Delhiites breathed cleaner air for the second consecutive day on Thursday as the AQI was recorded in the "moderate" category with a reading of 165. Restrictions under Stage 4 of the GRAP were lifted and replaced with the Stage-2 measures.

The 24-hour average Air Quality Index (AQI) of the city was recorded at 165 at 4 pm, registering a slight improvement from 178 on Wednesday.

It marks a significant improvement as on Tuesday, the AQI was recorded in the "poor" category with a reading of 268, compared to 280 on Monday and 285 on Sunday, according to the Central Pollution Control Board's (CPCB) data.

For the month of December, the city has not seen such clean air since December 14, 2022, when the AQI was 163.

In comparison, the lowest AQI recorded in December 2023 was 286, according to the CPCB.

The Supreme Court allowed the Commission for Air Quality Management (CAQM) to shift to the Stage-2 restrictions under the Graded Response Action Plan (GRAP) on Thursday.

As a result, several restrictions imposed under Stage 4, including a ban on the entry of diesel-run medium and heavy vehicles (BS-IV or below) registered in Delhi -- except those carrying essential goods -- have been lifted.

However, under Stage 2 of the GRAP, restrictions such as a ban on the use of coal and firewood, including tandoors at hotels, restaurants and open eateries, as well as the use of diesel generator sets -- except for emergency and essential services -- remain in force in the National Capital Region (NCR).

Of the 38 air quality monitoring stations in the city, five reported the air quality in the "poor" category on Thursday, while the remaining stations recorded "moderate" air quality, according to the Sameer app.

The CPCB classifies AQI levels between 0 and 50 as "good", between 51 and 100 as "satisfactory", between 101 and 200 as "moderate", between 201 and 300 as "poor", between 301 and 400 as "very poor", and above 400 as "severe".

On Thursday, the primary pollutants were PM2.5 and PM10, with PM2.5 levels recorded at 59.9 µg/m³ at 3 pm and PM10 levels at 138.7 µg/m³ during the same time.

These fine particles pose significant health risks as these can penetrate deep into the lungs and enter the bloodstream, experts warn.

The Decision Support System (DSS), the primary tool used to assess and estimate the sources of pollution in Delhi, has not been updated since Friday.

The latest data available on the website is from November 26 to November 29, and the data for the last six days is missing.

Officials at the Indian Institute of Tropical Meteorology (IITM), which operates the system, said the DSS model is still functional but technical difficulties with the website have caused delays.

"We are experiencing a technical glitch with the website, but the DSS model is still running. We have been using the model continuously, except for Monday, when we encountered some issues," a senior official at the IITM explained, adding that they are working to resolve the problem soon.

The last update on the DSS website, showing the contribution of stubble burning to Delhi's PM2.5 concentration, was recorded at 5.84 per cent on November 26, down from 6.99 per cent on November 25. No further update on stubble burning has been provided since then.

For November 27, November 28 and November 29, the DSS has estimated that Delhi's transport sector contributed between 21 per cent and 24.6 per cent to the city's PM2.5 levels, with "other sources" contributing between 22 per cent and 24.6 per cent.

The air quality in the national capital began deteriorating on October 30, when it entered the "very poor" category. Over the next 15 days, the AQI consistently remained in the "very poor" range, with readings of above 300.

The situation worsened further in the second half of November as the air turned toxic, with the AQI levels exceeding 400.

However, the air quality showed signs of improvement with the onset of December due to strong winds. Starting December 1, the AQI improved to the "poor" category and continued to get better.

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