Cirrus Temperatures Research Articles

Ice nucleation behavior of biomass combustion particles at cirrus temperatures

Measurements of ice formation by biomass combustion particles generated in controlled laboratory burns were made using a continuous flow diffusion chamber operating in the temperature range between −45 and −60°C. These measurements are the first of their kind to investigate the role that such particles may play in ice formation in upper tropospheric clouds. Measurements of aerosol water uptake were used to examine the role of hygroscopicity in low‐temperature ice nucleation by biomass combustion particles. Ice formation by the smoke particles at below −45°C and below water saturation followed the temperature and relative humidity dependencies predicted for water activity‐based homogeneous freezing within measurement uncertainties. As predicted, relative humidity dependence of homogeneous freezing fractions on hygroscopicity was weak in comparison with the temperature dependence for biomass particles of typical atmospheric sizes. The action of heterogeneous ice nucleation in or on these small smoke particles in the haze particle regime could not be distinguished within measurement uncertainties. Impedance of homogeneous freezing below −50°C was inferred for freezing higher number fractions of predominantly organic smoke particles separately reported to contain an abundance of large organic molecules, consistent with the recently predicted onset conditions for and impact of viscous or glassy aerosol phase states. Finally, we note that precision and accuracy requirements of instrumentation for resolving modest impacts (a few degrees less supercooling or a few percent lower relative humidity) of heterogeneous freezing, as compared with homogeneous freezing in the upper tropospheric temperature regime present difficulties for validating aerosol impacts on cirrus.

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.

Ice nucleation by surrogates for atmospheric mineral dust and mineral dust/sulfate particles at cirrus temperatures

Abstract. This study examines the potential role of some types of mineral dust and mineral dust with sulfuric acid coatings as heterogeneous ice nuclei at cirrus temperatures. Commercially-available nanoscale powder samples of aluminum oxide, alumina-silicate and iron oxide were used as surrogates for atmospheric mineral dust particles, with and without multilayer coverage of sulfuric acid. A sample of Asian dust aerosol particles was also studied. Measurements of ice nucleation were made using a continuous-flow ice-thermal diffusion chamber (CFDC) operated to expose size-selected aerosol particles to temperatures between -45 and -60°C and a range of relative humidity above ice-saturated conditions. Pure metal oxide particles supported heterogeneous ice nucleation at lower relative humidities than those required to homogeneously freeze sulfuric acid solution particles at sizes larger than about 50 nm. The ice nucleation behavior of the same metal oxides coated with sulfuric acid indicate heterogeneous freezing at lower relative humidities than those calculated for homogeneous freezing of the diluted particle coatings. The effect of soluble coatings on the ice activation relative humidity varied with the respective uncoated core particle types, but for all types the heterogeneous freezing rates increased with particle size for the same thermodynamic conditions. For a selected size of 200 nm, the natural mineral dust particles were the most effective ice nuclei tested, supporting heterogeneous ice formation at an ice relative humidity of approximately 135%, irrespective of temperature. Modified homogeneous freezing parameterizations and theoretical formulations are shown to have application to the description of heterogeneous freezing of mineral dust-like particles with soluble coatings.

Infrared Measurements of Jet-Stream Cirrus

Abstract An experimental investigation into the radiative characteristics of jet-stream cirrus was undertaken by using AFCRL's research instrumented U-2 aircraft. Radiation measurements in the 8–13 μ window region along with aerial photography were made by flying the aircraft at various altitudes above and through the cirrus regimes. The results of this investigation are described. The window radiation data obtained over and in jet-stream cirrus give little information as to the absolute cloud height or temperature. The results of direct observations from the U-2 aircraft suggets a very wide range of values of emission and transmission for cirrus cloud regimes in the 8–13 μ window region. The blackbody assumption in this region as it pertains to cirrus clouds must be abandoned. Deviations of radiometric from ambient cirrus temperatures range from 0–68K. These temperature deviations cause errors of 0–30,000 ft in estimating the height of the cirrus tops. It is possible to improve cirrus height and temperat...