Carynelisa Haspel

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.

The spectral and spatial distribution of light pollution in the waters of the northern Gulf of Aqaba (Eilat)

The urbanization of the shores of the Gulf of Aqaba has exposed the marine environment there, including unique fringing coral reefs, to strong anthropogenic light sources. Here we present the first in situ measurements of artificial nighttime light under water in such an ecosystem, with irradiance measured in 12 wavelength bands, at 19 measurement stations spread over 44 square km, and at 30 depths down to 30-m depth. At 1-m depth, we find downwelling irradiance values that vary from 4.6 × 10−4 μW cm−2 nm−1 500 m from the city to 1 × 10−6 μW cm−2 nm−1 in the center of the gulf (9.5 km from the city) in the yellow channel (589-nm wavelength) and from 1.3 × 10−4 μW cm−2 nm−1 to 4.3 × 10−5 μW cm−2 nm−1 in the blue channel (443-nm wavelength). Down to 10-m depth, we find downwelling irradiance values that vary from 1 × 10−6 μW cm−2 nm−1 to 4.6 × 10−4 μW cm−2 nm−1 in the yellow channel and from 2.6 × 10−5 μW cm−2 nm−1 to 1.3 × 10−4 μW cm−2 nm−1 in the blue channel, and we even detected a signal at 30-m depth. This irradiance could influence such biological processes as the tuning of circadian clocks, the synchronization of coral spawning, recruitment and competition, vertical migration of demersal plankton, feeding patterns, and prey/predator visual interactions.

Stratospheric response to intraseasonal changes in incoming solar radiation

Superposed epoch analysis of meteorological reanalysis data is used to demonstrate a significant connection between intraseasonal solar variability and temperatures in the stratosphere. Decreasing solar flux leads to a cooling of the tropical upper stratosphere above 7 hPa, while increasing solar flux leads to a warming of the tropical upper stratosphere above 7 hPa, after a lag of approximately 6–10 days. Late winter (February–March) Arctic stratospheric temperatures also change in response to changing incoming solar flux in a manner consistent with that seen on the 11 year timescale: 10–30 days after the start of decreasing solar flux, the polar cap warms during the easterly phase of the quasi-biennial oscillation. In contrast, cooling is present after decreasing solar flux during the westerly phase of the quasi-biennial oscillation (though it is less robust than the warming during the easterly phase). The estimated composite mean changes in Northern Hemisphere upper stratospheric (∼ 5 hPa) polar temperatures exceed 8 K and are potentially a source of intraseasonal predictability for the surface. These changes in polar temperature are consistent with the changes in wave driving entering the stratosphere.

Sensitivity study on the effects of hydrosol size and composition on linear polarization in absorbing and nonabsorbing clear and semi-turbid waters.

A full Mie scattering subroutine is employed to calculate what we call the linear polarization phase function (LPPF; percent polarization and e-vector orientation of radiation as a function of scattering angle) that results from refraction of the direct solar beam from air into water followed by single scattering by spherical hydrosols. The separate effects of refraction at the air-water interface, hydrosol size, the real and imaginary parts of the hydrosol refractive index, and absorption by the surrounding medium (water) on the LPPF are investigated. All of the above factors are found to alter the LPPF, changing the value of the maximum percent polarization (P(max)), the location of P(max), the number of fluctuations in the LPPF, or the location of the neutral points (points of 0 percent polarization), though absorption by the surrounding medium is found to have only a minimal effect. The character and extent of the influence on the LPPF is found to depend on the scattering regime (Rayleigh, Mie, or geometric optics). We conclude that in calculating underwater polarization, it is important to take into consideration Mie scattering even in relatively clear waters. We also find a coupling between the partial polarization and the e-vector orientation, which suggests that for some polarization-based visual tasks, only one of these would suffice. Other implications for aquatic animal polarization vision are discussed.

Linear Polarization Characteristics Within the Rosh HaNikra Mid-Littoral Cave, Israel

Light polarization characteristics, i.e., degree of linear polarization (DoLP) and angle of linear polarization (AoLP)E- vector angle, including annual and daily patterns, were documented in the depth of the littoral cave system of Rosh HaNikra on the northern Mediterranean shore of Israel (33° 5′ 35.24″ N, 35° 6′ 17.16″ E), based on light intensity sampled through polarizing filters at different hours of the day on different days of the year. . This is the first study to investigate the state of light polarization in such a unique habitat in which photosynthetic organisms, such as cyanobacteria, microalgae and macroalgae thrive. Such organisms play an essential ecological role as the energy base for the cave’s fauna. .Using these two methods, we found unique winter polarization characteristics within the cave, including high values of DoLP in the morning and at noon, reaching 50%, and nearly constant AoLP throughout the day. Given the low levels of light intensity that typically exist within the cave in the winter months, the relatively high DoLP and the nearly constant AoLP throughout the day may play a significant role in improving the ability of photosynthetic organisms within the cave to harvest light by orienting their light-harvesting receptors with respect to the AoLP. Using the polarization photograph analysis method, we were able to determine the polarization characteristics originating from the sky, reflection off of the far sea surface, and reflection off of (including refraction into followed by refraction out of) the cave wall separately. The maximum DoLP values originating from the sky, far ocean, and cave walls were found to be 27%, 50%, and 35%, respectively. The lowest daily variation in AoLP was that of light reflecting off the cave walls. The present pioneering study lays the foundation for any subsequent study of the role of light polarization in the distribution of the algal flora on the cave walls in and out of the water in the Rosh HanNikra cave and in sea caves in general.

The concept of apparent polarizability for calculating the extinction of electromagnetic radiation by porous aerosol particles

In the current study, the electromagnetic properties of porous aerosol particles are calculated in two ways. In the first, a porous target input file is generated by carving out voids in an otherwise homogeneous particle, and the discrete dipole approximation (DDA) is used to compute the extinction efficiency of the particle assuming that the voids are near vacuum dielectrics and assuming random particle orientation. In the second, an effective medium approximation (EMA) style approach is employed in which an apparent polarizability of the voids is defined based on the well-known solution to the problem in classical electrostatics of a spherical cavity within a dielectric. It is found that for porous particles with smaller overall diameter with respect to the wavelength of incident radiation, describing the voids as near vacuum dielectrics within the DDA sufficiently reproduces measured values of extinction efficiency, whereas for porous particles with moderate to larger overall diameters with respect to the wavelength of the radiation, the apparent polarizability EMA approach better reproduces the measured values of extinction efficiency.

Size selectivity during dip coating of sol–gel silica-based antireflective coatings and its effect on the porosity of the coatings

The behavior of the particle size distribution (PSD) during the preparation of sol–gel silica-based antireflective coatings (ARCs) by dip coating was examined. It was found that the PSD after deposition differs dramatically from the PSD in the sol–gel suspensions, with the PSD after deposition being much narrower. A correlation between a decrease in the porosity of the ARC and an increase in the dispersity of the deposited PSD was also found. Hence, controlling the PSD during deposition has a direct effect on the resulting porosity and thus on the reflectance of an ARC. It was found that the temperature and deposition speed during dip coating, respectively, have very little effect on the deposited PSD. It was found that the PSD of the sol–gel suspension control the content of the deposited PSD, but does not change the range of sizes deposited. Finally, it was found that random-uniform particle placement results in elongated sequences of four or more particles, which supports our previous conclusion that the linear permittivity mixing rule is most appropriate for Stöber-based ARCs.