Abstract
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 κ-Köhler theory, with the specific goal of this work being to determine the hygroscopic parameter (κ) 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 κ of the oxygenated factor so that the predicted and measured CCN concentrations are internally consistent and in good agreement, κ 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) ≈ 0, is completely non-hygroscopic, and we postulate a simple linear relationship between κorg and O/C. Under these assumptions, the κ of the entire organic component for bulk aerosols measured by the AMS can be parameterised as κorg=(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 κorg 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.
Highlights
Atmospheric aerosols can affect climate directly by scattering and absorbing incoming solar radiation, or indirectly by acting as cloud condensation nuclei (CCN), which form clouds and in turn can reflect light (Twomey, 1977)
It is difficult to reconcile the overprediction from this calculation of almost 20% to that calculated from a linear regression of the predicted and measured CCN concentrations, which resulted in a slope of 1.03
Case considers the changes in κox and a if the chemical composition is calculated from the size distribution data of the C-ToF aerosol mass spectrometer (AMS) for vacuum aerodynamic diameters 80–250 nm, as an indication of the composition of the smaller particles, while Case and consider the more general cases of the organic and inorganic components of the aerosol being 50% greater at smaller sizes compared to the bulk aerosol, respectively
Summary
Atmospheric aerosols can affect climate directly by scattering and absorbing incoming solar radiation, or indirectly by acting as cloud condensation nuclei (CCN), which form clouds and in turn can reflect light (Twomey, 1977). The efficiency of particles as CCN affects both aerosol particle and cloud droplet lifetimes (Albrecht, 1989). As such, understanding the hygroscopic properties of aerosols and the processes that govern cloud droplet activation are important. Kohler theory has been used to predict the CCN-activity of inorganic compounds for many years (Kohler, 1936). The focus has turned to the prediction of the CCN-activity of organic compounds in atmospheric particles.
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