18 January 2024
Hygroscopicity is the ability of a particle to take up moisture from the environment and is important for understanding the ability of particles to act as cloud condensation nuclei (CCN). The CCN Counter Derived Hygroscopicity Parameter Kappa (CCNKAPPA) products provide a means to represent the hygroscopicity properties of ambient aerosol in global climate models.
The ARM CCN particle counter measures the fraction of ambient aerosol that can become liquid droplets at five supersaturations. These CCN data and co-located dry aerosol particle size measurements from the scanning mobility particle sizer (SMPS) or ultra-high-sensitivity aerosol spectrometer (UHSAS) are used to calculate the critical diameter—the minimum particle diameter required to activate into a cloud droplet at each instrument supersaturation. For each critical diameter value, a single hygroscopicity parameter (kappa) is calculated using kappa-Köhler theory (Petters and Kreidenweis 2007).
The daily files of the CCNSMPSKAPPA and CCNUHSASKAPPA data products provide the frequency distribution of kappa; kappa values within critical diameter-supersaturation field space; and time series of kappa, CCN concentrations, and supersaturation.
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The kappa data products will improve our understanding of aerosol-cloud interactions and their treatment in global climate models. The parameter kappa can be used to calculate the amount of water associated with aerosol particles at sub- and supersaturated conditions. These calculations can be used in models to calculate the humidity dependent size of aerosol particles, which in turn is used to calculate aerosol scattering and direct radiative forcing. The kappa parameter can also be used to model the CCN activity of atmospheric aerosol of different sizes and composition. Researchers can compare these VAP kappa values with the kappa values calculated assuming other aerosol mixing states (e.g., external) by performing CCN closure studies to better model the activation properties of aerosol.
These value-added products use k-Köhler theory with fixed constants to derive the hygroscopicity parameter kappa from aerosol particle size and supersaturation measurements. These calculations assume the particles are all internally mixed and have a uniform chemical composition. The kappa is calculated based on the particle number size distribution and assumes that larger particles activate first, regardless of their chemical identity. Because of these assumptions, the derived kappa parameter is highly sensitive to any errors in the measured particle size and number concentration.
Reference: Petters MD and SM Kreidenweis. 2007. “A single parameter representation of hygroscopic growth and cloud condensation nucleus activity.” Atmospheric Chemistry and Physics, 7, 1961–1971, https://doi.org/10.5194/acp-7-1961-2007.