Heterogeneous ice nucleation of viscous secondary organic aerosol produced from ozonolysis of <i>α</i>-pinene

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.

The efficiency of secondary organic aerosol particles acting as ice-nucleating particles under mixed-phase cloud conditions

“…In addition to the different particle generation procedures, the resulting particle sizes vary, too, which certainly alters the ice nucleation capability (larger particles provide a larger surface area and are more likely to form ice). The findings of the above-mentioned studies can be summarized as follows: Wang et al (2012) and Ignatius et al (2016) found that atmospheric SOA particles are potentially important for ice nucleation due to their semi-solid or solid phase states by investigating SOA from naphthalene and α-pinene, respectively, whereas Ladino et al (2014) and Wagner et al (2017) found that α-pinene SOA at first is an inefficient INP at cirrus temperatures, but after precooling of the SOA particles, ice nucleation ability is increased. Schill et al (2014) found that semi-solid or glassy SOA from aqueous processing of methylglyoxal with methylamine is a poor depositional INP; however, Wilson et al (2012) found other aqueous glassy aerosol to nucleate ice heterogeneously at temperatures relevant for cirrus formation in the tropical tropopause layer.…”

“…Unfortunately, no bounce measurements with pure α-pinene were performed; however, several studies (e.g. Saukko et al, 2012a;Pajunoja et al, 2015;Ignatius et al, 2016) have shown that α-pinene SOA is semisolid under the conditions relevant here.…”

“…Ice nucleation has been tested e.g. in expansion chambers (Murray et al, 2010;Wilson et al, 2012;Wagner et al, 2017), in continuous-flow diffusion chambers and flow tubes (Ladino et al, 2014;Ignatius et al, 2016), and in microscope systems (Wang et al, 2012;Baustian et al, 2013). Whether these formation pathways and ice nucleation initiation methods have an impact on the ice nucleation ability of the SOA particles is not clear.…”

“…Several further studies have investigated the ice-forming capability of different SOA particles (e.g. Prenni et al, 2009;Wang et al, 2012;Ladino et al, 2014;Schill et al, 2014;Ignatius et al, 2016;Wagner et al, 2017) in the laboratory. The procedures for generation of the SOA particles as well as the methods of initiating ice nucleation do vary substantially among these experiments.…”

“…The procedures for generation of the SOA particles as well as the methods of initiating ice nucleation do vary substantially among these experiments. For example, SOA formation has been initiated by dark ozonolysis (Prenni et al, 2009;Wagner et al, 2017) or photochemical reactions (Ladino et al, 2014;Ignatius et al, 2016), using gas-phase reactions (Wang et al, 2012;Ignatius et al, 2016) or aqueous processing (Wilson et al, 2012;Baustian et al, 2013;Schill et al, 2014). Ice nucleation has been tested e.g.…”