in luence o he es o he campaign. Table1 summa izes hese wea he pa e ns o each day o he wo campaigns. 3.3. Plane a y Bounda y Laye Heigh Calcula ions and E alua ion We had adiosonde and ceilome e da a o calcula e he PBLH in one loca ion o he Me opoli an A ea o Ba celona, which we compa ed o he WRF-BEP BEM simula ed PBLH and es ablished ha he WRF model pe o med easonably well in e ms o bounda y laye de elopmen . Fu he mo e, WRF-BEP BEM was alida ed in e ms o empe a u e, wind di ec ion and speed, and ela i e humidi y. Thus, we we e able o employ WRF me eo ological ou pu such as wind di ec ion and speed o explain he obse ed OCS concen a ions, mainly in e m o hei accumula ion in o low o high PBLH and possible anspo pa e ns. Fo he same eason, he a mosphe ic s abili y classes, based on he adon ace me hod, may gi e mo e in o ma ion on he a mosphe ic episodes analyzed in o he s udy. The PBLH om he WRF BEP-BEM simula ions was compa ed agains ceilome e es ima ions and adiosonde obse a ions a 11:00 UTC o each day o he campaigns (see Figu e3). All da a ime se ies ollow he expec ed PBLH diu nal pa e n wi h a apid g ow h o he PBLH du ing he mo ning, a maximum ha was eached a midday, and a dec ease du ing he a e noon. The PBLH calcula ed by he WRF model is gene ally highe han ha es ima ed by he ceilome e and adiosonde. Du ing he i s pe iod o May and i s days o Oc obe when he e a e con inen al egional ai masses wi h p e ailing la ge-scale no he n winds, we no iced an impo an di e ence be ween he ceilome e , adiosonde, and WRF alues in some cases up o 150m di e ence a 11 UTC. In his si ua ion, he la ge-scale winds domina e he bounda y-laye scales, which limi he capaci y o he WRF model o es ima e he PBLH and leads o high unce ain y in he adiosonde obse a ions (S ull,1988). The in e se co ela ion be ween a mosphe ic adon concen a ions and he ceilome e -based PBLH is uni o mly main ained du ing he en i e Oc obe campaign as you can see in Figu e S5 in Suppo ing In o ma ionS1 how PBLH a e a is maximum a 00:00 UTC while a mosphe ic adon concen a ion a e a i s lowes , and he opposi e beha io is seen a ound 12:UTC (see Figu es S5 and S6 in Suppo ing In o ma ionS1), which u he indica es an o e es ima ion o he PBLH by WRF du ing s ong wind e en s in ea ly Oc obe . The highes adon ac i i y concen a ions (be ween 0.01 and 0.05 Bqm −3) a e obse ed du ing nigh ime (21–2h ) when he PBLH eaches i s minimum, whe eas he lowes alues (be ween 1 and 2Bqm −3) a e obse ed du ing diu nal hou s (11–16h ) when 222Rn is dilu ed due o a well-de eloped PBL. The o e es ima ed PBLH modeled alues in May a e consis en wi h an o e p edic ion o he 2m empe a u e seen o a s a ion loca ed 1.2km om he adiosonde launching loca ion. In con as , be e ag eemen be ween 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 9 o 21 he modeled and obse ed PBLH (bo h adiosonde and ceilome e ) is seen du ing he second hal o he May campaign om he 25 h o 28 h o May (Figu e3a), when he eas e ly winds lead o a mo e s able a mosphe e. Oc obe showed be e ag eemen o e all among he PBLH alues ob ained om he ceilome e , RSD and WRF model due o highe a mosphe ic s abili y. The sou h-eas e ly weak winds gene ally lead o s able condi ions and local scale dominance whe e he PBLH can be well es ima ed. 3.4. WRF-FLEXPART Foo p in s FLEXPART simula ions showed ha he sampling campaign co e ed di e en synop ic si ua ions and di e en ai mass o igins. Tex S4 in Suppo ing In o ma ionS1 shows ha du ing he May campaign, wo main di ec ions we e obse ed as ollows: one coming om he no hwes ollowing he Llob ega Ri e basin (days 18, 19, and 21 o May) and ano he om he no heas wi h mo e sea in luence (days 20 and 25 o May) and some o he s wi h mo e land in luence (such as days 26, 27, and 28 o May). Gene ally, he simula ion a 09:30 UTC shows mo e sea o igin o he ai masses a i ing a sampling poin s han he simula ions a 07:00 UTC in acco dance wi h he sea‒land b eeze egimes. Du ing he Oc obe campaign, he ai masses mainly came om he no hwes ollowing he Llob ega Ri e basin (days 15, 19, and 27 o Oc obe ), al hough a si ua ion wi h s ong winds coming om he sou h was obse ed be ween he 20 h and 21s o Oc obe . A s onge b eeze egime is obse ed du ing his mon h wi h gene al changes in he o igin o ai masses be ween he simula ions a 07:00 and 09:30 UTC. Figu e 3. The ime se ies o modeled PBLH (blue line) oge he wi h he Ceilome e ( ed line) and he Radiosonde (blue shade) o (a) May and (b) Oc obe campaigns. Shaded blue a eas indica e he PBLH using wo di e en de ini ions o i s calcula ion. Ve ical lines co espond o he 4 a.m., 7 a.m., 9:30 a.m., and 11 a.m. UTC sampling imes on a daily basis. 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 16 o 21 he no h whe e he ag icul u al ields a e loca ed. This indica es ha he OCS d awdown migh no be en i ely a local e ec o ag icul u e bu could also be due o a mo e egional e ec o OCS aken up by soil and ege a ion du ing he nigh all o e Ca alonia and hen ad ec ed by he land b eeze o e he AMB. Howe e , by 9:30 UTC, OCS concen a ions inc eased (up o an ΔOCS o +116 o he downwind si e) when ai masses we e coming om he u ban and po a eas and despi e PBLH de elopmen , which eached 1,380m ha mo ning. Du ing he Oc obe campaign, sou hwes e ly winds p edomina ed bo h a 7:00 and 9:30 UTC, which b ough in ai masses om he po and Ga à u ban a ea wi h high indus ial ac i i y esul ing in ΔOCS be ween +82 and +40 pp a 7:00 UTC. A 9:30 UTC, ΔOCS alues dec eased sligh ly and had less a iabili y anging om +49 o +55 pp , and his indica ed a mo e homogenous ai mix u e due o a mo e de eloped PBL, which inc eased om 21–84 o 380–519m o ha loca ion. Du ing Oc obe , ai anspo had much mo e in luence on he OCS budge han he ag icul u al land use as i did du ing he May campaign when he ields we e in ull pho osyn he ic ac i i y. The opposi e beha io was obse ed a he El P a ag icul u al si e, whe e a deep PBLH (1,200m) and egional ai mass in luence occu ed in May, while a mo e s able si ua ion and lowe PBLH (691m) domina ed in Oc obe . In gene al, he alues we e simila o he backg ound OCS o May wi h ΔOCS alues o +25, +37, and +22 pp a 7:00 UTC and e en highe enhancemen s a 9:30 (+17, +44 and +173 pp , Figu e7b). FLEXPART back ajec- o ies in May showed ai masses coming om he eas wi h longe esiden ial imes loca ed in he sea bu s ill ecei ed in luence om he ai po and he po o Ba celona. Du ing he Oc obe campaign, El P a had highe OCS enhancemen s a bo h sampling imes (ΔOCS o +70, +33, and +80 pp o 7:00 UTC; +7, +122, and +77 pp o 9:30 UTC), which, gi en he low PBLH, sugges s a local OCS sou ce. The FLEXPART showed ha ai masses came om he eas indica ing ma ine in luence and/o om he po o Ba celona, whe e ac i i ies, such as ai na iga ion and ma i ime a ic, had eco e ed 100% a e he lockdown. The lowe OCS concen a ions in May compa ed o Oc obe highligh he signi ican sink o ag icul u e du ing he g owing season as was seen o he Ga à loca ion. Figu e 7. OCS enhancemen (Δ OCS) in pp calcula ed as he measu e o OCS minus backg ound OCS (484 pp o May and 407 pp o Oc obe ) o each campaign a he wo u ban pa ks as ollows: (a) Ga à and (b) El P a . The black a ow shows he wind di ec ion and speed measu ed on si e, and he absence o an a ow indica es ha he e was no wind blowing a he momen o measu emen . A small map embedded in he lowe igh -hand co ne o each panel shows he esidence ime on a loga i hmic scale o he ai masses a i ing a he sampling si e as de e mined wi h he FLEXPART model. 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 17 o 21 To summa ize, he luc ua ions in OCS mixing a ios in u ban pa ks seemed o be mo e hea ily in luenced by he di ec ion and in ensi y o he wind. In u ban o es s, he e was a clea educ ion o OCS compa ed o u ban pa ks and buil en i onmen . Du ing he May campaign wi h he signi ican a ic educ ion due o he COVID es ic ions, we saw a educ ion o OCS concen a ions compa ed o Oc obe - leading us o he conclusion ha an h opogenic sou ces in u ban a eas a e signi ican . The pe i-u ban ag icul u al a eas did no show lowe alues o OCS as was expec ed, mos likely due o he high in luence o he ai po and he seapo ha we e close by. In gene al, i was di icul o de e mine i he e was any signi ican OCS d awdown by ege a ion, excep in u ban o es ed a eas, while OCS mixing a ios well abo e backg ound le els e idenced signi ican u ban sou ces o OCS. 4.2. Conside a ions o OCS S udies in U ban Ecosys ems An h opogenic emissions a e he p ima y sou ce o OCS on a plane a y scale, ollowed by oceanic lux (Remaud e al.,2023), hey a e signi ican a he local and u ban scales. In ou s udy, he peaks o OCS concen a ions we e mos ly due o ai masses coming om he po o he ci y, as was he case o Mon juic in Oc obe o Ga à du ing he May campaign, and eached alues o 665 pp o Mon juic and 600 pp o Ga à (up o ΔOCS o +258 and +193 pp , espec i ely). The e a e a ious po en ial sou ces o OCS in u ban a eas, such as he e osion o au omobile i es agains asphal , ehicle exhaus umes, na u al gas combus ion and indus ial p ocesses including oil e ine ies and aluminum p oduc ion (Yan e al.,2019). In ac , in hei in en o y, Yan e al. quan i ied he o al emissions in 2015 o China alone as 174GgSy −1 wi h he highes con ibu ions coming om indus ial OCS emission p ocesses (120GgSy −1). In ha same s udy, he o al ehicle exhaus was esponsible o 3GgSy −1, which is app oxima ely hal o he yea ly global es ima e o 6GgSy −1 gi en by Lee and B imblecombe(2016). The la e s udy also indica ed ha he hea y uel oil used in ships is a signi ican sou ce o he OCS budge and p oduces 30GgSy −1. The AMB quali ies o all hese po en ial sou ces and i has a hea ily ansi ed ma ine po ha also has a combined cycle gas u bine powe plan ha uns on na u al gas as well as an in ense use o p i a e ehicles wi hin he ci y. Figu e 7. (Con inued) 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 18 o 21 Addi ionally, a b oad a ie y o sul ides a e p oduced by was ewa e whe e ola ile o ganic compounds (VOC) such as me hane hiol (MeSH), dime hyl sul ide (DMS), ca bon disul ide (CS2), dime hyl disul ide(DMDS), dime- hyl isul ide (DMTS) and OCS a e gene a ed in anoxic en i onmen s o sewe s (Lee & B imblecombe,2016), and i has been no ed ha he mic obial ac i i y in hese anoxic sedimen s is closely linked o he p oduc ion o OCS (Ki z e al.,2019). Sul ide emissions om u ban was ewa e ne wo ks a e equen ly de ec ed due o hei bad odo as is he case o Ba celona (Eijo-Río e al.,2015) and Thessaloniki (Besis e al.,2021). In ou s udy, he sampling si es o Sag ada Familia in Oc obe (ΔOCS o +150 pp ) and Poble Nou (ΔOCS o +50 pp ) concu wi h loca ions o a combined sewe sys em p e iously epo ed o ha e high peaks o H2S, which indica ed a po en ial OCS sou ce om was ewa e . Un o una ely, cons aining his signal om o he po en ial u ban sou ces was ou o he scope o he p esen s udy. Howe e , he obse a ions p o ided by ou measu emen campaigns could se e o es ima e an h opogenic OCS sou ces when compa ed o simula ions o OCS mole ac ions combined wi h emission in en o ies and plan up ake luxes as was ecen ly done o he Ne he lands by Zanche a e al.(2023). In e ms o ege a ed land uses, we ound one case o a signi ican OCS d awdown in ag icul u al si es du ing he g owing season o May. In addi ion o pho osyn he ic up ake (Campbell e al.,2008,2017; Mon zka e al.,2007), wa e logged soils ha e been epo ed o ac as sinks o OCS when plan ed wi h c ops (Yi e al.,2008). The ag icul u al ields o he Llob ega basin a e mos ly i iga ed by inunda ion, which could u he in ensi y he OCS up ake and explain he OCS d awdown a 7:00 UTC when pho osyn he ic ac i i y was high. We also ound alues indica ing up ake du ing he g owing season in Collse ola Fo es , which is in ag eemen wi h o he s udies e alua ing o es OCS up ake (Bel iso e al.,2016; Campbell e al.,2008,2017). Finally, de e mining he in luence o land use on he o e all u ban OCS budge equi es a eliable backg ound measu emen . In gene al, backg ound le els o OCS a e associa ed wi h ai masses coming om he sea because he s abili y p o ided by oceans esul s in cons an OCS concen a ions, a leas a a seasonal scale (Mon zka e al.,2007), as was he case o San F ancisco (Villalba e al.,2021). Howe e , his is no applicable o he AMB because he con ec ion egime is qui e peculia . The AMB has a unique land‒sea ai mass exchange in which sea b eezes unde speci ic condi ions do no b ing new ai masses bu a he ecycle ai masses ha o igina ed o e he u ban a ea, howe e , a deepe esea ch should be done in his ega d, since ma ine con ibu ion canno be neglec ed. This ype o p ocess has been obse ed by Jaén e al.(2021), who desc ibed how ozone plumes s ay o e he sho e due o ad ec i e p ocesses du ing hea episodes such as hea wa es. The he mal componen o con ec i e p ocesses c ea ing episodes o ai eci cula ion on he Spanish eas e n coas has also been docu- men ed by Millán e al.(2000), whe e hey ou line how he sea b eeze can a o he o ma ion o laye s o s a - i ied ai ese oi s in heigh s whe e compounds accumula e ha can la e e u n o he land. This pa icula sea b eeze ci cula ion cons ic s local en ila ion and exace ba es he accumula ion o an h opogenic gases emi ed in he AMB. 5. Conclusions We pe o med wo measu emen campaigns o OCS mixing a ios in he Me opoli an A ea o Ba celona (AMB) o explo e o wha ex en di e en u ban land uses can ha e an impac on he OCS mixing a ios measu ed on lask samples ega ding a pa icula u ban clima e, geog aphy, and opog aphy o AMB. Ai samples we e aken a loca ions ep esen ing ou di e en land uses as ollows: u ban pa ks, o es s, ag icul u al ields and impe i- ous u ban landscapes. Based on he in e lask compa ison desc ibed in 2.3, we de e mined ha he OCS measu emen unce ain y can be as high as +53 pp , which e lec s ha nea -su ace measu emen s in u ban he e ogeneous en i onmen s esul in highe unce ain ies han in homogenous u al en i onmen s. We also compa ed a e age OCS concen a ions ±σ wi h alues obse ed om ICOS- LSCE in Gi -su -Y e e (GIF), which we e aken a he same momen as ou campaign (Figu e S3 in Suppo ing In o ma ionS1). We obse ed how mean OCS concen a ions and σ in he AMB we e sys ema ically highe han in GIF, which sugges s ha an h opogenic emissions could a ec he inc ease in unce ain y. The abili y o cap u e a pho osyn he ic signal appea s weak in ege a ed a eas wi hin he ci y, such as pa ks, and is elega ed o o es s and pe i-u ban ag icul u al a eas. U ban pa ks showed alues signi ican ly highe han he es ablished backg ound alue, bo h in May (i.e., up o +144 pp ) and in Oc obe (i.e., up o +150 pp ), which 21698996, 2023, 24, Downloaded om h ps://agupubs.onlinelib a y.wiley.com/doi/10.1029/2023JD039497 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [10/01/2024]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License Jou nal o Geophysical Resea ch: A mosphe es ESTRUCH ETAL. 10.1029/2023JD039497 19 o 21 shows he in luence o an h opogenic sou ces, possibly om he po and he ci y cen e , which masks he possi- bili y o cap u ing he signal om he ege a ion. The u ban o es sugges ed OCS up ake alues up o −33 pp , which indica es a signi ican pho osyn hesis sink in dense ege a ion masses e en wi hin ci y limi s. The u ban a ea showed a a ie y o signals om almos neu al o abo e he backg ound alue, which sugges s nea by emission sou ces. In ou case, we ound he explana ion o some p ocess alues ha may be aligned wi h u ban a ic bu abo e all wi h he indus ial ac i i y o he po and ai po (e.g., high OCS alues in Mon juic when he wind came di ec ly om he po ). We can also hink o o he biogenic p ocesses ha p oduce OCS p ocesses linked o human ac i i y on u ban land such as he was e sys em. These p ocesses should be s udied in mo e de ail in u u e wo ks. We ha e no been able o cap u e an e ec o u ban ege a ion on OCS up ake; ins ead, he pe i-u ban a ea o ag icul u e and o es s may ha e mo e impo an oles in he OCS dynamics in ci ies. Las , ag icul u al a eas showed a clea d awdown du ing he g owing season o he loca ion o Ga à, whe e we we e able o disca d egional in luences and in ense ai mixing. OCS enhancemen s in Oc obe e lec he loss o pho osyn he ic up ake and he e u n o business as usual a ic and nea by ai po and po ac i i ies. O e all, he a iabili y o OCS in he u ban ecosys em was highe han we expec ed. The URBAG campaigns o OCS measu emen s showed e idence ha he use o OCS as a ace gas o de e mine he con ibu ion o he u ban biosphe e o he CO2 budge is a complica ed ask and ha he ege a ion signal is di icul o de ec because local an h opogenic ac i i ies ha e a signi ican in luence. The u ban ecosys em is complex in e ms o land up ake, ma ine emissions, and an h opogenic emissions om he ci y po , and s ongly impac s he spa ial- empo al dis ibu ion o OCS in he AMB and impedes a clea cons ain o he biosphe e signal. Da a A ailabili y S a emen The da a acqui ed du ing he URBAG measu emen campaigns used o he analysis o land in luence in his s udy a e a ailable a ZENODO ia h ps://doi.o g/10.5281/zenodo.8072833 wi h C ea i e Commons A ibu ion 4.0 In e na ional license (Villalba, G.,2023). Re e ences Badia, A., Langemeye , J., Codina, X., Gilabe , J., Guile a, N., Vidal, V., e al. (2021). 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U ban Clima e, 37, 100863. h ps://doi.o g/10.1016/J.UCLIM.2021.100863 Acknowledgmen s This wo k has been made possible hanks o he inancial suppo o he Eu opean Resea ch Council (ERC) Consolida o p ojec : In eg a ed Sys em Analysis o U ban Vege a ion and Ag icul u e (818002-URBAG), he Spanish Minis y o Science, Inno a ion and Uni e si- ies, h ough he “Ma ia de Maez u” p og amme o Uni s o Excellence (CEX2019-000940-M), and he unding and ecogni ion awa ded o he esea ch g oup Sos enip a (2021 SGR 00734) by he Depa men o Resea ch and Uni e - si ies o he Gene ali a de Ca alunya. This wo k also has been g an ed by he Spanish Minis y o Science and Inno a- ion MCIN AEI/10.13039/501100011033 unde con ac PID2020-113614RB-C21, by he Spanish Go e nmen unde G an PRE2018-085425 and by he Ca a- lan Go e nmen unde con ac 2021 SGR 00574. 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