The impact of the Himalayan aerosol factory: results from high resolution numerical modelling of pure biogenic nucleation over the Himalayan valleys.

Observational data collected in December 2014 at the base camp of Mount Everest, Nepal, indicated frequent new particle formation events of pure biogenic origin. Those events were speculated to be controlled by the along-valley winds forming in the valley connecting the Indo-Gangetic plain to the observational site, the Nepal Climate Observatory-Pyramid. The valley winds funnel highly oxygenated organic molecules of biogenic origin to higher elevations where they nucleate. The mechanism was referred to as "The Himalayan aerosol factory". Its geographical extent and climate implications are currently unknown. In view of this, we conducted numerical chemical model simulations to corroborate the presence of the mechanism, and to quantify its geographical extent. Our numerical simulations confirmed that biogenic emissions located in the valleys can be converted into ultra-low volatility organic compounds, transported to the observational site by the along-valley winds, and therein nucleate. The overall time scale of the process, from the release of biogenic emissions to the conversion to ultra-low volatile organic compounds to the arrival time at the observational site, was found to be around 4 hours, consistent with the predicted along-valley winds intensity and the geographical distribution of biogenic emissions. A first estimation of the maximum injection height of biogenic particles, and highly oxygenated organic molecules, indicated the presence of efficient nucleating gases and biogenic particles at an elevation as high as 5000-6000 m a.s.l. These results suggest that the Himalayan chain, under specific weather conditions, is a main contributor to the biogenic aerosol loads in the free troposphere. Considering these findings, field campaigns, especially at the entrance of the valley's floors, and research consortia supporting atmospheric research in Asian mountain regions, are highly encouraged.