Light rain likely today but pol­lu­tion may shoot up again, cau­tions Met

Hindustan Times (Delhi) - - HTMETRO - HT Cor­re­spon­dent htre­porters@hin­dus­tan­

NEWDELHI: Delhi is likely to re­ceive a driz­zle on Wed­nes­day, which might help in tem­po­rar­ily bring­ing down the pol­lu­tion level of the city to some ex­tent .

Ex­perts, how­ever, fear that the air qual­ity is likely to be­come fouler af­ter the rain but was un­likely to hit the ‘se­vere’ level.

The rain will also en­sure that mois­ture level in the city’s air builds up again. The high mois­ture con­tent would trap the lo­cal pol­lu­tants hin­der­ing their dis­per­sal process.

This could help the pol­lu­tion level to shoot up again from Thurs­day.

The In­dian Me­te­o­ro­log­i­cal Depart­ment has fore­casted some shal­low fog and mist in the morn­ing till Satur­day.

“There can be a rise in pol­lu­tion lev­els af­ter the rain. This is be­cause the mois­ture level will in­crease which will help trap pol­lu­tants,” said D Saha head of the air qual­ity l ab­o­ra­tory of Cen­tral Pol­lu­tion Con­trol Board (CPCB).

Sci­en­tists said that fog and aerosol hazes likely am­plify each other and form a vi­cious cy­cle. “On one hand aerosol – a kind of pol­lu­tant – serves as ‘seeds’ that make it easier for fog to form, on the other hand the mois­ture droplets in the fog acts like small ‘chem­i­cal fac­to­ries’ that help gaseous pol­lu­tants to grad­u­ate into haze caus­ing aerosols,” said SN Tri­pathi, co­or­di­na­tor of the Cen­tre for En­vi­ron­men­tal Sci­ence & En­gi­neer­ing at IIT Kan­pur.

Ex­perts said that the rate of dis­per­sal of pol­lu­tants de­pends much on the type of rain.

“Heavy rain with large and high ve­loc­ity rain drops is able to clear pol­lu­tants more ef­fec­tively than a driz­zle with smaller wa­ter par­ti­cles fall­ing at a lower speed,” said Dilip Gan­guly, as­sis­tant pro­fes­sor at Cen­tre for At­mo­spheric Sciences in Iit-delhi.

Usu­ally rains wash away the par­tic­u­late mat­ter. But there are some par­tic­u­late mat­ters, whose size varies be­tween 0.1 mi­cron and 1 mi­cron. They are not cleaned and linger even af­ter heavy rains. They are hy­dropho­bic and tend to bounce away when­ever a rain drop hits them.

This phe­nom­e­non, ex­perts say, is known as Green Field Gap.

“Not all types of rain is ef­fec­tive. Longer du­ra­tions of driz­zle are more ef­fec­tive that short lived heavy show­ers,” said Ab­hi­jit Chat­ter­jee, as­sis­tant pro­fes­sor of en­vi­ron­men­tal sciences sec­tion at Bose In­sti­tute in Kolkata. Larger droplets means higher chances of wa­ter hit­ting par­tic­u­late mat­ter and re­duc­ing pol­lu­tion

High-ve­loc­ity rain will cap­ture more pol­lu­tants

Some pol­lu­tants are friendly with wa­ter and get eas­ily at­tached. When rain falls, they are washed away first Smaller rain droplets might miss par­ti­cles and fail to bring down pol­lu­tants

Low-ve­loc­ity rain drops might get de­flected by winds and may not be ef­fec­tive

Some par­ti­cles are scared of wa­ter and tend to bounce out of the way when wa­ter falls. They linger af­ter a shower

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