Delhi, through its pollution control committee, started monitoring air quality in real time only in 2010. Delhi, through its pollution control committee, started monitoring air quality in real time only in 2010.

Chief Minister Arvind Kejriwal said last week that pollution levels in Delhi, primarily the concentration of particulate matter, has reduced by 25% over a period of four years.

Five years ago, in 2014, a global study on air quality trends by the World Health Organisation had declared Delhi the most polluted city in the world. Since then, the Centre, states and courts have taken several steps to arrest pollution in the city.

Delhi air pollution: What the data show

Delhi, through its pollution control committee, started monitoring air quality in real time only in 2010. It started out with four stations — in R K Puram, Punjabi Bagh, Anand Vihar, and Mandir Marg. The number of stations was increased to 26 last year.

It was in 2012 that Delhi saw its worst air quality, a senior official of the Delhi Pollution Control Committee (DPCC) said. “We felt the full force of crop-residue burning that year, especially in October and November. It was the first time that this burning was seriously flagged. We realised this was a big reason for the sudden dip in air quality in Delhi,” he said.

But since 2012, the average annual concentration of particulate matter — the primary cause of pollution in the city — has been falling. Gradual in the beginning, the dip has been sharper between 2015 and 2018. (See chart)

Particulate matter, according to the United States Environmental Protection Agency (EPA), is a mixture of solid particles and liquid droplets in air. Some particles can be seen with the naked eye; others can only be detected under a microscope. In Delhi’s air, the primary pollutants are PM2.5 (inhalable particles of diameter 2.5 micrometres and smaller) and PM10 (10 micrometres and smaller).

PM2.5

DPCC data from 2012 to 2019 show 2018 saw the lowest average concentration of PM2.5. In 2012, the annual average was 160 micrograms per cubic metre; it came down 20% to 128 micrograms/cubic m in 2018. Despite a tough first three months, owing primarily to adverse weather, air quality improved consistently for the rest of the year.

The most polluted months of the year are November, December and January, with pollution peaking in November, monthly averages between 2012 and 2018 show.

It is in November that the highest volume of crop residue is burnt in Haryana, Punjab and UP. It is also when temperatures fall and humidity rises, aiding the increase in concentration of pollutants in the air. Locally, the burning of leaves picks up in November.

However, as the chart shows, PM2.5 concentrations have fallen over the years — in November as well as in the ‘cleaner’ months of July, August and September.

PM10

Between 2012 and 2018, the concentration of PM10 reduced by 21% from an average 351 micrograms/cubic m to 277 micrograms/cubic m. PM10 is more prominent in the air in winter, primarily because of open burning and road and construction dust.

Until August this year, Delhi’s performance in terms of PM10 concentration has been encouraging. In August, the average concentration fell to double digits for the first time since 2012; in 2013, this figure was as high as 288 micrograms/cubic m.

Delhi weather news: Seasonal variation, weather

Over the past five years, several studies have pointed to the fact that weather and seasons are among the biggest determinants of Delhi’s air quality.

“No matter how much we try, air quality in winter will be worse than in summer. It is difficult to imagine a day in Delhi when air quality is ‘good’ or ‘satisfactory’ on the air quality index. That said, the results that our efforts have borne over the past few years show that if we work on fixing what we can, things will improve,” another DPCC official said.

Localised weather conditions also have a major role in determining air quality. “On a sunny and windy winter day, air quality can improve several notches within hours. Weather conditions are also the reason why winters are more polluted than summers. Cold, foggy, windless days help in the accumulation of pollutants. Wind blowing from the direction of a major pollution event such as crop burning or dust storms, will pollute the city as well,” said a scientist at the India Meteorological Department (IMD).

What has worked in Delhi

In 2014, lawyer Vardhaman Kaushik approached the National Green Tribunal (NGT) against pollution levels. His petition became the basis of several NGT orders, upheld by the Supreme Court, including the ban on old diesel and petrol vehicles.

Between 2014 and 2017, the Delhi government, Central Pollution Control Board, and Environment Pollution (Prevention and Control) Authority carried out drives, issued orders, and implemented orders passed by NGT to curb air pollution, including the implementation of the odd-even road rationing scheme.

The biggest push came in 2017, when the Centre notified the Graded Response Action Plan (GRAP), which provided state governments in Delhi and the NCR with a roadmap for action. If the air was severely polluted for more than 48 hours, for example, the entry of trucks would be stopped, and all construction work halted. The GRAP also set roles for each agency, fixing accountability.

Shutting of the two thermal power plants in Delhi, completion of the eastern and western peripheral expressways for vehicles not destined for Delhi, a ban on PET Coke as industrial fuel, and the introduction of BS VI fuel have, experts believe, made a big difference.

There are, however, two things that experts believe have been done completely locally that have made a big difference.

“Open burning has been largely curtailed in the city. Earlier, as soon as autumn arrived, piles of leaves would be set on fire — but stringent fines now have meant the practice has almost disappeared. The second thing is the regulation of construction activity. While not as successful as the ban on open burning, regular enforcement drives have meant that whenever a ban is ordered, it is largely followed,” the first DPCC officer said.

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