Relative humidity-dependent viscosities of isoprene-derived secondary organic material and atmospheric implications for isoprene-dominant forests

M Song,Y J Li,P F Liu,S T Martin,S J Hanna,A K Bertram

doi:10.5194/acp-15-5145-2015

Abstract

Abstract. Oxidation of isoprene is an important source of secondary organic material (SOM) in atmospheric particles, especially in areas such as the Amazon Basin. Information on the viscosities, diffusion rates, and mixing times within isoprene-derived SOM is needed for accurate predictions of air quality, visibility, and climate. Currently, however, this information is not available. Using a bead-mobility technique and a poke-flow technique combined with fluid simulations, the relative humidity (RH)-dependent viscosities of SOM produced from isoprene photo-oxidation were quantified for 20–60 μm particles at 295 ± 1 K. From 84.5 to 0% RH, the viscosities for isoprene-derived SOM varied from ~ 2 × 10−1 to ~ 3 × 105 Pa s, implying that isoprene-derived SOM ranges from a liquid to a semisolid over this RH range. These viscosities correspond to diffusion coefficients of ~ 2 × 10−8 to ~ 2 × 10−14 cm2 s−1 for large organic molecules that follow the Stokes–Einstein relation. Based on the diffusion coefficients, the mixing time of large organic molecules within 200 nm isoprene-derived SOM particles ranges from approximately 0.1 h to less than 1 s. To illustrate the atmospheric implications of this study's results, the Amazon Basin is used as a case study for an isoprene-dominant forest. Considering the RH and temperature range observed in the Amazon Basin and with some assumptions about the dominant chemical compositions of SOM particles in the region, it is likely that SOM particles in this area are liquid and reach equilibrium with large gas-phase organic molecules on short time scales, less than or equal to approximately 0.1 h.

Highlights

Vegetation and urban environments emit large quantities of volatile organic compounds into the atmosphere (Guenther et al, 1995; Geron et al, 2000; Hakola et al, 2000; Henze et al, 2008)
Since the viscosities do not appear to vary with concentrations of the secondary organic material (SOM) in the oxidation flow reactor (OFR) over the range studied, we group the data in Fig. 6b by relative humidity (RH) to provide an overall summary from the bead-mobility technique
We investigated RH-dependent viscosities of micrometersized SOM particles produced from isoprene photooxidation and having mass concentrations of 100 to 1000 μg m−3 using a bead-mobility technique and a pokeflow technique combined with fluid simulations

Summary

Introduction

Vegetation and urban environments emit large quantities of volatile organic compounds (e.g., isoprene, α-pinene, and toluene) into the atmosphere (Guenther et al, 1995; Geron et al, 2000; Hakola et al, 2000; Henze et al, 2008) In the atmosphere these volatile organic compounds can be oxidized by OH radicals, NO3 radicals, and ozone, contributing to the burden of secondary organic material (SOM) in atmospheric particles (Hallquist et al, 2009). Particles containing SOM are important since they can affect the Earth’s energy budget directly by scattering and/or absorbing solar radiation and indirectly by serving as nuclei for cloud formation (IPCC, 2013). They can influence air quality and human health (Jang et al, 2006; Baltensperger et al, 2008). Researchers have shown that predictions of ultrafine par-

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Conclusion