Osku Kemppinen

Quasi-three-dimensional particle imaging with digital holography

In this work, approximate three-dimensional structures of microparticles are generated with digital holography using an automated focus method. This is done by stacking a collection of silhouette-like images of a particle reconstructed from a single in-line hologram. The method enables estimation of the particle size in the longitudinal and transverse dimensions. Using the discrete dipole approximation, the method is tested computationally by simulating holograms for a variety of particles and attempting to reconstruct the known three-dimensional structure. It is found that poor longitudinal resolution strongly perturbs the reconstructed structure, yet the method does provide an approximate sense for the structures longitudinal dimension. The method is then applied to laboratory measurements of holograms of single microparticles and their scattering patterns.

Solving the inverse problem for coarse-mode aerosol particle morphology with digital holography

Coarse mode atmospheric aerosol particles are abundant in agricultural, desert, and urban environments. Accurate characterisation of these particles’ morphology is an important need in scientific and applied contexts, especially to advance our understanding for how such aerosols influence solar radiative forcing of the atmosphere. Elastic light scattering is a standard method to study aerosol particles in a contact-free manner, wherein measured scattering patterns are interpreted to infer particle morphology. Due in part to the absence of wave-phase information in these measurements, the inference is not unique, a difficulty generally known as the inverse problem. An alternative approach is digital holography where wave-phase information is encoded in the measurements. We show that digital holography and spatial filtering can solve the inverse problem for free-flowing aerosol particles in the sense that a measured scattering pattern can be uniquely associated with the particle size, shape, and orientation producing it.

Imaging aerosols with digital holography

The shape and size of aerosol particles influence whether they heat or cool Earth and its atmosphere.

Two-dimensional scattering and digital holography from isolated aerosol particles

Non-spherical aerosols, particularly aggregates and those comprised of rough surfaces, produce complex light scattering patterns that deviate considerably from those of their spherical counterparts. Consequently, discerning particle morphology from the complex scattering pattern, i.e., the inverse problem, is difficult at best. Additional information is required to associate uniquely the interference pattern resulting from scattered light and the particles morphology (size, shape, etc.). This uniqueness challenge of the inverse problem may be overcome by incorporating digital holographic imaging into the light scattering apparatus. Using a color CCD camera, we demonstrate that two-dimensional light scattering patterns and digital holograms from individual owing aerosol particles may be recorded simultaneously at different wavelengths revealing the complex scattering pattern along with the size, shape, and orientation of the particle at the instant the scattering occurs. Knowing the exact scattering pattern associated with an exact particle morphology will improve the understanding the radiative characteristics of non-spherical atmospheric aerosols, and reduce uncertainties of important physical parameters such as radiative forcing of aerosols.

Retrieval of water vapor column abundance and aerosol properties from ChemCam passive sky spectroscopy

Abstract We derive water vapor column abundances and aerosol properties from Mars Science Laboratory (MSL) ChemCam passive mode observations of scattered sky light. This paper covers the methodology and initial results for water vapor and also provides preliminary results for aerosols. The data set presented here includes the results of 113 observations spanning from Mars Year 31 Lsxa0=xa0291° (March 30, 2013) to Mars Year 33 Ls=u202f127° (March 24, 2016). Each ChemCam passive sky observation acquires spectra at two different elevation angles. We fit these spectra with a discrete-ordinates multiple scattering radiative transfer model, using the correlated-k approximation for gas absorption bands. The retrieval proceeds by first fitting the continuum of the ratio of the two elevation angles to solve for aerosol properties, and then fitting the continuum-removed ratio to solve for gas abundances. The final step of the retrieval makes use of the observed CO2 absorptions and the known CO2 abundance to correct the retrieved water vapor abundance for the effects of the vertical distribution of scattering aerosols and to derive an aerosol scale height parameter. Our water vapor results give water vapor column abundance with a precision of ±0.6 precipitable microns and systematic errors no larger than ±0.3 precipitable microns, assuming uniform vertical mixing. The ChemCam-retrieved water abundances show, with only a few exceptions, the same seasonal behavior and the same timing of seasonal minima and maxima as the TES, CRISM, and REMS-H data sets that we compare them to. However ChemCam-retrieved water abundances are generally lower than zonal and regional scale from-orbit water vapor data, while at the same time being significantly larger than pre-dawn REMS-H abundances. Pending further analysis of REMS-H volume mixing ratio uncertainties, the differences between ChemCam and REMS-H pre-dawn mixing ratios appear to be much too large to be explained by large scale circulations and thus they tend to support the hypothesis of substantial diurnal interactions of water vapor with the surface. Our preliminary aerosol results, meanwhile, show the expected seasonal pattern in dust particle size but also indicate a surprising interannual increase in water–ice cloud opacities.