Brown Carbon "Tarballs" Detected In Himalayan Air Could Speed Up Glacial Melting

Mount Everest. Shrimpo1967 via Flicker CC BY-SA 2.0

The Himalayas has the highest peaks of any mountain range on Earth and is home to the largest reserve of glacial snow outside the Arctic and Antarctic circles, sometimes giving it the name the "third pole". But being so high up and isolated has not shielded the region from industrial air pollution.

A new paper analyzed the composition of particles in the air over the mountain range, which stretches across India, Pakistan, Afghanistan, ChinaBhutan, and Nepal, and discovered the abundant and alarming presence of brown carbon "tarballs". The study, published in Environmental Science & Technology Letters, reports that 28 percent of all particles in the air were tarballs, which is bad news for many different reasons.

Carbon substances in the atmosphere are divided into black carbon and brown carbon. Black carbon is a fine particulate matter that is mainly pure carbon. It is usually formed in incomplete combustion at high temperatures and is a major component of soot. Brown carbon is carbon mixed with oxygen but also traces of other elements such as nitrogen, sulfur, and potassium. This is mainly produced in the burning of biomass or vegetation and ends up creating tarballs, small, viscous spheres only a few hundred nanometers across.

Previous studies have long highlighted the effect of black carbon in the region, tracking how these aerosol particulates travel along air mass trajectories from China and India to the Tibetan Plateau and the Himalayas. Brown carbon has long been suspected to follow the same path.

The motion of black carbon around the Himalayas. NASA

The air samples were taken at a remote high-altitude recording station on the northern slope of the Himalayas and the team found that 28 percent of the thousands of particles detected were tarballs. They were able to link increased numbers of tarballs to days when elevated levels of pollution occurred. They were even able to track the creation of the brown carbon to the Indo-Gangetic plains, in particular to widespread wheat-burning, using satellite data and wind patterns.

The particles were likely pushed towards and then above the Himalayas by monsoons, before coming to ground on the ice of the Tibetan plateau where the Qomolangma research station is located.

These tarballs are both light- and heat-absorbing, making it harder for snow and ice to reflect the Sun's rays back and thus easier for glaciers to melt. The researchers suggest that based on this work, future climate models should include the long-range transport of tarballs for a more accurate picture.

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