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| Titel |
Quantitative estimates of the volatility of ambient organic aerosol |
| VerfasserIn |
C. D. Cappa, J. L. Jimenez |
| 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 ; 10, no. 12 ; Nr. 10, no. 12 (2010-06-21), S.5409-5424 |
| Datensatznummer |
250008561
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| Publikation (Nr.) |
 copernicus.org/acp-10-5409-2010.pdf |
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| Zusammenfassung |
| Measurements of the sensitivity of organic aerosol (OA, and its components)
mass to changes in temperature were recently reported by Huffman et al.~(2009)
using a tandem thermodenuder-aerosol mass spectrometer (TD-AMS)
system in Mexico City and the Los Angeles area. Here, we use these
measurements to derive quantitative estimates of aerosol volatility within
the framework of absorptive partitioning theory using a kinetic model of
aerosol evaporation in the TD. OA volatility distributions (or
"basis-sets") are determined using several assumptions as to the enthalpy
of vaporization (ΔHvap). We present two definitions of "non-volatile
OA," one being a global and one a local definition. Based on these
definitions, our analysis indicates that a substantial fraction of the
organic aerosol is comprised of non-volatile components that will not
evaporate under any atmospheric conditions; on the order of 50–80% when
the most realistic ΔHvap assumptions are considered. The sensitivity of
the total OA mass to dilution and ambient changes in temperature has been
assessed for the various ΔHvap assumptions. The temperature sensitivity
is relatively independent of the particular ΔHvap assumptions whereas
dilution sensitivity is found to be greatest for the low (ΔHvap = 50 kJ/mol)
and lowest for the high (ΔHvap = 150 kJ/mol) assumptions. This
difference arises from the high ΔHvap assumptions yielding volatility
distributions with a greater fraction of non-volatile material than the low
ΔHvap assumptions. If the observations are fit using a 1 or 2-component
model the sensitivity of the OA to dilution is unrealistically high. An
empirical method introduced by Faulhaber et al. (2009) has also been used to
independently estimate a volatility distribution for the ambient OA and is
found to give results consistent with the high and variable ΔHvap
assumptions. Our results also show that the amount of semivolatile gas-phase
organics in equilibrium with the OA could range from ~20% to
400% of the OA mass, with smaller values generally corresponding to the
higher ΔHvap assumptions. The volatility of various OA components
determined from factor analysis of AMS spectra has also been assessed. In
general, it is found that the fraction of non-volatile material follows the
pattern: biomass burning OA < hydrocarbon-like OA < semivolatile
oxygenated OA < low-volatility oxygenated OA. Correspondingly, the
sensitivity to dilution and the estimated amount of semivolatile gas-phase
material for the OA factors follows the reverse order. Primary OA has a
substantial semivolatile fraction, in agreement with previous results, while
the non-volatile fraction appears to be dominated by oxygenated OA produced
by atmospheric aging. The overall OA volatility is thus controlled by the
relative contribution of each aerosol type to the total OA burden. Finally,
the model/measurement comparison appears to require OA having an evaporation
coefficient (γe) substantially greater than 10−2; at this point
it is not possible to place firmer constraints on γe based on the
observations. |
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