Experimental determination of chemical diffusion within secondary organic aerosol particles

Abstract

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.

Global long-range transport and lung cancer risk from polycyclic aromatic hydrocarbons shielded by coatings of organic aerosol

“…At cool and/or dry conditions, however, we assume that highly viscous OA effectively shields BaP from heterogeneous oxidation, similar to that observed for eicosane (a highly viscous solid organic) coatings (18) (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). This approach is consistent with highly viscous SOA observed at dry conditions (27,28), persistence of highly viscous SOA at cooler temperatures in the free troposphere (26), and also higher viscosity of complex SOA mixtures (29) that are likely more atmospherically relevant compared with laboratory-generated SOA (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). We show that including the shielding of BaP by OA results in better agreement with BaP measurements in the field, promotes stronger long-range transport, and also produces greater global incremental lung cancer risk (ILCR) estimates due to exposure to PAHs, compared with the default model that neglects this shielding by OA.…”

“…S4B). This result suggests that more complex and aged atmospheric SOA, which is highly viscous at cool/dry conditions (24,26,29), is expected to be highly effective in shielding BaP from oxidation, as is assumed in our new modeling formulation.…”

Dynamic viscosity mapping of the oxidation of squalene aerosol particles

“…Namely (i) larger droplets can be studied allowing to better observe possible heterogeneities in the oxidised droplet composition and (ii) there is an opportunity for simultaneous measurement of more than one droplet under exactly the same experimental conditions and hence the possibility of better counting statistics. The latter advantage is depicted in Figure 4 where three droplets of different sizes (11,35 and 62 μm) are being imaged simultaneously while reacting with ozone. As seen in Figure 4B the total time required for all three droplets to reach at their maximum lifetime is 21 minutes, however individual droplets reach this maximum value at different times: 9 min for 11 μm droplet, 15 minutes for 35 μm droplet and 21 min for the largest 62 μm droplet.…”

“…An aerosol's viscous state also has direct implications on the particle morphology, phase behaviour, optical properties, aging processes and lifetime. At the same time, only few direct measurement methods are available for monitoring such properties 1, [6][7][8][9][10][11][12][13][14][15][16] . Recently, we have demonstrated that viscosity of model organic aerosols, such as oleic acid droplets, and secondary organic aerosols (SOA) produced from the oxidation of myrcene or -pinene can be measured and imaged in a spatially and time-resolved manner using Fluorescence Lifetime Imaging Microscopy (FLIM) of molecular rotors 17 .…”

Shikimic acid ozonolysis kinetics of the transition from liquid aqueous solution to highly viscous glass

“…This provides the basis for theoretical descriptions of secondary organic aerosols (SOA). However, recent studies of glass transition temperatures, 9 particle bouncing behaviour, 10 evaporation, [11][12][13] thermal desorption 14 and response to physical deformation 15 show that OM can adopt a semi-solid or amorphous solid (glassy) state under environmental conditions. Starting from aqueous organic liquids which exist in the atmosphere at high RH, the self-diffusion coefficient (of the organic solute) can decrease with decreasing RH from around 10 À8 cm 2 s À1 to less than 10 À20 cm 2 s…”