New Delhi, Oct 22 Air pollution in Delhi can persist for up to five days when atmospheric conditions are unfavourable for the dispersion of pollutants, with temperature emerging as the most influential meteorological factor affecting the city’s air quality over time, according to a study published in the journal “Environmental Pollution”.
The study, based on air quality and meteorological data from January 2017 to December 2024, found that pollution levels in Delhi were driven mainly by pollutant accumulation and subsequent dispersion, highlighting the need to reduce emissions at the source.
Researchers from Jamia Millia Islamia, the Delhi Pollution Control Committee and the Central Pollution Control Board analysed daily air quality index (AQI) readings alongside temperature, rainfall, relative humidity and wind speed to understand how weather conditions influence pollution levels.
The paper, titled “Impact of meteorological drivers on air quality index: A case study from Delhi”, was authored by Danish Ali, Mani Singh, Anwar Ali Khan, Ajit Kumar Vidyarthi and Sirajuddin Ahmed.
The study found that the average AQI in Delhi during the eight-year period was around 210, placing it in the ‘poor’ category under the Central Pollution Control Board’s classification.
The average AQI in 2024 was 8.14 per cent lower than in 2017, indicating some improvement over the period. However, severe pollution episodes continued, particularly during the winter months.
“Air pollution in Delhi primarily resulted from its own accumulation and diffusion patterns,” the researchers said, noting that when pollutant dispersion slows, much of the pollution can remain in the atmosphere for several days.
The study found that the contribution of AQI’s past values to its variation was around 93.3 per cent in the first five days of the analysis, declining to about 81.1 per cent by the 16th day. This points to the strong influence of existing pollution levels on subsequent air quality.
Among the meteorological variables examined, temperature accounted for 13.235 per cent of AQI variation over the 16-day analysis, followed by rainfall at 2.127 per cent. Wind speed contributed the most among these factors during the second period, accounting for 3.271 per cent, while relative humidity consistently accounted for less than 2 per cent.
The researchers noted that wind speed plays an important role in pollutant dispersion over the short term, particularly during the first two to four days. Rainfall helps remove pollutants from the atmosphere and limits the resuspension of road dust.
Temperature influences atmospheric conditions that determine whether pollutants accumulate near the surface or disperse. During winter, low temperatures, a shallow atmospheric boundary layer, and conditions such as temperature inversion can trap pollutants close to the ground, worsening air quality.
The monthly analysis showed that the number of ‘very poor’ and ‘severe’ AQI days increased between October and February. November recorded the highest proportion of ‘very poor’ days at 53.8 per cent and ‘severe’ days at 24.6 per cent.
In contrast, air quality improved during the monsoon months of July and August, when satisfactory AQI days accounted for 62.7 per cent and 60.5 per cent, respectively. The researchers attributed the improvement partly to rainfall washing pollutants out of the atmosphere.
The study also found that ‘moderate’ AQI days were the most frequent category throughout the study period, while ‘good’ AQI days accounted for less than 2 per cent annually. Notably, Delhi recorded no days classified as ‘good’ in 2018 or 2024.
The researchers stated that the study examined four meteorological variables and did not directly account for other factors such as traffic volume, industrial activity and crop residue burning. They recommended that future research incorporate these emission sources to better understand their contributions to AQI fluctuations.
The study called for sustained efforts to reduce emissions, including strengthening public transport, expanding the use of electric buses and taxis, and promoting coordination among states to address crop residue burning.
While existing pollution-control measures have yielded some improvements, the persistence of severe pollution episodes underscores the need for a more comprehensive approach, the researchers said.
(PTI News)
