Ice nucleating particles in the Saharan Air Layer

Abstract

Abstract. This study aims at quantifying the ice nucleation properties of desert dust in the Saharan Air Layer (SAL), the warm, dry and dust-laden layer that expands from North Africa to the Americas. By measuring close to the dust's emission source, before aging processes during the transatlantic advection potentially modify the dust properties, the study fills a gap between in situ measurements of dust ice nucleating particles (INPs) far away from the Sahara and laboratory studies of ground-collected soil. Two months of online INP concentration measurements are presented, which were part of the two CALIMA campaigns at the Izaña observatory in Tenerife, Spain (2373 m a.s.l.), in the summers of 2013 and 2014. INP concentrations were measured in the deposition and condensation mode at temperatures between 233 and 253 K with the Portable Ice Nucleation Chamber (PINC). Additional aerosol information such as bulk chemical composition, concentration of fluorescent biological particles as well as the particle size distribution was used to investigate observed variations in the INP concentration. The concentration of INPs was found to range between 0.2 std L−1 in the deposition mode and up to 2500 std L−1 in the condensation mode at 240 K. It correlates well with the abundance of aluminum, iron, magnesium and manganese (R: 0.43–0.67) and less with that of calcium, sodium or carbonate. These observations are consistent with earlier results from laboratory studies which showed a higher ice nucleation efficiency of certain feldspar and clay minerals compared to other types of mineral dust. We find that an increase of ammonium sulfate, linked to anthropogenic emissions in upwind distant anthropogenic sources, mixed with the desert dust has a small positive effect on the condensation mode INP per dust mass ratio but no effect on the deposition mode INP. Furthermore, the relative abundance of biological particles was found to be significantly higher in INPs compared to the ambient aerosol. Overall, this suggests that atmospheric aging processes in the SAL can lead to an increase in ice nucleation ability of mineral dust from the Sahara. INP concentrations predicted with two common parameterization schemes, which were derived mostly from atmospheric measurements far away from the Sahara but influenced by Asian and Saharan dust, were found to be higher based on the aerosol load than we observed in the SAL, further suggesting aging effects of INPs in the SAL.

The Horizontal Ice Nucleation Chamber (HINC): INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch

“…For conditions below water saturation, dust events yielding higher INP concentrations at the JFJ have been reported before (Chou et al, 2011) and in the Saharan air layer by aircraft sampling (DeMott et al, 2003b). At water-saturated conditions, this is the first study to clearly show that during a SDE at the JFJ an increase in INPs is observed at 242 K. This agrees with the findings of an increase in INP concentrations observed during minor influence of Saharan dust (Boose et al, 2016b). We note that at 265 K an increase in immersion INPs at the JFJ was not observed during SDEs , indicating that dust contributes to ice nucleation at colder temperatures as has been previously suggested in numerous studies (see references in Hoose and Möhler, 2012;Murray et al, 2012;Kanji et al, 2017).…”

Comparative measurements of ambient atmospheric concentrations of ice nucleating particles using multiple immersion freezing methods and a continuous flow diffusion chamber

“…To quantify the initial stage of ice nucleation in the atmosphere, multiple sampling techniques are now being used in field studies (Hader et al, 2014;Mason et al, 2015;DeMott et al, 2015;Stopelli et al, 2015;Boose et al, 2016;Schrod et al, 2016Schrod et al, , 2017. Since these various measurements are being used as bases for developing numerical model parameterizations for different emission sources, their comparability should be assessed.…”

Uncertainty in counting ice nucleating particles with continuous flow diffusion chambers

“…Continuous flow diffusion chambers (CFDCs) have provided a means to understand ice cloud formation by measuring INP concentrations in the field. By exposing an ambient aerosol population to controlled humidity and temperature conditions, the ability of natural aerosols to act as INPs can be quantified (DeMott et al, 2003a, b;Chou et al, 2011;Boose et al, 2016).…”