Abstract
Abstract. The influence of aerosols on cloud properties is an important modulator of the climate system. Traditional Köhler theory predicts the equilibrium concentration of cloud condensation nuclei (CCN); however, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States during the summer of 2006: Great Smoky Mountain National Park, TN; Bondville, IL; Houston, TX; and the Atmospheric Radiation Measurement Program Southern Great Plains site near Lamont, OK. We express droplet growth rates with the mass accommodation coefficient (α), and report values of α measured in the field normalized to the mean α measured for lab-generated ammonium sulfate (AS) particles (i.e., α'=α/αAS). Overall, 59% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were "low-α' " (α'<10−1, corresponding to α<1.5×10−2). Of the 16 days during which these measurements were made, 8 had relatively few low-α' CCN (<16%), 6 had moderate low-α' fractions (27% to 59%), and 2 had large low-α' fractions (>82% during at least one ~30 min period). Day to day variability was greatest in Tennessee and Illinois, and low-α' particles were most prevalent on days when back trajectories suggested that air was arriving from aloft. The highest fractions of low-α' CCN in Houston and Illinois occurred around local noon, and decreased later in the day. These results suggest that for some air masses, accurate quantification of CCN concentrations may need to account for kinetic limitations.
Highlights
After several decades of research attempting to quantify the influence of human activities on the Earth’s climate, the largest single source of uncertainty in the total anthropogenic radiative forcing of the atmosphere remains the effect of atmospheric particles on cloud properties, i.e., the indirect aerosol effects on climate (IPCC, 2007)
Cloud droplets form on pre-existing aerosol particles, which can act as cloud condensation nuclei (CCN) when the ambient partial pressure of water vapor exceeds the saturation vapor pressure resulting in a supersaturation (S)
The purpose of this study was to measure α distributions for various ambient aerosols to determine the extent to which potential kinetic limitations to droplet formation exist in the atmosphere
Summary
After several decades of research attempting to quantify the influence of human activities on the Earth’s climate, the largest single source of uncertainty in the total anthropogenic radiative forcing of the atmosphere remains the effect of atmospheric particles on cloud properties, i.e., the indirect aerosol effects on climate (IPCC, 2007). Often discrepancies between predictions and observations can only be reduced by assuming that the aerosol soluble fraction is unrealistically low (e.g., Snider and Brenguier, 2000), or alternatively that all aerosol organic matter is insoluble (e.g., Cantrell et al, 2001), despite observations that some organic aerosol is CCN active (e.g., Novakov and Penner, 1993) These discrepancies could be due to kinetic limitations to droplet growth, a possibility that seems more likely considering that most identified chemical effects on droplet activation lower Sc (e.g., surface tension reduction and dissolution of gases). The purpose of this study was to measure α distributions for various ambient aerosols (urban, regional polluted, and background) to determine the extent to which potential kinetic limitations to droplet formation exist in the atmosphere
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