Meteorological conditions in Delhi have been “extremely unfavourable” in September and so far in October as compared to last year, which has helped in the rise of pollution levels, Central Pollution Control Board member secretary Prashant Gargava said on Friday.Data shared by Gargava during a press briefing showed that the main meteorological factors that help in dispersion of pollutants have been weaker between September 1 and October 14 this year as compared to the same period in 2019.
“Concentration of PM2.5 and PM10 in October and September was much higher than the same period in 2019,” Gargava said.The meteorological factors that determine accumulation and dispersion of pollutants include the number of rainy days, ventilation index, wind speed and mixing height. The total amount of rainfall in the period between September 1 and October 14 was 121 mm last year in seven rainy days, whereas this year it was just 21 mm in three rainy days.
Rain helps in settling down dust and other pollutants suspended in the air. Ventilation index — the product of mixing depth and wind speed, which is used to forecast the atmosphere’s potential to disperse pollutants — also remained low this year.
A ventilation index below 6,000 sqm/second with average wind speed less than 10 kmph is considered unfavourable for dispersion of pollutants. In September this year, the average ventilation index was 1,334 sqm/second as compared to 1,850 sqm/second last year.
Between October 1 and 14, the average ventilation index was 1,389 sqm/second as compared to 1,626 sqm/second last year.
Mixing depth is the vertical height in which pollutants are suspended in the air. On cold days with calm wind speed, this height reduces and leaves pollutants suspended closer to ground.
“If we have more mixing height and more winds, that’s good. If one of them reduces, then that creates an impact… In a landlocked area like Delhi, in winter, you have absolutely no winds, and the mixing depth also, which normally is much higher, upto 3,000 metres, gets reduced to 100-300 metres. So the (atmospheric) volume in which dispersion happens gets suppressed. In winter, these two things have an extremely adverse impact, ” Gargava said.
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