"Pollution in India, Pakistan and China has become a worldwide issue," Paolo Laj of the University of Blaise Pascal in France, told environmentalresearchweb. "Determining the air concentration of aerosol particles in free troposphere air masses over the Hindu Kush Himalaya area is crucial to understanding the impact of anthropogenic aerosols on the atmospheric radiative forcing and on monsoon circulation, but measurements of the aerosol properties in this area are scarce, due to severe logistic and technical problems for long-term studies."
Laj and colleagues from the University of Blaise Pascal and Italy’s Institute for Atmospheric Sciences and Climate and EvK2-CNR studied a 16-month record of aerosol size distribution taken at the Nepal Climate Observatory. This measurement station is at Pyramid in Nepal’s Khumbu Valley, 5079 m above sea level. The researchers also examined data for ion cluster measurements taken over a two week period.
"We believed that information on aerosol and gas concentration in a remote area such as the Khumbu valley could be an ideal place to monitor long-rang transported pollution from India and China," said Laj.
Ground-based studies of ultrafine particles at high elevations have previously taken place at Izana in the Canary Islands (3200 m above sea level) and Jungfraujoch in Switzerland (3580 m). These experiments weren’t able to distinguish whether new particles had nucleated in situ or had been travelled up the mountain from lower down, however.
Laj says the study provides the first direct evidence that pollution aerosols from India and Nepal can be transported by mountain breezes up to the 5000 m line and, episodically, can further penetrate over the Tibetan Plateau above the frontal line of many Himalayan glaciers.
The number concentration of particles was surprisingly high for such a high altitude, say the researchers, peaking at 2000/cubic cm during the post-monsoon season at noon. For comparison, the monthly averaged concentrations at the free tropospheric site at Jungfraujoch were less than 1000/cubic cm independent of the season.
It's also the first time scientists have found production of new particles taking place at high altitude with a high frequency. This formation of secondary aerosol particles takes place when conditions are right for their precursor gases to react. In this case, it led to a 10 to 20 times increase in the concentration of small (10 to 20 nm diameter) particles with respect to background levels.
The researchers say the frequency of ultrafine particle events was surprisingly similar to that reported in urban environments.
"The presence of polluted aerosols and black carbon in regionally transported aerosols affects the energy budget of the atmosphere, leading to warming of the atmosphere, but also deposits to snow surfaces and absorbs light, thus decreasing the albedo of the snow and modifying the energy budget of snow surfaces," said Lao. "This affects snow melting, the duration of the snow cover and the seasonal availability of water."
Now the team plans to extend its measurements to identify chemical mechanisms involved in the process of particle formation. It’s also possible that researchers will install an additional measuring site higher up in the mountain – at 8000 m, close to the SHARE-EVEREST weather station – to monitor export of the newly formed particles to the free troposphere.
The researchers reported their work in PNAS.