GAIA-ARBOL

Atmospheric aerosols are a two-phase system of solid or liquid particles dispersed in ambient air, of natural (desert dust, volcanic emissions, or sea salt) or anthropogenic (particulate matter emitted by combustion reactions) origin. These particles have a significant impact at the global scale on climate. At the local scale, they impact human health and visibility. Aerosols interact with direct solar radiation and act as condensation nuclei for cloud formation. Additionally, they can carry toxic substances that are inhaled and absorbed by the human body.

The ARBOL instrument is composed of seven parallel telescopes, one of which functions as a pointing telescope. The pointing telescope compensates for the alignment of the system by means of a four-quadrant photodiode connected to a GPS system, which enables the tracking of the Sun even in suboptimal weather conditions.

The remaining six multispectral telescopes measure the incoming solar radiance through bandpass filters centered at specific wavelengths: 400, 500, 675, 870, 940 and 1020 nm. The ARBOL system functions by automatically acquiring direct radiance measurements at a rate of 5 seconds, with a total sampling rate of 600 Hz. This enables the acquisition of a substantial volume of data with high temporal resolution. Subsequently, the raw data are averaged to reduce electronic noise and obtain more stable radiance values. Direct radiance measurements are utilized to calculate aerosol optical thickness (AOD), a parameter that quantifies the contribution of aerosols to light extinction in the atmosphere. The determination of AOD is achieved through the Lambert-Beer law,

I/ I₀ = e^{-m τ}

which describes the attenuation of solar radiation in relation to total optical thickness (τ), which encompasses extinction attributable to atmospheric gases, and airmass m, denoting the optical thickness of the atmosphere traversed by solar radiation along the line of sight. To obtain the AODs, it is necessary to calibrate the ARBOL photometer and calculate the extraterrestrial irradiance (I0), and then to subtract the gas contributions from the total optical thickness τ. Information on the optical thickness of aerosols at multiple wavelengths can then be exploited to better characterize the component particles, primarily by determining their size distribution. inversion algorithms such as GRASP (Generalized Retrieval of Aerosol and Surface Properties). Consequently, ARBOL represents a state-of-the-art tool that can enhance our comprehension of the physicochemical and radiation-matter interaction processes occurring in the atmosphere, thereby providing essential information to facilitate a more profound understanding of the impact of aerosols on climate change and air quality.