Ice-nucleating particles near two major dust source regions
Authors: Beall Charlotte M. , Hill Thomas C. J. , DeMott Paul J. , Köneman Tobias , Pikridas Michael , Drewnick Frank , Harder Hartwig , Pöhlker Christopher , Lelieveld Jos , Weber Bettina , Iakovides Minas , Prokeš Roman , Sciare Jean , Andreae Meinrat O. , Stokes M. Dale , Prather Kimberly A. 
Published: September 2022

Abstract. Mineral dust and sea spray aerosol represent important sources of
ice-nucleating particles (INPs), the minor fraction of aerosol particles
able to trigger cloud ice crystal formation and, consequently, influence
multiple climate-relevant cloud properties including lifetime, radiative
properties and precipitation initiation 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 in 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 radiative properties are
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-active surface 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 (PM10), as simulated by the Modern-Era
Retrospective analysis for Research and Application, version 2
(MERRA-2). The impacts of heat and peroxide treatments indicate that
organics dominated the observed ice nucleation (IN) activity at
temperatures ≥ −15 ∘C with proteinaceous (heat-labile)
INPs frequently observed at high freezing temperatures >−10 ∘C. INP concentrations in seawater samples ranged between 3 and
46 mL−1 at −19 ∘C, demonstrating the relatively low INP
source potential of seawater in the region as compared to seawater from
multiple other regions reported previously. Overall, our 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). Future
efforts to develop or improve representations of dust INPs at modest
supercooling (≥-15 ∘C) would benefit from a characterization
of the specific organic species associated with dust INPs. More generally,
an improved understanding of the organic species associated with increased
IN activity and their variability across dust source regions would directly
inform efforts to determine whether ns-based parameterizations are
appropriate for faithful representation of dust INPs in this sensitive
temperature regime, whether region-specific parameterizations are required,
or whether an alternative to the ns approach is necessary.