High-resolution air quality maps for Bucharest using a mixed-effects modeling framework

Abstract. High-resolution mapping of pollutants based on mobile observation facilitates deep understanding of air pollutant distributions within a city. This approach fosters science-based decisions to improve air quality, by adding 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 cold seasons. Maps were generated using a mixed-effects 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 d 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 a higher gradient during the warm period.