Custom Particles for Improved Aerosol Field Calibrations

Abstract:

Uncertainty surrounding the effect of atmospheric aerosols has been a persistent obstacle for climate modeling and forecasting. In situ characterization of particles is critical for reducing this uncertainty. Instruments used for aerosol characterization require regular calibration to provide accurate results. Currently many calibrations are performed in the lab rather than the field due to cumbersome deployment equipment, which introduces calibration drift from differing environmental conditions. Outside a few standard particles such as polystyrene latex spheres (PSLs), particles presently available for optical calibrations also suffer from inhomogeneity, unknown form factors, and reactivity; attributes which contribute additional uncertainty or burden with non-specific corrections and preprocessing.

This work presents three production techniques and prototypes for novel calibration particles: arbitrarily shaped particles of SU-8 via dynamic maskless holographic lithography (DMHL), mineral dust analogs produced with planar lithography, and inorganic absorptive nanoparticles. These particles have known composition and high monodispersity, and can be deployed from simple containers at the push of a button.

DMHL allows for rapid fabrication of custom 3D structures with submicron precision. This work leverages this capability by direct writing particles into SU-8 with 405nm light. Non-spherical particles with well-characterized form factors have been produced which require minimal processing to be used in instrument calibration.

Planar lithography similarly allows for repeated production of structures with extremely high precision. After coating a thin substrate with SU-8 and exposing a pattern of various diameter dot arrays, the substrate is etched away to leave disks with very well controlled diameters.

Extinction (absorption + scattering) calibrations are challenged by the choice of particles with regular diameters but irregular absorption, or vice versa. Inorganic nanoparticles which have been doped for specific absorption allow for both and have been produced in this effort.

Characterizations of each particle type, as well as comparisons against current techniques for the calibration of a cavity ring-down spectrometer will be presented.