Angular properties of the solar light scattering in the terrestrial atmosphere observed by the ScanPol instrument of the AEROSOL-UA project

Abstract

Angular characteristics of solar light scattering in the Earth’s atmosphere and parameters of the atmosphere volume and the surface part observed by the ScanPol instrument of the “Aerosol-UA” project are estimated in order to determine properties of aerosol particles from a satellite orbit. Estimations were obtained for the scattering angles ranges in the lower troposphere which can be observed by the ScanPol instrument from the polar sun-synchronous orbit with altitude 705 km and inclination 98.1° crossing the ascending knot on the equator at 21h 30m, 22h 30m, and 23h 30m of local mean solar time. The estimations of the scattering angles were obtained at each of the orbit realizations for sites on the Earth’s surface, were the zenith angle of the Sun is 80°, 50°, and minimal one for each of the orbit realizations. The calculations were performed for the dates of 2020 close to the summer solstice, the autumnal equinox, and the winter solstice (namely 21 June, 21 September, and 21 December, respectively). The scattering angles range was computed for the range of the ScanPol scanning angles equal to 110° along the sub-satellite trace. The range of scattering angles is maximal at the considered here maximal zenith angle of the Sun equal to 80°, which occurs in the polar latitudes of the northern hemisphere during the period between the spring and the autumnal equinoxes and in the middle latitudes during the rest of a year. The maximal range of scattering angles is approximately 51.1°...149.5° close to the summer solstice for the satellite orbit with local time of crossing the ascending knot (TBB) equal to 21h 30m and 41.9°...172.9° for the orbit with ТВВ = 23h 30m. The minimal range of scattering angles occurs at minimal zenith angles of the Sun close to the winter solstice and takes on the values approximately 103.2°...142.8° and 108.2°...170.4° at mentioned ТВВ, respectively. For the rest of the zenith angles of the Sun, the range of scattering angles takes on the intermediate values. The range of scattering angles decreased mainly at the cost of small scattering angles. In summary, the range of observed scattering angles is maximal for the orbit with local time close to the noon, to be precise at TBB between 22h 30m and 24h 00m for the orbit with inclination assumed here. Comparison of the ranges of scattering angles observed by the ScanPol instrument and data of simulations showed that measurements by ScanPol from the considered orbit allow us to retrieve microphysical and optical properties of aerosol particles. Linear size of the scene observed by instrument along the sub-satellite trace increases during the scanning process from approximately 6 km at nadir to almost 60 km at maximal scattering angle equal to 60°, and simultaneously the longitude of the observed scene decreases by 1.55° that corresponds to linear shift along the parallel from DS »172 km on the equator to DS » 24.5 km on the latitude 82°. That is why data measured by the ScanPol can be used after mesoscale averaging.