A Correlation between Ambient Depositional Ice Nucleating Particle Concentration and Isoprene-Derived Secondary Organic Aerosol Concentration

Atmospheric ice nucleation influences the global climate by altering cloud radiative impact, lifetime, and precipitation efficiency. Ice nucleating particles (INPs) promote ice formation at warmer temperatures and lower relative humidities than otherwise required for homogeneous ice nucleation, yet more studies are needed to investigate their sources in the ambient atmosphere. Here, we present measurements of depositional INP concentrations at the Puy de Dome observatory in central France. Online INP concentration measurements were made using the Spectrometer for Ice Nuclei, while bulk aerosol chemical composition was determined with liquid chromatography mass spectrometry in filter samples collected during INP measurements. Ambient concentrations of INPs ranged from 0.1 to 70 L^-1 at -46 ˚C and at an ice supersaturation of 30%. INPs were most abundant during periods of high secondary organic aerosol (SOA) concentration. Specifically, concentrations of depositional INPs were strongly correlated (R² > 0.9) with increased mass loadings and mass fractions of isoprene-derived SOA, while moderately correlated with abundances of α‐pinene (R² < 0.7) and uncorrelated anthropogenic (R² < 0.1) SOA. Fractional ice activation was elevated with increasing SOA concentration but not with total aerosol concentration. Aerosol source apportionment indicates INP concentration was elevated when sampling air from terrain with high isoprene and other biogenic volatile organic compound emission rates. Isoprene-derived SOA, or other particle types originating from similar sources, warrant further exploration as a source of heterogeneous ice nuclei at cirrus-relevant conditions.