Pollution Transport from South Asia into the Tibetan Plateau

It is very common to think that the great barrier of the Himalayas protect the Tibetan Plateau from air pollution in the south. However, this is not true. When the transport pattern at higher levels is favorable, the transport of pollution from the South Asia to the Tibetan Plateau is evident.

South Asia is dominated by the periodical monsoon system. During the monsoon season, aloft winds are blowing towards east, hereby enabling the air pollution transport into Tibet. On the contrary, during non-monsoon season, the westerly winds are suppressed.

The first paper ‘Enhanced trans-Himalaya pollution transport to the Tibetan Plateau by cut-off low systems’ analyzes the aromatic measurements in Tibet during 2010 October. We use a 3D Chemistry Transport Model (CTM) and found that the cut-off low (enclosed low pressure system) can enable the pollution transport pathway even during non-monsoon season. Also, we found that the aromatic emissions are underestimated by current emission inventories.

The second paper ‘Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry’ studies the measurements during 2015 summer, when the monsoon occurs. I again simulated the pollution using a CTM and found that the ridge (high pressure system) will suppress the prevalent eastward transport during monsoon season. This complements our conclusion from the first paper.

But, why would the pollution into Tibet every matter? The transported pollution levels are quite low (say, ten times if not one hunder times lower than those in mega-cities) and hereby may not be a huge health threat there. However, one thing is more vulnerable than humans, that is, the glaciers.

Pollution such as black carbon will follow the same transport pathway and deposit on the glaciers which have been peacefully sleeping there for centuries. The blackened glaciers will absorb more sun light (shortwave radiation in scientific terms) and melt more during the summer time. This poses a huge threat to South Asia as the melt water might become a flood in lower altitudes.

Well, increased flooding might be hard to directly related to melt glaciers. Here is one thing more obvious: avalanches. If we heat the glaciers, they may slide or move more easily like our melt ice cream. But if glaciers move, they become avalanches.

Small mistake, big problem.