Hygroscopic growth factor (GF) distributions of 13, 25, 50, 100, 200, and 400 nm particles measured with a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) from 2009 to 2012 at the Southern Great Plains (SGP) site in Oklahoma, U.S. were used to describe time of day-and annually-averaged hygroscopicity parameters (κ). A diel pattern was often observed with an average daytime κ that was higher than that at other times, especially on days with new particle formation (NPF) events. The average hygroscopicity of the smaller and larger particles at the tails of the measured size range was higher than that in between, with the minimum for each of the 4 years at 50 nm. This pattern is thought to result in part from addition of soluble inorganic and organic compounds formed through gas phase and aqueous phase reactions for the smaller and larger particles, respectively. The size dependence is reflected in the averaged κ and in the frequency with which GF distributions possessed modes categorized as nearly-hydrophobic, less hygroscopic, and hygroscopic. A hygroscopicity-based mixing state parameter, MS hyg , defined as the ratio of the standard deviation (SD) of a measured GF distribution to the size specific threshold SD roughly separating internal and external mixtures, was used to study the diel and seasonal variation in particle mixing state. Internal mixtures were found to be more common during the daytime and during the summer, likely reflecting more rapid photochemical processing and growth at those times.

Abstract:A 4-year record of aerosol size and hygroscopic growth factor distributions measured at the Department of Energy's Southern Great Plains (SGP) site in Oklahoma, U.S. were used to estimate supersaturation (S)-dependent cloud condensation nuclei concentrations (N CCN ). Baseline or reference N CCN (S) spectra were estimated using κ-Köhler Theory without any averaging of the measured distributions by creating matrices of size-and hygroscopicity-dependent number concentration (N) and then integrating for S > critical supersaturation (S c ) calculated for the same size and hygroscopicity pairs. Those estimates were first compared with directly measured N CCN at the same site. Subsequently, N CCN was calculated using the same dataset but with an array of simplified treatments in which the aerosol was assumed to be either an internal or an external mixture and the hygroscopicity either assumed or based on averages derived from the growth factor distributions. The CCN spectra calculated using the simplified treatments were compared with those calculated using the baseline approach to evaluate the error introduced with commonly used approximations.

Abstract.A 4-year record of aerosol size and hygroscopic growth factor distributions measured at the Department of Energy's SGP ARM site in Oklahoma, U.S. were used to estimate supersaturation (S)-dependent cloud condensation nuclei concentrations (NCCN). Baseline or reference NCCN(S) spectra were estimated by using the data to create a matrix of size-and 10 hygroscopicity-dependent number concentration (N)and then integrating for S > critical supersaturation (Sc) calculated for the same size and hygroscopicity pairs using -Ko hler Theory. The accuracy of those estimates was assessed through comparison with the directly measured NCCN at the same site. Subsequently, NCCN was calculated using the same dataset but with an array of simplified treatments in which the aerosol was assumed to be either an internal or an external mixture and the hygroscopicity either assumed or based on averages derived from the growth factor distributions. The CCN spectra calculated using the 15 simplified treatments were compared with those from the baseline approach to evaluate the impact of commonly used approximations. Among the simplified approaches, assuming the aerosol is an internal mixture with size-dependent hygroscopicity parameter () resulted in estimates closest to those from the baseline approach over the range in S considered.

The role of meteorology in facilitating the formation and accumulation of ground-level ozone is of great theoretical and practical interest, especially due to emissions shifts and changing global climate.

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