According to the CPCB, on Monday, also ozone levels remained high with five of nine stations including RK Puram, Punjabi Bagh and AnandVihar recording Ozone as the dominant pollutant on Monday.

The second phase of the odd-even scheme vehicle has come exactly a year after a series of orders by the National Green Tribunal (NGT) on a spate of issues concerning air pollution in Delhi. These orders came in the backdrop of a series of articles published in The Indian Express, titled `Death by Breath’ which investigated the particulate matter curves in the capital from the pre-CNG days to 2015.

One year later, how far has the air quality changed in the capital?

If we compare the period April 15-25 the CPCB station at Punjabi Bagh recorded three “moderate days” last year and not a single moderate day this year. Compared to four “very poor” days last year, this year, we’ve had six days `very poor’ days this year.

At another station, RK Puram, there were three moderate days between April 15 and 17 last year while this year no moderate air quality day has been recorded during the same period. The number of `very poor’ days at this station, however, fell from four to two. The number of “poor” days increased from four to six this year.

At the two stations, the gaseous pollutant ozone has been recorded as the prominent pollutant for two days April 17 and April 25 for RK Puram, and April 25 for Punjabi Bagh this year. Last year, only PM 10 and PM 2.5 were recorded as the dominant pollutants. Scientists say ozone is among the dominant summer pollutants which is formed by a photochemical reaction between different pollutants including the finer particles from vehicular pollution. “Ozone formation is also linked to weather because high temperature is critical to its formation. The heat seen over the last few days could also be responsible for the spike in ozone levels this year compared to the previous year during the same period,” a scientist explained.

AnumitaRoychowdhury from the Centre for Science and Environment (CSE) said meteorological conditions this year were starkly different from last year. “Summer pollutant patterns as such are different from winter pollutants. Last year there were a lot of rainy days during this period which could have led to the apparently low pollutant levels,” Roychowdhury explained.

She added that rather than short term data, the impact of the odd-even scheme could be assessed scientifically only after comparing the 15 days with the whole season. “Not only are there wide fluctuations in data between days, but even during the same day the levels may vary. In January we had to compare the data of the odd-even fortnight with the entire winter season to understand its impact on air quality,” she said.

A comparison of PM 2.5, data between 8 am and 8 pm on April 15, 2015 and on April 15, 2016, highlighted that at RK Puram PM 2.5 peaked on April 15 last year at 85 micrograms per cubic metre at 10 am, followed by 75 microrgams per cubic metre at 9 am, and 73 micrograms per cubic metre at 6 pm. On April 15 this year, at the same station a peak of 298 micrograms per cubic metre was recorded at 8.30 am, followed by 213 micrograms per cubic metre at 9.30 am. In the evening, the peak was lower at 173 micrograms per cubic metre at 6 pm. At Mandir Marg in 2015, a peak of 67 micrograms per cubic metre was observed at 9 am, followed by 99 micrograms per cubic metre at 12.30 pm. This year, on the same date, a peak of 136 micrograms per cubic metre was recorded at 9 am followed by 94 micrograms per cubic metre at 10.55 am.

An official in the IMD department said despite the high temperatures, air quality dipped on April 15 this year due to a reduction in wind speeds compared to the last few days. “The last few days, the high temperatures and wind speeds had kept air quality at moderate levels. On Friday, the wind speeds were comparatively lower, so more particulate matter could collect in the air,” an official said.