Ice nucleating particles at a coastal marine boundary layer site: correlations with aerosol type and meteorological conditions

R H Mason,W R Leaitch,L A Ladino,C Pöhlker,R Dickie,M Si,C Chou,C L Schiller,K Jones,J P D Abbatt,D Toom-Sauntry,A K Bertram,J A Huffman,J D Yakobi-Hancock,J Li

doi:10.5194/acp-15-12547-2015

Abstract

Abstract. Information on what aerosol particle types are the major sources of ice nucleating particles (INPs) in the atmosphere is needed for climate predictions. To determine which aerosol particles are the major sources of immersion-mode INPs at a coastal site in Western Canada, we investigated correlations between INP number concentrations and both concentrations of different atmospheric particles and meteorological conditions. We show that INP number concentrations are strongly correlated with the number concentrations of fluorescent bioparticles between −15 and −25 °C, and that the size distribution of INPs is most consistent with the size distribution of fluorescent bioparticles. We conclude that biological particles were likely the major source of ice nuclei at freezing temperatures between −15 and −25 °C at this site for the time period studied. At −30 °C, INP number concentrations are also well correlated with number concentrations of the total aerosol particles ≥ 0.5 μm, suggesting that non-biological particles may have an important contribution to the population of INPs active at this temperature. As we found that black carbon particles were unlikely to be a major source of ice nuclei during this study, these non-biological INPs may include mineral dust. Furthermore, correlations involving chemical tracers of marine aerosols and marine biological activity, sodium and methanesulfonic acid, indicate that the majority of INPs measured at the coastal site likely originated from terrestrial rather than marine sources. Finally, six existing empirical parameterizations of ice nucleation were tested to determine if they accurately predict the measured INP number concentrations. We found that none of the parameterizations selected are capable of predicting INP number concentrations with high accuracy over the entire temperature range investigated. This finding illustrates that additional measurements are needed to improve parameterizations of INPs and their subsequent climatic impacts.

Highlights

The formation of ice in the atmosphere can occur by two primary mechanisms: homogeneous and heterogeneous ice nucleation
To determine which aerosol particles are the major source of ice nucleating particles (INPs) in the immersion mode at a coastal site in western Canada, we investigate correlations between INP number concentrations and both concentrations of different atmospheric particle types and meteorological conditions
At −30 ◦C, INP number concentrations associated with air masses from the coastal SE appear to be higher than INP number concentrations associated with other air masses, but the statistics are low for the coastal SE air masses, especially at −30 ◦C

Summary

Introduction

The formation of ice in the atmosphere can occur by two primary mechanisms: homogeneous and heterogeneous ice nucleation. Homogeneous nucleation can only occur at temperatures below approximately −37 ◦C. Heterogeneous nucleation can occur at all temperatures below 0 ◦C. Heterogeneous nucleation occurs on solid or partially solid aerosol particles termed ice nucleating particles (INPs). INPs are a small subset of the total aerosol population (Rogers et al, 1998) whose unique surface properties make them capable of lowering the energy barrier to ice nucleation and cause freezing at warmer temperatures or lower supersaturations with respect to ice compared to homogeneous nucleation. Four modes of nucleation have been identified (Vali, 1985; Vali et al, 2015): deposition nucleation, where ice forms on the INP directly from the gas phase; condensation freezing, where ice forms during

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Conclusion