Airborne Observations of Ice-Nucleating Particles Associated with Deep Convective Clouds in New Mexico

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. We present INP concentration spectra from the Deep Convective Microphysics Experiment (DCMEX) in New Mexico, USA in summer 2022. Filter sampling was done over a 4-week period during the monsoon season from aircraft flights around the summit of the Magdalena Mountains, where and when deep convective clouds frequently form. We collected aerosol on filters and analysed for INPs offline, aiming to characterise the INP concentration, spatial distribution and origin in airmasses participating in deep convective cloud formation. We observed INP concentrations that were consistently at the upper end previously reported in continental air with ~1 L^-1 at -13 °C and ~ 0.1 L^-1 at -8 °C. Vertically resolved sampling at this location revealed a deep aerosol layer extending 3-4 km above ground level. INP concentrations in this layer were within about 1 order of magnitude. However, above this layer, low temperature INP (< -15 °C) decreased, but high temperature INP (> -15 °C) were abundant, despite having been collected in air with a lower aerosol concentration than below 3-4 km. Composition analysis of filter aerosol suggested mineral dusts dominated the aerosol surface area. We suggest that these mineral dusts are either internally mixed with ice-nucleating biogenic components, as they would be in fertile carbon containing soils, or there is an additional biogenic/biological ice-nucleating component(s) that strongly enhances the INP population over what would be expected for mineral dust alone. Finally, our INP observations were compared with primary ice crystal number concentrations measured in updrafts at cloud tops in clouds forming immediately after filter samples were collected, revealing agreement in some cases or absence of primary ice crystals when expected in other cases.