What caused Delhi air pollution this year

What kept Delhi’s air polluted from October 21 to 26, the week that the city recorded the first ‘very poor’ air day of the season? The Centre for Science and Environment (CSE), a Delhi-based organisation that focuses on environment-related research and advocacy, analysed data to indicate that much of the air pollution over the week from within the city was caused by vehicular emissions.

What were CSE’s findings?

CSE’s analysis of data from October 21 to 26 found that of the local sources of pollution, vehicular emissions may have contributed around 51 per cent to the PM2.5 levels in Delhi. Local sources refer to the sources within the city. Among these, local sources, the next largest contribution was 13 per cent from residential sources, and 11 per cent from industries.

Construction activities contributed around 7 per cent to PM2.5, followed by 5 per cent each from the burning of waste and the energy sector. Road dust contributed around 4 per cent to PM2.5 levels.

The analysis also found that Delhi’s local sources caused around 32.9 per cent of the pollution in the city. The remaining share came from NCR districts (32.8 per cent), other districts (25.8 per cent) and biomass burning in the neighbouring states (9.5 per cent).

The analysis also used data from Google Maps to look at hourly traffic speed on 15 arterial roads in the city over the week. “High level of congestion” was noted on all of these roads with average speed ranging from 27 kmph to 32 kmph, speed being used as an indicator for congestion. The traffic build-up was found to be highest on October 21 and 22.

With vehicles contributing to the nitrogen dioxide levels in the city, the hourly nitrogen dioxide level was also found to peak around the time that congestion was maximum on the roads. The analysis noted: “Hourly NO2 levels during evenings could be high ranging between 73 µg/m³ to 86 µg/m³. This is the time when congestion is also high.”

What data did the analysis use?

The analysis used data from the Decision Support System (DSS) developed by the Indian Institute of Tropical Meteorology (IITM). The DSS provides hourly information of the sources of PM2.5 in Delhi – the contribution of different sectors to emissions, and the contribution of emission from Delhi and 19 districts in the neighbourhood.
The model developed by IITM uses an emissions inventory prepared by The Energy and Resources Institute (TERI) in 2018, data from the Central Pollution Control Board monitoring stations, and data from NASA satellites to generate forecasts on emission sources for five days.

What suggestions does the analysis make?

It asks for a “verifiable and measurable shift” to public transport by improving access — improving bus services and integration of metro stations with other modes of transport, besides providing an extensive network of walking and cycling infrastructure to connect neighbourhoods.

“Delhi needs congestion and pollution pricing and other restraint measures to control the traffic volume,” the analysis stated. Targets set for the electrification of the new vehicle fleet should be accelerated and met, it added.

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