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New Evidence on Himalayan Pollution

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HERVE´ VENZAC, KARINE SELLEGRI, PAOLO LAJ et al. report new evidence of pollution in the Himalayas.

Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the Himalayas may lead to substantial radiative forcing in South Asia, resulting in potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat.

An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. And a better understanding of aerosol sources and their variability is needed for predicting the future evolution of the Earth’s climate. In particular, the formation of secondary particles through nucleation of their gaseous precursors is not well constrained although it potentially represents a significant source in specific areas of the troposphere. This is because new particle formation is a complex process that depends on the nature of gaseous precursor species, which differ according to the environment; on meteorological factors such as UV-radiation, temperature, and relative humidity; and on boundary layer dynamics.

Thanks to recent advances in measurement techniques, new particle formation has now been observed in rural, marine, urban, and background environments. However, the spatial extent of new particle formation events, in particular, their occurrence at high altitude, has rarely been documented on a long-term basis.

Elevated concentrations of ultrafine particles have sporadically been observed during airborne campaigns, and over longer time periods during ground-based measurements at the high altitude stations of Izana [3,200 m above sea level (asl), Canary Islands], and Jungfraujoch (3,580 m asl, Switzerland) (9). Although quite scarce, these studies provided indications that the process of new particle formation may not only be surfacelinked, but may constitute an entire atmospheric column. However, they did not allow differentiation between in situ nucleation of new particles and transport of newly formed particles from the boundary layer.

Our study shows the first evidence of very frequent new particle formation events occurring up to high altitudes. Our analysis is based on a 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid at Khumbu Valley (Nepal, 5,079 m above sea level), the highest atmospheric research station. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys.

During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.

Measurements of aerosol-size distributions at the NCO-P site were continuously performed by using the scanning mobility particles sizer (SMPS) technique (see Methods). The mean diurnal variation of the particle size distribution was calculated for each of the four distinct periods related to monsoon circulation: pre-monsoon (April–June), monsoon (July–September), postmonsoon (October– December), and dry (January–March) seasons. The dataset is available at here. To our knowledge, this database represents the first long-term record of particle size distribution at high altitude. Event frequency over the Himalayas is higher than the few reported so far at Mt Norikura, Japan (17% of the days over a limited period of 23 days) and at Puy de Doˆme, France.

The observed concentrations are surprisingly high for this high. Average size distribution observed at high altitude in the Himalayas is significantly driven by the occurrence of ultrafine particle events. Throughout the 511-day observation period, 200 unambiguous ultrafine particle events were detected. During these seasons, the ultrafine particle formation frequency is close to 50%. It should be noted that the simultaneous appearance of ultrafine particle events and increase in accumulation mode particle concentration results from the averaging process and does not necessarily document the occurrence of both events during the same days. Our results provide an important confirmation of the relevance of ultrafine particle events to influence aerosol number concentration at high altitude.

We note four interesting features related to the ultrafine particle concentration increase. One, the increase and following growth of the new ultrafine particle number concentration takes place during several hours, which indicates that the events are extended, at least, to the valley scale. Two, the occurrence of new particle formation is linked to meteorological conditions at the station. New particle formation events are inhibited under cloudy and favored under clear-sky conditions. Three, factors such as the condensational sink (CS), which is related to the amount of surface available for semivolatile gases to condense on preexisting aerosols, plays a significant role as well. Four, a marked diurnal variability of CN10–20 (condensation nuclei) is found for days with ultrafine particle events, but not on days without events. Clearly, the morning peak in particle concentration does not coincide with the maximum concentration of primary aerosol tracers, represented by the black carbon concentration. These factors strongly indicate that the increase in particle number concentration is decoupled from the primary pollution aerosols transported from the valley.

Cluster ions (diameter smaller than 1.4 nm) were observed to grow into larger sizes before each ultrafine particle event on 11 of 13 measurement days. Ions and particles continue to grow throughout the day while being transported by upslope winds from locations further down in the valley where they have nucleated. During February–March 2007, stable wind conditions were encountered, allowing the calculation of a growth rate.

Our results show that very frequent in situ nucleation takes place at high altitude along the whole Khumbu valley. The number concentration of ultrafine particle linearly increases during the first 2 h after the onset of new particle formation events, indicating that new particle formation lasts a few hours. During this period, the new particle source rate fluctuates. The cause for higher source rate during the monsoon and dry season may be linked to the lower preexisting aerosol background concentration previous to ultrafine particle events, as mentioned above.

The vertical wind component in mountain areas plays a crucial role in transporting gases and aerosols to higher elevations. The change of air masses between day and night can be followed with the evolution of a conservative variable such as the water vapor mixing ratio. The mixing ratio shows a clear diurnal variation with lower values during the night and maximum values in the afternoon connected to a transport of moist air from the valley. We found that nucleation (intermediate ion concentration increase) is detectable at the interface between the free troposphere and the polluted boundary layer air still containing diluted anthropogenic precursor gases. Considering that particle concentrations are locally increased by a factor of 3 or more during nucleation events which are likely taking place on a larger scale in the Himalayan valleys, new particle formation events potentially provide a significant additional source of particles to the free troposphere. Because of the very low aerosol number concentration background in the free troposphere, this source may potentially control the aerosol number concentration in the free troposphere all year round. Additional modeling studies are needed to quantify the export of particle from the Himalaya valleys to the free troposphere.

The nature of the gaseous or ionic precursors involved in the new particle formation events is still unknown and will be the object of future studies. If anthropogenic precursors, possibly those emitted by domestic wood combustion, are involved in events, the predicted increase in emissions in Nepal may lead to changes of the free tropospheric aerosol background driven by both mountain slope circulation and new particle formation during upslope transport.

Abridged from "High frequency new particle formation in the Himalayas," (PDF/HTML) published in the U.S. journal Proceedings of the National Academy of Sciences.


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