Ice Nucleation by Mineral Dusts

Heterogeneous ice nucleation in the immersion mode plays a vital, yet poorly understood, role in the development of ice in mixed-phase clouds. From recent work it is thought that mineral dusts are a major, globally important source of ice-nucleating particles (INP). However, field observations to support this are sparse and the prediction of INP concentrations is challenging. This thesis expands on our current knowledge of the ice-nucleating characteristics of various components of mineral dust with the ultimate goal of being able to better predict INP concentrations globally and improve our fundamental understanding. These laboratory based experiments are then used to guide fieldwork studies and develop an interpretation of the observations. In addition to this, presented here is the development and characterisation of a new immersion mode assay for the detection of low concentrations of INP. From previous work K-feldspar is thought to be the dominant component in mineral dusts for ice nucleation. However, the quartzes have not been extensively surveyed although they make up a much larger proportion of mineral dusts than K-feldspar (in most instances). The investigation of 10 α-quartz samples show quartz to be ice active with a large variability in activity and sensitivity to time spent in water and air. The development of four new parameterisations supports evidence that K-feldspar is the dominant mineral INP in the atmosphere and we can predict field collected dust sample measurements well with the proposed K-feldspar parameterisation. We also make the first field measurements of INP at Barbados where desert dust aerosol is present which has been transported for many days after being emitted from dust sources in Africa. We show the activity of the dust to have decreased on transport across the Atlantic. Using XRD, SEM and INP modelling we attribute the deactivation to the preferential loss of the K-feldspar component on transport. The last section of this project describes the development of a 50 µL drop assay which is more sensitive to the detection of low concentrations of INP active at warm temperatures (> -15°C) and lends itself to future use in field campaigns. This instrument is novel in that it uses an infrared camera to make temperature measurements and to automatically detect freezing.