Gurugram: Air quality data from four official monitoring stations in the city shows that particulate matter pollutants, PM2.5 and PM10, are present in far higher quantities during the night than during the day. Peak nighttime concentrations of these pollutants, on several days over the past two weeks, have been double or nearly double the peak daytime concentration, according to the data from the Haryana State Pollution Control Board (HSPCB).
For example, data from the HSPCB’s air quality monitor at Vikas Sadan in Sector 11 shows that between 2am and 6am on November 1, the concentration of PM2.5 in the region was as high as 442ug/m3, indicating ‘severe’ quality air. However, this improved during the day, with PM2.5 levels dropping to as much as 182ug/m3 by 2pm.
Similarly, data from another monitor located on the Gurugram-Faridabad road shows that, between 1am and 10am on November 1, PM2.5 levels were as high as 464ug/m3, while the concentration of PM10 was as high as 439ug/m3. By 4pm, however, the concentration of PM2.5 had dropped to as little as 68ug/m3, while that of PM10 dropped to 126ug/m3. However, consistently high levels of particulate matter were observed at both locations over multiple hours during the night.
A third monitor, also located on the Gurugram-Faridabad road, shows that PM10 levels on November 1 were as high as 381ug/m3 in the early hours of the morning, before reducing to about 200ug/m3 during the day. Meanwhile, after touching 348ug/m3 at 12am, the level of PM2.5 dropped the subsequent morning to as little as 132ug/m3.
The trend holds true for most dates prior to November 5, when pollution levels hit the ‘severe’ threshold during the day itself. On November 3, for example, data from a station in Sector 51 shows that PM2.5 levels between 12am and 10am were in excess of 500ug/m3. At 4pm, however, the concentration was just 162ug/m3, shows the HSPCB data.
Kuldeep Singh, regional officer, HSPCB, Gururgam, confirmed the veracity of these observations, saying they are not surprising. “We are aware that PM2.5 and PM10 levels increase significantly at night, which is why most of our enforcement teams and patrolling happen at night itself. Even water sprinkling and road sweeping activities happen at night for the same reason,” he said.
This trend can also be observed on dates going back to early October, when air quality in the region had not yet deteriorated to current levels, though the variation is less stark around that time.
“There are three parameters which influence this day and night difference, including wind velocity, atmospheric temperature, and density of air. What happens during the night is that wind speeds are calmer, allowing pollutants to accumulate. Along with this, nighttime temperatures are also lower, making air more dense and less prone to dispersal, which happens later in the day when the sun is out,” said Dr Mukesh Khare, an air pollution scientist at IIT-Delhi and former member of the recently disbanded Environment Pollution (Prevention & Control) Authority (Epca).
However, experts also said that the general public is less likely to know of these differences in air quality, due to the manner in which the same is quantified. For example, on November 1, when the peak nighttime load of particulate matter exceeded the 500ug/m3 threshold in some parts of the city, Gurugram has an air quality index (AQI) reading of 306 — indicating ‘very poor’ air. Similarly, on November 3, Gurugram had recorded ‘poor’ quality air (with 270 on the AQI bulletin, the lowest reading in at least two weeks), even though data shows that air quality during the night time remained extremely hazardous.
“So, even if you’ve had eight hours of extremely hazardous air at night, which puts at risk the health of many people, including delivery boys, construction workers, the homeless, and so on… officially the data will still record the date as a ‘very poor’ air day, because the daily AQI value is calculated on a 24-hour average,” explained Abhishek Shrivastava, a city resident and environmental engineer.
A fundamental issue with reporting the AQI daily, in this manner, is that it does not take into account these diurnal variations. “The point of the AQI is to show exigencies of pollutants over a certain period. The way it is done in India looks at exigencies over a 24-hour period, which is done for the sake of conveying to the public a sense of how things were on a given date. While this method may suffice at other times of the year, we are now in a critical period of the pollution cycle, and regulatory authorities should monitor these episodic spikes,” said Khare.
Responding to these observations, HSPCB’s Singh said that authorities at the district level are well aware of the issue. “The AQI is calculated as per a Centrally prescribed method, but at our level we are looking at raw data from all the monitors. We plan our enforcement drives accordingly, deploying more manpower where pollution levels are higher,” said Singh.
According to Singh, biomass and garbage burning, and dumping of garbage and C&D waste — the activities that are harder to carry out during the day without going unnoticed — are the primary violations of air quality norms that have been observed by inspection teams during night patrols.
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