In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 18, No. 1 ( 2018-01-04), p. 65-79
Abstract:
Abstract. It was recently shown by the CERN CLOUD experiment that biogenic highly
oxygenated molecules (HOMs) form particles under atmospheric conditions in
the absence of sulfuric acid, where ions enhance the nucleation rate by 1–2 orders of magnitude. The biogenic HOMs were produced from ozonolysis of
α-pinene at 5 ∘C. Here we extend this study to compare the
molecular composition of positive and negative HOM clusters measured with
atmospheric pressure interface time-of-flight mass spectrometers (APi-TOFs),
at three different temperatures (25, 5 and −25 ∘C). Most negative HOM clusters include a
nitrate (NO3-) ion, and the spectra are similar to those seen in the nighttime
boreal forest. On the other hand, most positive HOM clusters include an
ammonium (NH4+) ion, and the spectra are characterized by mass bands
that differ in their molecular weight by ∼ 20 C atoms,
corresponding to HOM dimers. At lower temperatures the average oxygen to
carbon (O : C) ratio of the HOM clusters decreases for both polarities,
reflecting an overall reduction of HOM formation with decreasing temperature.
This indicates a decrease in the rate of autoxidation with temperature due to
a rather high activation energy as has previously been determined by quantum
chemical calculations. Furthermore, at the lowest temperature
(−25 ∘C),
the presence of C30 clusters shows that HOM monomers start to
contribute to the nucleation of positive clusters. These experimental
findings are supported by quantum chemical calculations of the binding
energies of representative neutral and charged clusters.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-18-65-2018
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2018
detail.hit.zdb_id:
2092549-9
detail.hit.zdb_id:
2069847-1
