High ice-nucleating particle concentrations associated with Arctic haze in springtime cold-air outbreaks

Abstract. The global variation of ice-nucleating particle (INP) concentrations is an important modulator of the cloud-phase feedback, where the albedo of mixed-phase clouds increases in a warming climate. Shallow clouds such as those observed in cold-air outbreaks (CAOs) are particularly important for cloud-phase feedbacks and highly sensitive to INPs. To investigate the sources and concentrations of INPs in CAOs, we made airborne measurements over the Norwegian and Barents seas as part of the March 2022 Arctic Cold-Air Outbreak (ACAO) field campaign. Aerosol samples were collected on filters at locations above, below and upwind of CAO cloud decks. Throughout the campaign, INP concentrations were comparable to the highest previously observed in the Arctic. Scanning electron microscopy analysis of samples taken upwind of cloud decks showed that super-micron aerosol was dominated by mineral dusts. Analysis of aerosol particle size measurements to obtain an INP active site density suggested sea spray was unlikely to be the dominant INP type. These site densities were also too great for mineral components alone to be the dominant INP type above -20 °C. Accordingly, it is likely that the dominant INP type was mineral dust mixed with other ice nucleating materials, possibly of biogenic origin. Back-trajectory analysis and meteorological conditions suggested a lack of local INP sources. We therefore hypothesise that the high INP concentration is most likely to be associated with aged aerosol in Arctic haze that has undergone long-range transport from lower latitude regions.