AMBIENT SMALL ION COMPOSITION DURING NEW PARTICLE FORMATION EVENTS
MEASURED WITH THE APi-TOF
M. Ehn1, H. Junninen1, T. Petäjä1, T. Kurtén1, V.-M. Kerminen1,2, S. Schobesberger 1, H. E. Manninen1 , I. K.
Ortega1, H. Vehkamäki1, M. Kulmala1, and D. R. Worsnop1,2,3
1
2
Department of Physics, P.O.Box 64, FI-00014, University of Helsinki, Finland.
Finnish Meteorological Institute, Research and Development, P.O. Box 503, FI-00101 Helsinki, Finland
3
Aerodyne Research Inc, Billerica, MA 01821, USA
Keywords: Atmospheric ions, mass spectrometry, nucleation, ion clusters.
INTRODUCTION
Ions in the atmosphere are mainly produced by cosmic radiation and radioactive decay of radon. To a first
approximation, when an ion collides with a neutral molecule, the charge will be transferred if energetically
favorable. Typical lifetimes of ions are on the order of several minutes, giving the ions time to reach a
pseudo steady state with most of the negative charges on the strongest acids (e.g. sulfuric, nitric, malonic
acid), and the positive charges on the strongest bases (e.g. ammonia, amines, pyridines, quinolines),
although these species may only be present in trace amounts (Eisele et al., 1989). This feature makes the
analysis of the ambient ions particularly interesting, as sulfuric acid and amines are often mentioned as
main precursors for new atmospheric new particle formation (e.g. Kurtén et al., 2008). Understanding the
exact mechanisms behind the nucleation and growth processes in the atmosphere is important for a
detailed understanding of the climate system via the role of aerosol particles in cloud formation.
In this study, a recently developed high-resolution mass spectrometer (APi-TOF, Junninen et al, 2010)
was deployed at the SMEAR II station in Hyytiälä, Finland in spring 2009. The chemical composition and
temporal behavior of both ambient negative and positive ions were measured with higher sensitivity and
mass resolution than in any previous studies.
METHODS
The APi-TOF inlet is a critical orifice drawing a sample flow of 0.8 l min-1, and the aim is to transport all
the ions into the TOF to determine their mass per charge (m/Q). This is done by using two guiding
quadrupoles and an ion lens assembly, in three separate differentially pumped chambers, leading into the
TOF. The instrument measured negative and positive naturally charged ions in Hyytiälä during April-May
2009. Simultaneous measurements of the total sulfuric acid concentration were conducted with a chemical
ionization mass spectrometer (CIMS, Petäjä et al., 2009).
RESULTS
A typical negative ion mass spectrum measured by the APi-TOF is presented in Figure 1. It is clearly
dominated by sulfuric acid compounds (plotted in red), with the largest peak being de-protonated sulfuric
acid, HSO4-, followed by the H2SO4 dimer and trimer. Although small, also the tetramer and its cluster
with ammonia are visible in the spectrum, as can be seen in the inset in Figure 1. The observation of the
sulfuric acid/ammonia cluster is a significant result, as the evaporation rate of NH3 from ion clusters with
low H2SO4/NH3 ratio is very high (Hanson and Lovejoy, 2006) which explains why we have not measured
ammonia clustered with the H 2SO4 dimer or trimer. The detection of a charged sulfuric acid/ammonia
cluster strongly implies the presence of many more of the corresponding neutral clusters. This gives an
indication of the new particle formation pathway in Hyytiälä.
Figure 1. Typical anion mass spectrum during a nucleation event in Hyytiälä. The largest peaks
correspond to sulfuric acid and its clusters.
The time trends of the main sulfuric acid peaks are shown in Figure 2, together with the total sulfuric acid
concentration measured by the CIMS. The overall agreement between the sulfuric acid compounds
measured by the APi-TOF and CIMS is very good. This is to be expected since sulfuric acid is one of the
strongest acids in the atmosphere, which means that it will transfer a proton to practically any ion that it
collides with. Therefore the total concentration of sulfuric acid ions will be dependent on the total ion
concentration and the neutral H2SO4 concentration. The concentration of sulfuric acid tracks the radiation
intensity very closely during this period.
Figure 2. Temporal behavior of sulfuric acid related negative ions during six measurement days in
Hyytiälä. Also plotted in black is the concentration of neutral sulfuric acid measured by the CIMS.
CONCLUSIONS
The recently developed atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) has
been shown to provide important information on trace gases and clustering during new particle formation
events. Sulfuric acid and its clusters dominate the negative ion spectrum during nucleation events, and
also ion clusters containing sulfuric acid and ammonia were observed. This strongly implies that these
clusters are present in large concentrations in the neutral molecules, and indicates an important nucleation
pathway through sulfuric acid clusters being stabilized by ammonia.
ACKNOWLEDGEMENTS
This work has been supported by European Commission 6th Framework program projects: EUCAARI,
contract no 036833-2 (EUCAARI), and by the Academy of Finland Center of Excellence program (project
number 1118615).
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