AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Sensitivity of Estimated CCN Concentration at a Rural Site to Common Assumptions Regarding Aerosol Composition and Mixing State
MANASI MAHISH, Anne Jefferson, Don Collins, Texas A&M University
Abstract Number: 605 Working Group: Aerosols, Clouds, and Climate
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 NCCN(S) spectra were first estimated by using the data to create matrices of size- and hygroscopicity-dependent number concentration (N) and critical supersaturation (Sc) and then integrating N over S > Sc. The accuracy of those estimates was assessed through comparison with the directly measured NCCN at the same site. Subsequently, NCCN(S) 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 from the baseline approach to evaluate the impact of commonly used approximations. NCCN was calculated using κ-Köhler Theory for all mixing state and solution type assumptions. The average Normalized Root Mean Square Errors (NRMSE) between NCCN derived from the baseline approach and those measured directly are 0.39, 0.25, 0.27 and 0.35 for data from 2009, 2010, 2011 and 2012, respectively. Among the simplified approaches, assuming the aerosol was an internal mixture with size-dependent hygroscopicity parameter (κ) resulted in estimates closest to those from the baseline approach, with a goodness of fit (r2) of 0.86 and the smallest variation in best fit slope between 0.25% and 0.85% S (0.002).