Ice nuclei properties in the Saharan Air Layer close to the source - Results from the CALIMA2013 campaign

Mineral dust has been observed in many laboratory studies as effective ice nuclei (IN) in all four heterogeneous freezing modes from temperatures below -10°C. Measurements of the chemical composition of ice residues in clouds have shown that it is also predominant in atmospheric ice crystals. Despite the importance of mineral dust in ice nucleation of clouds only few in-situ measurements of atmospheric IN concentrations and onset conditions for freezing of airborne dust exist up to date. None of the previously reported studies was conducted close to one of the main dust sources such as the Sahara.

In the current work we present results from the CALIMA (Cloud Affecting particLes In Mineral dust from the sAhara) 2013 campaign which aimed at quantifying IN concentrations and onset conditions of ice nucleation in the Saharan Air Layer (SAL) close to the source. CALIMA 2013 was conducted at the Izańa Atmospheric Observatory, located west of the African shore on Tenerife, Canary Islands at 2400m a.s.l. for four weeks in August 2013. During the time of the campaign the station was frequently located in the Saharan Air Layer (SAL) with PM10 (particulate matter below 10µm) 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.

Ice nuclei concentrations were measured with the continuous flow thermal gradient Portable Ice Nucleation Chamber (PINC) at temperatures between 232K and 253K and relative humidity w.r.t. ice (RHi) between 100% and 148%, i.e. in the deposition (below water saturation) and condensation (above water saturation) freezing mode. Apart from measurement at single temperature and RHi conditions, the high dust loadings allowed conducting RHi – scans at a constant temperature from which the onset of ice nucleation is derived. Aerosol particles with a diameter below 1µm were sampled. An aerodynamic lens concentrator was applied upstream of the PINC chamber which enriched the sample air by a factor of 7 in particles between 0.5 to 1µm. This allowed higher counting statistics and thus lowered the detection limit of PINC. Airborne dust samples were collected with a cyclone in parallel to the in-situ IN experiments for offline investigations in the laboratory under similar temperature and relative humidity conditions as in the field.

The results show clearly that in the condensation mode the IN number concentration during the dust events is up to 50 times higher than during low dust periods. Also, ice nucleation onset is at lower RHi for the same temperature during dust events than during low dust periods. This suggests that mineral dust nucleated ice more efficiently than the background aerosol.