Cloud nuclei properties in the Saharan Air Layer close to the source

Mineral dust, a highly abundant aerosol in the atmosphere, plays an important role in cloud processes. Several laboratory studies have shown the effectiveness of mineral dust particles to act as ice nuclei (IN) at temperatures below -10°C and with that its importance for understanding cold cloud properties. Nevertheless, only few in-situ studies measured atmospheric IN, and none close to one of the main global dust sources such as the Sahara. Investigations of warm cloud nuclei properties show a higher activated fraction of cloud condensation nuclei (CCN) in air masses coming from the Sahara, however, the collected data is also limited.

We present data from the CALIMA (Cloud Affecting particLes In Mineral dust from the sAhara) 2013 campaign. The study aimed at quantifying atmospheric IN as well as CCN concentrations and onset conditions of ice nucleation and droplet formation in the Saharan Air Layer (SAL) close to the source. Furthermore, correlation between the results and the chemistry and mineralogy of the dust aerosol sampled was investigated. CALIMA 2013 was conducted in August 2013 at the Izaña Atmospheric Observatory, located at 2373 m a.s.l. on Tenerife, Canary Islands, west off the African shore. The campaign lasted for four weeks. During the measurement period the station was located in the Saharan Air Layer (SAL) most of the time with PM10 (particulate matter below 10 μm) values reaching more than 500 μg/m³ during a strong dust event. If not exposed to a dust event, the location is considered typical for being in the free troposphere especially during night times.

IN concentrations were measured with the continuous flow thermal gradient Portable Ice Nucleation Chamber (PINC) at temperatures between 232 K and 253 K and relative humidities w.r.t. ice (RHi) between 100 % and 148 %. For our sampling conditions we infer deposition nucleation below water saturation and condensation freezing above water saturation. Aerosol particles with an aerodynamic diameter below 1 μm were sampled. A CCN counter was continuously measuring the polydisperse CCN number concentration at 10 different supersaturations between 0.1 – 1 %. Airborne dust samples were collected with a cyclone in addition to the in-situ IN and CCN experiments for offline investigations in the laboratory under similar temperature and relative humidity conditions as in the field. Filter samples were taken to investigate the chemical composition of the dust particles. For the CALIMA period, the DREAM8 dust model extended with mineral fractions carried by dust was run to simulate meteorological and dust aerosol conditions for ice nucleation. Further information on dust transport was obtained from the LAGRANTO back trajectory analysis.

The results show that during dust events the IN number concentration strongly increases by up to a factor of 50. The activated fractions are larger for both IN and CCN in comparison to low dust periods. Also, for the same temperature, during dust events the ice nucleation onset was observed at lower RHi compared to low dust periods. This suggests that mineral dust nucleated ice more efficiently than the background aerosol.