Haze Could be Fuelling South Asia's Warming

Since brown carbon in South Asia seems to have a longer bleaching half-life, it has implications for warming in the region.

New Delhi:  The haze over the national capital region and Northwest India seen during winters is not only causing problems to people on the ground but also impacting atmospheric climate.

A new study by an international group of researchers has found that the composition of a class of carbon-carrying aerosols in the haze changes as it travels from above the landmass to the oceans.

The light absorption capacity of brown carbon – a key component of the haze – decreases during transport but its bleaching half-life is 3.6 days compared to 9-15 hours in other regions. This means that brown carbon stays in the South Asian atmosphere for a longer duration and could contribute to the region’s warming, according researchers.

The haze, also called an atmospheric brown cloud (ABC), contains several types of particles from vehicular pollution, biomass burning and other sources. This layer of polluted air plays a critical role in shaping the regional climate as it absorbs as well as scatters incoming solar radiation.

Two important light-absorbing components of ABC are black carbon and brown carbon. While black carbon comes mainly from sources like diesel combustion, brown carbon is a result of biomass burning. The new study focused on brown carbon, as little was known about its light absorption capacity as it traveled over the oceans.

Researchers measured air samples in Delhi, located within the India-Gangetic plain; Bhola in south Bangladesh, which is at the edge of the plains; and at the Maldives Climate Observatory, Hanimaadhoo, where the brown cloud reaches after a long journey across the ocean.

The researchers found that the chemical composition of the aerosols changes between sites and over time. The concentration of brown carbon, which is water-soluble, was the highest over Delhi, lower over the site in Bangladesh and lowest over the Maldives.

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When scientists studied the water-soluble extracts of brown carbon, they found notable differences in light absorption characteristics between the three locations. Specifically, brown carbon molecules lost their warming capacity during transport, according to the study. It was published in the journal Science Advances on January 31.

“Through measurements at three locations, we were able to determine the ability of brown carbon to absorb solar light, and thus warm the surrounding atmosphere, and how fast this ability was decreasing due to a photochemical oxidation – a process we call bleaching,” Örjan Gustafsson, of the department of environmental science and analytical chemistry, Stockholm University, and a senior member of the research team, told India Science Wire.

Since brown carbon in South Asia seems to have a longer bleaching half-life, it has implications for warming in the region.

“As of now, we cannot comment on what is the ‘true’ warming capacity of brown carbon,” Sanjeev Dasari, a PhD student at Stockholm University and first author of the study, told India Science Wire. “We can, however, say that previous global modelling studies relying on ‘set’ values of brown carbon light-absorption might have wrongly estimated the warming effect.”

According to Raghu Murtugudde, a professor of atmospheric and oceanic science at the University of Maryland and a visiting professor at IIT Bombay, “This study is for winter months but still has crucial information considering the leading hypothesis for the reduction in monsoon over the past few decades has been the solar dimming by aerosols.

“It is clear that both climate prediction and projection models need to understand impacts of such drastic transformation of brown carbon properties.” He was not associated with the study.

Dinesh C. Sharma writes at India Science Wire and tweets @dineshcsharma.