Ice nucleation by combustion products at conditions relevant to mixed-phase clouds

Quantifying the ice nucleation activity of combustion aerosols is crucial in understanding their impact on cloud properties, and consequently, on climate. This study investigates the ice nucleation abilities of soot and combustion ashes in the immersion mode at conditions relevant to mixed-phase clouds. Some physical and chemical properties of these aerosols, which aided the interpretation of their ice nucleation activities, were also explored. Soot generated from eugenol and n-decane compounds were used as proxies for atmospheric soot from biomass and hydrocarbon combustion, respectively. Combustion ashes studied were coal fly ash (CFA), wood bottom ash, domestic bottom ash, and coal bottom ash. The ice nucleation experiments were performed with droplet freezing assay instruments; the results obtained are summarised below: (1) Eugenol and n-decane soot particles in suspension were found to nucleate ice between -16.5 and -28 ºC. To describe the ice nucleation efficiency of these particles, a singular model of ice nucleation that yields the ice active nucleation sites density (ns) was applied. Both soot types showed ns in the range: 1 - ~ 107 cm-2 at a temperature range of -16 to ~ -28 °C. An estimation of potential ice nuclei (IN) number based on the ice nucleation efficiency of these soot types indicated that soot substantially contributes to primary ice formation in mixed-phase clouds from temperatures below -22 ºC. At ~ - 25 ºC, soot showed a competition with mineral dust IN. This study suggests that soot is an important IN in mixed-phase clouds especially in regions where mineral dust is not a dominant IN. (2) The freezing temperatures for combustion ashes were between -15 to -36 °C. The fraction of droplets frozen showed that the freezing temperatures were in this order of significance: CFA>>wood ash>domestic ash>coal ash. The ns values estimated for all ashes were between 10-2 and 107 cm-2 for freezing temperatures between -15 and -36 °C. The best estimate suggests that combustion ashes can account for global primary ice nuclei number up to 1 cm-3, and this could impact on primary ice formation budget in mixed-phase clouds.