Q. Why does air pollution rise in October each year?
October usually marks the withdrawal of monsoons in Northwest India. During monsoons, the prevalent direction of wind is easterly. These winds, which travel from over the Bay of Bengal, carry moisture and bring rains to this part of the country. Once monsoon withdraws, the predominant direction of winds changes to north westerly. During summers, too, the direction of wind is north westerly and storms carrying dust from Rajasthan and sometimes Pakistan and Afghanistan. According to a peer reviewed study conducted by scientists at the National Physical Laboratory, 72 per cent of Delhi’s wind in winters comes from the northwest, while the remaining 28 per cent comes from the Indo-Gangetic plains.
In 2017, a storm that originated in Iraq, Saudi Arabia and Kuwait led to a drastic dip in Delhi’s air quality in a couple of days.
Along with the change in wind direction, the dip in temperatures is also behind the increased pollution levels. As temperature dips, the inversion height — which is the layer beyond which pollutants cannot disperse into the upper layer of the atmosphere – is lowered. The concentration of pollutants in the air increases when this happens.
Also, high-speed winds are very effective at dispersing pollutants, but winters bring a dip in wind speed over all as compared to in summers. The combination of these meteorological factors makes the region prone to pollution. When factors such as farm fires and dust storms are added to the already high base pollution levels in the city, air quality dips further.
Q. What is the role of farm fires?
Farm fires have been an easy way to get rid of paddy stubble quickly and at low cost for several years. With the use of combine harvesters, the practice became more common as the harvester leaves behind tall stalks, which have to be removed before replanting. But the practice gained widespread acceptance starting 2009, when the governments of Punjab and Haryana passed laws delaying the sowing of paddy. The aim of passing this law was to conserve groundwater as the new sowing cycle would coincide with monsoons and less water would be extracted. This, however, left very little time for farmers to harvest paddy, clear fields and sow wheat for the next cycle. The paddy straw and stalks have high silica content and are not used to feed livestock. The easiest, but the least productive, way to get rid of it is to set it on fire.
Over the past 11 years, the practice has thrived despite efforts made by the Centre and state governments primarily because the alternatives, like the happy seeder machine which helps mulch the residue, are seen as unavailable, and money and time consuming by smaller farmers.
A 2015 source-apportionment study on Delhi’s air pollution conducted by IIT-Kanpur also states that 17-26% of all particulate matter in Delhi in winters is because of biomass burning. Over the years, the System of Air Quality and Weather Forecasting and Research (SAFAR) has developed a system to calculate the contribution of stubble burning to Delhi’s pollution.
Last year, during peak stubble burning incidents, its contribution rose to 40%. Over the past few days, it has been 2%-4%, indicating that a variety of factors, not just stubble burning, are responsible for the dip in quality. As November draws closer, the percentage contribution is set to go up.
The stubble burning season is around 45 days long. Air in Delhi, however, remains polluted till February.
Q. What are the other big sources of pollution in Delhi?
Dust and vehicular pollution are the two biggest causes of dipping air quality in Delhi in winters. Dry cold weather means dust is prevalent in the entire region, which does not see many rainy days between October and June. Dust pollution contributes to 56% of PM 10 and and the PM2.5 load at 59 t/d, the top contributors being road 38 % of PM 2.5 concentration, the IIT Kanpur study said.
Vehicular pollution is the second biggest cause of pollution in winters. According to the IIT Kanpur study, 20 % of PM 2.5 in winters comes from vehicular pollution. Over the years, governments have taken several steps to address pollution from vehicles. The introduction of BS VI (cleaner) fuel, push for electric vehicles, Odd-Even as an emergency measure, and construction of the Eastern and Western Peripheral Expressways are all part of the effort to reduce vehicular pollution, which experts say is more harmful as it is released at breathing level.
During the lockdown, this year, Delhi saw among the cleanest air since comprehensive records have been kept since 2015. It also saw above average temperatures in September, which meant the air remained cleaner for longer.
With vehicles back on the road, temperature dipping and stubble burning starting, Delhi’s air is set to get worse.