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