Correction to “African dust aerosols as atmospheric ice nuclei”

Abstract

[1] In the paper “African dust aerosols as atmospheric ice nuclei” by P. J. DeMott et al. (Geophysical Research Letters, 30(14), 1732, doi:10.1029/2003GL017410, 2003), it was found that a standard processing procedure was incorrectly performed. The consequence of that error is a reduction in the previously reported ice nuclei (IN) number concentrations associated with aerosol measurements within the Saharan Aerosol Layer (SAL). While these results do not affect the basic conclusions of the study, the data have been frequently cited and used as a basis for numerical simulations of dust impacts on ice phase formation in Florida storms during the NASA CRYSTAL-FACE project [Phillips et al., 2008; van den Heever et al., 2006]. We therefore provide some corrected data and Figures 2 and 4 in this note. [2] The continuous flow diffusion chamber (CFDC) used for the measurements reported by DeMott et al. [2003] uses the growth in optical size of ice crystals nucleated on ambient aerosol particles as the basis for determining ice nuclei concentrations, following exposure to ice supersaturated conditions at one processing temperature. Standard procedure also involves use of an aerosol impactor upstream of the CFDC to limit aerosol aerodynamic sizes introduced into the diffusion chamber to a size that is approximately one-half of the size used to determine the presence of ice crystals nucleated and grown in the CFDC. The size separation of liquid aerosol or activated cloud droplets from grown ice crystals is assured by passing the flow through an ice saturated section in the lower one-third of the CFDC. For CRYSTAL-FACE, the inlet aerosol impactor had a cut-size of 1 μm for the volumetric flow used. The intended threshold (lower) ice crystal size limit for this project was 2 μm. [3] Although the optical particle counter used for sizing particles exiting the CFDC was calibrated before and after the field deployment to assure correct size attribution of nucleated ice crystals versus aerosols, during processing, the calibration data were not applied correctly. Instead of setting the lower size limit for ice crystals to 2 μm, it was set to 1 μm or the same value as the 50% cut-point of the inlet aerosol impactor. Since a substantial number of supermicron particles were present in the SAL during CRYSTAL-FACE (Figure 2, revised herein), the IN concentrations reported were rather strongly “contaminated” by aerosol influences. Reprocessed data using the correct lower size limit for nucleated ice crystal attribution are now shown in revised Figure 2, and in revised Figure 4, which compares the IN number concentration vertical profiles on three days during CRYSTAL-FACE. Peak IN number concentrations at −36°C and 123% ice relative humidity (RHice) on July 28 and July 29 are lowered from originally reported values of 1.3 cm−3 to approximately 0.3 cm−3, about a factor of four times lower. Corrected data for the July 29 case are applicable to the profile shown in Figure 2 of Sassen et al. [2003], but have no impact on the conclusions of that paper, or the still strong contribution of dust particles as ice nuclei compared to the background atmosphere. Nevertheless, corrections impact the entire archived data set from the field study. Corrected data corrections will be re-archived. An additional brief correction will be published regarding the cirrus anvil ice nuclei data presented by Prenni et al. [2007]. Revised data affects only the magnitude of dust impacts noted or implemented in models and should not affect the conclusions for any of these studies. Corrections were applied already for CRYSTAL-FACE data used for parameterization development in the recent paper of Phillips et al. [2008].