As pol­lu­tion soared, major­ity of Delhi’s air mon­it­ors went off­line

As Delhi’s sky­line lit up unabated on Diwali night, the levels of pol­lut­ants skyrock­eted far bey­ond per­miss­ible lim­its across the city. But at the very moment when track­ing dan­ger­ous pol­lu­tion peaks was most crit­ical, the major­ity of Delhi’s 39 con­tinu­ous ambi­ent air qual­ity mon­it­or­ing sta­tions (CAAQMS) went off­line for hours, cre­at­ing sig­ni­fic­ant data gaps.

An ana­lysis of Cent­ral Pol­lu­tion Con­trol Board (CPCB) and Delhi Pol­lu­tion Con­trol Com­mit­tee (DPCC) data by HT revealed that only nine sta­tions – barely 23% of the city’s mon­it­or­ing net­work – recor­ded con­tinu­ous data. The remain­ing sta­tions logged black­outs ran­ging from one to nine hours dur­ing the cru­cial 36-hour period between mid­night on Monday and 11am Tues­day.

An ana­lysis of Cent­ral Pol­lu­tion Con­trol Board (CPCB) and Delhi Pol­lu­tion Con­trol Com­mit­tee (DPCC) data by HT revealed that only nine sta­tions – barely 23% of the city’s mon­it­or­ing net­work – recor­ded con­tinu­ous data. The remain­ing sta­tions logged black­outs ran­ging from one to nine hours dur­ing the cru­cial 36-hour period between mid­night on Monday and 11am Tues­day.

Dwarka Sec­tor 8 recor­ded the poorest per­form­ance, with data avail­able for only 27 of the 36 crit­ical hours. Mon­it­or­ing sta­tions at Jawa­harlal Nehru Sta­dium, Nehru Nagar, Pat­par­ganj, and RK Puram each lost approx­im­ately eight hours of read­ings.

Four other sta­tions missed only one hour, but 10 cru­cial mon­it­or­ing points went dark for six hours or more dur­ing the period when fire­cracker emis­sions typ­ic­ally peak.

Data gaps, pol­lu­tion spikes

Experts stressed that the miss­ing data is not a new phe­nomenon. Every year, sev­eral sta­tions stop report­ing when pol­lu­tion levels spike dur­ing Diwali, sug­gest­ing the sys­tem’s inab­il­ity to handle pol­lu­tion loads bey­ond a cer­tain threshold.

Experts stressed that the miss­ing data is not a new phe­nomenon. Every year, sev­eral sta­tions stop report­ing when pol­lu­tion levels spike dur­ing Diwali, sug­gest­ing the sys­tem’s inab­il­ity to handle pol­lu­tion loads bey­ond a cer­tain threshold.

“The observed pat­tern shows that most sta­tions stopped provid­ing data when they approached 1,000 µg/m³, while there were sta­tions such as Anand Vihar, Nehru Nagar, Mun­dka, that recor­ded val­ues above 1,000 µg/m³, indic­at­ing that the tech­no­logy of mon­it­or­ing isn’t an issue, at least in terms of being avail­able. We have observed this over the past years too,” said Sunil Dahiya, founder and lead ana­lyst of think tank EnviroCata­lysts.

Dahiya said the prob­lem cuts across agen­cies. He referred to pre­vi­ous years, which had sim­ilar data gaps observed not only for sta­tions under the Delhi Pol­lu­tion Con­trol Com­mit­tee (DPCC), but also those under the Indian Insti­tute of Trop­ical Met­eor­o­logy (IITM), the Cent­ral Pol­lu­tion Con­trol Board (CPCB) and the India Met­eor­o­lo­gical Depart­ment (IMD).

Dahiya said the prob­lem cuts across agen­cies. He referred to pre­vi­ous years, which had sim­ilar data gaps observed not only for sta­tions under the Delhi Pol­lu­tion Con­trol Com­mit­tee (DPCC), but also those under the Indian Insti­tute of Trop­ical Met­eor­o­logy (IITM), the Cent­ral Pol­lu­tion Con­trol Board (CPCB) and the India Met­eor­o­lo­gical Depart­ment (IMD).

Where data was avail­able, the pic­ture was alarm­ing. DPCC’s Mandir Marg sta­tion recor­ded PM2.5 con­cen­tra­tions of 1,066 µg/m³ at mid­night before going off­line from 1am to 4am. At Nehru Nagar – Delhi’s most pol­luted sta­tion on Diwali night – PM2.5 touched 1,763 µg/m³ at 10pm, with no data for the next hour and a four-hour black­out between 1am and 5am.

CPCB’s ITO sta­tion registered PM2.5 levels of 923 µg/m³ at 1am and went dark. It worked again at 7am when it logged 967 µg/ m³. At Dwarka’s NSIT sta­tion, data van­ished from mid­night to 5am, while read­ings at IMD’s Ayanagar were unavail­able for up to five hours.

CPCB’s ITO sta­tion registered PM2.5 levels of 923 µg/m³ at 1am and went dark. It worked again at 7am when it logged 967 µg/ m³. At Dwarka’s NSIT sta­tion, data van­ished from mid­night to 5am, while read­ings at IMD’s Ayanagar were unavail­able for up to five hours.

An expert, well acquain­ted with the CAAQMS across NCR, said for hourly read­ings – sta­tions spend close to 40 minutes to col­lect par­tic­u­late mat­ter, with the remain­ing time spent to cal­ib­rate and ana­lyse it. “Sta­tions are designed to record extremely high val­ues – even up to 10,000µg/m³. With the rise in con­cen­tra­tion, accur­acy may reduce but it does not mean val­ues are not released. It simply means the accur­acy is reduced mar­gin­ally – by 1% or 2%. In this case, it is plaus­ible a cer­tain threshold has been set bey­ond which val­ues will not be cap­tured.”

An expert, well acquain­ted with the CAAQMS across NCR, said for hourly read­ings – sta­tions spend close to 40 minutes to col­lect par­tic­u­late mat­ter, with the remain­ing time spent to cal­ib­rate and ana­lyse it. “Sta­tions are designed to record extremely high val­ues – even up to 10,000µg/m³. With the rise in con­cen­tra­tion, accur­acy may reduce but it does not mean val­ues are not released. It simply means the accur­acy is reduced mar­gin­ally – by 1% or 2%. In this case, it is plaus­ible a cer­tain threshold has been set bey­ond which val­ues will not be cap­tured.”

The cost of miss­ing data

These gaps, experts warned, under­mine pub­lic health response and sci­entific under­stand­ing of pol­lu­tion’s real impact. “Such miss­ing data hampers insights, makes it impossible to assess expos­ure accur­ately, and gives a false sense that air qual­ity is slightly bet­ter than it actu­ally is,” said Dahiya.

Act­iv­ist Bhavreen Kand­hari called it a “wor­ry­ing breach of trans­par­ency.”

Anu­mita Roy­chow­dhury, exec­ut­ive dir­ector for research and advocacy at the Centre for Sci­ence and Envir­on­ment (CSE), said such lapses were par­tic­u­larly con­cern­ing dur­ing “extreme pol­lu­tion epis­odes” like Diwali. “High pol­lu­tion peaks are expec­ted dur­ing such extreme pol­lu­tion epis­odes. It is thus highly import­ant to keep the data open and trans­par­ent even if extreme peaks are recor­ded, to under­stand the mag­nitude of the impacts and to inform policy and people for adequate and effect­ive health safe­guards,” she said.

When asked about the miss­ing data, Delhi’s envir­on­ment min­is­ter Manjinder Singh Sirsa dis­missed the find­ings. “All data is avail­able on CPCB and on DPCC web­sites. There is no miss­ing data. You have to check. Those who are say­ing there is miss­ing data, you know what their inten­tions are,” he said at a press con­fer­ence on Tues­day.

Offi­cials at CPCB and DPCC did not respond to HT’s quer­ies, nor did the Com­mis­sion for Air Qual­ity Man­age­ment (CAQM).

Offi­cials at CPCB and DPCC did not respond to HT’s quer­ies, nor did the Com­mis­sion for Air Qual­ity Man­age­ment (CAQM).

Delhi’s 39 CAAQMS sta­tions — run by mul­tiple agen­cies — are designed to col­lect real-time data on par­tic­u­late mat­ter, gases such as NO2 and SO2, and met­eor­o­lo­gical factors like humid­ity and wind speed.

This net­work forms the back­bone of CPCB’s daily Air Qual­ity Index (AQI) bul­let­ins and helps determ­ine emer­gency responses such as bans on con­struc­tion or traffic restric­tions under the Graded Response Action Plan (GRAP).

But without reli­able data, sci­ent­ists said, both short-term inter­ven­tions and long-term plan­ning become blind­folded. “Such miss­ing data hampers data-driven insights, under­stand­ing the con­tri­bu­tion of cer­tain activ­it­ies, their impact on health, as well as skews the daily pol­lu­tion level record­ings, giv­ing a false sense of lower air pol­lu­tion levels,” Dahiya said.

But without reli­able data, sci­ent­ists said, both short-term inter­ven­tions and long-term plan­ning become blind­folded. “Such miss­ing data hampers data-driven insights, under­stand­ing the con­tri­bu­tion of cer­tain activ­it­ies, their impact on health, as well as skews the daily pol­lu­tion level record­ings, giv­ing a false sense of lower air pol­lu­tion levels,” Dahiya said.