Influence of Common Assumptions Regarding Aerosol Composition and Mixing State on Predicted CCN Concentration

Manasi Mahish,Anne Jefferson,Don Collins

doi:10.3390/atmos9020054

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 (NCCN). Baseline or reference NCCN(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 (Sc) calculated for the same size and hygroscopicity pairs. Those estimates were first compared with 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 simplified treatments were compared with those calculated using the baseline approach to evaluate the error introduced with commonly used approximations.

Highlights

Indirect forcing by aerosols involves complex interactions between the aerosol and clouds that affect the droplet number concentration, albedo, precipitation efficiency, and lifetime of clouds and the chemical processing and scavenging of aerosol particles
Data from the scanning mobility particle sizer (SMPS)/hygroscopicity tandem differential mobility analyzer (HTDMA) system were used for most of the analyses described here
An initial estimate of NCCN that served as a basis for comparison used all of the information in the

Summary

Introduction

Indirect forcing by aerosols involves complex interactions between the aerosol and clouds that affect the droplet number concentration, albedo, precipitation efficiency, and lifetime of clouds and the chemical processing and scavenging of aerosol particles. Mediating this interaction are cloud condensation nuclei (CCN), which are those particles that activate to form droplets at a given supersaturation (S), known as the critical supersaturation (Sc ). Estimates of CCN concentration (NCCN ) using detailed size-dependent aerosol chemical composition are computationally intensive and simplifications used to minimize computation introduce additional uncertainty

Methods

Discussion

Conclusion