Wind, rain, lockdown keep pollution levels in check

Strong winds, frequent showers and vehicular and industrial restrictions during the two-and-a-half month-long lockdown have helped keep pollution levels in the national

Strong winds, frequent showers and vehicular and industrial restrictions during the two-and-a-half month-long lockdown have helped keep pollution levels in the national Capital in check.

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On Wednesday, the average Air Quality Index reading of the city was recorded at 152, in the ‘moderate’ range. This, however, dipped further to reach 128 in lower end of the ‘moderate’ range after some parts of the city witnessed thunderstorm and heavy rain, with wind speed touching 40-50kmph. Light rainfall was also recorded in parts of the city on June 3, June 4, June 5 and June 7, which also helped keep pollution under control.

On Wednesday, the maximum temperature at the Safdarjung observatory, considered the official recording of the city, was 41.5 degrees Celsius. At the Palam observatory, the maximum temperature recorded during the day was 43.6 degrees Celsius. However, after the evening’s rain, the temperature at Safdarjung fell by four degrees and at Palam, the temperature fell by seven degrees, according to IMD. The temperature is expected to fall further by 2-3 degrees in the next few days, the IMD forecast said.

“Cyclonic circulation over north Rajasthan and a trough from north Pakistan to the low-pressure area over the Bay of Bengal was observed on Wednesday, which resulted in the thunderstorm and rain in the Delhi-NCR region. This not only helped bring the temperature under control, but it also helped blow away pollution particles,” Kuldeep Srivastava, head of India Meteorological Department’s regional weather forecasting centre, said.

Recordings maintained by the Central Pollution Control Board (CPCB) show that since March 25, when a nationwide lockdown was announced, the AQI readings of Delhi have largely remained in the ‘satisfactory’ category. Even though the pollution levels in the city are generally lower in summers when compared to winters, this year’s levels were much lower compared to previous years.

Data from 2015 to 2019 shows that in the period between March and June, the pollution levels usually remain in the latter end of the ‘moderate’ or in the middle range of the ‘poor’ category.

Scientists at IMD said that along with the restrictions on human activities since March 25, the national Capital has also benefitted from the increased number of western disturbances (WDs) that have crossed the city this year. Even if the WDs did not bring rain on several occasions, they did result in increased wind speed that caused pollution particles to dissipate. From June 1, when the lockdown restrictions were lifted, the AQI levels have remained in the ‘moderate’ zone.

Data shows that six WDs passed Delhi in March and six in April, which is unlikely for these months. This trend, however, began in January, when nine WDs passed Delhi. Seven WDs passed the city in February. Usually, on an average, the months of January to June see four WDs each.

Pawan Gupta, a senior scientist (earth sciences) at the National Aeronautics and Space Administration’s (NASA) Universities Space Research Association (USRA), agreed that there has been a visible dip in the pollution levels all over northern India, including Delhi. Gupta is heading a study at NASA to examine the effect of the lockdown on pollution levels in India by studying the aerosol optical depth (AOD) levels, recorded through satellite images.

AOD is an indirect proxy for air quality that shows how solid and liquid particles in polluted air block sunlight. It represents particle loading in the entire atmosphere from the surface to top-of-the-atmosphere as opposed to PM2.5 levels, which are measured at the surface.

“There are a lot of factors behind the reduction in pollution levels, along with restrictions in human activities such as vehicular movement and industrial operations and fires. The weather has also played an important role,” Gupta said.