Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol

Abstract

Field measurements of secondary organic aerosol (SOA) find significantly higher mass loads than predicted by models, sparking intense effort focused on finding additional SOA sources but leaving the fundamental assumptions used by models unchallenged. Current air-quality models use absorptive partitioning theory assuming SOA particles are liquid droplets, forming instantaneous reversible equilibrium with gas phase. Further, they ignore the effects of adsorption of spectator organic species during SOA formation on SOA properties and fate. Using accurate and highly sensitive experimental approach for studying evaporation kinetics of size-selected single SOA particles, we characterized room-temperature evaporation kinetics of laboratory-generated α-pinene SOA and ambient atmospheric SOA. We found that even when gas phase organics are removed, it takes ∼24 h for pure α-pinene SOA particles to evaporate 75% of their mass, which is in sharp contrast to the ∼10 min time scale predicted by current kinetic models. Adsorption of "spectator" organic vapors during SOA formation, and aging of these coated SOA particles, dramatically reduced the evaporation rate, and in some cases nearly stopped it. Ambient SOA was found to exhibit evaporation behavior very similar to that of laboratory-generated coated and aged SOA. For all cases studied in this work, SOA evaporation behavior is nearly size-independent and does not follow the evaporation kinetics of liquid droplets, in sharp contrast with model assumptions. The findings about SOA phase, evaporation rates, and the importance of spectator gases and aging all indicate that there is need to reformulate the way SOA formation and evaporation are treated by models.single-particle mass spectrometry | morphology A tmospheric particles have a strong, yet poorly characterized effect on climate (1). Organic aerosols (OA) comprise 20-90% of atmospheric dry particles mass (2), the majority and least understood of which is secondary organic aerosol (SOA), formed from oxidation of gas phase organic vapors in the atmosphere (3-6). Despite an ongoing intense research effort aimed at understanding the formation and atmospheric evolution of OA, current models severely underestimate the formation of SOA in the atmosphere (5, 7). The effort to resolve the persistent discrepancy between field measurements and the amount of SOA predicted by atmospheric chemistry models has mostly focused on improving the understanding of SOA formation yields and finding new sources (8-11). In contrast, present models maintain the following fundamental assumptions: (i) Gas-particle partitioning is modeled assuming that all organics form a pseudoideal solution in the condensed particle phase, (ii) SOA particles remain liquid-like throughout their lifetime in the atmosphere, (iii) reversible thermodynamic equilibrium exists between gas and particle phases, and (iv) adsorption of other organic species and their effects on SOA properties and evaporation are ignored.The assumption that particles are liquid is central to ...

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

“…The mass loss for AIDA experiments (Fig. 2B) was because of semivolatile vapor loss, consistent with other isothermal observations (14,38). At PSI and CMU, however, particle diameters were stable after SOA formation, and mass losses were associated with particle number loss via deposition to the walls.…”

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.…”

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…We report here an expanded investigation of the ozonolysis of trans-3-hexene in which particle composition is probed as a function of size. In addition, the influence of water vapor, OH and SCI scavengers, as well as the phase state of SOA particles (which plays critical roles in various physical and chemical processes of aerosols) [47][48][49][50][51][52][53][54][55][56] are elucidated and possible mechanisms are examined in light of predictions of a box model for this system. Initial studies of the ozonolysis of the much larger and complex biogenic alkene a-cedrene are also reported, and it is shown that the mechanism of particle formation for this sesquiterpene is different from that for trans-3-hexene.…”