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 ...

The Weather Research and Forecasting model coupled with chemistry (WRF-Chem) is modified to include a volatility basis set (VBS) treatment of secondary organic aerosol formation. The VBS approach, coupled with SAPRC-99 gas-phase chemistry mechanism, is used to model gas-particle partitioning and multiple generations of gas-phase oxidation of organic vapors. In addition to the detailed 9-species VBS, a simplified mechanism using 2 volatility species (2-species VBS) is developed and tested for similarity to the 9-species VBS in terms of both mass and oxygen-to-carbon ratios of organic aerosols in the atmosphere. WRF-Chem results are evaluated against field measurements of organic aerosols collected during the MILAGRO 2006 campaign in the vicinity of Mexico City. The simplified 2-species mechanism reduces the computational cost by a factor of 2 as compared to 9-species VBS. Both ground site and aircraft measurements suggest that the 9-species and 2-species VBS predictions of total organic aerosol mass as well as individual organic aerosol components including primary, secondary, and biomass burning are comparable in magnitude. In addition, oxygen-to-carbon ratio predictions from both approaches agree within 25 %, providing evidence that the 2-species VBS is well suited to represent the complex evolution of organic aerosols. Model sensitivity to amount of anthropogenic semi-volatile and intermediate volatility (S/IVOC) precursor emissions is also examined by doubling the default emissions. Both the emission cases significantly under-predict primary organic aerosols in the city center and along aircraft flight transects. Secondary organic aerosols are predicted reasonably well along flight tracks surrounding the city, but are consistently over-predicted downwind of the city. Also, oxygen-to-carbon ratio predictions are significantly improved compared to prior studies by adding 15 % oxygen mass per generation of oxidation; however, all modeling cases still under-predict these ratios downwind as compared to measurements, suggesting a need to further improve chemistry parameterizations of secondary organic aerosol formation

[1] We investigate issues related to volatility and multi-generational gas-phase aging parameterizations affecting the formation and evolution of secondary organic aerosol (SOA) in models. We show that when assuming realistic values for the mass accommodation coefficient, experimentally observed SOA evaporation rates imply significantly lower "effective volatility" than those derived from SOA growth in smog chambers, pointing to the role of condensed phase processes and suggesting that models need to use different parameters to describe the formation and evolution of SOA. We develop a new, experimentally driven paradigm to represent SOA as a non-absorbing semi-solid with very low "effective volatility." We modify both a box model and a 3D chemical transport model, to include simplified parameterizations capturing the first-order effects of gas-phase fragmentation reactions and investigate the implications of treating SOA as a non-volatile, non-absorbing semi-solid (NVSOA). Box model simulations predict SOA loadings decrease with increasing fragmentation, and similar SOA loadings are calculated in the traditional, semi-volatile (SVSOA) approach and with the new paradigm (NVSOA) before evaporation reduces loadings of SVSOA. Box-model-calculated O:C ratios increase with aging in both the SVSOA and the NVSOA paradigms. Consistent with box model results, 3D model simulations demonstrate that predicted SOA loadings decrease with the addition of fragmentation reactions. The NVSOA paradigm predicts higher SOA loadings compared to the SVSOA paradigm over nearly the entire 3D modeling domain, with larger differences close to the surface and in regions where higher dilution favors SVSOA evaporation.Citation: Shrivastava, M., A. Zelenyuk, D. Imre, R. Easter, J. Beranek, R. A. Zaveri, and J. Fast (2013), Implications of low volatility SOA and gas-phase fragmentation reactions on SOA loadings and their spatial and temporal evolution in the atmosphere,

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.