Abstract
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.
Highlights
Aerosols, whether anthropogenic or natural, are ubiquitous in the environment and there is need to accurately characterize their physical form
With the exception of spherical and elipsoidal particles, which constitute a minority of coarse-mode aerosols (CMAs) particles[40], this ability has not been demonstrated in a flowing aerosol stream
The optical arrangement begins with an aerosol nozzle that delivers a stream of particles to a sensing region
Summary
Whether anthropogenic or natural, are ubiquitous in the environment and there is need to accurately characterize their physical form. One source of this uncertainty is the use of unrealistically simple particle shapes in climate models, which is partly due to the lack of accurate in-situ observations of the particles present[2, 5,6,7,8] The need for such observations is especially relevant for coarse-mode aerosols (CMAs), which are complex in morphology and include airborne mineral dust[8] (MD), bioaerosols of pollens and plant fragments[9], large combustion particles from wildfires[10], and volcanic ash[11]. The size-distributions of these aerosols continue to be poorly understood, and while some conventional microscopy[16] has been done, a systematic quantitative description of particle morphology is lacking[13] These issues underpin the need for methods to ascertain CMA particle morphology without measurement-based shape-distortions, as is the case, e.g., when particles are collected on substrates for later analysis. This “solves” the inverse problem in the sense that one is able to confidently correlate a measured pattern to the particle properties of size, shape, and orientation free of assumptions
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