Speaker
Description
The Angstrom coefficient characterises the wavelength dependence of aerosol extinction and allows the vertical aerosol optical depth (VAOD) measured in one waveband to be extrapolated to others. We determine the Angstrom coefficient using the FRAM (F/Photometric Robotic Atmospheric Monitor) telescopes — small robotic instruments designed for wide-field stellar photometry, currently deployed at sites of astroparticle physics observatories. VAOD is derived simultaneously in the Johnson B (440 nm), V (540 nm), and R (620 nm) filters. Shorter wavelengths are expected to yield higher VAOD. However, we observe a filter-dependent ordering VAOD(V) > VAOD(R) > VAOD(B) inconsistent with this expectation. We determine an intercept for each filter pair by fitting a linear relation between VAOD measured in one filter versus another — a relationship that would otherwise be expected to pass through the origin — and use it to correct the Angstrom coefficient calculation. We present results from FRAM and validate against independent AERONET sun-photometer measurements (2021–2022) at the Roque de los Muchachos. The B–V combination shows the best agreement with AERONET. The derived Angstrom coefficient distribution shows a bimodal structure: the low-α peak corresponds to Calima periods —
intrusions of Saharan dust dominated by large particles — while the
high-α peak reflects clean, non-dusty conditions.