High-resolution air quality maps for Bucharest using Mixed-Effects Modeling Framework

Abstract. Fine-scale mapping of pollutants based on mobile observations facilitates deep understanding of air pollutants distribution within a city and fosters science-based decisions to improve air quality, by adding up to the existing but not optimally distributed permanent monitoring stations. In this study, we developed high-resolution concentration maps of nitrogen dioxide (NO₂), particulate matter (PM₁₀) and ultrafine particles (UFP) for Bucharest, Romania, to evaluate the spatial variation of pollutants across the city during the warm and the cold seasons. Maps were generated using a mixed-effect method applied to a Land-use Regression (LUR) model. The approach relies on multiple land-use and traffic predictor variables, and assimilation of data collected by mobile measurements over 30 days in the periods May–July 2022 and January–February 2023. Cross-validation was done against in-situ data extracted from the same collection, while validation was organized by comparison with standard measurements at fixed reference sites. Our study shows that this combined method has a good performance for all pollutants (R² > 0.65), the highest performance being observed for the cold season. PM₁₀ concentration maps indicate multiple sources of particles during the warm season, the most important source being traffic. During the cold season PM₁₀ concentration maps show a more uniform distribution of sources in Bucharest. The city’s principal roads, particularly the Bucharest ring road, are also highlighted in the NO₂ maps, with higher gradient during the warm period.