The UNAM-MicroOrifice Uniform Deposit Impactor-Droplet Freezing Technique: Size-Resolved Ice Nucleating Particle Concentrations

The presence of ice in clouds affects their microphysical and radiative properties, lifetime, electrification, and precipitation formation; however, ice particle formation within mixed-phase clouds (MPC) is highly uncertain. Despite the fact that primary ice particles and some secondary ice production mechanisms in MPC are strongly related to the presence of ice nucleating particles (INPs), what makes an aerosol particle an efficient INP remains unclear. Although significant advances in identifying the most important physicochemical properties of the most common INPs has been made in the last two decades, most of the currently available INP observations has focused on particles smaller than 1-2 μm, mainly because of technical limitations in the distinction of super-micron particles and small ice particles by the used optical sensors.

Up to date, there is not a perfect INP sensor; however, the UNAM-MicroOrifice Uniform Deposit Impactor-Droplet Freezing Technique (UNAM-MOUDI-DFT) is an alternative apparatus to overcome the aforementioned limitation. It measures the INP concentration, via immersion freezing, as a function of particle size (from 0.32 μm to 10 μm) at six different size bins. In the present work, the importance of particle size in the ice nucleating abilities of different aerosol types (e.g., urban, biomass burning, African dust, marine aerosol, volcanic, and agricultural dust) will be discussed. Our observations demonstrate that super-micron aerosol particles represent the largest fraction (>70%) of the INP population, indistinct of the aerosol type. This information is useful for the development of new INP parametrizations.