Kan rook uit een schoorsteen bepalen of jij kleddernat wordt?
Stel je voor dat je fietst naar een sollicitatiegesprek. Je weerapp belooft droog weer, dus je laat je regenjas thuis. Tien minuten later ben je doorweekt. Vreemd genoeg kan een fabrieksschoorsteen enkele kilometers verderop hiermee te maken hebben. Zijn rookpluim kan namelijk meebepalen hoe hard het regent. Soms zorgt die rook voor minder regen, en soms net voor een intense stortbui. Hoe kan dat nu? Dat wilde ik uitzoeken in mijn onderzoek.
Niet zomaar water
Wanneer we spreken over regen, hebben we het meestal over millimeters neerslag, oftewel de hoeveelheid water die op de grond terechtkomt. Maar voor dat water de grond raakt, bestaat het uit een verzameling druppels met uiteenlopende groottes, van bijna een tiende van een millimeter tot ruim een halve centimeter. Hoewel die verschillen op het eerste gezicht misschien onbelangrijk lijken, bepalen ze mee of de buienradar op je gsm de bui over een kwartier juist inschat, of een rivier het water nog aankan en hoeveel schade de regen aan de bodem aanricht.
Wat bepaalt dan juist de grootte van regendruppels? Voor een groot deel gewoon het weer: de wind, de temperatuur, het seizoen en het type neerslag. Maar wij mensen spelen ook een rol, bijvoorbeeld via steden en luchtvervuiling. Vooral de invloed van luchtvervuiling op regen is nog niet volledig begrepen. Daarom ging ik er zelf mee aan de slag.
Van wolkje tot stortbui
Om te snappen hoe luchtvervuiling een rol speelt, moeten we eerst stilstaan bij de manier waarop regendruppels ontstaan. Een wolk is in essentie een groep minuscule waterdruppels die te klein en licht zijn om te vallen. Ze zweven dus gewoon in de lucht. Om door te groeien tot regendruppels moeten die druppeltjes eerst herhaaldelijk met elkaar botsen en samensmelten tot ze groot genoeg zijn om naar beneden te vallen.
Luchtvervuiling, en met name fijnstof, kan hierbij roet in het eten gooien. Deze stofdeeltjes die we uitstoten via verkeer, industrie en verwarming kunnen hoog in de atmosfeer terechtkomen. Daar gedragen de deeltjes zich als kiemen waaraan waterdamp zich kan vastklampen. Zo ontstaan dus wolken. Het probleem is dat er maar een beperkte hoeveelheid waterdamp aanwezig is. Dus hoe meer kiemen, hoe minder water aan elke kiem gaat blijven hangen, wat resulteert in kleinere druppels. En kleinere druppels botsen minder makkelijk tegen elkaar aan, dus groeien ze trager tot regendruppels. De wolk blijft dan langer hangen voordat het gaat regenen. Als voorspellingen hier dus geen rekening mee houden, dan sleur je misschien je paraplu nodeloos mee. Maar zien we dat effect ook terug in de regendruppels die uiteindelijk op de grond vallen?
Fijnstof: rem- of gaspedaal?
Om dat effect concreet te meten, gebruikte ik drie meetinstrumenten nabij Brussel, Gent en Luik. Deze zogenaamde “disdrometers” meten van elke individuele druppel die erin valt hoe groot het is en hoe snel het valt. Door de waarnemingen te combineren met gegevens over het weer en de hoeveelheid fijnstof in de lucht, kon ik zien of er een verband was.
En wat bleek? De impact van fijnstof was tegenstrijdig. Op dagen met veel fijnstof in de lucht regende het tot de helft minder vaak, en als het toch regende, was de regen gemiddeld 40% minder hevig dan op de minst vervuilde dagen. Maar tijdens onweer en kortstondige stortbuien was het effect juist omgekeerd. Bij Luik, waar de metingen het langst duurden, sloeg de regen tijdens de meest vervuilde buien met zo’n 18% meer kracht in op de grond, en waren de druppels gemiddeld 10% groter dan in de schoonste buien.
Hoe kan hetzelfde fijnstof twee tegengestelde dingen doen? De meest gangbare verklaring: omdat de kleine druppeltjes trager samenklonteren, kunnen ze gemakkelijker naar boven geblazen worden. Hoger in de atmosfeer is het ijskoud en bevriezen de waterdruppels. Hierbij komt extra warmte vrij die de groei van mooiweerwolk tot onweerswolk aanwakkert. Dan groeien de inmiddels bevroren regendruppels opeens razendsnel. Kortom, fijnstof remt de regen eerst af, maar áls de wolk de kans krijgt om door te groeien tot onweerswolk, is de daaropvolgende stortbui des te sterker.
Kip of ei?
Toch moeten we hier voorzichtig zijn. Fijnstof hangt namelijk ook samen met het weer. Bij mooi, windstil weer stapelt fijnstof zich gemakkelijker op, en tegelijk valt er dan vaak toch al minder neerslag. Het zou dus kunnen dat niet het fijnstof zelf de regen beïnvloedt, maar gewoon het rustige weer dat toevallig ook met veel fijnstof gepaard gaat. Om dat uit te sluiten, hield ik in mijn berekeningen rekening met wind en luchtdruk. Het effect van fijnstof werd daardoor kleiner, maar verdween niet. Dat is een aanwijzing dat fijnstof zelf wel degelijk meespeelt.
Klein versus grof
Niet elk stofdeeltje heeft trouwens dezelfde invloed op regen. De allerkleinste stofdeeltjes, kleiner dan 2,5 micrometer, hingen vooral samen met het afremmen van gewone regen. De grovere deeltjes, tot 10 micrometer groot, hingen juist samen met het versterkend effect van de onweersbuien. Waarom dat precies zo is, weten we nog niet helemaal. Mogelijk werken die grotere deeltjes beter als startpunt waarrond een druppel sneller kan groeien. Verder onderzoek moet dat nog uitwijzen.
Er is dus wel degelijk een rol weggelegd voor fijnstof als het aankomt op neerslag. Deze is misschien eerder bescheiden ten opzichte van het weer, maar is zonder twijfel relevant. Voorspellingen van klimaatverandering vertrouwen namelijk op aannames over hoe fijnstof en wolken op elkaar inwerken. En dit blijft tot op de dag van vandaag één van de grootste bronnen van onzekerheid voor het toekomstige klimaat. Preciezer inschatten hoe fijnstof regendruppels beïnvloedt, kan dus voorspellingen verbeteren en ons beter voorbereiden op extreme neerslag en overstromingen in de toekomst. Dus als conclusie: de rook uit een schoorsteen is er misschien niet helemaal verantwoordelijk voor of jij doorweekt aankomt op je sollicitatie, maar ze beslist wel subtiel mee over de grootte van elke druppel die valt.
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