| dc.description.abstract | Cloud condensation nuclei (CCN) concentrations were measured at Egbert, a rural site in
Ontario, Canada during the spring of 2007. The CCN concentrations were compared to
values predicted from the aerosol chemical composition and size distribution using
kappa-Kohler theory, with the specific goal of this work being to determine the
hygroscopic parameter (kappa) of the oxygenated organic component of the aerosol,
assuming that oxygenation drives the hygroscopicity for the entire organic fraction of
the aerosol. The hygroscopicity of the oxygenated fraction of the organic component, as
determined by an Aerodyne aerosol mass spectrometer (AMS), was characterised by two
methods. First, positive matrix factorization (PMF) was used to separate oxygenated and
unoxygenated organic aerosol factors. By assuming that the unoxygenated factor is
completely non-hygroscopic and by varying kappa of the oxygenated factor so that the
predicted and measured CCN concentrations are internally consistent and in good
agreement, kappa of the oxygenated organic factor was found to be 0.22 +/- 0.04 for the
suite of measurements made during this five-week campaign. In a second, equivalent
approach, we continue to assume that the unoxygenated component of the aerosol, with a
mole ratio of atomic oxygen to atomic carbon (O/C) approximate to 0, is completely
non-hygroscopic, and we postulate a simple linear relationship between kappa(org) and
O/C. Under these assumptions, the kappa of the entire organic component for bulk
aerosols measured by the AMS can be parameterised as kappa(org)=(0.29 +/- 0.05).(O/C),
for the range of O/C observed in this study (0.3 to 0.6). These results are averaged
over our five-week study at one location using only the AMS for composition analysis.
Empirically, our measurements are consistent with kappa(org) generally increasing with
increasing particle oxygenation, but high uncertainties preclude us from testing this
hypothesis. Lastly, we examine select periods of different aerosol composition,
corresponding to different air mass histories, to determine the generality of the
campaign-wide findings described above. | en_US |
