Competing Effects of Organic Matter and Sulphur Species on the CCN activation of combustion aerosol particles - Results from the PartEmis Experiment

Abstract

During the European PartEmis project (Measurement and prediction of emissions of aerosols and gaseous precursors from gas turbine engines) which was focussed on the characterisation and quantification of exhaust emissions from a gas turbine engine, a comprehensive suite of aerosol, gas and chemi-ion measurements were conducted under different combustion conditions and fuel sulphur concentrations. Data sets on aerosol mass and number concentration, size distribution, mixing state, thermal stability of internally mixed particles, hygroscopicity, cloud condensation nuclei (CCN) activation potential and aerosol chemical composition were collected simultaneously from the exhaust of a gas turbine combustor. The composition of the carbonaceous particle fraction was determined using multi-step combustion methods for the determination of total carbon (TC) and elemental carbon (EC), and evolved gas analysis methods (EGA) for measuring thermograms of the thermal stability of carbonaceous compounds. Targeted scientific objectives were aerosol microphysics and aerosol dynamics of combustion particles, formation of condensation particles from the gas phase, interaction of combustion particles with gaseous and particulate sulphuric acid, speciation of the organic fraction of combustion particles, hygroscopic particle growth factors at water-subsaturated conditions, and CCN activation at well-defined water-supersaturated conditions. Modelling of CCN activation of combustion particles was conducted using microphysical and chemical properties as measured in the experiment. Based on this unique data set, the importance of the chemical composition of the organic particle fraction and of the mass transfer of water-soluble compounds from the gas and particle phase to the combustion aerosol surface was investigated with respect to the resulting CCN activation potential of the emitted carbonaceous combustion aerosol particles. Two major scientific questions are addressed in the presented study: 1/ Which role play volatile condensation particles forming in the cooling exhaust gas from sulphuric acid in the CCN activation of carbonaceous combustion particles? 2/ Which role plays the organic fraction of the carbonaceous combustion particles in the CCN activation process? It was found that particles containing a large fraction of nonvolatile organic compounds grow significantly lower than particles with a lower content of nonvolatile OC. Also the effect of the nonvolatile OC fraction on the potential CCN activation is significant. While a coating of water-soluble sulphuric acid increases the potential CCN activation, or lowers the activation diameter, respectively, the organic compounds partially compensate this sulphuric acid-related improvement in CCN activation of carbonaceous combustion aerosol particles.