Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors

Abstract

The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA particles spanning a wide range of oxygen to carbon ratios (O/C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O/C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS). <br><br> The main findings of the study are: (1) Biogenic and anthropogenic SOA particles are found to be solid or semi-solid until a relative humidity of at least 50 % RH at impaction is reached. (2) Long-chain alkanes produce liquid SOA particles when generated at low oxidation level of O/C<0.2, but at higher oxidation levels they solidify. (3) Increasing sulphuric acid (H<sub>2</sub>SO<sub>4</sub>) within the SOA particles reduces the threshold of humidity-induced phase changes. (4) The bounce behavior of the various SOA systems did not show a consistent linear relationship with the particle O/C. Rather, the molar mass of the gas-phase VOC precursor showed a positive correlation with the resistance to the RH-induced phase change of the formed SOA particles

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

“…Our study suggests that this mechanistic interaction between climate-relevant OA and healthrelevant PAHs should be explicitly represented in PAH chemical transport models. We show that the OA coating is likely more effective in shielding PAHs in the middle/high latitudes compared with the tropics because of differences in OA properties [semisolid when cool/dry (42)(43)(44) vs. liquid-like when warm/humid, as shown by OA measurements (27,28)]. Thus, the effectiveness of shielding depends on the viscosity of OA that varies with temperature, RH, and the atmospheric aging of complex OA coatings.…”

Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

“…The model also accounts for the loss of gas-phase SVOCs to the chamber Teflon wall (14,15) based on measurements for representative compounds in separate experiments (SI Materials and Methods, Gas-phase wall loss, Table S1 and S2). The physical state of the particle bulk is assumed to be semisolid with an average bulk diffusivity of 10 −12 cm 2 ·s −1 [a typical value for a semisolid state (16) (SI Materials and Methods)], consistent with observations that long-chain alkane-derived SOA particles bounce moderately on the smooth plates of an inertial impactor indicating behavior between that of liquid and glassy particles (17).…”

Hydrolysis of Organonitrate Functional Groups in Aerosol Particles

“…First, molecular diffusion coefficients for ON and water molecules in particles may be slower than in solutions, potentially reducing the hydrolysis rate. Recent studies suggest that laboratory and ambient SOA (both anthropogenic and biogenic) exist in an amorphous solid (glassy) state when RH < 30% (Virtanen et al 2010;Vaden et al 2011;Saukko et al 2012). One likely consequence of this phase state is that molecular diffusion through the particle would be hindered.…”

“…One likely consequence of this phase state is that molecular diffusion through the particle would be hindered. A moistureinduced phase transition of pure SOA from a solid state to a semi-solid or liquid state is suggested to occur at RH > 40% (Shiraiwa et al 2011;Saukko et al 2012). In the semisolid or liquid state, organic would diffuse to the organic interface much more rapidly (Vaden et al 2011).…”