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| Titel |
Hygroscopic and chemical characterisation of Po Valley aerosol |
| VerfasserIn |
J. Bialek, M. Dall Osto, P. Vaattovaara, S. Decesari, J. Ovadnevaite, A. Laaksonen, C. O'Dowd |
| 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 ; 14, no. 3 ; Nr. 14, no. 3 (2014-02-12), S.1557-1570 |
| Datensatznummer |
250118369
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| Publikation (Nr.) |
 copernicus.org/acp-14-1557-2014.pdf |
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| Zusammenfassung |
| Continental summer-time aerosol in the Italian Po Valley was characterised in
terms of hygroscopic properties and the influence of chemical composition
therein. Additionally, the ethanol affinity of particles was analysed. The
campaign-average minima in hygroscopic growth factors (HGFs, at 90%
relative humidity) occurred just before and during sunrise from 03:00 to
06:00 LT (all data are reported in the local time), but, more generally, the
hygroscopicity during the whole night is very low, particularly in the
smaller particle sizes. The average HGFs recorded during the low HGF period
were in a range from 1.18 (for the smallest, 35nm particles) to 1.38 (for the
largest, 165 nm particles). During the day, the HGF gradually increased to
achieve maximum values in the early afternoon hours 12:00–15:00, reaching
1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting
case scenarios were encountered during the measurement period: Case 1 was
associated with westerly air flow moving at a moderate pace and Case 2 was
associated with more stagnant, slower moving air from the north-easterly
sector. Case 1 exhibited weak diurnal temporal patterns, with no distinct
maximum or minimum in HGF or chemical composition, and was associated with
moderate non-refractory aerosol mass concentrations (for 50% size cut at
1 μ) of the order of 4.5 μg m−3. For Case 1,
organics contributed typically 50% of the mass. Case 2 was characterised
by >9.5 μg m−3 total non-refractory mass
(<1 μ) in the early morning hours (04:00), decreasing to
~3 μg m−3 by late morning (10:00) and exhibited strong
diurnal changes in chemical composition, particularly in nitrate mass but
also in total organic mass concentrations. Specifically, the concentrations
of nitrate peaked at night-time, along with the concentrations of
hydrocarbon-like organic aerosol (HOA) and of semi-volatile oxygenated
organic aerosol (SV-OOA). In general, organic growth factors (OGFs) followed
a trend which was opposed to HGF and also to the total organic mass as
measured by the aerosol mass spectrometer. The analysis of the HGF
probability distribution function (PDF) reveals an existence of a predominant
"more hygroscopic" (MH) mode with HGF of 1.5 around noon, and two
additional modes: one with a "less hygroscopic" (LH) HGF of 1.26, and
another with a "barely hygroscopic" (BH) mode of 1.05. Particles sized
165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at
night to 95% of "more hygroscopic" growth factors (i.e. HGFs 1.35–1.9)
around noon. The diurnal changes in HGF progressively became enhanced with
decreasing particle size, decreasing from 95% "more hygroscopic" growth
factor fraction at noon to 10% fraction at midnight, while the "less
hygroscopic" growth factor fraction (1.13–1.34) increased from 5% at
noon to > 60% and the "barely hygroscopic" growth factor
fraction (1.1–1.2) increased from less than 2% at noon to 30% at
midnight. Surprisingly, the lowest HGFs occurred for the period when nitrate
mass reached peak concentrations (Case 2). We hypothesised that the low HGFs
of nitrate-containing particles can be explained by a) an organic coating
suppressing the water-uptake, and/or by b) the existence of nitrates in a
less hygroscopic state, e.g. as organic nitrates. The latter hypothesis
allows us to explain also the reduced OGFs observed during the early morning
hours (before dawn) when nitrate concentrations peaked, based on the evidence
that organic nitrates have significant lower ethanol affinity than other
SV-OOA compounds. |
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