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| Last Updated:14/11/2018

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Heat, emissions raise Delhi's ozone levels

 NEW DELHI: Air pollution is once again at worrying levels in the capital. But this time it is not particulate matter but ground-level ozone, associated with severe health impacts, that has breached the safe limit.

On Friday, the average ozone level went over 71 parts per billion in Lodhi Road and Delhi University, compared to the standard of 50ppb. Interestingly, ozone levels are way higher around certain monitoring stations than others. Scientists say these spots could be experiencing the urban heat island effect where a mix of soaring temperatures, built-up area and emissions from various sources like traffic cause ozone levels to go up.

This ozone is different from stratospheric ozone that protects the earth from harmful ultra-violet rays. The ground-level ozone that's currently wreaking havoc in certain parts of Delhi is created due to chemical reactions between oxides of nitrogen (NOX), volatile organic compounds (VOC) and carbon monoxide (CO) in the presence of sunlight and heat.

Sources of NOX and VOCs are usually industrial facilities, motor vehicle exhausts, power plants and others. Ozone levels are usually measured in eight-hourly averages (during daylight) and hourly averages. When Indian Institute of Tropical Meteorology under ministry of earth sciences calculated the average for eight-hourly averages from May 11 to May 20, ozone levels were significantly high around Delhi University and Lodhi road. The temperature during this phase has been hovering around 40-43 degrees. Hourly average of ozone at 2 pm at these two spots peaked to about 103ppb on Thursday and Friday when the standard for hourly average is 90ppb.

"The variability of ozone levels is very high in Delhi. Certain areas are more affected than others which is why we are calling them urban heat islands. The temperature in these areas is also possibly higher than other parts of the city," said Gufran Beig, chief project scientist at System of Air Quality Weather Forecasting and Research (SAFAR), IITM.

Every summer between April and June, ozone levels go up in Delhi. "We see a clear trend of ozone levels rising around this time every year. Ozone has an immediate effect on people suffering from asthma. Which is why ozone levels are always given as part of smog alerts as school children and vulnerable population need to aware of it and stay indoors if required. Ozone is the only gas which has two sets of standards-hourly and eight hourly because high levels of ozone can have very serious health impacts," said Anumita Roychowdhury, head of the clean air programme at Centre for Science and Environment.

Last year the eight-hourly average for ozone had peaked to 80-85ppb according to records with IITM. This year, too, ozone levels may increase next month.

This toxic gas which is "temperature dependent" according to scientists can have a slew of health impacts. It is associated with breathing difficulty, coughing, and sore throat, aggravate lung disease like chronic bronchitis, increased frequency of asthma attacks, damage lungs says environment protection agency.

Many urbanized parts of US and Europe are known to suffer due to high ozone levels. While PM2.5 (fine, respirable particles) levels go up in Delhi during winter, the city is now experiencing high ozone levels every summer making the population vulnerable in both seasons.