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 solor 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.
The heterogeneous reaction between ozone and oleic and linoleic acids, prevalent components of both marine and urban organic aerosol, were studied in a flow reactor using electron impact and chemical ...ionization mass spectrometry. Liquids and frozen liquids were used as proxies for atmospheric aerosol. The reactive uptake coefficients, γ, were determined to be (8.3 ± 0.2) × 10-4 and (1.2 ± 0.2) × 10-3 for liquid oleic and linoleic acid respectively and (5.2 ± 0.1) × 10-5 and (1.4 ± 0.1) × 10-4 for frozen oleic and linoleic acid, respectively. Although, the reacto-diffusive length is estimated to be rather small in the liquid experiments, <10 nm, a clear indication of the participation of subsurface layers in the uptake is observed. This is in contrast to uptake by the frozen acids where the reaction is limited to the surface. Aldehydes were identified as the major volatile reaction products: 1-nonanal was detected following reaction with oleic acid, 2-nonenal, 4-nonenal, and 1-hexanal were detected following reaction with linoleic acid. The aldehyde yield, defined as the amount of the volatile product released relative to the ozone consumed, is dictated by its solubility in the liquid and frozen liquid acids. Azelaic acid was identified as a liquid-phase reaction product following the reaction with oleic acid. The implications regarding the atmospheric aging of aerosols with a fatty acid component are discussed.
Ambient fine particulate matter (PM2.5) samples were collected from January to December 2007 to investigate the sources and chemical speciation in Palestine, Jordan, and Israel. The 24-h PM2.5 ...samples were collected on 6-day intervals at eleven urban and rural sites simultaneously. Major chemical components including metals, ions, and organic and elemental carbon were analyzed. The mass concentrations of PM2.5 across the 11 sites varied from 20.6 to 40.3 μg/m3, with an average of 28.7 μg/m3. Seasonal variation of PM2.5 concentrations was substantial, with higher average concentrations (37.3 μg/m3) in the summer (April–June) months compared to winter (October–December) months (26.0 μg/m3) due mainly to high contributions of sulfate and crustal components. PM2.5 concentrations in the spring were greatly impacted by regional dust storms. Carbonaceous mass was the most abundant component, contributing 40% to the total PM2.5 mass averaged across the eleven sites. Crustal components averaged 19.1% of the PM2.5 mass and sulfate, ammonium, and nitrate accounted for 16.2%, 6.4%, and 3.7%, respectively, of the total PM2.5 mass. The results of this study demonstrate the need to better protect the health and welfare of the residents on both sides of the Jordan River in the Middle East.
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.
The actual number of particles in formulations of nanoparticles (NP) is of importance for quality assurance, comprehensive physicochemical characterization, and pharmacodynamics. Some calculation ...methods that have been previously employed are limited because they rely on several assumptions and are not applicable for certain preparations. Currently there are no validated experimental methods for determining the particle number-concentration (
N
c) of liposomal and polymeric nanoparticulate preparations (<500
nm). This study examines a new empirical method for counting the number of particles in nanoparticulate formulations including drug-containing liposomes and polymeric NP. In the new method, suspended NP are nebulized to form aerosol droplets which are dried and counted using a scanning mobility particle sizer (SMPS). Experiments were conducted with three different preparations, empty liposomes (200 and 400
nm), drug-loaded liposomes (200
nm), and polymeric NP (150
nm). It was verified that no detrimental morphological or structural changes of the formulations have been induced by the SMPS technique, and that the obtained
N
c values represent the original particles. It is concluded that nano-formulations with concentrations of up to 10
7 particles per 1
cm
3 air, corresponding to approximately 10
12 particles per 1
ml solution, can be directly counted within the size range of 30–900
nm. The measured values are compared to newly developed theoretical calculations to assess the viability of these calculations.
Thirty-six springs and wells from the Dead Sea Rift Valley were periodically sampled and analyzed. The latter included full chemical analyses,
222Rn and
226Ra (by α-counting emanometry), as well as
...228Ra/
226Ra and
224Ra/
228Ra activity ratios (γ spectrometry). Sampling stretched over almost 2 yr. Several hundred Rn-Ra analyses and close to a hundred isotopic ratios were measured. Most waters in the Rift Valley have both elevated Ra (several to 750 dpm/L) and Rn (a few hundred to almost 60,000 dpm/L) content. In practically all samples
222Rn activity considerably exceeds that of its parent
226Ra. The Ra content is the result of all Dead Sea Rift Valley waters being mixtures of fresh water with saline brines. Ra is efficiently extracted from surrounding rocks into the brine end member.
228Ra/
226Ra ratios are exceptionally low −0.07 to 0.9, mostly less than 0.2. This apparently reflects the U over Th enrichment in the source rocks that contribute the Ra. Ra enrichment (both
228Ra and
226Ra) is locally correlated throughout the Rift Valley with water salinity. This correlation can be used to constrain the age of the brine-freshwater mixing process. In one of the hydrologic subsystems studied (the Fuliya block), the mixing of the shallow ground water must have occurred in less than 200 to 300 yr ago, probably before no more than some 30 yr. High radon activities in surface waters along the Dead Sea Rift Valley result from the formation of radium-enriched linings on the aquifer rock surface. The mixing of the radium-extracting brines with fresh water leads to continual adsorption of radium as water salinity decreases. The unique combination of fast upward flow and continual mixing in the Dead Sea Rift Valley accounts for a constant replenishment of radium in the waters. This causes a gradual buildup of a radium lining on the aquifer walls until eventually a steady-state surface activity is established.
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.
Key Points
Optical extinction of aerosol is reduced following ice cloud processing
Extinction reduction is more significant for glassy porous aerosol
Measured and modeled extinction best match when using a fixed aerosol porosity
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, gamma, is shown to be due to reaction with the double bond. For the monolayers, gamma 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, gamma is time variant. We define a parameter gamma-prime as the surface uptake probability per reactive site and determine its value as 9 x 10 exp -19 sq cm/molecule. This enables the modeling of surface reactions as surface site concentrations diminish following interaction with the gaseous species. (Author)
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 (V
d
) 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 V
d
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 V
d
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 publisher's online edition of Aerosol Science and Technology to view the free supplementary files.
Copyright 2013 American Association for Aerosol Research
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