NEW DELHI: Satellite data show that the levels of airborne particles (aerosols) over northern India dropped significantly since the Covid-19 lockdown began in the country, said the United States space agency, Nasa, on Tuesday. Aerosols are tiny solid and liquid particles, suspended in air, that reduce visibility and can damage human lungs and heart.The Nasa satellite sensors observed “aerosol levels at a 20-year low for this time of year in northern India” after just a week of reduced human activities such as closure of many factories and severely reduced vehicular and airplane traffic — a situation which allowed some in Punjab to report seeing a few Himalayan mountain ranges with the naked eye for the first time in many years.The space agency, however, noted that the story in southern India is a little hazier as satellite data show that the aerosol levels have not yet decreased to the same extent and the levels, in fact, seem to be slightly higher than in the past four years. It said though the reasons are unclear, it could be related to recent weather patterns, wind or other factors.“By all accounts, the 2020 lockdown reduced those human-made emission sources,” said a Nasa Earth Observatory report that analysed the data, retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) on Nasa’s Terra satellite.“We knew we would see changes in atmospheric composition in many places during the lockdown. But I have never seen aerosol values so low in the Indo-Gangetic Plain at this time of year,” said Pawan Gupta, a Universities Space Research Association (USRA) scientist at Nasa’s Marshall Space Flight Center Though some aerosols have natural sources (dust storms, volcanic eruptions, and forest fires), the others come from human activities, such as burning of fossil fuels and agriculture fields.The Nasa Earth Observatory’s report on its portal by its science write Kasha Patel noted that the human-made aerosols “tend to contribute most of the smaller particles that have greater potential for damaging human health”. The report said such aerosols contribute to unhealthy levels of air pollution in many Indian cities.Scientists, however, expect the aerosol levels to increase slightly in upcoming weeks in parts of India as seasonal dust storms begin from the Thar Desert and Arabian Peninsula.“The hard part with understanding aerosols is that particles can move based on wind patterns and other meteorology. You have to disentangle what is caused by the human fingerprint versus a meteorological factor,” said Robert Levy, programme leader for Nasa’s MODIS aerosol products.On the lockdown and its effects on pollution, Levy said, “We have a unique opportunity to learn how the atmosphere reacts to sharp and sudden reductions in emissions from certain sectors. This can help us separate how natural and human sources of aerosols affect the atmosphere.”Earlier, the Central Pollution Control Board (CPCB) - India’s national pollution watchdog - too in its report on ‘impact of Janta curfew and lockdown on air quality’ on March 31 noted substantial improvement in air quality in many towns and cities across the country. It attributed this improvement to stringent travel restrictions and shutting down of non-essential activities including those of air polluting sectors. The Board had analysed the real time air quality data of 115 cities from March 16 to March 29 to assess impact of lockdown.