The concentration of poisonous gases such as nitrogen dioxide and carbon monoxide rose up to seven times their safe limit in November, when Delhi was covered by a choking haze for several days.
Since pollution control agencies were busy with the exceptionally high levels of particulate matter — PM2.5 and PM10, the most dominant pollutants in Delhi’s air, these gases escaped scrutiny.
The Supreme Court-appointed Environment Pollution (Prevention and Control) Authority considers the part matter levels to enforce the Graded Response Action Plan, which was for the first time this year. The EPCA is planning to take into account other pollutants from next year.
Levels of ozone and ammonia also fluctuated drastically.
While SO2 and carbon monoxide can linger in the air from 10 days to a month, prolonged exposure to high levels of these gases could trigger a range of respiratory and cardiac diseases.
SO2, NO2 and volatile organic carbon can also trigger the formation of secondary pollutants such PM2.5 – ultra fine particles that can go deep inside the lungs.
High level of NO2 in the air has become a concern for experts and scientists. Data from air monitoring stations show that NO2 levels shot up by more than seven times above the safe standards in areas such as Punjabi Bagh over the last four November days. It had shot up to 588ug/m3 on November 28.
It is not just particulates
Concentration of choking gases shot up in the national Capital in November
“NO2 is primarily the result of burning diesel. It can’t be said for certain why it increased to such levels in Punjabi Bagh without a detailed analysis, but it could be because of the high density of vehicular population,” said Anumita Roychowdhury executive director at Centre for Science and Environment.
The gas, one of the key elements of smog, has dropped well below the safe standards in Delhi primarily because of use of cleaner fuels. Though it did not breach the safe standards for a single day, in NCR cities such as Ghaziabad and Bhiwadi, it increased beyond the safe standard.
“Monitoring SO2 is important because it triggers the formation of secondary pollutants. SO2 often condense and convert into other pollutants, reflecting a reduction in its own level,” said SN Tripthi head of the Centre for Environmental Science and Engineering at IIT Kanpur.
Ozone is a threat to asthma patients and can cause premature death, if it is high even for a short duration during the day. It forms primarily when nitrogen oxides, carbon monoxide (CO) and volatile organic compounds (VOCs), react in the atmosphere in the presence of sunlight.
The level of O3 shot up above the safe standards on some days at places such as Anand Vihar, Delhi University North campus and Lodhi Road. The maximum level was recorded at DU where it had shot up to 143 above the permissible limit of 100.
On Lodhi Road, it had shot up at least three times above the safe standards on certain days. Once in the air, this gas can linger for more than a month and its presence affects concentrations of other greenhouse gases, including methane, ozone and carbon dioxide.
“Carbon monoxide is usually emitted during burning of garbage or incomplete combustion,” said Dilip Ganguly assistant professor at Centre for Atmospheric Sciences at IIT Delhi.
Other gases: Other pollutants such as Benzene and Ammonia have also spiked sometimes in Delhis air in certain places. But on most of the days they found to be under safe limits.
Benzene, which is usually found in emissions, also escapes into the atmosphere when petroleum products are transported or handled. A fuel station (petrol pump) in the vicinity of an air monitoring station could push up levels of this gas. Its level often shot up in places such as Shadipur, Punjabi Bagh and Anand Vihar.
Levels of Ammonia also shot up in Delhi on some days but have never crossed the safe standards.
“Some pollutants may spike momentarily at some stations for various reasons but their overall levels are under control. This time, we experienced a far better November as far as pollution levels are concerned. Efforts like GRAP has started to show its effects and despite unfavourable weather conditions, pollution is still in very poor category,” said D Saha, head of the air quality laboratory at CPCB.