Heterogeneous ice nucleation of viscous secondary organic aerosol produced from ozonolysis of &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene

Karoliina Ignatius,Urs Baltensperger,Claudia Fuchs,Markku Kulmala,Frank Stratmann,Annele Virtanen,Sarvesh Garimella,Emma Järvinen,Joachim Curtius,Chao Yan,Ottmar Möhler,Leonid Nichman,Neil M Donahue,Jonathan Duplissy,Carla Frege,Hamish Gordon,Paul Herenz,Jasmin Tröstl,Thomas Bjerring Kristensen ,António Dias ,Martin Gallagher ,N Höppel ,R J Wagner ,Harald Saathoff ,A Amorim ,C R Hoyle ,António Tomé ,J Kirkby ,M Schnaiter ,Douglas R Worsnop

doi:10.5194/acp-16-6495-2016

Abstract

Abstract. There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −39.0 and −37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.

Highlights

Atmospheric aerosol particles are known to influence the Earth’s radiative balance and climate directly by reflecting and absorbing sunlight, and indirectly through their influence on clouds, e.g. when the particles act as cloud condensation nuclei (CCN) and/or ice nucleating particles (INPs) (Yu et al, 2006)
A prominent droplet mode can be seen in the third panel (c), when the RHw was greater than 100 %, and it is possible that the secondary organic aerosol (SOA) particles liquefied and froze homogeneously, as we have observed homogeneous freezing of diluted ammonium sulphate droplets at similar conditions
We produced viscous α-pinene SOA particles at 10 % relative humidity (RH) at four different atmospherically relevant subzero temperatures, −10, −20, −30 and −38 ◦C, and measured their ice nucleation capability with a new portable INP counter

Summary

Introduction

Atmospheric aerosol particles are known to influence the Earth’s radiative balance and climate directly by reflecting and absorbing sunlight, and indirectly through their influence on clouds, e.g. when the particles act as cloud condensation nuclei (CCN) and/or ice nucleating particles (INPs) (Yu et al, 2006). Homogeneous ice nucleation requires temperatures below approximately −37 ◦C and high supersaturations, typically ice saturation ratios Sice of 1.4 or larger (Koop et al, 2000), and can contribute to cirrus cloud formation. Heterogeneous ice nucleation is considered to be an important pathway for ice formation in the troposphere, especially in mixed-phase clouds (Hoose and Möhler, 2012; Murray et al, 2012), and in cirrus clouds (Krämer et al, 2009; Cziczo et al, 2013). The surface area of the seed particles typically plays an important role, so that larger particles tend to be more efficient INPs (Connolly et al, 2009; Welti et al, 2009)

Methods

Results

Conclusion