Arctic Haze and New Particle Formation Influences on Enhanced Riming Processes in Mixed-Phase Stratiform

Mixed-phase clouds on the Northern Slope of Alaska are critical for radiative balance but seem to exist in a delicate balance—often persisting for several days, despite their inherent instability, followed by a sudden dissipation (Morrison et al., 2011). Riming is a highly efficient process for removal of cloud mass and is surprisingly common in the Arctic despite frequent low levels of liquid water path (LWP< 50 g m^-2; Fitch & Garrett, 2022). In this work, we evaluate such “enhanced riming” cases in the context of two competing hypotheses: 1) “clean” clouds, with relatively few, larger cloud droplets—leading to a higher riming efficiency (e.g, Tridon et al., 2022); and 2) “polluted” clouds, where a larger number of smaller droplets leads to amplified cloud-top radiative cooling—in turn leading to more intense cloud-scale circulations and lofting of riming particles. We analyze these hypotheses using ground-based Multi-Angle Snowflake Camera (MASC) data coupled with aerosol and LWP measurements at Utqiagvik, Alaska. At cloud level, we use cloud and aerosol measurements from the Chemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) Field Campaign (Fuentes et al., 2025). We first show results from the CHACHA period, 21 February to 16 April of 2022, during which time there was a transition from low-level, long-range transport of “Arctic haze” particles to a more photochemical-dominant new particle formation regime. Beyond the CHACHA period, we also show results from surface-based measurements only for 2021-2024 at Utqiagvik and 2016-2018 at Oliktok Point, Alaska. Initial results suggest that enhanced riming is more common for the polluted clouds cases of the second hypothesis. Additional analysis will help to shed more light on a poorly understood yet important microphysical process that needs more accurate representation in numerical climate and weather models.