Measured and modeled equilibrium sizes of NaCl and (NH4)2SO4 particles at relative humidities up to 99.1%

Abstract

The Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to measure equilibrium sizes of particles at relative humidities (RHs) up to 99.1%. Particles of substances with well-known hygroscopic behavior (NaCl and (NH4)2SO4) were used for the measurements. The intention was to check the proper functionality of LACIS for this measurement mode. Equilibrium sizes were also simulated on the basis of Köhler theory, by (1) assuming ideal solutions and (2) accounting for the nonideality of the droplets by using osmotic coefficients from the open literature. Measured and simulated equilibrium diameters were compared to measured values published in the open literature. For sodium chloride, model approaches 1 and 2 and all measured values were in good agreement. For ammonium sulfate, the results of model approach 2 (nonideal solution) were in good agreement with the measured diameters, whereas assuming an ideal solution yields equilibrium diameters that are too large by up to 20%. Measured hygroscopic growth factors were used to derive scattering coefficients and visibilities for two exemplary atmospheric dry number size distributions at different RHs. For both salts, at 90% RH the scattering coefficient increased by about a factor of 5 compared to that of the dry aerosol; at 99% RH the increase was about 21-fold. If instead the hygroscopic growth was assumed to follow the growth factors that were simulated for an ideal ammonium sulfate solution, the increase of the scattering coefficient is overestimated by ∼40%. This highlights the need to account for the nonideal solution behavior of droplets when calculating equilibrium diameters, even at high RHs. Copyright 2005 by the American Geophysical Union.