A compartmental model POSEIDON-G of the transport and fate of radionuclides in the Global Ocean has been developed. The marine environment is a system of 3D compartments (boxes) representing the water column, bottom sediment, and marine organisms forming food chains. Water exchange between boxes was calculated using the monthly circulation data from the ocean general circulation model. Model equations are solved numerically using the high-accuracy matrix exponential method. The efficiency of the numerical algorithm was substantially improved relative to the previous version (POSEIDON-R). We account for all important global sources of 137Cs to reproduce the 137Cs concentration for the period 1945–2030. The simulation agrees well with measurements of 137Cs concentration in the ocean basins. Across the Global Ocean, before 2011 the geometric mean and geometric standard deviation for simulated-to-observed ratios were 1.10 and 1.79, respectively, whereas after 2011, they were 0.97 and 1.73, respectively. The 137Cs inventory in the World Ocean reached its maximum of 491 PBq in 1971, and then decreased to 181 PBq in 2026. Inventory in bottom sediments increased from 1

A web system for the analyzing unknown sources of radioactive pollution in the air was developed. With inverse modeling of atmospheric transport using the atmospheric transport model FLEXPART and the newly developed minimization module SIMFLEX, it allows to automate the solving of the inverse problem when radioactive pollution in the air is detected. The probability density distribution of the source location, estimates of release start time, duration, and inventory are visualized on a cartographic basis. Testing of the system was performed using measurement data collected during an incident with the detection of ruthenium-106 in the atmosphere in the fall of 2017. With the developed system, the estimated source location that minimized the cost function was closer to the most probable source (Mayak, Chelyabinsk region of the Russian Federation) than in previous calculations, which were based on the use of an identical set of measurements and Eulerian long-range atmospheric transport model. Presently the system is implemented for use by the radiology experts of the Ukrainian Hydro-meteorological Center.

After accidents at the Chornobyl NPP in 1986 and Fukushima Daiichi NPP in 2011, it became clear that there are many causes that can lead to a nuclear accident, including techno-genic and natural disasters. There is a danger of damage to the Zaporizhzhia NPP, with the subsequent release of radioactivity into the environment, as a result of the Russian invasion of Ukraine. The coastline of the Yellow Sea and East China Sea(YSECS) is a place where 9 NPPs are in operation in China and Korea. Since they are semi-enclosed seas with a very high density of population, any potential nuclear accident in the region can significantly contaminate the marine environment and affect the health of many people. In the current study, a set of numerical models for the first time was applied to simulate the spreading of radionuclides in the environment as a result of the hypothetical accident at the Haiyang Nuclear Power Plant in China. The scenario of accidental release with containment-bypass was considered in this work. The atmospheric transport and deposition of radionuclides on the sea surface were simulated by the FLEXPART model. The set of 1450 dispersion scenarios following hypothetical accidental releases with different start dates were calculated for the next 120 h after release start, thus covering meteorological conditions from 1 Mar 2020 to 28 Feb 2021. Scenario with the heaviest deposition densities on the Yellow Sea was selected. These results were used as a source term for three different marine dispersion model simulating the transfer and fate of Cs-137 in YSECS: the grid-based Eulerian model THREETOX, Lagrangian radionuclide transport model and compartment model POSEIDON-R. Such approach emulates the application of various models with their own settings in the event of an unexpected accidental release, similar to the Fukushima accident. For THREETOX model setup, 3D current velocities with 30 vertical layers were extracted from the KIOST-MOM model, results of which are monthly averaged and cover North Pacific. The Lagrangian radionuclide transport model used regional currents and suspended sediments concentrations from circulation model adopted for the YSECS taking into account tides and multi-fractional sediments. These two models were applied for emergency and post-emergency phases for the period from half a year to one year after deposition. The POSEIDON-R model already had a system of boxes for the North-Western Pacific covering the YSECS, East/Japan Sea and Eastern coastal area of Japan. It was applied for a long-term assessment of several decades. Obtained concentrations of Cs-137 in water, bottom sediments and partly in marine organisms were compared and the differences were analysed. Application of three marine dispersion models provides the possible ranges of radionuclide concentrations on the one hand and increases the reliability of results on the other.

To cope with the increasing threat of radioactivity release accidents in the Yellow Sea a Lagrangian radionuclide transport model in the region was recently developed coupled in off-line manner with current-wave-suspended sediment modeling system (Brovchenko et al, 2022). The radionuclide model included as an essential feature the fast adsortion-desorption processes of dissolved and particulate radionuclides in the presence of multi-ftactional sediments. Upgrade is made in this work by including fast and slow adsorption-desorption processes of radionuclides and a novel approach for lagrangian simulation of the radionuclide exchange between near-bottom water-layer and bed sediments. Lagrangian particles in the model can possess several states: dissolved in the water column, adsorbed on suspended sediment of particular size, dissolved in the pore water, adsorbed on the bed sediments of particular size. Note that, If particles are adsorbed on the sediments then it can be in two different states, namely fast and slow reversible forms; if there are Nsed sediment size classes then we have Ntot =2+4Nsed total states of the radionuclide. Throughout the numerical integration the model calculates the probabilities to transfer into each possible state (that depends on the current state and time step) during the next time step and then chooses the new particular state by comparing with the generated uniformly distributed random number. Hypothetical accident at the Haiyang NPP in China, which is located at the coast of Yellow Sea close to Korea is considered as a scenario of accident. The atmospheric transport and deposition of radionuclides on the sea surface was simulated by the FLEXPART model. The obtained deposition fluxes were used as a source term in the Lagrangian radionuclide transport model. 3D fields of currents, suspended sediment concentration and turbulent diffusion coefficient as well as bed sediment fractional composition are identical to the previous results of the Yellow Sea (Brovchenko et. al. 2022). Computational domain of the FLEXPART model includes bigger outer area, which covers Yellow and East China Sea, with spatial resolution of 0.15 deg, and inner area, which covers only Yellow Sea with better spatial resolution of 0.05 deg. The source term of 137Cs released due to hypothetical accident at the Haiyang NPP was obtained from the 6-day simulations of the FLEXPART model. The total amount of radioactivity that deposited on the calculation area is approximately 55 PBq. The radioactivity budget analysis reveals that almost near 50% of the 137Cs was deposited to the bottom sediments and approximately half remained in the dissolved form. About 4% of the total amount remains on the suspended sediments in one-step modelling and about 9% with the use of two-step model. The total bed contamination changed only 1% because for this period bottom contamination fluxes dominated over the bed cleaning process. More differences are expected for simulation with duration of several years when dissolved 137Cs concentration in water will decrease and bed cleaning process become more significant.

The paper describes one of the many possible scenarios of the hypothetical accident at the Zaporizhzhia Nuclear Power Plant (ZNPP), located in southern part of Ukraine not far from the Black Sea coast. The special scenario with atmospheric release of a large amount of activity with the subsequent transport to the south and predominant deposition on the Black Sea is considered. In the study, the atmospheric dispersion model, marine dispersion model and dose models are applied within the EU nuclear emergency response system JRODOS. According to model results, under the selected conditions the whole western part of the Black Sea will be radioactively contaminated. The values of atmospheric deposition will reach 400 kBq/m2 at distances up to 290 km from ZNPP. The maximum concentrations of radionuclides in the surface water could exceed 1000 Bq/m3 immediately after the deposition, while 20 years later the concentration of 137Cs in the Black Sea will be around 25 Bq/m3, which is several times higher than the current concentration. The concentration of 137Cs in fish could exceed 100 Bq/kg. The doses from terrestrial and marine pathways could reach considerable levels in large territories. For example, the isoline of 1 mSv of the calculated total effective dose received by 1-yr children from terrestrial exposure pathways during 1 year after the accident covers large parts of the territories of southern Ukraine, Romania and Bulgaria. The calculated annual individual dose for adults due to seafood consumption will also exceed 1 mSv for large areas of the Black Sea.

In this paper, software tools for automating the inverse calculation of the atmospheric transport model were developed as a part of the pilot version of the system for analyzing unknown sources of atmospheric pollution in the case of their detection by monitoring networks. By min-imizing the defined quality function, the probability of the source location at a certain geo-graphical point, its duration, and time of onset depending on the location, together with the vol-ume of the emission, can be analyzed. The source-receptor function is calculated by using the well-known atmospheric transport model FLEXPART in the inverse calculation mode. Auto-mated calculations of the atmospheric transport model are carried out on the Cloud Computing Platform of the Ukrainian National Grid Infrastructure by creating a virtual machine for the se-ries of FLEXPART calculations. In the future, it is planned to automate the creation and dele-tion of virtual machines performing calculations. Testing of the developed algorithms was car-ried out based on meteorological conditions during the wildfires in the Chornobyl Exclusion Zone in 2020 and data generated from measurements taken at one of the stations near Kyiv on April 18–19, 2020. During the test simulation, the coordinates of the source and the amount of Cs-137 emissions were considered unknown. Satisfactory results were obtained by comparing the estimated coordinates of the source and the volume of emissions with the corresponding real values. It is shown that the system can timely and sufficiently accurately analyze the most im-portant characteristics of possible unknown sources of atmospheric emissions. The results of the study confirm the potential importance of the obtained results for use in real-life situations and help in the identification and analysis of possible sources of radioactive contamination. The developed methods and algorithmic tools have no limitations regarding the geographical region of calculations and can be used both in the case of emissions in Ukraine and abroad.

Long-term consequences of radionuclide contamination of the Arabian Gulf as a result of hypothetical accidents at the Bushehr and Barakah nuclear power plants (NPPs) were studied using a chain of models including the atmospheric dispersion model RIMPUFF, the marine compartment model POSEIDON-R, and the dose model. The compartment model POSEIDON-R is complemented by a dynamic model of the biota food chain that includes both pelagic and benthic organisms. The source terms for the hypothetical releases of the selected radionuclides (134Cs, 137Cs, 106Ru, and 90Sr) in the atmosphere were defined as a fraction of respective reactor inventories available in the literature. Conservative meteorological scenarios for the calculation of the initial depositions of radionuclides were selected. Because the Gulf is shallow, a significant portion of the reactive radionuclides (134Cs, 137Cs, 106Ru) remain in the bottom sediments and continue to contaminate water and benthic organisms for a long period of time. The annual dose due to the consumption of marine products can exceed 1 mSv, whereas the annual dose due to drinking the water from desalination plants is expected to be an order less. The contribution of elements to the dose depends on the type of reactor. This is manifested in differences between the contributions of different marine organisms to the dose.

Since 24 of February 2022, military activities following the large-scale invasion of russian troops into Ukrainian territory lead to degradation of agricultural landscapes affecting 20-30 % of Ukrainian territory and 182 900 km2 (during the first 8 months) of arable soil. Main ecosystem damage include soil and water pollution (including Pb, Cd, Cr, As, Hg, Сu, Ni, Zn, W), soil compaction, breaking soil structure and reducing its water holding capacities, mixing and rotating soil layers, creation of craters, increased erosion and desertification, damage to soil life, regional fauna, and natural and semi-natural vegetation that increases effects of droughts and frosts and detrimental impact of climate change on agriculture. Consolidated actions of Ukrainian and international researchers and experts are needed to designing strategies for restoring agricultural landscape with the aims of increasing their multifunctionality and agroecosystem service provisioning capacities, promoting agroecological transition and support small, medium, and family farming, regionalizing green energy production, protecting and increase biodiversity. Our conference promoted the use of agroforestry, cultivation of bioenergy crops in diversified systems, and the application of microbial biopreparations for accelerated bioremediation, for transition to regenerative and carbon farming, and for increasing regional environmental resilience and economic and energy self-sufficiency. Prioritizing these solutions which are in line to major agrifood strategies in Germany and in the EU is not only important for the European integration of Ukraine, but also for (over)compensating detrimental effects of military activities on carbon emission (31 mln t during the first 7 month) and carbon sequestration (3 mln ha of affected forests). The aim of our conference is to facilitate knowledge exchange and networking between Ukrainian, German, and international researchers and experts and to present approaches and tools for sustainable landscape restoration to the range of institutions and initiatives involved in the restoration projects in Ukraine.

The scenarios of deposition on the sea surface around the Korean Peninsula following hypothetical release at Kori NPP were studied by application of the atmospheric transport model FLEXPART. The series of 1460 dispersion scenarios was calculated, covering the time period of 1 year. The total depositions on the surfaces of the East/Japan Sea (EJS), Yellow Sea (YS), East China Sea (ECS), and near-coast zone of 200-km vicinity around the Korean Peninsula, normalized on the total emission inventory were analyzed. The normalized deposition on the total area of 3 seas varied by more than 2 orders of magnitude from 0.003 to 0.59. The fraction of wet deposition in total deposition varied from 0 to 0.99. The selected most heavy deposition scenarios were divided into those dominated by dry deposition and wet deposition. The most conservative release scenarios (7 August 2020 and 2 September 2020) were dominated by wet deposition and were caused by the monsoon rainband and passage of the typhoon. In the scenario of 7 August 2020, 96 D_r = 0.055 ). Minimum average deposition on all seas was obtained for the ‘Winter’ cluster.

In the current work, there has been developed for the first time a geo-information web-system for presenting the results of the inventory of air pollution emissions in Kyiv from industrial en-terprises on the basis of the processing of data from the State Statistics Service of Ukraine, as well as estimates of geo-distributed emission fields from vehicles. The system contains infor-mation on emissions of 21 pollutants, including those that are regularly monitored in Kyiv by governmental agencies. The system is available via the link http://env.kiev.ua:8080/pollutionsystem/ pollutionsystem.html. Its main features – viewing on the map the locations of industrial emission sources for the selected substance, and the corre-sponding amount of emissions – can be used even by unregistered users. The developed system can be used in combination with the existing public and state systems for monitoring of air pol-lution in Kyiv for preliminary (screening) analysis of possible causes of high levels of air pollu-tion during their observing with the monitoring systems. The paper presents a relevant example of the system application to explain the high levels of hydrogen sulfide concentrations at one of the stations in Kyiv. Another purpose of the system may be to detect shortcomings in the exist-ing inventory of industrial emissions, namely the lack of information on certain sources, as well as verification and adjustment of data on emissions from the registered sources. To fully use the system potential, it is necessary to expand its functionality in the direction of mathematical modeling of the spread of pollution due to emissions from industrial enterprises and vehicles. For this purpose, it is also necessary to integrate hydrometeorological parameters and levels of air pollution in Kyiv into the data monitoring system.

The capabilities of the ensemble Kalman filter (EKF) data assimilation (DA) method to reduce errors in simulations of concentration distributions following the accidental release of a contaminant in the ocean were evaluated. The method was tested in an idealized setting where the contaminant was released in the ocean described by a simple linear Stommel model that includes the main features of two-dimensional (2D) wind-driven circulation in the ocean on the β-plane. The wind stress curl in the right-hand side of the equation for stream function was randomly perturbed to generate an ensemble of the perturbed fields of currents. The velocity fields obtained from the ensemble of stream functions were then used for the calculation of the ensemble of concentration fields following short-duration point release. On day 1000 of the simulation, correlation coefficients of the members of the concentration ensemble and the unperturbed concentration distribution fell to 0.087. The ensemble member with the maximum deviation from the unperturbed concentration distribution was selected to be used as “truth” in data assimilation experiments. Due to the high inhomogeneity of the concentration fields, the free regularization parameter had to be defined and tuned using the L-curve approach. Different DA scenarios were considered with different topologies of measurement networks and different source locations. In all cases, data assimilation gradually brought ensemble-averaged concentration fields close to the true distribution. The root mean square errors of the analyzed concentrations on day 1000 decreased by the factors varying from 3 to 4 in different DA scenarios as compared to the simulation without DA.

The previously developed system Povitrya allows the calculation of the atmospheric distribu-tion of 36 chemicals using the CALPUFF atmospheric dispersion model. In this paper, after the literature review and according to the requirements of the CALPUFF model, the values of the parameters for the calculation of dry and wet depositions of pollutants in the Povitrya system have been defined. Based on the analysis of the model parameters, the parameters that depend on the type of substance have been identified. For pollutants that spread in the form of aerosols, only one parameter – the scavenging coefficient included in the formula for the calculation of wet deposition – depends on the type of substance. Other deposition parameters for aerosols are a function of the characteristics of the particle size distribution, so they are not considered in this paper. The scavenging coefficient is specified for all types of chemicals – aerosols and gas-es modeled by the Povitrya system depending on the solubility of the substance (for gases) or miscibility (for aerosols). For gases, several other parameters in deposition calculation formu-las also depend on the type of substance, namely: molecular diffusivity, solubility enhancement factor, reactivity, mesophyll resistance, and Henry’s law. The values of the corresponding pa-rameters are set for 17 types of gaseous pollutants. To determine Henry’s law constant, there have been established correspondences between the definitions of this parameter in the litera-ture and the CALPUFF model. For mesophyll resistance, an interpolation formula is proposed to calculate this parameter as a function of solubility. The least data was found for reactivity and solubility enhancement coefficient. Therefore, with some exceptions, the values of the rele-vant parameters are set the same for all substances. A more detailed definition of these parame-ters requires a separate study.

The emission inventories of cesium-137 resulting from the wildfires in the Chernobyl Exclusion Zone (3-24 April 2020) and from the dust storm (16-17 April 2020), which resuspended contaminated ash, were estimated using inverse modeling. The goal of this work was to take into account uncertainties of inexactly known source term and meteorological input data for evaluation of emission inventories and their confidence limits, by developing Ensemble Iterative Source Inversion Method (EISIM). A set of source receptor matrices (SRMs) was calculated using FLEXPART atmospheric transport code with varying source term parameters (size distribution of emitted particles, height distribution of emissions) and meteorological input data. The covariance matrix of model errors was estimated by ensemble averaging of model results obtained after multiplication of the pre-calculated SRMs by the estimated emission inventories at the current iteration step. The emission inventories at each iteration step were evaluated for each of the ensemble members by solving the conventional variational source inversion problem. With iterations, the variance of model error was reduced by an order of magnitude. The estimated total emission of Cs-137 from wildfires was 448 GBq, close to the first guess estimation. By using emission inventories within the obtained confidence limits (from 39 to 1530 GBq), different combinations of source term parameters and input meteorological data, FLEXPART could fit observations with a correlation coefficient of more than 0.6 and a normalized mean squared error of less than 10. The obtained estimate of the total emission resulting from the wind resuspension during the dust storm was 27 GBq. The respective estimated confidence interval was from 3 to 93 GBq. By analyzing model error statistics, some of the source term parameters could be reliably evaluated. The fraction of the fine particles (0.25 mu m) in total emissions W-sf(0.25)asymptotic to 0.1 and the fraction of emission below a bottom height of convective plume W-h(1)asymptotic to 0.5.

Досліджено можливість комбінованого застосування скринінгових моделей для оцінювання характеристик джерел у разі аварій на об’єктах зберігання небезпечних речовин зі складними моделями атмосферного перенесення у складі сучасних систем підтримання рішень для розрахунку атмосферного забруднення у широкому діапазоні просторових і часових масштабів. Час випаровування у разі аварійного розливу, оцінений скринінговими моделями, використовується для задання інтенсивності емісії і розрахунку атмосферного поширення системою ядерного аварійного реагування RODOS. Для аварії в Чернігові 23.03.2022 оцінено, що перевищення граничної допустимої концентрації аміаку 0.2 мг/м3 відбувається на відстанях до 75 км від джерела. Залежність розрахованих максимальних концентрацій від часу має асимптотичний характер близький до cmax ~ t-4.5 до 15 год після викиду, що узгоджується з асимптотичним співвідношенням σ ~ t3/2 для часової залежності розмірів хмар у разі турбулентної дисперсії миттєвих викидів.

The OpenFoam CFD code was adapted to simulate atmospheric transport of pollutants. By performing calculations on unstructured grids and using the modifications of turbulence parameterizations to account for the influence of Earth's rotation on PBL structure, the presented model can take into account the influence of urban obstructions and complex topography on atmospheric dispersion from a city to street-scale. The normalized mean squared error of simulated results for the conditions of the known MUST experiment was comparable with the results of other models (NMSE approximate to 0.6). The value of turbulent Schmidt number in this experiment was estimated to be Sc approximate to 0.4. The example of model application for the assessment of atmospheric pollution created by the industrial site of uranium production was presented.

The dynamics of emissions of radioactive aerosols during powerful wildfires (3-23 April 2020) and dust storm (16-17 April 2020) in the Chernobyl Exclusion Zone (ChEZ) was estimated using an ensemble inverse method. The unique feature of this event is that the wildfires of unprecedented power in ChEZ were combined with the dust storm on 16-17 April 2020, which covered the Northern-West and Central Ukraine. Due to both events, the levels of Cs-137 concentrations in air were increased significantly above the background levels. In our study, the ensemble covariance matrices of model errors were calculated by a series of runs of the FLEXPART atmospheric transport model using different input meteorological data (22 meteorological datasets produced by Global Ensemble Forecasting System GEFS) and different sets of model parameters describing the size distribution of particles and height distribution of releases. Simulations covered the period from 3rd to 27th of April 2020. The prior estimates for the temporal dynamics of emissions were taken from [1]. Measurements of Cs-137 concentration in air collected by different countries and presented in [2] were used for source inversion. The vertical extensions of releases from different sources were estimated based on the data of the CAMS Global Fire Assimilation System. The fractions of emissions below plume bottom and between plume bottom and plume top heights were allowed to vary in different ensemble runs. It is shown that varying all the mentioned parameters (meteorological data, particle size distribution, and the parameters of emission distribution by height) significantly affected the results of the calculated temporal dynamics of emissions during the wildfires. However, the variability of meteorological data had the largest overall influence on the results. Confidence intervals for emissions from wildfires and dust storm (16-17 April) were obtained by processing the ensemble of estimates. The estimated total emissions of Cs-137 from the wildfires ranged from about 200 to about 1000 GBq. The total estimates of Cs-137 emissions due to the dust storm estimated by inverse modeling appeared to be considerably less than the emissions from the wildfires on the same days. At the same time, the levels of air pollution by common contaminants (PM2.5 and ash) observed in Kyiv were strongly dominated by the dust storm because the area covered by the dust storm was much greater than the area of ChEZ. References * Talerko, M., Kovalets, I., Lev, T., Igarashi, Y., Romanenko, O. (2021) Simulation study of the radionuclide atmospheric transport after wildland fires in the Chernobyl Exclusion Zone in April 2020. Atmospheric Pollution Research, 12(3) 193-204. DOI:1016/j.apr.2021.01.010 * Masson O., Romanenko O., Saunier O., Kirieiev S., Protsak V., Laptev G., Voitsekhovych O., Durand V., Coppin F. [et al.] (2021) Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020. Environmental Science & Technology, 55(20) 13834-13848. DOI: 10.1021/acs.est.1c03314

We studied the possibility of the combined application of screening models to assess the characteristics of sources in accidents at storage facilities for hazardous substances with complex models of atmospheric transport as part of modern decision support systems to calculate air pollution in a wide range of spatial and temporal scales. The evaporation time following an emergency spill, estimated by screening models, is used to set the emission intensity and calculate the atmospheric transport by the RODOS nuclear emergency response system. For the accident in Chernihiv on March 23, 2022, it was estimated that the maximum permissible concentration of ammonia 0.2 mg/m3 was exceeded at distances up to 75 km from the source. The dependence of the calculated maximum concentrations on time is close to asymptote cmax ~ t-4.5 up to 15 h after emission, which is consistent with the asymptote σ ~ t3/2 for the time dependence of the sizes of puffs following turbulent dispersion of instantaneous releases.

A computationally efficient source inversion algorithm was developed and applied with the Lagrangian atmospheric dispersion model DIPCOT. In the process of source location estimation by minimizing a correlation-based cost function, the algorithm uses only the values of the time-integrated concentrations at the monitoring stations instead of all of the individual measurements in the full concentration-time series, resulting in a significant reduction in the number of integrations of the backward transport equations. Following the source location estimation the release start time, duration and emission rate are assessed. The developed algorithm was verified for the conditions of the ETEX-I (European Tracer Experiment—1st release). Using time-integrated measurements from all available stations, the distance between the estimated and true source location was 108 km. The estimated start time of the release was only about 1 h different from the true value, within the possible accuracy of estimate of this parameter. The estimated release duration was 21 h (the true value was 12 h). The estimated release rate was 4.28 g/s (the true value was 7.95 g/s). The estimated released mass almost perfectly fitted the true released mass (323.6 vs. 343.4 kg). It thus could be concluded that the developed algorithm is suitable for further integration in real-time decision support systems.

The evaluation of the previously developed one-dimensional model of radiocarbon atmospheric transport in vegetated canopies against C-14 concentration data collected at the site of SMEAR-II research station was presented. In most cases, the simulated vertical profiles of C-14 concentrations within the canopy layer agreed reasonably with measurements, the correlation coefficient of simulated vs. observed concentrations was 0.72. The developed model could be used to evaluate vertical variations of C-14 concentrations in vegetated canopy layers.

In April 2020, the largest forest fire occurred in the Chernobyl Exclusion Zone (ChEZ) in its history. The results of modeling the atmospheric transport of radioactive aerosols released into the atmosphere as a result of wildland fires in the ChEZ and around it are presented. The atmospheric transport model LEDI, developed at the Institute for Safety Problems of NPPs, and the Atmospheric Dispersion Module of the real -time online decision support system for offsite nuclear emergency RODOS, which development was funded by the EU, were used. The 137Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The 137Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m−3 during the period April 3–20. The modeling results are generally consistent with measured data pertaining to radioactive contamination in Kyiv, within the ChEZ, and areas around four operating nuclear power plants in Ukraine.A method for estimating the radionuclide activity emissions during wildland fires in radioactively contaminated areas is proposed. This method is based on satellite data of the fire radiative power (FRP), the radionuclide inventory in the fire area, and an emission factor for radioactive particles. A method was applied for forest fires in the ChEZ in April 2020. Preliminary estimations of an emission factor are made using FRP values obtained from NASA's MODIS and VIIRS active fire products.On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional 137Cs activity raised by the dust storm from burned areas in the meadow biocenoses was estimated to be about 162 GBq, i.e. up to 20% of the total activity emitted into the air during the entire period of forest fires on April 3-20, 2020. According to the modeling results, during April 16-17, the input of resuspension of radioactive particles due to a dust storm was up to 80-95% of the total 137Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%.The total effective dose to the population of Kyiv during the fire period is estimated to be 5.7 nSv from external exposure and the inhalation of 137Cs and 90Sr, rising to 30 nSv by the end of 2020. This is about 0.003% of the annual permissible level of exposure of the population. A committed effective dose up to 200-500 nSv is estimated for the personnel of the Chernobyl NPP from the radioactive aerosol inhalation during the 2020 forest fires, which is not more than 0.05% of the established control levels of internal exposure for them.