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

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 × 10 5 Pa s, implying that isoprenederived 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 cm 2 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 isoprenedominant 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.

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities

“…This growth and coagulation process resulted in larger but fewer SOM particles on the hydrophobic slides. Details of this procedure are given by Renbaum-Wolff et al (2015) and Song et al (2015). This procedure was used for samples 1, 2, 5, and 6 shown in Table 1.…”

“…The calibration line was developed by Song et al (2015) from measurements of bead speeds in sucrose-water particles over a range of RH values. The RH within the flow cell was measured using a hygrometer with a chilled mirror sensor (General Eastern, Canada), which was calibrated by measuring the deliquescence RH for pure ammonium sulfate particles (80.0 % RH at 293 K, Martin, 2000).…”

“…Most qualitative and quantitative measurements of viscosity and diffusion rates within organic particulate matter have focused on SOM generated from biogenic VOCs such as α-pinene and isoprene (Virtanen et al, 2010;Cappa and Wilson, 2011;Perraud et al, 2012;Saukko et al, 2012;Abramson et al, 2013;Robinson et al, 2013;RenbaumWolff et al, 2013a;Bateman et al, 2015;Kidd et al, 2014;Pajunoja et al, 2014;Wang et al, 2015;Grayson et al, 2015;Song et al, 2015). Recently, the viscosity and diffusion rates within SOM particles generated from anthropogenic VOCs have also been investigated.…”

Nanoparticle growth by particle-phase chemistry

“…for constraining aerosol formation models. Future modeling of size-dependent molecular composition should include physico-chemical parameters such as diffusion, phase separation, and reaction reversibility (Liu et al, 2014;Mai et al, 2015;Riipinen et al, 2012;Shiraiwa et al, 2012;Song et al, 2015;Trump and Donahue, 2014;Zaveri et al, 2014) that may influence the contribution of particlephase chemistry to nanoparticle growth.…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…For example, experimental study demonstrated that when exposed to SO 2 , NO 2 , and NH 3 , the growth factor of the seed particles increased with RH, with the values of near unity at RH < 20% and 2.3 at 70% RH [33]. We admit that the particles collected by impaction may potentially be flattened and the secondary particles at different RH might show different viscosity [34], having different flattened ability. However, the larger secondary particles in the atmosphere will generally have a larger flatten size on the filter.…”