Coating of Soot and (NH4)2SO4 Particles by Ozonolysis Products of α-Pinene

H. Saathoff*¹, K.-H. Naumann¹, M. Schnaiter¹, W. Schöck¹, O. Möhler¹, U. Schurath¹,

E. Weingartner², M. Gysel², U. Baltensperger².

1 Institute of Meteorology and Climate Research, Forschungszentrum Karlsruhe, POB 3640, D-76021 Karlsruhe, Germany

2 Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland

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Abstract

The ozonolysis of α-pinene in a large aerosol chamber was used to generate secondary organic aerosol (SOA) mass by homogeneous nucleation, or by heterogeneous nucleation, either on soot, or on (NH4)2SO4 seed aerosols. The rate of the α-pinene + ozone reaction and the aerosol yield of ~19% are in good agreement with literature data. The organic coating of soot particles leads to a compaction of the fractal agglomerates expressed by an increase in fractal dimension from 1.9 to 2.1 for Diesel soot, and from 2.0 to 2.3 for spark generated "Palas" soot. The dielectric coating of the soot particles with SOA layers between 2 to 11 nm gives rise to a substantial enhancement of their single scattering albedo, from about 0.2 to 0.5, and increases the effective absorption coefficients of both soot types by ca. 30%. The coating of both soot types increases the hygroscopic growth factors (HGF) to values close below the HGF measured for pure SOA material (d/do ~ 1.12 at 90% RH).