Here is the lowdown on ozone and levels of particulate matter during the second phase of odd-even scheme, where they were in the run-up to the curbs being put in place, and a comparison with trends during the corresponding period last year.
The levels of ozone, which scientists describe as the most critical summer pollutant in Delhi, between April 15 and 30, peaked at 88 parts per billion or 176 micrograms per cubic metre (µg/m³) on April 30, the last day of the odd-even scheme, according to SAFAR data. In comparison, during April 1-15, the levels peaked at around 60 parts per billion or 120 µg/m³ on April 6.
On April 24, ozone levels hit 75 parts per billion or 150 µg/m³, the first sharp rise seen during the scheme, from around 62 parts per billion or 124 µg/m³ the preceding day. Between April 19-22 ozone levels remained around 51-52 parts per billion or 102-104 µg/m³. According to scientists, the level of the pollutant spiked consistently between April 17 and 19, ranging between 75 and 80 µg/m³. An initial dip was reported between April 15-17.
Comparing this with data from April last year, the trends are quite similar, according to scientists. On April 24 last year, ozone levels had peaked at 90 parts per billion or 180 µg/m³, the highest between April 15-30. Between April 1-15 last year, a peak of around 70 parts per billion or 140 µg/m³ was observed in ozone levels on April 5, the highest during the first fortnight of the month. This year, the peak was observed on April 6.
“Last year between April 24-30, a drop was observed after the peak, with ozone levels dropping to around 55 parts per billion or 110 µg/m³ on April 30. This year after dropping to 68 parts per billion or 136 µg/m³ on April 29, the levels peaked again on April 30. This could be due to lower wind speeds, and higher temperatures, critical to the accumulation of other pollutants and the photochemical reaction which triggers their combination to form ozone,” explained a scientist.
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Where drops were noticed
Delhi Pollution Control Committee (DPCC) data shows peak levels of ozone dropped this year at some stations in central Delhi. According to DPCC’s Mandir Marg station data, between April 15-20, ozone levels peaked at 57 µg/m³ on April 20. Last year during the same period, ozone peak levels stood at 166 µg/m³.
In the week after April 20 this year, the levels of the gaseous pollutant peaked at 105 µg/m³ on April 25, compared to a peak of 188 µg/m³ during the corresponding period last year. Between April 25-30, the peak levels dropped to 81 µg/m³. Last year, the peak levels, while recording a drop in the last week of April, stood at 138 µg/m³.
System of Air Quality and Weather Forecasting And Research ( SAFAR) also monitored the finer PM 1 particles, or particles of size less than 1 micron, which are known to penetrate deep into the lung tissues and the cardiovascular tract. Between April 15-30, the data shows, PM 1 levels peaked on April 30 at around 78 µg/m³. After recording less than 60 µg/m³, till April 23 — the safe limit for the larger PM 2.5 particles — the PM 1 levels escalated between April 25-28 at around 75 µg/m³. After dropping to 60 µg/m³ again on April 29, the levels shot up on the last day of the scheme. “The safe limits for PM 1 are not defined as yet in India. But considering that these are finer than PM 2.5 and they can penetrate deeper into the respiratory and cardiac organs, the safe limits should be far lower,” a scientist said.
In comparison, between April 1-15, PM 1 levels peaked at around 70 µg/m³ on April 3. Three peaks of around 60 µg/m³ each were observed on April 4, 7 and 9, before the levels dropped to between 30 and 40 µg/m³ till April 14. On April 15, levels again hit 60 µg/m³. PM 1 levels were not being monitored last year.
PM 2.5 is the pollutant most directly associated with combustion in vehicles, according to scientists. It includes all particles of size less than 2.5 microns. According to SAFAR data, between April 15-30, PM 2.5 levels peaked at 190 µg/m³ on April 30, almost double the level on the same date last year. Between April 26-28 this year, PM 2.5 levels spiked from 129 to 179 µg/m³. Last year, between April 25-29, the levels dropped from 140 µg/m³ to about 70 µg/m³.
Between April 1-15 this year, just before the odd-even scheme started, PM 2.5 levels peaked at 137 µg/m³ on April 3. After dropping to 81 µg/m³ on April 14, the levels rose on April 15 to 120 µg/m³. On April 9, another peak of 117 µg/m³ was observed. Last year during the same fortnight, levels of PM 2.5 peaked at 120 µg/m³ on April 11. On April 15 last year, the levels were recorded at 90 µg/m³.
According to data from DPCC’s 24-hour monitoring station at RK Puram , PM 2.5 levels peaked at 458 µg/m³ on April 30 and 430 µg/m³ on April 29, in comparison to a peak of 225 µg/m³ between April 26 and 28. Between April 21-25, the levels of the same pollutant peaked at 101 µg/m³.
Last year, in the last week of April, the levels peaked at 113 µg/m³ on April 30, according to DPCC data. Between April 15-20 last year, PM 2.5 levels at the station peaked at 119 µg/m³. Between April 20-25, the peaks increased to 229 µg/m³ on April 21 and 222 µg/m³ on April 23. On April 25, peak levels dropped to 105 µg/m³, nearly comparable to this year’s peak of 101 µg/m³ during the same period, according to scientists. Levels were also high in the first week between April 1-6, when a peak of 170 µg/m³ was observed. Last year, during the same period, a peak of 126 µg/m³ was observed.
At Punjabi Bagh station, DPCC data shows between April 14-20, peak levels of the pollutant were seen at 115 µg/m³. Last year during the same period, the levels peaked much higher at 231 µg/m³ on April 16.
Over April 20-25 this year, levels peaked at 225 µg/m³. Between April 25-30, the PM 2.5 levels peaked at 475 µg/m³ on April 29, and dropped to a peak of 406 µg/m³ on April 30. Last year, during April 25-30, PM 2.5 peaked at 54 µg/m³ on April 27. Between April 20-25, levels peaked at 181 µg/m³ on April 21.
DPCC data for larger particles of size less than 10 microns which are associated with vehicular and other types of pollution such as road dust, smoke emitted from garbage and other kind of fires shows levels showed the sharpest spikes during the last week of April.
At RK Puram station, between April 1-6, PM levels peaked at 328 µg/m³. Between April 7-11 and April 12-16, the peak levels of the large sized particles of size less than 10 microns dropped to 194 and 199 µg/m³, respectively. Between April 17 and 22, the peak levels spiked again to 346 µg/m³. Between April 23-27, the levels of PM 10 observed the sharpest peak of 941 µg/m³. On April 28, the peak levels dropped to 428 µg/m³. On April 30, another spike was seen in the peak levels at 799 µg/m³. Last year, between April 20 and 25, PM 10 peaks recorded at the station were much lower at 226 µg/m³.
At Punjabi Bagh, PM 10 level peaks showed a nearly similar trend, with the sharpest spike in levels observed between April 25-30 at 656 µg/m³ on April 30 this year. Last year, during the same period, peak levels recorded were lower at 233 µg/m³ on April 27. The peak levels saw a sharp rise between April 14 and 20 this year, at 343 µg/m³, compared to the preceding weeks, when peaks of 164 and 219 µg/m³, respectively, were seen.
Dr B Sengupta, former Central Pollution Control Board (CPCB) member secretary, said PM 10 levels could also be associated with an increase in spontaneous fires observed this year due to the spike in temperatures. “When these fires are created, they generate smoke which comprises largely coarse particles and some small particles. They definitely contribute to the particulate load, but the extent of that contribution needs to be studied compared to other sources.”
The CPCB is yet to release a detailed report on its analysis of air quality during the second phase of the odd-even scheme. However, according to the daily summary of the AQI from seven stations in Delhi, between April 23-28 this year, AQI increased consistently from 262 to 332. On April 29 and 30, the AQI dipped marginally to 314 and 316 respectively. On April 23, 25, 26, and 29, ozone and PM 2.5 were recorded as the dominant pollutants. On the rest of the days in the last week of April, particulates of both sizes were the dominant pollutants. Between April 15-23, AQI levels, after spiking at 293 on April 20, dropped to 270, 272 and 262, respectively over the next few days.
At CPCB’s Punjabi Bagh station, between April 15-30 this year, compared to three “moderate days” last year, not a single moderate day was observed this year. Compared to four “very poor” days last year, this year, six days of the same category were observed. The number of “poor days” came down from four to three. At RK Puram station, again compared to three moderate days between April 15-17 last year, this year no moderate air quality day was recorded. The number of very poor days at this station, however, fell from four to two. The number of “poor” days increased from four last year to six this year.
What experts say
Scientists caution meteorological conditions need to be taken into account while comparing levels of pollutants between this year and the last . “Summer pollutant patterns as such are different from winter pollutants and not contributed by particulates alone. Last year there were also a lot of rainy days during this period, which could have led to the apparently low pollutant levels,” said Anumita Roychowdhury from the Centre for Science and Environment (CSE).
She added that rather than short-term data, the impact of the scheme could be assessed scientifically only after comparing the 15 days with the whole summer 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.”
Roychowdhury said, “The agenda behind this policy needs to be understood. Our aim was to arrest the peaks of pollution as was done in the winter. Even if the ambient air quality does not indicate that, it needs to be understood that actual human exposure has come down.”
Scientists also said that due to high rains last year during this period, which would have dispersed pollutants, short-term comparison may not give an accurate comparison of air quality. Dr Sengupta said load of pollutants from vehicles would have come down because a significant number stayed off the roads, but its impact on pollution concentration in the air warranted study.