Ci eravamo già occupati tempo fa di una teoria secondo la quale l’inquinamento su una regione geografica non necessariamente era stato prodotto in quella zona stessa. Anzi, una grossa fetta di inquinamento prodotto veniva trasportata dai venti in altre aree “innocenti”.
Ora arrivano delle immagini satellitari molto interessanti in cui si nota come circa un terzo del piombo recentemente raccolto in due siti nella San Francisco Bay proveniva dall’Asia. In uno studio, primo nel suo genere, gli scienziati dello US Department of Energy’s Lawrence Berkeley National Laboratory e del California Air Resources Board hanno registrato variazioni della quantità di piombo trasportate attraverso il Pacifico. Hanno usato la firma isotopica delle particelle di piombo che ha permesso loro di risalire ad alcune delle origini che conducono al carbone e a minerali metallici che si trovano solo in Asia.
La concentrazione totale del piombo nei nostri campioni era piccola, ma una parte significativa di essa proveniva dall’Asia
spiega John Christensen, uno degli scienziati della ricerca pubblicata sulla rivista Environmental Science & Technology. E’ noto che le particelle di aerosol e altre riescono a coprire lunghe distanze attraverso l’atmosfera terrestre. Ma i dettagli di questo trasporto, come il viaggio sui circa 11.000 km che separano le ciminiere della Cina e la costa Occidentale del Nord America, sono in gran parte sconosciuti. Ma qualcosa potrebbe cambiare, anche grazie ad immagini come questa qui di fianco che dimostrano, dal satellite, enormi nuvole contenenti gas tra i più disparati che si spostano in tutto il mondo.
Questo lavoro dimostra che possiamo usare il piombo come tracciante per le particelle sospese nell’aria all’interno del crescente inquinamento industriale asiatico. Possiamo usarlo per comprendere più a fondo le condizioni sopra l’Oceano Pacifico che agevolano il trasporto di aerosol dall’Asia agli Stati Uniti.
La ricerca potrebbe aiutare gli scienziati a migliorare i modelli che descrivono come le polveri sottili e le altre sostanze che compongono l’inquinamento atmosferico riescono a “cavalcare il vento” attraverso i Continenti e gli oceani. Il lavoro potrebbe anche aiutare le autorità nella regolamentazione dell’inquinamento atmosferico degli Stati Uniti, e spiegare come mai, nonostante alcune regioni facciano immensi sforzi per ridurre le proprie emissioni, non riescano a rispettare gli standard di qualità dell’aria.
Spiega Stephanie Ewing, autore principale dello studio e assistente alla cattedra di pedologia e biogeochimica del suolo alla Montana State University
I modelli di trasporto sono incerti. Quando siamo in grado di distinguere tra le fonti con uno strumento isotopico, abbiamo un enorme potere quantitativo in quanto le misurazioni sono precise e non dobbiamo fare affidamento su un modello di trasporto. In effetti, possiamo usare le misure per verificare e migliorare i modelli.
La composizione isotopica del piombo varia in vaste regioni della crosta terrestre. In alcuni casi, porta una firma che può essere quasi unica in una regione. In Asia, ad esempio, le polveri minerali e altre fonti di piombo come il carbone ed i minerali metallici hanno proporzioni particolarmente elevate di 208Pb, l’isotopo che si forma dal decadimento radioattivo del torio. In questo modo si può distinguere, come una sorta di impronta digitale, a chi “appartiene” una sostanza che si trova a migliaia di chilometri di distanza.
Già una decina di anni fa degli scienziati australiani hanno catalogato le concentrazioni di piombo isotopico in vari punti intorno al mondo, e tra i molti risultati, avevano scoperto una distinzione tra aerosol raccolti in Cina e quelli trovati in Nord America, ed è proprio da qui che gli scienziati americani oggi sono voluti ripartire.
[Fonte: Physorg]
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