In Delhi, heatwave pushes ozone levels past safe limit
According to DPCC data for the last week of April (April 24-30), temperatures shot up to 43-47°C in parts of Delhi (against a normal average of around 38°C during the period), with locations such as Jawahar Lal Nehru (JLN) Stadium, Dr Karni Singh Shooting Range, Nehru Nagar, Mandir Marg, RK Puram and Narela all breaching the eight-hour ozone limit of 100 micrograms per cubic metre six out of the seven days.
With mercury in the National Capital Region (NCR) rising earlier than usual this year and roads choking with vehicles, the level of ground-level ozone, a highly reactive gas that can be particularly dangerous to those suffering from asthma and respiratory conditions, is on the rise and has already breached permissible standards at several spots across the city, data from Delhi Pollution Control Committee (DPCC) shows.
The gas is formed as a result of a mixture of heat and gases such as oxides of nitrogen and volatile organic compounds, largely occurring during the day in areas where there is traffic congestion or the presence of several industries.
According to DPCC data for the last week of April (April 24-30), temperatures shot up to 43-47°C in parts of Delhi (against a normal average of around 38°C during the period), with locations such as Jawahar Lal Nehru (JLN) Stadium, Dr Karni Singh Shooting Range, Nehru Nagar, Mandir Marg, RK Puram and Narela all breaching the eight-hour ozone limit of 100 micrograms per cubic metre six out of the seven days. At Mundka, a busy industrial area, it breached the eight-hour limit on five of the seven days, while at Aurobindo Marg, where traffic congestion is an issue, it failed to meet the standards on three of the seven days.
During this period, the highest eight-hourly values were recorded at JLN stadium, where it touched 199.8 micrograms per cubic metre, followed by the Dr Karni Singh Shooting Range (188.4) -- both nearly two times the safe limit.
Unlike PM2.5, PM 10 or NO2 which have 24-hour standards, ozone levels are measured by eight-hour standards and one-hour standards, largely owing to how dangerous the gas can be in a short span of time. While the eight-hour standard of ozone is 100 micrograms per cubic metre, the hourly standard is 180 micrograms per cubic metre.
DPCC data shows the gas’s hourly values touched 251 micrograms per cubic metre at JLN stadium during the last week of April close to noon every day when there is substantial traffic and the temperature is high. At Nehru Nagar, next to Lajpat Nagar, it reached 238 micrograms per cubic metre, primarily due to the traffic.
According to experts, while the levels of PM2.5 (particulate matter with diameters of 2.5 microns) and PM10 (suspended coarse particulate matter, either solid or liquid, with a diameter of 10 micrometres or less) may reduce, dangerous gases can still pollute the air, with ozone generally becoming the lead pollutant on those days.
“Ozone levels are highest on days when there are clear skies, and if the level of particulate matter is low, the sun’s heat penetrates deeper into the atmosphere, reacting with oxides of nitrogen to form ozone,” said Dipankar Saha, former head of the Central Pollution Control Board’s air laboratory. He added that ozone levels are likely to increase further in May and June, when the heat peaks.
A study by the Centre for Science and Environment (CSE) last year found that high levels of ozone have become a year-round problem for Delhi — particularly during the summers.
Tanushree Ganguly, programme lead at the Council on Energy, Environment and Water (CEEW) said since emissions from vehicles are a significant source of NOx, there is a need to focus on the vehicular sector to reduce ozone levels, which will also aid in the reduction of particulate matter.
“Multiple spots, including Anand Vihar, Ashok Vihar, Chandni Chowk, Patparganj, Narela and Jahangirpuri, reported ozone concentrations higher than the permissible 180 µg/m3 over the last few days,” said Ganguly.