Decoding Delhi’s Diwali data: How 24-hour average hides real-time pollution peaks

On Diwali night, despite a widespread cracker ban, Delhi witnessed massive violations across the city, leaving residents puzzled when the Air Quality Index (AQI) readings reflected only “Very Poor” levels rather than “Severe.”
Understanding the intricacies of AQI measurement helps explain this apparent discrepancy. The AQI reported by pollution monitoring bodies, such as the Delhi Pollution Control Committee (DPCC), Central Pollution Control Board (CPCB), and the India Meteorological Department (IMD), represents a 24-hour average, rather than real-time data. Consequently, the intense pollution spikes between 8 pm to 1 am, during the height of Diwali festivities, are diluted by the cleaner air data from earlier and later hours.
A detailed analysis by the monitoring stations reveals that night-time pollution levels on Diwali were alarmingly high, with PM2.5 and PM10 particles soaring up to 30 times the standard limit. These pollutants pose significant health risks, underscoring the failure of the cracker ban enforcement.
However, various climatological and meteorological factors played a crucial role in dispersing these pollutants. After midnight, conditions such as horizontal and vertical dispersal—where pollutant particles are carried away by wind and atmospheric movement—contributed to a rapid improvement in air quality.
Explaining the real-time and 24-hour average AQI, former additional director with the DPCC, Mohan P George, said, “AQI is for the common man hence we use a 24-hour average because it evens out the extreme events which is of no use for the common man. In India, we release AQI data but in Western countries, the AQI is just a colour code, like Green, Yellow, and Red. Hence, in those countries, there is no difference between an AQI of 215 and 225 because colour coding for any data between 200-300 range will be the same.”
Every year the pollution levels during Diwali have become a significant concern, with data often failing to accurately reflect the real-time worsening situation. Observers frequently note that pollution reports do not isolate the critical hours of Diwali when firecrackers are predominantly used, thus lacking a straightforward annual comparison.
This oversight is compounded by variables like meteorological conditions, which differ each festival season, influencing pollution readings and health impacts. Meteorological conditions play a crucial role in pollution dispersion and accumulation. For instance, in years with low temperatures and minimal wind speeds, pollutants from firecracker combustion can linger in the atmosphere longer, leading to worse air quality even if fewer crackers are used.
Conversely, in years with a significant rise in firecracker usage and increased violations of regulations like those enacted by the Supreme Court, favourable weather conditions, such as higher temperatures and stronger winds, can mitigate the immediate pollution impact, thus presenting a distorted picture in the 24-hour air quality data.
The current year’s scenario exemplifies this complexity. Despite a substantial increase in cracker bursting and regulatory non-compliance, the pollution levels have not escalated to expected levels due to beneficial weather patterns. This discrepancy in data underscores the need for a nuanced approach to monitoring pollution—one that takes into account real-time data and adjusts for meteorological influences.
The air quality in Nehru Nagar took a significant hit. By 5 pm on October 31st, the concentration of PM2.5 particles measuring at 101 micrograms per cubic meter. The scenario worsened dramatically as the evening progressed and firecrackers began to light up the sky. At 8 pm, the PM2.5 levels skyrocketed to 324 micrograms per cubic meter. By 9 pm, a further surge was observed, with levels reaching a hazardous 539 micrograms per cubic meter. The situation spiralled by 10 pm, with PM2.5 concentrations climbing to 898 micrograms per cubic meter and peaking at an alarming 1,527 micrograms per cubic meter by midnight.
This means, the air pollution increased by more than 15 times between 5 pm to midnight on Diwali when firecrackers were burnt. As the festivities wound down post-midnight, the pollutant levels began to decline. By 1 AM on November 1st, the concentration saw a marginal reduction to 1,172 micrograms per cubic meter. By 11 AM the next day, the levels decreased significantly but still remained at 145 micrograms per cubic meter.
The Diwali night of heightened air pollution in Vivek Vihar, East Delhi due to the bursting of firecrackers led to a dramatic spike in PM2.5 levels. Prior to the onset of the celebrations, air quality measurements indicated a PM2.5 concentration of 101 micrograms per cubic meter at 5 PM on October 31st. However, the situation rapidly deteriorated as night fell and the festivities began in full swing.
At 8 pm, the PM2.5 concentration in the atmosphere had more than doubled to 222 micrograms per cubic meter, signalling the start of a significant air quality decline. By 9 pm, the readings escalated to 296 micrograms per cubic meter, and an hour later, the concentration surged to an alarming 677 micrograms per cubic meter.
The crescendo of air pollution was reached at 11 pm, with the PM2.5 levels skyrocketing to 1210 micrograms per cubic meter, almost doubling in just an hour. The peak was observed at midnight when the readings hit a staggering 1853 micrograms per cubic meter, marking an increase of more than 17 times the level recorded earlier at 5 pm. As the crackers subsided post-midnight, the air quality showed gradual signs of improvement.
By 1 AM on November 1st, the concentration decreased to 1416 micrograms per cubic meter as the peak period subsided. The following morning brought further relief, with readings falling to 119 micrograms per cubic meter by 11 am, hinting at a return to a semblance of normalcy.