Ice‐Nucleating Particle and Cloud Ice Crystal Concentrations Associated With Developing Summertime Deep Convective Clouds in South‐Western USA

Ice-nucleating particles (INPs), essential for initiating primary ice production in many mixed-phase clouds, have only rarely been measured in air directly relevant for deep convective clouds. In July–August 2022 we used an aircraft to sample aerosol near developing deep convective clouds over Magdalena Mountain, New Mexico, USA. We observed INP concentrations consistently at the upper end of those typically observed in continental air with ∼0.01–0.5 L ‾¹ at −8°C. Vertically resolved sampling revealed a deep aerosol layer extending 3–4 km above ground level, in which INP concentrations only varied within an order of magnitude. Above this layer, low temperature INP (<−15°C) decreased, but high temperature INP (>−15°C) remained abundant, despite having been collected in air with a lower aerosol concentration. Compositional analysis of filter aerosol showed mineral dusts dominated the surface area of aerosol larger than ∼800 nm, but the activity of the mineral alone cannot account for the observed INP concentrations. We suggest that these mineral dust particles are internally mixed with ice-nucleating biogenic components, as they would be in organic containing fertile soils, or there is an additional biogenic/biological ice-nucleating component(s) strongly enhancing the INP population over what would be expected for mineral dust alone. Finally, ice particle measurements in rising cloud turrets at progressively colder temperatures reveal that the temperature at which we first observed ice was consistent with the measured INP spectra.