Ice nucleation under cirrus cloud conditions

Cirrus are thin upper tropospheric clouds that are composed entirely of ice particles. Due to their extensive coverage of the Earth’s surface, they have a significant impact on the Earth’s radiative balance. In the absence of a foreign surface, cirrus ice particles can form via homogeneous ice nucleation in liquid aqueous solution aerosol droplets. Ice can also nucleate heterogeneously when catalysed by the presence of solid particles, which can affect the physical properties of the cloud, potentially resulting in high in-cloud humidity and low optical depth. In order to better quantify the impact of cirrus on future climate it is vital to understand the microphysical processes that lead to their formation. This project focuses on the investigation of heterogeneous ice nucleation in the deposition mode under cirrus conditions. It can be divided into two sections:

1) The investigation of the ice nucleation behaviour of glassy aerosols. Atmospheric secondary organic aerosol (SOA) is likely to exist in a semi-solid or glassy state, particularly at low temperatures and humidities. Over the course of two experimental campaigns at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud simulation chamber, the ice nucleation properties of aerosols of five different glass forming compositions (citric acid, raffinose, 4-hydroxy-3-methoxy-mandelic acid (HMMA), levoglucosan, and a mixture – raffinose/M5AS) were studied. These organic compounds have similar functionality to oxidised organic material found in atmospheric aerosol and have estimated temperature/humidity induced glass transition thresholds that fall within the range predicted for atmospheric SOA. A small fraction of aerosol particles of all compositions were found to nucleate ice heterogeneously in the deposition mode at temperatures < 200 K, which is relevant to the tropical tropopause layer (TTL). Raffinose and HMMA, which form glasses at higher temperatures, nucleated ice heterogeneously at temperatures as high as 214.6 and 218.5 K respectively. Using a 1D cirrus model, it was shown that nucleation on glassy aerosols may explain low ice crystal numbers and high in-cloud humidity in the TTL. It was also found that the ice nucleation efficiency of glassy aerosol particles became enhanced after having been frozen homogeneously at temperatures close to their glass transition thresholds.
2) The design and development of a benchtop instrument for the study of deposition mode ice nucleation. After characterisation, the deposition mode chamber was used to investigate ice nucleation by a natural sample of kaolinite clay mineral dust (KGa-1b, Clay Minerals Society). The surface area of kaolinite present was found to affect the onset humidity of ice nucleation. Evidence was also found for the enhancement of kaolinite particles as ice nuclei after they had nucleated ice in an initial experiment.