Sources, Occurrence and Characteristics of Fluorescent Biological Aerosol Particles Measured Over the Pristine Southern Ocean.

In this study, we investigate the occurrence of primary biological aerosol particles (PBAP) over all sectors of the Southern Ocean (SO) based on a 90-day data set collected during the Antarctic Circumnavigation Expedition (ACE) in austral summer 2016-2017. Super-micrometer PBAP (1-16 µm diameter) were measured by a wide band integrated bioaerosol sensor (WIBS-4). Low (3σ) and high (9σ) fluorescence thresholds are used to obtain statistics on fluorescent and hyper-fluorescent PBAP, respectively. Our focus is on data obtained over the pristine ocean, that is, more than 200 km away from land. The results indicate that (hyper-)fluorescent PBAP are correlated to atmospheric variables associated with sea spray aerosol (SSA) particles (wind speed, total super-micrometer aerosol number concentration, chloride and sodium concentrations). This suggests that a main source of PBAP over the SO is SSA. The median percentage contribution of fluorescent and hyper-fluorescent PBAP to super-micrometer SSA was 1.6% and 0.13%, respectively. We demonstrate that the fraction of (hyper-)fluorescent PBAP to total super-micrometer particles positively correlates with concentrations of bacteria and several taxa of pythoplankton measured in seawater, indicating that marine biota concentrations modulate the PBAP source flux. We investigate the fluorescent properties of (hyper-)fluorescent PBAP for several events that occurred near land masses. We find that the fluorescence signal characteristics of particles near land is much more variable than over the pristine ocean. We conclude that the source and concentration of fluorescent PBAP over the open ocean is similar across all sampled sectors of the SO.

Ice residual properties in mixed-phase clouds at the high-alpine Jungfraujoch site.

Ice residual (IR) and total aerosol properties were measured in mixed-phase clouds (MPCs) at the high-alpine Jungfraujoch research station. Black carbon (BC) content and coating thickness of BC-containing particles were determined using single-particle soot photometers. The ice activated fraction (IAF), derived from a comparison of IR and total aerosol particle size distributions, showed an enrichment of large particles in the IR, with an increase in the IAF from values on the order of 10-4 to 10-3 for 100 nm (diameter) particles to 0.2 to 0.3 for 1 µm (diameter) particles. Nonetheless, due to the high number fraction of submicrometer particles with respect to total particle number, IR size distributions were still dominated by the submicrometer aerosol. A comparison of simultaneously measured number size distributions of BC-free and BC-containing IR and total aerosol particles showed depletion of BC by number in the IR, suggesting that BC does not play a significant role in ice nucleation in MPCs at the Jungfraujoch. The potential anthropogenic climate impact of BC via the glaciation effect in MPCs is therefore likely to be negligible at this site and in environments with similar meteorological conditions and a similar aerosol population. The IAF of the BC-containing particles also increased with total particle size, in a similar manner as for the BC-free particles, but on a level 1 order of magnitude lower. Furthermore, BC-containing IR were found to have a thicker coating than the BC-containing total aerosol, suggesting the importance of atmospheric aging for ice nucleation.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions.

Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10(-9) fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

Infrared optical constants of crystalline sodium chloride dihydrate: application to study the crystallization of aqueous sodium chloride solution droplets at low temperatures.

Complex refractive indices of sodium chloride dihydrate, NaCl·2H(2)O, have been retrieved in the 6000-800 cm(-1) wavenumber regime from the infrared extinction spectra of crystallized aqueous NaCl solution droplets. The data set is valid in the temperature range from 235 to 216 K and was inferred from crystallization experiments with airborne particles performed in the large coolable aerosol and cloud chamber AIDA at the Karlsruhe Institute of Technology. The retrieval concept was based on the Kramers-Kronig relationship for a complex function of the optical constants n and k whose imaginary part is proportional to the optical depth of a small particle absorption spectrum in the Rayleigh approximation. The appropriate proportionality factor was inferred from a fitting algorithm applied to the extinction spectra of about 1 µm sized particles, which, apart from absorption, also featured a pronounced scattering contribution. NaCl·2H(2)O is the thermodynamically stable crystalline solid in the sodium chloride-water system below the peritectic at 273.3 K; above 273.3 K, the anhydrous NaCl is more stable. In contrast to anhydrous NaCl crystals, the dihydrate particles reveal prominent absorption signatures at mid-infrared wavelengths due to the hydration water molecules. Formation of NaCl·2H(2)O was only detected at temperatures clearly below the peritectic and was first evidenced in a crystallization experiment conducted at 235 K. We have employed the retrieved refractive indices of NaCl·2H(2)O to quantify the temperature dependent partitioning between anhydrous and dihydrate NaCl particles upon crystallization of aqueous NaCl solution droplets. It was found that the temperature range from 235 to 216 K represents the transition regime where the composition of the crystallized particle ensemble changes from almost only NaCl to almost only NaCl·2H(2)O particles. Compared to the findings on the NaCl/NaCl·2H(2)O partitioning from a recent study conducted with micron-sized NaCl particles deposited onto a surface, the transition regime from NaCl to NaCl·2H(2)O is shifted by about 13 K to lower temperatures in our study. This is obviously related to the different experimental conditions of the two studies. The partitioning between the two solid phases of NaCl is essential for predicting the deliquescence and ice nucleation behavior of a crystalline aerosol population which is subjected to an increasing relative humidity.

Ice crystal habits from cloud chamber studies obtained by in-line holographic microscopy related to depolarization measurements.

We investigate hydrometeor habits at the AIDA chamber with a newly developed in-line holographic microscope HOLographic Imager for Microscopic Objects (HOLIMO). Sizes and habits of ice crystals and droplets in a mixed-phase cloud experiment are related to relative humidity with respect to ice (RH(ice)), temperature (T), and experiment time. This experiment is initiated with supercooled water drops. As a result, ice crystals within a maximum particle diameter size range of 2 to 118 microm (average size of 19 microm) are detected and 63% of them reveal regular habits. The observed particle habits match those predicted for a given RH(ice) and T. Two different growth modes emerge from this cloud. The first one appears during water injection and reveals mainly optical particle sizes in the range of 5 to 250 microm. The second mode grows to sizes of 5 to 63 microm, just after the particles of the first one fall out. It is found that an increasing aspect ratio chi of maximum length over thickness from 2 to 20 as obtained by HOLIMO corresponds to a decreasing linear depolarization ratio from 0.1 to 0.04, as independently obtained by depolarization measurements.

Infrared optical constants of highly diluted sulfuric acid solution droplets at cirrus temperatures.

Complex refractive indices for supercooled sulfuric acid solution droplets in the mid-infrared spectral regime (wavenumber range 6000-800 cm(-1)) have been retrieved for acid concentrations ranging from 33 to 10 wt % H2SO4 at temperatures between 235 and 230 K, from 36 to 15 wt % H2SO4 at temperatures between 225 and 219 K, and from 37 to 20 wt % H2SO4 at temperatures between 211 and 205 K. The optical constants were derived with a Mie inversion technique from measured H2SO4/H2O aerosol extinction spectra that were recorded during controlled expansion cooling experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The new data sets cover a range of atmospherically relevant temperatures and compositions in the binary sulfuric acid/water system for which infrared refractive indices have not been published so far, namely, the regime when supercooled H2SO4/H2O solution droplets at T < 235 K are subjected to an environment that is supersaturated with respect to the ice phase. With increasing ice supersaturation, the H2SO4/H2O aerosol particles will continuously dilute by the uptake of water vapor from the gas phase until freezing of the solution droplets eventually occurs when the acid concentration has dropped below a critical, temperature-dependent threshold value. With the aid of the new measurements, the homogeneous freezing process of supercooled H2SO4/H2O solution droplets at cirrus temperatures can be quantitatively analyzed by means of Fourier transform infrared spectroscopy, thereby overcoming a major drawback from previous studies: the need to use complex refractive indices that were measured at temperatures well above 235 K to deduce the composition of the low-concentrated H2SO4/H2O aerosol particles. As in the case of the complex refractive indices for sulfuric acid solutions with acid concentrations greater than 37 wt % H2SO4, the new low-temperature optical constants for highly diluted droplets also reveal significant temperature-induced spectral variations in comparison with the refractive indices for higher temperatures, which are associated with a change in the equilibrium between sulfate and bisulfate ions.

Influence of particle aspect ratio on the midinfrared extinction spectra of wavelength-sized ice crystals.

We have used the T-matrix method and the discrete dipole approximation to compute the midinfrared extinction cross-sections (4500-800 cm(-1)) of randomly oriented circular ice cylinders for aspect ratios extending up to 10 for oblate and down to 1/6 for prolate particle shapes. Equal-volume sphere diameters ranged from 0.1 to 10 microm for both particle classes. A high degree of particle asphericity provokes a strong distortion of the spectral habitus compared to the extinction spectrum of compactly shaped ice crystals with an aspect ratio around 1. The magnitude and the sign (increase or diminution) of the shape-related changes in both the absorption and the scattering cross-sections crucially depend on the particle size and the values for the real and imaginary part of the complex refractive index. When increasing the particle asphericity for a given equal-volume sphere diameter, the values for the overall extinction cross-sections may change in opposite directions for different parts of the spectrum. We have applied our calculations to the analysis of recent expansion cooling experiments on the formation of cirrus clouds, performed in the large coolable aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe at a temperature of 210 K. Depending on the nature of the seed particles and the temperature and relative humidity characteristics during the expansion, ice crystals of various shapes and aspect ratios could be produced. For a particular expansion experiment, using Illite mineral dust particles coated with a layer of secondary organic matter as seed aerosol, we have clearly detected the spectral signatures characteristic of strongly aspherical ice crystal habits in the recorded infrared extinction spectra. We demonstrate that the number size distributions and total number concentrations of the ice particles that were generated in this expansion run can only be accurately derived from the recorded infrared spectra when employing aspect ratios as high as 10 in the retrieval approach. Remarkably, the measured spectra could also be accurately fitted when employing an aspect ratio of 1 in the retrieval. The so-deduced ice particle number concentrations, however, exceeded the true values, determined with an optical particle counter, by more than 1 order of magnitude. Thus, the shape-induced spectral changes between the extinction spectra of platelike ice crystals of aspect ratio 10 and compactly shaped particles of aspect ratio 1 can be efficiently balanced by deforming the true number size distribution of the ice cloud. As a result of this severe size/shape ambiguity in the spectral analysis, we consider it indispensable to cross-check the infrared retrieval results of wavelength-sized ice particles with independent reference measurements of either the number size distribution or the particle morphology.

Mid-infrared extinction spectra and optical constants of supercooled water droplets.

Complex refractive indices of supercooled liquid water have been retrieved at 269, 258, 252, and 238 K in the 4500-1100 cm(-1) wavenumber regime from series of infrared extinction spectra of micron-sized water droplets. The spectra collection was recorded during expansion experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. A Mie inversion technique was applied to derive the low-temperature refractive index data sets by iteratively adjusting the room-temperature optical constants of liquid water until obtaining the best agreement between measured and calculated infrared spectra of the supercooled water droplets. The new optical constants, revealing significant temperature-induced spectral variations in comparison with the room-temperature refractive indices, proved to be in good agreement with data sets obtained in a recent study. A detailed analysis was performed to elaborate potential inaccuracies in the retrieval results when deriving optical constants from particle extinction spectra using an iterative procedure.

Experiments on light scattering and extinction by small, micrometer-sized aggregates of spheres.

We performed experiments to study the extinction, scattering, and polarization of light by ensembles of fractal dust aggregates that consist of spherical monomers large compared with the wavelength. Extinction was measured on a homogeneous dust cloud. Scattering and polarization were measured on a collimated dust beam. We found that polarization and extinction are determined only by a small size scale that is defined by a monomer and its closest neighbors in an aggregate. The scattering function might also depend on the overall size of the aggregate or the total number of monomers in an aggregate.