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

Abstract. 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 center to suburban and forest sites, and on–board aircraft, during the exceptionally hot and dry summer 2022. 3D–modelling represents an important tool in ACROSS to disentangle processes such as emissions, transport and physico–chemical transformations. Here we use the available measurements from the ACROSS campaign in addition to observations from air quality and meteorological networks to evaluate the coupled WRF–CHIMERE model simulation. We find that the WRF model is able to reproduce the meteorological variability during the campaign, in particular two heat waves at the beginning and at the end. The model reproduces the daily ozone maxima well, but overestimates PM_2.5 by a factor of 1.5–2, partly due to an overestimation of secondary aerosol, both organic and inorganic. This overestimation was unexpected, and could be related to the specific hot summer conditions. For organic aerosol in the Ile–de–France area, the biases are reduced to about ±20 %. The model allows to explain how the interplay of different processes affects the fine aerosol variability and chemical composition over the campaign sites during two heatwave days: biogenic secondary organic aerosol formation in different forests around Paris, advection of wildfire aerosols, and long-range transport of Saharan dust.