Abstract
Hygroscopicity measurements of secondary organic aerosol (SOA) particles often show inconsistent results between the supersaturated and subsaturated regimes, with higher activity as cloud condensation nucleus (CCN) than indicated by hygroscopic growth. In this study, we have investigated the discrepancy between the two regimes in the Lund University (LU) smog chamber. Various anthropogenic SOA were produced from mixtures of different precursors: anthropogenic light aromatic precursors (toluene and m-xylene), exhaust from a diesel passenger vehicle spiked with the light aromatic precursors, and exhaust from two different gasoline-powered passenger vehicles. Three types of seed particles were used: soot aggregates from a diesel vehicle, soot aggregates from a flame soot generator and ammonium sulphate (AS) particles. The hygroscopicity of seed particles with condensed, photochemically produced, anthropogenic SOA was investigated with respect to critical supersaturation (sc) and hygroscopic growth factor (gf) at 90% relative humidity. The hygroscopicity parameter κ was calculated for the two regimes: κsc and κgf, from measurements of sc and gf, respectively. The two κ showed significant discrepancies, with a κgf /κsc ratio closest to one for the gasoline experiments with ammonium sulphate seed and lower for the soot seed experiments. Empirical observations of sc and gf were compared to theoretical predictions, using modified Köhler theory where water solubility limitations were taken into account. The results indicate that the inconsistency between measurements in the subsaturated and supersaturated regimes may be explained by part of the organic material in the particles produced from anthropogenic precursors having a limited solubility in water.
Highlights
The Earth’s atmosphere and the global climate are today greatly perturbed by anthropogenic emissions to the air – gases as well as particulates (e.g. IPCC 2014)
A decreasing trend for the hygroscopicity of the particles in the ammonium sulphate (AS) seed experiments (G and P) was observed, which can be explained by the fact that the size of the AS seed particles, connected with a given total size, decreases as more organic material condenses in the aerosol, i.e. the mass fraction of organic coating material increases and the mass fraction AS decreases in the particles
Anthropogenic secondary organic aerosol (SOA) produced from four different precursor mixtures ((1) toluene and mxylene, (2) Euro II diesel vehicle exhaust in addition to the precursors (1), (3) Euro II and (4) Euro IV gasoline-powered passenger vehicle exhaust), were photochemically processed in a smog chamber
Summary
The Earth’s atmosphere and the global climate are today greatly perturbed by anthropogenic emissions to the air – gases as well as particulates (e.g. IPCC 2014). With respect to the global production of anthropogenic SOA, benzene (C6), toluene (C7) and xylenes (C8) are estimated to be among the important aromatic hydrocarbon precursors (Henze et al 2008) These three species are commonly found in gasoline exhaust and are present in diesel exhaust at lower levels (Schauer et al 1999, 2002; Nordin et al 2013). The SOA production in the gasoline car experiments is described in detail by Nordin et al (2013) and the cloud droplet formation ability of soot agglomerates in the initial phases of SOA condensation is discussed by Wittbom et al (2014) The data from these measurements has been used together with ambient data to discuss time scales for soot transformation from hydrophobic to potential CCN (Eriksson et al 2017). We evaluate and show that the solubility of the organic fraction in the particle may explain the inconsistency between the measured observation from the subsaturated and supersaturated regimes
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