Explained: How Cyclone Sitrang helped Delhi record its cleanest post-Diwali air since 2015

Delhi’s air quality slipped to ‘very poor’ on Tuesday, a day after Diwali as Delhiites ignored the ban and celebrated the festival by setting off firecrackers across the Capital and NCR region.

As per data provided by the Central Pollution Control Board (CPCB), the AQI one day after Diwali (Tuesday) was 323 at 6.30 am. The 24-hour average AQI at 4 pm on Tuesday stood at 303, the lowest for the day after Diwali since 2015.

An AQI between zero and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, and 401 and 500 severe.

Delhi Chief Minister Arvind Kejriwal said the air levels had improved and it was the results of efforts made by Delhi residents to curb pollution in the city. However, he added that while the outcome was encouraging there was still a long way to go.

प्रदूषण के क्षेत्र में दिल्लीवासी काफ़ी मेहनत कर रहे हैं। काफ़ी उत्साहजनक नतीजे आये हैं। पर अभी लंबा रास्ता तय करना है। दिल्ली को दुनिया का सबसे बेहतरीन शहर बनायेंगे https://t.co/VVoLxBrGd4

— Arvind Kejriwal (@ArvindKejriwal) October 25, 2022

On Wednesday morning, Delhi’s air became a bit easier to breathe as the AQI moved from the ‘very poor’ category to the ‘poor’ category — the AQI stood at 262 at 6 am.

While Chief Minister Kejriwal credited residents of the Capital for their fight against pollution, experts cite that there were multiple meteorological conditions that played a role in the improved air quality this time. And one of the biggest factors was Cyclone Sitrang.

Cyclone Sitrang and the dirty air

According to experts, the movement of Cyclone Sitrang towards Bangladesh helped in clearing Diwali pollution over Delhi-NCR. Experts observed that since Cyclone Sitrang moved away from the northern plains of India, it pulled winds and moisture towards it. This led to moderately strong westerly wind flows and dry conditions, both of which enabled dispersal of pollutants.

India Meteorological Department (IMD) chief Mrutyunjay Mohapatra explained that when Sitrang moved towards Bangladesh, winds from north India moved in that direction. Therefore, it exacerbated the air flow in north India. It also drained out moisture from the region.

Mohapatra said that a deep trough in the mid and upper atmosphere over east Uttar Pradesh also reinforced the westerly wind flows in the region.

Mohapatra was quoted as saying to Times of India, “Had the cyclone hit the Odisha coast and moved northwestward towards Chhattisgarh, the situation in north India would have been very different. A cyclone moving towards the region would have led to an incursion of moist, easterly winds which would have led to cloudy skies. Under such conditions, pollution levels may have shot up.”

Former IMD scientist DS Pai echoed similar sentiments, attributing the cyclone for lower pollution levels in Delhi. He explained the pollution did not stay because after the remnant of the cyclone moved towards northeast India and the northwesterly winds strengthened.

Private forecaster Skymet Weather Services vice president (climate change and meteorology) Mahesh Palawat also noted the same that the pollution build-up wasn’t too high and that Sitrang’s strong winds moved it away from over Delhi.

Incidentally, it wasn’t just Delhi-NCR that benefited from Cyclone Sitrang. News agency IANS had reported that Cyclone Sitrang had also brought down pollution levels on Kali Puja night of Monday and the morning after in Kolkata.

Other factors at play

While Cyclone Sitrang was a major factor in bringing down pollution levels in Delhi a day after Diwali, there were other reasons too at play.

A delay in peak stubble burning incidents and also that Diwali fell in October when temperatures are warmer helped control air pollution, according to Vijay Kumar Soni, head of the environment monitoring and research centre at the IMD.

Data released by Consortium for Research on Agroecosystem Monitoring and Modeling from Space (CREAMS) — an initiative of the Indian Agricultural Research Institute — reveals that the country recorded 1,519 instances of stubble burning on the day of Diwali. This is far lower than the 3,525 fires that were detected last year on the same day.

Another factor that needs to be considered is the delayed withdrawal of monsoon from northwest India. Delhi and surrounding areas witnessed widespread rain in early October, which washed out accumulated air pollution. This ensured that the build-up of pollutants was not high.

Moreover, measures under the Graded Response Action Plan (GRAP) also kicked in earlier than previous years — actions such as mechanical and vacuum-based sweeping of roads, stopping the use of diesel generator sets had already been implemented.

How other cities fared

While Delhi recorded a ‘very poor’ AQI a day after Diwali, it wasn’t the only city suffering from air pollution. In Mumbai, the country’s financial capital, most stations were in the “moderate category” with the AQI below 200. However, the air quality was in the “poor category” with AQI above 200 at Malad West and Deonar stations.

In Chennai, most weather stations recorded AQI in the “poor” category. In Bengaluru, the AQI ranged between “moderate” and “satisfactory”. The AQI stood in the “poor” category at the Silkboard monitoring station.

Kolkata had a different story to share. Most monitoring stations recorded an AQI in the ‘good’ category.

With inputs from agencies

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