Water surfaces in satellite imagery are often not quite what they actually are. The culprit is atmospheric aerosol, which consists of small particles of dust, smoke, and other pollutants in the air. They distort the image, so when processing remote sensing data from water, it is important to understand what kind of aerosol is present in the atmosphere.
This problem is especially relevant for inland water bodies, which are affected by particles brought by the wind from land more strongly. These include the largest reservoirs of the Volga — the Gorky and Cheboksary ones. In order to understand the optical impact of the atmosphere on these reservoirs, scientists from the Marine Hydrophysical Institute of RAS (Sevastopol) and the Institute of Applied Physics of RAS (Nizhny Novgorod) carried out a special study. The publication of its results in the authoritative international journal Remote Sensing confirms their importance for the problems of remote sensing of inland water bodies.
"The Gorky and Cheboksary reservoirs are inland eutrophic water bodies, the geographical location and size of which in many cases do not allow to obtain objective satellite optical and hydrophysical characteristics. Both reservoirs are strongly affected by continental aerosol as they are surrounded by land on all sides. Our goal was to obtain optical characteristics of the atmosphere above these reservoirs for determining the background regional characteristics of aerosol and identify events associated with the transfer of absorbing aerosol to the regions under study for the period 2022-2023," says the first author of the article, research associate at Marine Optics and Biophysics Department of FSBSI FRC MHI Daria Kalinskaya.
To obtain photometric data, the scientists conducted several expeditions on r/v Geofizik. Measurements were carried out in exceptionally clear, cloudless weather. The results made it possible to determine the background "clean" aerosol level for the reservoir region. The authors analyzed temporal variability of aerosol optical thickness and other parameters, and applied satellite data to identify the aerosol activity sources.
“We identified dust transfer cases (April 13, 2022 and April 14, 2023), as well as the presence of smoke aerosol in the atmosphere over the Gorky Reservoir region (in the summer of 2022) during the study. Cases of desert dust presence over the Gorky Reservoir were confirmed by natural, satellite, and model data,” explained Daria Kalinskaya.
According to the researchers, the work results are regional in nature but are important for testing standard algorithms for processing satellite imagery.
"Our study has shown that the quality of regional atmospheric correction algorithms, considered using the example of the Gorky and Cheboksary reservoirs, depends on taking into account the absorbing and scattering properties of aerosol in the atmosphere above the region under study, as well as on the size estimate of aerosol particles. Desert dust aerosol comprises both fine and coarse particles, which can directly affect the temperature of surface waters in freshwater bodies and the absolute humidity of the air above inland water areas," the scientist concluded.
Thus, a comprehensive study of aerosol characteristics of the atmosphere above the Volga reservoirs provides valuable regional data for monitoring and satellite sounding of water bodies. Further research will be aimed at improving atmospheric correction methods taking into account the identified aerosol features.
The study was carried out within the framework of the grant No. 23-17-00071 of Russian Science Foundation and the Federal Academic Leadership Program "Priority-2030" of Lobachevsky State University of Nizhny Novgorod.