Ice Nucleation Efficiency of SOA Particles from Boreal Forests

Secondary organic aerosol (SOA) particles are produced in the atmosphere from oxidation of volatile compounds (VOCs) and subsequent condensation of the reaction products, SOA constitutes a high portion of the submicron particulate mass in the lower troposphere with most of the VOC precursors emitted from biogenic sources.

There is evidence about ice nucleation (IN) activity of SOA: oxidized organic matter has been found in ice particle residuals from cirrus clouds, and modelling studies suggest that viscous SOA could be an important ice nucleating particle. The phase state of SOA, which depends on temperature and relative humidity, seems to be a key factor responsible for their IN properties. However, the role of SOA particles in cloud activation and especially their importance in ice cloud formation remains poorly understood.

We report preliminary results from the SINE campaign (SOA Ice Nucleation Experiment) carried out at the University of Eastern Finland Aerosol Physics Laboratory. The SINE campaign was focused on studying the IN properties of SOA formed from boreal forest emissions. SOA was produced from α-pinene (one of the most common VOCs found in boreal forests) with different oxygen-to-carbon (O:C) ratios (~0.5, 0.8, 1.0) in an oxidation flow reactor by ozonolysis or photo oxidation. The size distribution and composition of the generated particle population was monitored by a scanning mobility particle sizer and an aerosol mass spectrometer. As a proxy for real trees emissions, SOA particles were also generated from pine-needle oil.

The ice nucleation efficiency of the produced SOA particles was explored between 210 and 240K and from ice to water saturation using the Spectrometer for Ice Nuclei (SPIN). SOA particles were size selected with a differential mobility analyzer at 100nm and the relative humidity of the sample entering the SPIN was set at 40%, 10%, or <1% RH.

To investigate the effect of SOA formation conditions on IN activity, we also performed IN experiments using an atmospheric simulation chamber to ensure more atmospheric relevant conditions. In these experiments real plant emitted VOCs were used in addition to the α-pinene and pine-needle oil experiments.

Preliminary results show that the relative humidity of the SOA sample is the major factor controlling the phase state and therefore enabling ice activation. Regarding the O:C ratio, studied precursor VOCs, and SOA formation conditions (flow tube vs atmospheric simulation chamber) results indicate that they had a negligible effect on ice formation from α-pinene SOA.