Experimental determination of chemical diffusion within secondary organic aerosol particles.

Formation, properties, transformations, and temporal evolution of secondary organic aerosol (SOA) particles depend strongly on SOA phase. Recent experimental evidence from both our group and several others indicates that, in contrast to common models' assumptions, SOA constituents do not form a low-viscosity, well-mixed solution, yielding instead a semisolid phase with high, but undetermined, viscosity. We find that when SOA particles are made in the presence of vapors of semi-volatile hydrophobic compounds, such molecules become trapped in the particles' interiors and their subsequent evaporation rates and thus their rates of diffusion through the SOA can be directly obtained. Using pyrene as the tracer molecule and SOA derived from α-pinene ozonolysis, we find that it takes ~24 hours for half the pyrene to evaporate. Based on the observed pyrene evaporation kinetics we estimate a diffusivity of 2.5 × 10(-21) m(2) s(-1) for pyrene in SOA. Similar measurements on SOA doped with fluoranthene and phenanthrene yield diffusivities comparable to that of pyrene. Assuming a Stokes-Einstein relation, an approximate viscosity of 10(8) Pa s can be calculated for this SOA. Such a high viscosity is characteristic of tars and is consistent with published measurements of SOA particle bounce, evaporation kinetics, and the stability of two reverse-layered morphologies. We show that a viscosity of 10(8) Pa s implies coalescence times of minutes, consistent with the findings that SOA particles formed by coagulation are spherical on the relevant experimental timescales. Measurements on aged SOA particles doped with pyrene yield an estimated diffusivity ~3 times smaller, indicating that hardening occurs with time, which is consistent with the increase in SOA oligomer content, decrease in water uptake, and decrease in evaporation rates previously observed with aging.

Synergy between secondary organic aerosols and long-range transport of polycyclic aromatic hydrocarbons.

Polycyclic aromatic hydrocarbons (PAHs), known for their harmful health effects, undergo long-range transport (LRT) when adsorbed on and/or absorbed in atmospheric particles. The association between atmospheric particles, PAHs, and their LRT has been the subject of many studies yet remains poorly understood. Current models assume PAHs instantaneously attain reversible gas-particle equilibrium. In this paradigm, as gas-phase PAH concentrations are depleted due to oxidation and dilution during LRT, particle-bound PAHs rapidly evaporate to re-establish equilibrium leading to severe underpredictions of LRT potential of particle-bound PAHs. Here we present a new, experimentally based picture in which PAHs trapped inside highly viscous semisolid secondary organic aerosol (SOA) particles, during particle formation, are prevented from evaporation and shielded from oxidation. In contrast, surface-adsorbed PAHs rapidly evaporate leaving no trace. We find synergetic effects between hydrophobic organics and SOA - the presence of hydrophobic organics inside SOA particles drastically slows SOA evaporation to the point that it can almost be ignored, and the highly viscous SOA prevents PAH evaporation ensuring efficient LRT. The data show the assumptions of instantaneous reversible gas-particle equilibrium for PAHs and SOA are fundamentally flawed, providing an explanation for the persistent discrepancy between observed and predicted particle-bound PAHs.

Viscosity of carbon dioxide measured to a pressure of 8 GPa and temperature of 673 K.

Shear viscosities of supercritical carbon dioxide have been measured to 673 K and 8 GPa (80 kbar). Measurements were made in a diamond-anvil cell with a rolling-ball technique. Individual isotherms are well fit by a modified free-volume equation. The data demonstrate a close relation between viscosity and residual entropy.

Viscosity of nitrogen measured to pressures of 7 GPa and temperatures of 573 K.

Shear viscosities of supercritical nitrogen have been measured to 573 K and 7 GPa (70 kbars). Measurements were made in a diamond-anvil cell with a rolling-ball technique. Individual isotherms are well fitted by a modified Doolittle equation. The data demonstrate a close relation between viscosities and excess entropy; this relation is further explored for the systems argon, oxygen, carbon dioxide, sodium, cesium, and a Lennard-Jones fluid.

Viscosity of water measured to pressures of 6 GPa and temperatures of 300 degrees C.

Shear viscosities of fluid water have been measured to 300 degrees C and 6 GPa (60 kbar). Measurements were made in a diamond-anvil cell with a rolling-ball technique. Enskog's equation for viscosity, coupled with an ad hoc assumption that increased collision rates are due to an "excluded volume", yield excellent matches to the data at temperatures of 100 degrees C and over, without any freely variable parameter. The data overlap the pressure-temperature range in which experiments on shocked water have previously been interpreted to indicate extremely high viscosities. It is shown conclusively that viscosities in this region are very close to those at ambient temperature. Further, it is argued that explanations of high apparent viscosities which rely on the putative formation of ice behind the shock front are probably incorrect.

Viscosity of fluid nitrogen to pressures of 10 GPa.

Shear viscosities of supercritical nitrogen have been measured in the high-pressure diamond-anvil cell, to 673 K and pressures in excess of 10 GPa, using a rolling-sphere technique. The entire set of data, along with lower pressure data from the literature, can be fit to a two-parameter expression in reduced viscosity and reduced residual entropy. The fit spans densities from the dilute gas to 5x the critical density, and two orders magnitude in temperature and in viscosity, with a maximum deviation of 20%. Reduced viscosities scale as ρ(4)/T and comport with the theory of state "isomorphs" for "Roskilde-simple" systems. The new data allow direct comparison with results of molecular dynamic simulations at high densities.

Viscosity of methane to 6 GPa and 673 K.

A rolling-sphere technique has been used to measure shear viscosities of (supercritical) fluid methane in a diamond-anvil cell between temperatures of 294 and 673 K, up to a pressure of 6 GPa. A correlation between a reduced viscosity and reduced residual entropy is shown to give a good account of much of the extant data, both from this study and the literature.