Weather systems turn one region's waste into another's pollutant

  • Snehal Rebello, Hindustan Times, Mumbai
  • Updated: Sep 09, 2015 11:30 IST

The soot lurking in the air you breathe may have travelled from thousands of kilometres away, a new study has shown, underlining the importance of a united effort to reduce pollution.

According to a study by the US-based National Center for Atmospheric Research (NCAR), soot or black carbon (BC) – released mainly by the burning of agricultural waste and the use of dung as cooking fuel – are being transported by weather systems to places far away from their source.

The study also said an estimated 60% BC emissions are from the residential sector.

“The Indian government providing subsidies on LPG to the poor and its focus on boosting solar energy will immensely help bring down black carbon emissions,” said Rajesh Kumar, principal investigator, NCAR.

“If we can bring down emissions levels of this short-lived pollutant in the next 20 years, it will help not only to improve quality of the air we breathe, but also help slow down global warming in the short-term,” Kumar said.

The study measured BC mass concentration from 21 sites across India to validate the US-based air quality model they used for simulating BC.

The validated model results were used to show around 30% of BC in peninsular India comes from the north Indian plains. Similarly, 30% BC released in south India travels north during the summer and monsoon months.

In addition to biomass burning, BC particles – known as soot – are also emitted during vehicular combustion. This has impacts on both the climate, as well as health, as the particles are so fine, they can cross cell walls.

BC particles also efficiently absorb heat from sunlight, making these pollutants the second most individual warming agent in the atmosphere, after carbon dioxide.

Black carbon participles also affect health as it constitutes nearly 10% of the particulate matter (PM) with a diameter of 2.5 micrometres or less, with a potential to be breathed into and lodged in the lungs.

“Our analysis shows neighbouring states have to act together to control BC emissions. For example, Bihar cannot mitigate its air pollution effectively, without the support of Uttar Pradesh and West Bengal,” said Kumar.

India’s contribution to global BC emissions is 7% to 14%, while BC pollution from the rest of the world accounts for about 5% to India’s BC pollution.

The major problem is the burning of agriculture waste, according to Harish Gadhavi, head, aerosol radiation and trace-gases group, National Atmospheric Research Laboratory, Andhra Pradesh.

Gadhavi, who has studied BC emissions over Gadanki, a rural site in AP, said, “Previously, sugarcane waste was burnt, but today it is used to make paper. Similarly, if there are ways to reduce other agriculture waste, it would help reducing black carbon,” said.

The team also studied distribution of BC emissions near the earth’s surface – in the lower troposphere (below 3kms) and free troposphere (above 3kms). This study is important because elevated levels of BC in the lower troposphere is dangerous for human health, and high BC levels in the free troposphere can worsen global warming.

The analysis found the highest concentration of BC is near the earth’s surface in winter, while it is highest during summer in the free troposphere.

“Most of these changes are driven by changes in regional weather patterns, which means the monsoon season not only provides relief from the heat, but also brings healthier air to breath,” the study said.

During winter and monsoon transition months, local sources contribute about 94% to 96% to the BC emissions near the earth’s surface in north India, while their contribution to other parts of India ranges between 68% and 81%.

In contrast, local sources contribute about 95% to BC emissions during the summer in south India, while regional sources account for between 26% and 28% of these emissions in the rest of the country.

Additionally, emissions from north and as far as east India have been found to reach the Himalayas.

BC can contribute 3-15% to total particulate matter pollution with a diameter of 2.5 micrometer or less in India depending on the season and location.

The study was conducted by US-based National Center for Atmospheric Research, Vikram Sarabhai Space Center, Thiruvananthapuram, Indian Institute of Tropical Meteorology, Pune and Max Planck Institute for Chemistry, Germany. It was published in the Journal of Geophysical Research - Atmospheres.

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