Water-soluble SOA from Alkene ozonolysis: Composition and droplet activation kinetics inferences from analysis of CCN activity

Cloud formation characteristics of the water-soluble organic fraction (WSOC) of secondary organic aerosol (SOA) formed from the ozonolysis of alkene hydrocarbons (terpinolene, 1-methlycycloheptene and cycloheptene) are studied. Based on size-resolved measurements of CCN activity (of the pure and salted WSOC samples) we estimate the average molar volume and surface tension depression associated with the WSOC using Köhler Theory Analysis (KTA). Consistent with known speciation, the results suggest that the WSOC are composed of low molecular weight species, with an effective molar mass below 200 g mol -1. The water-soluble carbon is also surface-active, depressing surface tension 10-15% from that of pure water (at CCN-relevant concentrations). The inherent hygroscopicity parameter, κ, of the WSOC ranges between 0.17 and 0.25; if surface tension depression and molar volume effects are considered in Î°, a remarkably constant "apparent" hygroscopicity ∼0.3 emerges for all samples considered. This implies that the volume fraction of soluble material in the parent aerosol is the key composition parameter required for prediction of the SOA hygroscopicity, as shifts in molar volume across samples are compensated by changes in surface tension. Finally, using "threshold droplet growth analysis", the water-soluble organics in all samples considered do not affect CCN activation kinetics.

Investigation of cloud condensation nuclei properties and droplet growth kinetics of the water-soluble aerosol fraction in Mexico City

Athanasios Nenes

We present hygroscopic and cloud condensation nuclei (CCN) relevant properties of the water-soluble fraction of Mexico City aerosol collected upon filters during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign. Application of κ-Kohler theory to the observed CCN activity gave a fairly constant hygroscopicity parameter (κ = 0.28 ± 0.06) regardless of location and organic fraction. Kohler theory analysis was used to understand this invariance by separating the molar volume and surfactant contributions to the CCN activity. Organics were found to depress surface tension (10-15%) from that of pure water. Daytime samples exhibited lower molar mass (∼200 amu) and surface tension depression than nighttime samples (∼400 amu); this is consistent with fresh hygroscopic secondary organic aerosol (SOA) condensing onto particles during peak photochemical hours, subsequently aging during nighttime periods of high relative humidity. Changes in surface tension partially compensate for shifts in average molar volume to give the constant hygroscopicity observed, which implies the amount (volume fraction) of soluble material in the parent aerosol is the key composition parameter required for CCN predictions. This finding, if applicable elsewhere, may explain why CCN predictions are often found to be insensitive to assumptions of chemical composition and provides a very simple way to parameterize organic hygroscopicity in atmospheric models (i.e., κorg = 0.28εWSOC). Special care should be given, however, to surface tension depression from organic surfactants, as its nonlinear dependence with organic fraction may introduce biases in observed (and predicted) hygroscopicity. Finally, threshold droplet growth analysis suggests the water-soluble organics do not affect activation kinetics. Copyright © 2010 by the American Geophysical Union.

Blackwell Publishing Ltd2010

Molecular phenomena in organic aerosol: water interaction

Gözde Bitlislioglu

There is still lack of knowledge about the aerosols physics and chemistry and consequently their impact on climate and health. The complexity of involved chemical interactions makes the characterization of these particles with experimental studies challenging. We discuss the dynamic and thermodynamic properties of organic aerosol by using molecular dynamics (MD) simulations. MD has been successfully used in atmospheric research applications due to its ability to predict macroscopic thermodynamic and dynamic properties for a wide variety of systems using the rules of classical statistical mechanics. In this research, organic-water interaction in organic aerosol is analyzed by investigating the parameters such as mass accommodation coefficient of water on organic coated surfaces, surface tension of organic aerosol, partitioning of organic between surface-bulk and activities of components in organic-water solution. The condensational growth of aerosol particles is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. Growth processes of organic aerosols remain highly uncertain due to the numerous individual species involved. Understanding of the effect of organic on the aerosol hygroscopicity is the first goal of this research. The mass accommodation coefficient of water is estimated on organic-coated water surfaces using MD simulations. We study the effect of straight-chain and branched alcohols on water uptake process. Different methods are tested and discrepancy between applied methods are explained according to their underlying assumptions. It is found that an increase in the carbon density captures the effect of tail-group chain length and extent of branching on lowering the mass accommodation coefficient. Second, we explore the possible synergistic interactions among surfactants on the aerosol surface by MD simulations. The synergism in surface tension reduction efficiency describes the ability of the mixture to lower surface tension at a concentration level that is lower than the concentration levels required to create the same effect with the individual components. Since synergy depends on intermolecular interaction between surfactants, the possibility of having synergistic mixture on anionic-cationic organic aerosols is quite high. The main motivation of the study is to study reasons for inconsistencies in surface tension values of atmospheric particles in laboratory experiments and field studies. The study is extended to understand the relation of synergism with molecular structure by simulating the surfactants that have the same head but different tail groups. The synergistic effect of the considered surfactants is validated and it is found that not only head-group but also tail-group interactions are important to consider for determining the occurrence of synergism. Organic aggregate formation in aerosols can change both the physical and chemical properties of atmospheric particles. Therefore, lastly, we study how the formation of aggregates can change thermodynamic properties relevant for aerosol-water interactions. We use MD to simulate the aggregated and fully-solved organic+water systems at the same molality level. Possible effects of aggregation on the water, solute activities and supersaturation of aerosols are explored.

EPFL2018

Investigation of cloud condensation nuclei properties and droplet growth kinetics of the water-soluble aerosol fraction in Mexico City

Athanasios Nenes

Blackwell Publishing Ltd2010

Molecular phenomena in organic aerosol: water interaction

Gözde Bitlislioglu

EPFL2018