Lack of bright sunshine in last six days spiked Delhi’s pollution levels
Delhi did not receive any ‘bright’ sunshine over the last six days at least. The pale sun, with its diffused rays, failed to heat up the atmosphere. This trapped pollutants near the ground, pushing up pollution to record levels similar to that of 2016..
It was only on Monday that the city received around three hours of ‘bright’ sunshine after six days. According to the India Meteorological Department’s (IMD) sunshine recorder at Safdarjung, the last time Delhi received some ‘bright’ sunshine was on October 28—just for 1.4 hours.
“Air when heated up by the sun’s rays moves up vertically, which helps to disperse pollutants. But as the sunlight was diffused, this process was hampered and pollutants got trapped close to the ground, pushing up pollution levels. This is just one of the many factors behind the spike,” said an IMD official.
On October 23, when Delhi received around 8.4 hours of bright sunshine, Delhi AQI was 242 (poor category). But as the sun disappeared from October 29, the city’s AQI started increasing. On October 29, the AQI touched 400, just one notch less than the severe level. On November 3, it touched 494.
Experts said it was this diffused sunlight, along with other factors such as low wind speed, northwesterly winds and high moisture, that spiked pollution levels since Diwali. Manmade factors such as smoke from stubble burning in Punjab and Haryana and emission from Diwali crackers worsened the condition.
IMD officials said in the initial period, the clouds blocked the sun’s rays. Later, the blanket of smoke and dust that was enveloping the city further cut off the sun’s rays.
“Sunshine and pollution are closely interlinked and sometimes form a vicious cycle. If the sunshine is cut off, the atmosphere fails to heat up and pollutants don’t get dispersed. And when pollution spikes, there is an envelope of smoke and dust. This cuts off the sun’s rays, which spikes pollution,” said a senior official of the Delhi environment department.
Aerosols in the air and the clouds seeded by them have been known to reflect about a quarter of the Sun’s energy back to space. Different aerosols scatter or absorb sunlight to varying degrees, depending on their physical properties.
“Sunshine helps to bring down the moisture level, which otherwise makes the air heavy and traps the pollutants close to the ground level. This makes the air lighter and the pollutants rise upwards. The free movement of pollutants make them to disperse faster,” said D Saha, former head of the CPCB’s air quality laboratory.
How IMD measures ‘bright sunshine hours’
An instrument named sunshine recorder is used for measurement. The sun’s rays pass through a glass sphere in the instrument and falls on a card board strips graduated in terms of time. Whenever there is sunshine, only that portion of the card is burnt, just like children burn a piece of paper with a magnifying glass using sunlight. Whenever the sun is covered by cloud, that portion of the card is not burnt. As the sun moves, its focused image burns a trace on the card. By measuring the trace at sunset, the duration of ‘bright sunshine’ may be recorded.