Chemical Characterization Of Atmospheric Ions At The High-Alpine Station Jungfraujoch (Switzerland)

The ion composition at high-altitude (3454 m a.s.l.) was measured with an Atmospheric<br> Pressure interface Time of Flight mass spectrometer (APi-TOF) during a period of nine<br> months, from August 2013 to April 2014. The negative mass spectra were dominated by the<br> ions of sulfuric, nitric, malonic and methanesulfonic acid (MSA) as well as SO5- . The most<br> prominent positive ion peaks were from amines. The other cations were mainly organic compounds<br> clustered with a nitrogen-containing ion, which could be either NH4+ or an aminium.<br> Occasionally the positive spectra were characterized by groups of compounds each differing<br> by a methylene group. In the negative spectrum, sulfuric acid was always observed during<br> clear sky conditions following the diurnal cycle of sun irradiation. On many occasions we<br> also saw a high signal of sulfuric acid during night time when clusters up to the tetramer<br> were observed. A plausible reason for these events could be evaporation from particles at low<br> relative humidity. A remarkably strong correlation between the signals of SO5- and CH3SO3-<br> was observed for the full measurement period. The presence of these two ions during both<br> the day and the night suggests a non-photochemical channel of formation which is possibly<br> linked to halogen chemistry. Halogenated species, especially Br– and IO3- , were frequently<br> observed in air masses that originated mainly from the Atlantic Ocean and occasionally from<br> continental areas based on back trajectory analyses. We found I2O5 clustered with an ion, a<br> species that was proposed from laboratory and modelling studies. All halogenated ions exhibited<br> an unexpected diurnal behaviour with low values during day time. New particle formation<br> (NPF) events were observed and characterized by 1) highly oxygenated molecules (HOMs) and<br> low sulfuric acid or 2) ammonia-sulfuric acid clusters. We present characteristic spectra for<br> each of these two event types based on 26 nucleation episodes. The mass spectrum of the<br> ammonia-sulfuric acid nucleation event compares very well with laboratory measurements<br> reported from the CLOUD chamber. A source receptor analysis indicates that new particle<br> formation events at the Jungfraujoch take place within a restricted period of time of 24-48<br> hours after air masses have had contact with boundary layer. This time frame appears to be<br> crucial to reach an optimal oxidation state and concentration of organic molecules necessary<br> to facilitate nucleation.