Modelling of atmospheric variability in gas and aerosols during the ACROSS campaign 2022 of the greater Paris area: evaluation of the meteorology, dynamics and chemistry

Understanding the interaction of anthropogenic and biogenic emissions around large urban agglomerations remains an important question for atmospheric research and the key question of the ACROSS (Atmospheric Chemistry of the Suburban Forest) project. ACROSS is based on an intensive field campaign in the Paris area, including ground-based measurements in the urban inner centre and suburban and forested sites and measurements made on board aircraft, during the exceptionally hot and dry summer of 2022. In addition, 3D modelling represents an important tool in ACROSS, and here we use the available measurements from the campaign together with observations from air quality and meteorological networks to evaluate the WRF-CHIMERE model simulation for the ACROSS period. We find that the WRF model is able to reproduce the meteorological variability during the campaign, in particular during two heatwaves. The model reproduces the daily ozone maxima well but overestimates PM2.5 by a factor of 1.5-2, partly due to an overestimation of secondary aerosol, both organic and inorganic. For organic aerosol in the Île-de-France area, the biases are reduced to about ± 20%. These differences are in contrast with those of the existing literature and might have been increased by the hot conditions of summer 2022. For case studies during two heatwave days, the model shows the sources for two organic aerosol peaks above 20-30 μg m^-3, on one occasion due to biogenic secondary organic aerosol formation in different forests around Paris and on another occasion due to the advection of wildfire aerosols in combination with secondary formation mainly from forest-emitted biogenic volatile organic compounds (BVOCs).