A method for single particle mass spectrometry of ice nuclei

High altitude cirrus clouds play an important role in the terrestrial radiation budget. Cirrus clouds are composed of ice particles that generally form on only a small subset, from 1 in 10 to 1 in 10 5, of the background aerosol. Ice particles may form due to the homogeneous freezing of aqueous aerosols or by the action of heterogeneous ice nuclei (IN). IN possess the ability to form ice at a higher temperature for a given vapor pressure of water than is required for homogeneous freezing. Apart from a few studies of refractory components, the chemical composition of these climatically important particles remains largely unknown. Almost nothing has been reported about the semivolatile and volatile components of IN. One of the principal reasons is that collection of cirrus precursors ideally should take place immediately after ice formation, before significant alteration of the crystals due to particle and gas-phase scavenging. Here we describe a method to measure the concentration and activation conditions of aerosols by exposure to temperatures and relative humidities (RH) similar to those that initiate cirrus cloud formation in the atmosphere. Laser mass spectrometry was subsequently used to investigate only those particles that nucleated ice. With this technique we were able to differentiate particles known to act as IN from those that entered the ice phase homogeneously. Deployment to study aerosol effects on ice formation in cirrus clouds is presented, although this method is applicable to the entire tropospheric mixed-phase and ice-phase regimes.