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| Titel |
Stable carbon isotope ratios of ambient secondary organic aerosols in Toronto |
| VerfasserIn |
M. Saccon, A. Kornilova, L. Huang, S. Moukhtar, J. Rudolph |
| 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. 18 ; Nr. 15, no. 18 (2015-09-29), S.10825-10838 |
| Datensatznummer |
250120063
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| Publikation (Nr.) |
 copernicus.org/acp-15-10825-2015.pdf |
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| Zusammenfassung |
A method to quantify concentrations and stable carbon isotope ratios of
secondary organic aerosols has been applied to study atmospheric
nitrophenols in Toronto, Canada. The sampling of five nitrophenols, all with
substantial secondary formation from the photooxidation of aromatic volatile
organic compounds (VOCs), was conducted in the gas phase and particulate matter (PM)
together and in PM alone. Their concentrations in the atmosphere
are in the low ng m−3 range and, consequently, a large volume of air
(> 1000 m3) is needed to analyze samples for stable carbon
isotope ratios, resulting in sampling periods of typically 24 h. While
this extended sampling period increases the representativeness of average
values, it at the same time reduces possibilities to identify meteorological
conditions or atmospheric pollution levels determining nitrophenol
concentrations and isotope ratios.
Average measured carbon isotope ratios of the different nitrophenols are
between −34 and −33 ‰, which is
well within the range predicted by mass balance. However, the observed
carbon isotope ratios cover a range of nearly 9 ‰ and
approximately 20 % of the isotope ratios of the products have isotope
ratios lower than predicted from the kinetic isotope effect of the first
step of the reaction mechanism and the isotope ratio of the precursor. This
can be explained by isotope fractionation during reaction steps following
the initial reaction of the precursor VOCs with the OH radical.
Limited evidence for local production of nitrophenols is observed since
sampling was done in the Toronto area, an urban center with significant
anthropogenic emission sources. Strong evidence for significant local
formation of nitrophenols is only found for samples collected in summer. On
average, the difference in carbon isotope ratios between nitrophenols in the
particle phase and in the gas phase is insignificant, but for a limited
number of observations in summer, a substantial difference is observed. This
indicates that at high OH radical concentrations, photochemical formation or
removal of nitrophenols can be faster than exchange between the two phases.
The dependence between the concentrations and isotope ratios of the
nitrophenols and meteorological conditions as well as pollution levels
(NO2, O3, SO2 and CO) demonstrate that the influence of
precursor concentrations on nitrophenol concentrations is far more important
than the extent of photochemical processing. While it cannot be excluded
that primary emissions contribute to the observed levels of nitrophenols,
overall the available evidence demonstrates that secondary formation is the
dominant source for atmospheric nitrophenols in Toronto. |
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