Tamar Moise

Reactive uptake of ozone by proxies for organic aerosols : Surface versus bulk processes

Uptake measurements of ozone were conducted with two types of proxies for atmospheric organic aerosols: organic liquids and self-assembled organic monolayers. Alkanes and terminal alkenes were used. The monolayer surface was characterized, prior to and after reaction, using IR spectroscopy. Uptake experiments were conducted using a flow tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficient, γ, is shown to be due to reaction with the double bond. For the monolayers, γ is composed solely of a surface reactive component and is smaller by at least an order of magnitude than values obtained for a liquid of the same chain length. Uptake by the liquids is higher due to solubility and reaction in the bulk. The phase of the atmospheric organic aerosol will determine the appropriate use of a bulk or surface uptake probability in atmospheric models. Since the aerosol surface is processed and sites are consumed, γ is time variant. We define a parameter γ as the surface uptake probability per reactive site and determine its value as 9×10−19 cm2 molecule−1. This enables the modeling of surface reactions as surface site concentrations diminish following interaction with the gaseous species.

Uptake of Cl and Br by organic surfaces—A perspective on organic aerosols processing by tropospheric oxidants

The reactive uptake of Cl and Br atoms by closely packed organic thin films was studied in a flow reactor. For Cl, the reactive uptake coefficient, γ, was near collision rate for alkane and alkene surfaces. For Br, γ =(3±1) × 10 -2 for alkane and γ=(5±2) × 10-2 for alkene surfaces. The processing of the surface was monitored using FTIR, XPS and contact angle measurements. Oxidized surface-bound products and a concurrent increase in hydrophilicity were observed. The probability of a reactive collision between Br, Cl, O( 3 P), O 3 and NO 3 and surface-bound organics is compared with that of comparable gas-phase reactions, showing that reactions with a high activation energy in the gas-phase have an enhanced surface reaction probability. The uptake coefficients for these tropospheric oxidants are used to estimate the processing time for an organic coated aerosol.

Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds.

Significance Aerosols cycling through clouds affect particle morphological and chemical properties, thus modifying aerosol effects on cloud microphysics and climate. Previous studies have focused on aerosol processing in warm clouds via aqueous-phase reactions. Here we investigate the physical modifications of aerosols following processing within ice clouds using a unique laboratory setup that simulates ice cloud processes. The processed particles have a porous structure due to phase separation upon freezing, subsequent glass transition, and ice sublimation. Such modified aerosols can be better ice and cloud condensation nuclei and scatter less light. These changes have implications for aerosol–cloud interactions and optical properties of aerosols in the vicinity of clouds. The cycling of atmospheric aerosols through clouds can change their chemical and physical properties and thus modify how aerosols affect cloud microphysics and, subsequently, precipitation and climate. Current knowledge about aerosol processing by clouds is rather limited to chemical reactions within water droplets in warm low-altitude clouds. However, in cold high-altitude cirrus clouds and anvils of high convective clouds in the tropics and midlatitudes, humidified aerosols freeze to form ice, which upon exposure to subsaturation conditions with respect to ice can sublimate, leaving behind residual modified aerosols. This freeze-drying process can occur in various types of clouds. Here we simulate an atmospheric freeze-drying cycle of aerosols in laboratory experiments using proxies for atmospheric aerosols. We find that aerosols that contain organic material that undergo such a process can form highly porous aerosol particles with a larger diameter and a lower density than the initial homogeneous aerosol. We attribute this morphology change to phase separation upon freezing followed by a glass transition of the organic material that can preserve a porous structure after ice sublimation. A porous structure may explain the previously observed enhancement in ice nucleation efficiency of glassy organic particles. We find that highly porous aerosol particles scatter solar light less efficiently than nonporous aerosol particles. Using a combination of satellite and radiosonde data, we show that highly porous aerosol formation can readily occur in highly convective clouds, which are widespread in the tropics and midlatitudes. These observations may have implications for subsequent cloud formation cycles and aerosol albedo near cloud edges.

Optical extinction of highly porous aerosol following atmospheric freeze drying

Porous glassy particles are a potentially significant but unexplored component of atmospheric aerosol that can form by aerosol processing through the ice phase of high convective clouds. The optical properties of porous glassy aerosols formed from a freeze-dry cycle simulating freezing and sublimation of ice particles were measured using a cavity ring down aerosol spectrometer (CRD-AS) at 532 nm and 355 nm wavelength. The measured extinction efficiency was significantly reduced for porous organic and mixed organic-ammonium sulfate particles as compared to the extinction efficiency of the homogeneous aerosol of the same composition prior to the freeze-drying process. A number of theoretical approaches for modeling the optical extinction of porous aerosols were explored. These include effective medium approximations, extended effective medium approximations, multilayer concentric sphere models, Rayleigh-Debye-Gans theory, and the discrete dipole approximation. Though such approaches are commonly used to describe porous particles in astrophysical and atmospheric contexts, in the current study, these approaches predicted an even lower extinction than the measured one. Rather, the best representation of the measured extinction was obtained with an effective refractive index retrieved from a fit to Mie scattering theory assuming spherical particles with a fixed void content. The single-scattering albedo of the porous glassy aerosols was derived using this effective refractive index and was found to be lower than that of the corresponding homogeneous aerosol, indicating stronger relative absorption at the wavelengths measured. The reduced extinction and increased absorption may be of significance in assessing direct, indirect, and semidirect forcing in regions where porous aerosols are expected to be prevalent.

Fluxes of Fine Particles Over a Semi-Arid Pine Forest: Possible Effects of a Complex Terrain

Semi-arid forests are of growing importance due to expected ecosystem transformations following climatic changes. Dry deposition of atmospheric aerosols was measured for the first time in such an ecosystem, the Yatir forest in southern Israel. Size-segregated flux measurements for particles ranging between 0.25 μm and 0.65 μm were taken with an optical particle counter (OPC) using eddy covariance methodology. The averaged deposition velocity (Vd ) at this site was 3.8 ± 4.5 mm s−1 for 0.25–0.28 μm particles, which is in agreement with deposition velocities measured in mid and northern latitude coniferous forests, and is most heavily influenced by the atmospheric stability and turbulence conditions, and to a lesser degree by the particle size. Both downward and upward fluxes were observed. Upward fluxes were not associated with a local particle source. The flux direction correlated strongly with wind direction, suggesting topographical effects. We hypothesize that a complex terrain and a patchy fetch affected the expected dependence of Vd on particle size and caused the observed upward fluxes of particles. The effect of topography on the deposition velocity grows greater as particle size increases, as has been shown in modeling and laboratory studies but had not been demonstrated yet in field studies. This hypothesis is consistent with the observed relationship between Vd and the friction velocity, the topography in the area of the flux tower, and the observed correlation of flux direction with wind direction. [Supplementary materials are available for this article. Go to the publishers online edition of Aerosol Science and Technology to view the free supplementary files.] Copyright 2013 American Association for Aerosol Research