Air quality impacts of war detected from the Sentinel-5P satellite over Ukraine

The general aim of this research is to analyze spatial-temporal changes of air quality, which are the consequences of the military invasion of the Russian on the territory of Ukraine based on open remote sensing data, geoinformation and cloud technologies. The density of NO2, SO2, CO and HCHO as well as the density of O3 and the absorbing aerosol index (AAI) extracted from the Sentinel-5P satellite using Google Earth Engine (GEE) showed reduced emission of primary air pollutants and a high level of atmospheric restoration compared to 2021, the year before the war. At the same time values of the AAI increased significantly, indicating the presence of smoke and dust in the air. Obtained results showed one more aspect of the drastic effect of the war on the economic and social processes and environmental conditions. The consequences of this ecological effect can have a long-term negative impact on the health of populations.


Introduction
The full-scale war unleashed by Russia on Ukraine's territory caused irreparable loss of population, damage to the infrastructure and destruction of the economy, but also caused a significant deterioration of the environment and the formation of significant ecological problems.Thus, as preliminary estimated [1], 900 protected natural territories of Ukraine have been affected and more than 1.2 million hectares (30% of all protected areas of Ukraine) suffer from the effects of the war.As a result of shelling, uncontrolled forest fires occur, and the amount of waste has increased catastrophically.As a result of damage to the water supply infrastructure, 1.4 million people in Ukraine currently have no access to safe water.
The state of atmospheric air is an important indicator of the ecological situation [2].The direct and indirect impact of active military operations through the bombing and burning of forests and industrial and energetic objects causes the formation of large volumes of pollutants in the atmospheric air.According to the data of the Ministry of Environmental Protection and Natural Resources, as of January 11, 2022, estimated losses due to air pollution amounted to UAH 1,000 billion [3].It is obvious that for a general analysis and assessment of the impact of military operations on air quality, it is necessary to use background monitoring data to display an entire perspective of air quality changes during the war.The solution to this problem is complicated by the difficult or completely closed access to environmental data due to security issues or the lack of funding for systematic monitoring.
The purpose of this study is to assess the change in the volume of the main atmospheric air pollutants over the territory of Ukraine compared to the pre-war period using open data of

Data and methodology
Global monitoring results with a spatial resolution near 1 km 2 which were obtained with the Sentinel-5P TROPOMI satellite instruments during 2021 and 2022 were used as input data for the study.The overall results of the previous studies [4,5,6] confirmed the capability of Sentinel-5P data to be used in monitoring the air quality.
The offline (OFFL) versions of air quality datasets of TROPOMI products, including the density of nitrogen dioxide NO 2 , sulfur dioxide SO 2 , ozone O 3 , formaldehyde HCHO, carbon monoxide CO in the unit of mol/m 2 and unitless aerosol absorbing index (AAI), which represent pollution by the absorbing aerosols were used in this study.The general characteristics and sources of the input data are shown in the table 1. Google Earth Engine, a cloud computing platform, which is a modern powerful open tool for solving environmental monitoring problems, was used as an environment for data preparation and analysis [7,8,9].This platform has been successfully used in various scale research [10,11,12,13,14] to analyze changes in atmospheric air quality during the lockdown due to the COVID pandemic in 2020-2021.
Additionally, for the territory of Ukraine it was investigated the changes in NO 2 , CO, O 3 , SO 2 , and PM 2 .5 atmospheric pollution during the first months of the full-scale war, compared to the similar periods of 2019, 2020 and 2021 [15].The aforementioned study found a high correlation between NO 2 and PM 2 .5 with the intensity of hostilities, an increase in CO and O 3 levels, and a significant decrease in SO 2 concentrations as the war intensified.
Therefore, taking into account previous research experience, Google Earth Engine is chosen as the optimal tool for working with large-scale observational data.
Visualization of the obtained analysis results were carried out using QGIS 3.22.14"Białowieża".
This study included the following stages: • downloading observed data; • filtering and clipping data for the research area (the whole Ukraine);

Results and discussions
The spatial distribution of the air pollutants is shown in figure 1.
The obtained results showed a decrease in the average annual density value for NO 2 (1.99%), CO (11.72%),HCHO (1.54%), and SO 2 (32.77%) in 2022 compared to 2021.For the average annual value of O 3 density, an increase of 1.55% is observed (figure 2).The average annual value of AAI in 2022 also increased by 55.84% compared to 2021 (figure 3), which is evidence of a huge concentration of aerosols, smoke and dust in the atmosphere, resulting from explosions and fires.
Average monthly values were calculated separately for 2021 and 2022 using JavaScript language tools in Google Earth Engine to evaluate and compare the temporal dynamics of the considered variables.The created time series of the seasonal dynamics of the air quality are presented in figure 4. It is obvious that with a general tendency to decrease the amount of the considered pollutants, the seasonal patterns of its variability were preserved during 2022.
Evidently, the decrease in emissions of pollutants into the atmosphere, and therefore their content in the air, is connected with the shutdown of enterprises and is a reflection of the negative impact of military aggression on the Ukrainian economy.It should also be realized that pollution has become more dispersed in space.Emissions of specific substances occur as a result of explosions, which settle in soils, surface waters, migrate into bottom sediments and groundwater [16,17,18].Thus, a necessary step will be to conduct a diagnostic assessment and analysis of the specific pollutants content (in particular, heavy metals, carcinogens, etc.) with the involvement of the developed investigated methods of individual environmental components [19,20,21,22].

Conclusions
The study of changes in the values of NO 2 , CO, O 3 , SO 2 , HCHO density in atmospheric air and the absorbing aerosol index (AAI) as a result of military operations for the territory of Ukraine were conducted using the Google Earth Engine cloud geoinformation platform and remote sensing data of the Sentinel-5P TROPOMI.
The study of the distribution of pollutants showed declining annual mean values of NO 2 , CO, SO 2 , and HCHO in 2022 compared to the 2022 year.In contrast, overall higher O 3 was observed.The AAI increasing significantly.
The obtained results are a clear indication of the armed conflict's impact on air quality, the health of the population and the environment in general.They are important for conducting audits and designing an environmental monitoring system for assessing losses due to military aggression and post-war environmental restoration.

Figure 1 . 4 Figure 2 .
Figure 1.Spatial distribution of air pollutants over Ukraine in 2021 and 2022.

Figure 3 .
Figure 3. Spatial distribution of AAI values over Ukraine in 2021 and 2022.

Table 1 .
List of datasets used in the study.
Loading and saving spatio-temporal data for further research.