Pollution tracks rising fire graph
The 24-hour average air quality index (AQI) recorded till 4pm on Saturday was 345, down from Friday’s 366. The air quality continued to be in the ‘very poor’ category, a notch below what is considered ‘severe’ – a threshold beyond which exposure could be hazardous for those with vulnerabilities.Updated: Oct 24, 2020, 23:56 IST
Air pollution relented slightly in the national capital region, helped by stronger winds and marginally warmer day temperature on Saturday, although a sustained number of farm fires in neighbouring states meant the situation could take a turn for the worse at any point and the region’s 30 million people were largely at the mercy of weather conditions.
The 24-hour average air quality index (AQI) recorded till 4pm on Saturday was 345, down from Friday’s 366. The air quality continued to be in the ‘very poor’ category, a notch below what is considered ‘severe’ – a threshold beyond which exposure could be hazardous for those with vulnerabilities.
The situation comes at a time when farm fires in Punjab and Haryana recorded their highest numbers yet – it was 1,676 on Thursday and 1,257 on Friday – according to satellite thermal imaging data from United States’s National Aeronautics and Space Administration (Nasa).
Farmers and environmental experts expect more of such fires, which send plumes of micro fine PM2.5 into the atmosphere before it descends and settles over cities, making their way into people’s lungs and leading to breathing difficulties and chronic illnesses.
“About 50% of harvesting is done but another half is left. Though fires have been fewer compared to previous years, I think still they haven’t stopped completely because a large number of farmers do not have straw management machinery. We are expecting that harvest will be complete by November 15,” said Harinder Singh Lakhowal, Bharatiya Kisan Union general secretary, Punjab.
The System of Air Quality and Weather Forecasting and Research (SAFAR) model used by Central Pollution Control Board estimated that farm fires contributed roughly 9% to Delhi’s pollution on Saturday, although this estimated is difficult to make.
“It’s very difficult to say if farm fires have peaked. If you compare last year’s data, peak is yet to come,” said Vijay Soni, scientist, IMD (air pollution).
This now leaves the Capital at the mercy of how the weather – the wind speed and the wind direction – changes. “Air quality has improved in Delhi for two reasons — good mixing height and better wind speed. Wind speed was around 8-12kmph on Saturday, even though wind direction was northwesterly. Our models are showing slight improvement in air quality in the next couple of days owing to better mixing height,” Soni added.
Union Ministry of Earth Sciences’s air quality monitoring centre, System of Air Quality and Weather Forecasting and Research (Safar), also credited Saturday’s improvement to weather conditions. “The boundary layer wind direction (the layer where surface friction plays a role in slowing the wind and changing the wind direction) is westerly and wind speed is low. Hence, pollutant transport towards the Delhi region was low,” Safar analysis showed.
Environmental experts said the recurring problem underscores the flaw in focussing on pollution control measures only when the situation has already worsened. Anumita Roychowdhury, executive director (research and advocacy), Centre for Science and Environment, said for results to show, agencies will have to work around the year, instead of focussing only on the winter phase of Grap.
“The result of your work will be seen now. Last moment measures will not work. Winter action is only fire fighting,” Roychowdhury said, referring to Grap (graded response action plan – a set of curbs on activities such as construction and personal cars that become stricter as the air quality worsens).
Northwesterly winds are what sweep the PM2.5 particles in from Punjab and Haryana. And since air moves in different ways in different layers, there are also complicated local wind patterns that determine what happens to construction dust, tailpipe emissions and gases from any other kind of fires generated in the city.
This has two components: the wind speed and what is known as the mixing height of air and suspended particles above the ground. This height is primarily determined by temperature. When temperature increases, the air expands and pushes the mixing height further, giving pollutants closer to the ground more space to mix with the atmosphere. When the opposite happens, the pollutants get trapped closer to settlements, becoming more concentrated and, thus, raising pollution levels.
Together, these local factors are used to calculate the ventilation index. Saturday’s ventilation index was 12,000 m2/s. A ventilation index below 2,350 sq metres/second is considered poor, and it effectively means local pollutants are being trapped.
In Delhi’s worst bouts of air pollution recorded till now, it is these local pollutants that mix with pollutants from fires to create a deadly cocktail that is linked to drastically reduced life expectancy in the country.
That situation might still play out, since temperatures are expected to only get colder. IMD data shows that on Saturday, the city’s maximum temperature at the Safdarjung observatory, which is considered the official recording of Delhi, was 33.2 degree Celsius — a notch above the season’s normal. On Friday, the maximum temperature was 32 degree Celsius.