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| Titel |
On the composition of ammonia–sulfuric-acid ion clusters during aerosol particle formation |
| VerfasserIn |
S. Schobesberger, A. Franchin, F. Bianchi, L. Rondo, J. Duplissy, A. Kürten, I. K. Ortega, A. Metzger, R. Schnitzhofer, J. Almeida, A. Amorim, J. Dommen, E. M. Dunne, M. Ehn, S. Gagné, L. Ickes, H. Junninen, A. Hansel, V.-M. Kerminen, J. Kirkby, A. Kupc, A. Laaksonen, K. Lehtipalo, S. Mathot, A. Onnela, T. Petäjä, F. Riccobono, F. D. Santos, M. Sipilä, A. Tomé, G. Tsagkogeorgas, Y. Viisanen, P. E. Wagner, D. Wimmer, J. Curtius, N. M. Donahue, U. Baltensperger, M. Kulmala  , D. R. Worsnop |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 1 ; Nr. 15, no. 1 (2015-01-07), S.55-78 |
| Datensatznummer |
250119286
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| Publikation (Nr.) |
 copernicus.org/acp-15-55-2015.pdf |
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| Zusammenfassung |
| The formation of particles from precursor vapors is an
important source of atmospheric aerosol. Research at the Cosmics Leaving
OUtdoor Droplets (CLOUD) facility at CERN tries to elucidate which vapors
are responsible for this new-particle formation, and how in detail it
proceeds. Initial measurement campaigns at the CLOUD stainless-steel aerosol
chamber focused on investigating particle formation from ammonia (NH3)
and sulfuric acid (H2SO4). Experiments were conducted in the
presence of water, ozone and sulfur dioxide. Contaminant trace gases were
suppressed at the technological limit. For this study, we mapped out the
compositions of small NH3–H2SO4 clusters over a wide range of
atmospherically relevant environmental conditions. We covered [NH3] in
the range from < 2 to 1400 pptv, [H2SO4] from 3.3 × 106 to 1.4 × 109 cm−3 (0.1 to 56 pptv),
and a temperature range from −25 to +20 °C. Negatively and
positively charged clusters were directly measured by an atmospheric
pressure interface time-of-flight (APi-TOF) mass spectrometer, as they
initially formed from gas-phase NH3 and H2SO4, and then grew
to larger clusters containing more than 50 molecules of NH3 and
H2SO4, corresponding to mobility-equivalent diameters greater than
2 nm. Water molecules evaporate from these clusters during sampling and are
not observed. We found that the composition of the NH3–H2SO4
clusters is primarily determined by the ratio of gas-phase concentrations
[NH3] / [H2SO4], as well as by temperature. Pure binary
H2O–H2SO4 clusters (observed as clusters of only
H2SO4) only form at [NH3] / [H2SO4] < 0.1 to
1. For larger values of [NH3] / [H2SO4], the composition of
NH3–H2SO4 clusters was characterized by the number of
NH3 molecules m added for each added H2SO4 molecule n (Δm/Δ n),
where n is in the range 4–18 (negatively charged clusters) or
1–17 (positively charged clusters). For negatively charged clusters,
Δ m/Δn saturated between 1 and 1.4 for
[NH3] / [H2SO4] > 10. Positively charged clusters
grew on average by Δm/Δn = 1.05 and were only observed at
sufficiently high [NH3] / [H2SO4]. The H2SO4
molecules of these clusters are partially neutralized by NH3, in close
resemblance to the acid–base bindings of ammonium bisulfate. Supported by
model simulations, we substantiate previous evidence for acid–base reactions
being the essential mechanism behind the formation of these clusters under
atmospheric conditions and up to sizes of at least 2 nm. Our results also
suggest that electrically neutral NH3–H2SO4 clusters,
unobservable in this study, have generally the same composition as ionic
clusters for [NH3] / [H2SO4] > 10. We expect that
NH3–H2SO4 clusters form and grow also mostly by Δm/Δn > 1 in the atmosphere's boundary layer, as
[NH3] / [H2SO4] is mostly larger than 10. We compared our
results from CLOUD with APi-TOF measurements of NH3–H2SO4
anion clusters during new-particle formation in the Finnish boreal forest.
However, the exact role of NH3–H2SO4 clusters in boundary
layer particle formation remains to be resolved. |
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