New particle formation in the free troposphere: from the Alps to the Everest Base Camp.

Atmospheric aerosols can affect the climate by absorbing or scattering incoming radiation and also by acting as cloud condensation nuclei (CCN). A recent study estimates that the major fraction of CCN comes from gas to particle conversion (Merikanto et al., 2009). During the last decade, several nucleation studies have been published based on field observations mainly in the planetary boundary layer. Therefore, we have only little information about the free tropospheric case. The aim of these studies is to understand what species contribute to new particle formation at high altitude. In order to characterize NPF processes, advanced instrumentation was deployed at the Swiss station Jungfraujoch (3580 m asl - Bianchi et al., 2016) and at the Himalayan Nepal Climate Observatory Pyramid site, not far from Everest base camp (5079 m asl). Previous studies have already showed that at both of these locations NPF takes place frequently. However, no chemical information of the vapours was retrieved. At the Nepal Observatory, we deployed an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF), a particle size magnifier (PSM) and a neutral cluster and air ion spectrometer (NAIS). The APi-TOF measured the chemical composition of either the positive or negative ions during the nucleation events and when equipped with a chemical ionization source it provided information on the chemical composition of the neutral species. Figure 1 shows the first APi-TOF mass spectrum running in negative mode recorded above 5000 m asl during a nucleation event. The main ions that have been identified so far are all deprotonated acids: sulfuric acid, nitric acid, malonic acid, methanesulfonic acid and iodic acid. Larger ions are formed by different combinations of these acids (i.e. H2SO4°HSO4-, CH3SO3H°HSO4-, etc.). We will present a detailed analysis of the particle evolution during NPF and also the chemical composition of the small clusters measured with these advanced mass spectrometers. We will also show that even at this altitude highly oxidised molecules (HOM) have been detected, confirming the CLOUD laboratory experiments (Kirkby et al., 2016). Bianchi, F., et al. (2016) Science, 6289, 1109-1112.Kirkby, J., et al. (2016) Nature, 10.1038/nature17953. Merikanto, J., et al. (2009) Atmos. Chem. Phys. 9 (21), 8601-8616.