Data from: Ice-nucleating particles near two major dust source regions. In Center for Aerosol Impacts on Chemistry of the Environment (CAICE)

Publication abstract: Mineral dust and sea spray aerosol are important sources of ice nucleating particles (INPs), the small fraction of aerosol particles able to trigger cloud ice crystal formation and consequently, influence multiple climate-relevant cloud properties including lifetime, reflectivity and precipitation efficiency. Mineral dust is considered the dominant INP source in many parts of the world due to its ice nucleation efficiency and its sheer abundance, with global emission rates of up to 4700 Tg a-1. However, INPs emitted from the ocean surface via sea spray aerosol frequently dominate INP populations in remote marine environments, including parts of the Southern Ocean where cloud-resolving model simulations have demonstrated that cloud reflectivity is likely strongly controlled by INPs. Here we report INP concentrations measured in aerosol and seawater samples during Air Quality and Climate Change in the Arabian BAsin (AQABA), a shipborne campaign that spanned the Red Sea, Gulf of Aden, Arabian Sea, Arabian Gulf and part of the Mediterranean. In aerosol samples collected within a few hundred kilometers of the first and second ranked sources of dust globally, the Sahara and Arabian Peninsula, INP concentrations ranged from 0.2 to 11 L-1 at -20 °C with observed ice nucleation site densities (ns) 1-3 orders of magnitude below levels predicted by mineral dust INP parameterizations. Over half of the samples (at least 14 of 26) were collected during dust storms with average dust mass concentrations between 150 and 490 μg m-3. The impacts of heat and peroxide treatments indicate that the observed INPs were organic in composition at temperatures ≥ -15 °C with proteinaceous (heat-labile) INPs frequently observed at higher freezing temperatures > -10 °C. Overall, results demonstrate that despite proximity to the Sahara and the Arabian Peninsula and the dominance of mineral dust in the aerosol sampled, existing mineral dust parameterizations alone would not skillfully represent the near-surface ns in the observed temperature regime (-6 to -25 °C). This study underscores the need for measurements of ns and organic composition in characteristic soil dust samples from N. Africa and the Arabian Peninsula. Future efforts to develop representations of dust INPs at high temperatures (> -15 °C) would benefit from improved understanding of the organic species associated with IN-activity, the extent to which the organic contribution to IN-activity varies acros