Delhi’s air quality deteriorates despite rains as toxic smog blankets city

New Delhi: The national capital experienced relentless rains on Friday, but remained enveloped in a thick blanket of smog, rendering the city landscape rather gloomy.

The dense fog was a result of condensation of moisture that trapped pollution, hurting visibility and worsening air quality in the city, according to experts.

The air quality index (AQI) in the national capital region deteriorated to 353 on Friday, from 345 a day earlier, remaining in the ‘very poor’ category, according to data from the Central Pollution Control Board (CPCB). This is despite the moderate to slightly intense rain in the city since morning.

An AQI reading of 0-50 is categorized as ‘good’, 51-100 as ‘satisfactory’, 101-200 ‘moderate’, 201-300 ‘poor’, 301-400 very poor’ and 401-500 ‘severe’.

The air quality is likely to remain in the ‘poor’ to ‘very poor’ category until 5 January due to unfavourable meteorological conditions, including wind speed.

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Meanwhile, the India Meteorological Department (IMD) expects moderate to intense rainfall for another two hours in the national capital and adjoining areas and issued a warning for people planning to go outside.

It rains in NCR

“During winter, if there is intense rain, it is expected to break down the particle like a washout effect and increase moisture levels. Because of cold conditions, a lot of moisture condenses, and fog gets created. Because of the increase in the moisture level and condensation of water in the air, the pollution is trapped. We are in a bad air quality situation because of the inefficient natural ventilation process in the city,” Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment (CSE), told Mint.

“The pollution is trapped because of inversion, and when the rain is pouring, there may be some washout effect, but at the same time because of higher moisture level in the air and the moisture that is getting condensed because of the cold becomes the trapper of the pollution,” Roychowdhury explained.

Whether the rain, which is predicted to continue until Saturday, will bring down pollution levels in the national capital is hard to anticipate, she said CSE. “It depends on whether, after the rain stops, we are going to have a lighter condition, whether the sun will be out or not, and most importantly on wind speed."

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Due to the interaction of western disturbance with easterly winds over central parts of India, light to moderate rainfall or thunderstorm accompanied by hailstorm is likely over Northwest (includes Delhi) and Central India till Saturday, the weather bureau said.

According to the air quality early warning system for Delhi, monitored by the ministry of earth sciences, the predominant surface wind is likely to be variable direction with wind speed less than 8 kmph till Friday evening and decrease thereafter, becoming less than 4 kmph from northeast direction during night. As a result, smog or shallow fog is likely in the evening and night. This means cloudy sky, light to moderate rain or thunderstorm and lightning accompanied with gusty winds (speed 30-40 kmph) are likely. This is going to be persistent till Monday.

Anti-pollution measure

The Delhi-National Capital Region (NCR) on 16 December enforced graded response action plan (GRAP) stage 4, the strictest level of anti-pollution measure that the city follows, which comes into effect when the air quality index crosses the 400-mark, indicating that air quality is “severe”.

This was the second time this winter that these restrictions were enforced; the first time was in November, when stubble burning was a large contributor to pollution.

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The GRAP restrictions, however, do not address the problem of transport as a whole. Under GRAP 4, entry of trucks is banned into Delhi, except those carrying essential commodities – but it includes no other measures to control the use of private vehicles. Construction and demolition activities are also banned, though data shows that on 17 December, construction was responsible for only around 2% of PM 2.5 levels in the National Capital Region (NCR).

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Delhi's AQI typically has multiple peaks each year, with the most severe pollution occurring between October and February.

According to a paper by IIT Delhi, there are two major peaks; one obviously coincides with the stubble burning period and the second peak is always in December-January, which is mostly meteorology driven.

“December-January pollution levels are always high in winter because of the weather. Whether the situation was worse or better during the corresponding period last time can be said once the emission data is out,” said Sagnik Dey, professor at the Centre for Atmospheric Sciences, IIT Delhi. “Although we do not have updated emission data, pollution levels are frequently exceeding. Once the year ends, only then can one make a comparison. There is progress, but quite slow.”

“For considerable progress, we need to accelerate all the emission control actions for all the sources, not just stubble burning. You must also investigate the meteorology or conditions, for example, if the boundary is set over this year, so obviously it will trap more pollution closer to the surface. So, you have to isolate the meteorological effect to understand what's happening in terms of emissions. Otherwise, it's always the mixed signal,” Dey suggested. “It is important to know the major sources and reduce emissions from those sources.”