Factors controlling the ice nucleating abilities of α‐pinene SOA particles

Abstract

The ice nucleation abilities of fresh, water-soluble, internally mixed, and photochemically oxidized -pinene secondary organic aerosol (SOA) particles were investigated at cirrus cloud temperatures in a continuous flow diffusion chamber. SOA sampled from a flow tube (SOA-fresh-FT) mimicked freshly generated particles, while the water-soluble organic compound fraction from a FT and smog chamber (SOA-WSOC-FT, SOA-WSOC-SC) mimicked cloud-processed particles. SOA-fresh-FT, SOA-WSOC-FT, and SOA-WSOC-SC particles were not highly active at nucleating ice between 233 K and 213 K, with activation onsets (i.e., 0.1% of particles forming ice) at or slightly above the theoretical homogeneous freezing line. A significant increase in the O/C of SOA-WSOC-SC via aqueous phase OH oxidation did not modify the ice nucleation abilities, indicating that the detailed composition of the particles is not of paramount importance to their ice nucleating abilities. Instead, precooling the SOA-WSOC-FT and SOA-WSOC-SC particles to 233 K dropped their ice nucleation onsets by up to 20% relative humidity with respect to ice, with lower temperatures likely driving the particles to be more viscous and solid-like. However, it is possible that preactivation contributed to the reduction of the ice nucleation onsets. Particles composed of both SOA-WSOC and ammonium sulfate (AS) were significantly less active in the deposition nucleation mode than pure, solid AS particles.

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities

“…Simulations show that total SOM mass concentrations can be overestimated or underestimated depending on what diffusion rates are used (Shiraiwa and Seinfeld, 2012). Chemical aging of atmospheric particles by heterogeneous reactions can depend on diffusion rates within SOM Kuwata and Martin, 2012;Zhou et al, 2013;Steimer et al, 2014;Houle et al, 2015) and heterogeneous ice nucleation may be influenced by the viscosity of SOM particles (Murray et al, 2010;Wang et al, 2012;Ladino et al, 2014;Schill et al, 2014;Wilson et al, 2012). Moreover, long-range transport of polycyclic aromatic hydrocarbons can depend on diffusion rates in a particle Zhou et al, 2013) and the efflorescence of crystalline salts can be hindered for highly viscous SOM (Murray, 2008;Murray and Bertram, 2008;Bodsworth et al, 2010;Song et al, 2013).…”

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

“…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.…”

“…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.…”

“…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.…”