CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol

The ability of secondary organic aerosol (SOA) produced from the ozonolysis of α-pinene and monoterpene mixtures (α-pinene, β-pinene, limonene and 3-carene) to become cloud droplets was investigated. A static CCN counter and a Scanning Mobility CCN Analyser (a Scanning Mobility Particle Sizer coupled with a Continuous Flow counter) were used for the CCN measurements. Consistent with previous studies monoterpene SOA is quite active and would likely be a good source of cloud condensation nuclei (CCN) in the atmosphere. A decrease in CCN activation diameter for α-pinene SOA of approximately 3 nm hr−1 was observed as the aerosol continued to react with oxidants. Hydroxyl radicals further oxidize the SOA particles thereby enhancing the particle CCN activity with time. The initial concentrations of ozone and monoterpene precursor (for concentrations lower than 40 ppb) do not appear to affect the activity of the resulting SOA. Köhler Theory Analysis (KTA) is used to infer the molar mass of the SOA sampled online and offline from atomized filter samples. The estimated average molar mass of online SOA was determined to be 180±55 g mol−1 (consistent with existing SOA speciation studies) assuming complete solubility. KTA suggests that the aged aerosol (both from α-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 65%). CCN activity measurements of the SOA mixed with (NH4)2SO4 suggest that the organic can depress surface tension by as much as 10 N m−1 (with respect to pure water). The droplet growth kinetics of SOA samples are similar to (NH4)2SO4, except at low supersaturation, where SOA tends to grow more slowly. The CCN activation diameter of α-pinene and mixed monoterpene SOA can be modelled to within 10g-15% of experiments by a simple implementation of Köhler theory, assuming complete dissolution of the particles, no dissociation into ions, a molecular weight of 180 g mol−1, a density of 1.5 g cm−3, and the surface tension of water.

Biomass-burning impact on CCN number, hygroscopicity and cloud formation during summertime in the eastern Mediterranean

Athanasios Nenes

This study investigates the concentration, cloud condensation nuclei (CCN) activity and hygroscopic properties of particles influenced by biomass burning in the eastern Mediterranean and their impacts on cloud droplet formation. Air masses sampled were subject to a range of atmospheric processing (several hours up to 3 days). Values of the hygroscopicity parameter, κ, were derived from CCN measurements and a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). An Aerosol Chemical Speciation Monitor (ACSM) was also used to determine the chemical composition and mass concentration of non-refractory components of the submicron aerosol fraction. During fire events, the increased organic content (and lower inorganic fraction) of the aerosol decreases the values of κ, for all particle sizes. Particle sizes smaller than 80 nm exhibited considerable chemical dispersion (where hygroscopicity varied up to 100% for particles of same size); larger particles, however, exhibited considerably less dispersion owing to the effects of condensational growth and cloud processing. ACSM measurements indicate that the bulk composition reflects the hygroscopicity and chemical nature of the largest particles (having a diameter of ∼ 100 nm at dry conditions) sampled. Based on positive matrix factorization (PMF) analysis of the organic ACSM spectra, CCN concentrations follow a similar trend as the biomass-burning organic aerosol (BBOA) component, with the former being enhanced between 65 and 150% (for supersaturations ranging between 0.2 and 0.7%) with the arrival of the smoke plumes. Using multilinear regression of the PMF factors (BBOA, OOA-BB and OOA) and the observed hygroscopicity parameter, the inferred hygroscopicity of the oxygenated organic aerosol components is determined. We find that the transformation of freshly emitted biomass burning (BBOA) to more oxidized organic aerosol (OOA-BB) can result in a 2-fold increase of the inferred organic hygroscopicity; about 10% of the total aerosol hygroscopicity is related to the two biomass-burning components (BBOA and OOA-BB), which in turn contribute almost 35% to the fine-particle organic water of the aerosol. Observation-derived calculations of the cloud droplet concentrations that develop for typical boundary layer cloud conditions suggest that biomass burning increases droplet number, on average by 8.5%. The strongly sublinear response of clouds to biomass-burning (BB) influences is a result of strong competition of CCN for water vapor, which results in very low maximum supersaturation (0.08% on average). Attributing droplet number variations to the total aerosol number and the chemical composition variations shows that the importance of chemical composition increases with distance, contributing up to 25% of the total droplet variability. Therefore, although BB may strongly elevate CCN numbers, the impact on droplet number is limited by water vapor availability and depends on the aerosol particle concentration levels associated with the background. © 2016 Author(s).

Copernicus GmbH2016

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

Evaluation of global simulations of aerosol particle and cloud condensation nuclei number, with implications for cloud droplet formation

Athanasios Nenes, Julia Schmale

A total of 16 global chemistry transport models and general circulation models have participated in this study; 14 models have been evaluated with regard to their ability to reproduce the near-surface observed number concentration of aerosol particles and cloud condensation nuclei (CCN), as well as derived cloud droplet number concentration (CDNC). Model results for the period 2011–2015 are compared with aerosol measurements (aerosol particle number, CCN and aerosol particle composition in the submicron fraction) from nine surface stations located in Europe and Japan. The evaluation focuses on the ability of models to simulate the average across time state in diverse environments and on the seasonal and short-term variability in the aerosol properties. There is no single model that systematically performs best across all environments represented by the observations. Models tend to underestimate the observed aerosol particle and CCN number concentrations, with average normalized mean bias (NMB) of all models and for all stations, where data are available, of −24 % and −35 % for particles with dry diameters >50 and >120 nm, as well as −36 % and −34 % for CCN at supersaturations of 0.2 % and 1.0 %, respectively. However, they seem to behave differently for particles activating at very low supersaturations (