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. Aerosols play an important role in Europe and the Mediterranean area where differentsources of natural and anthropogenic particles are present. Among themammonium and nitrate (A&N) aerosols may have a growing impact on regionalclimate. In this study, their representation in coarse and fine modes hasbeen introduced in the prognostic aerosol scheme of the ALADIN-Climateregional model. This new aerosol scheme is evaluated over Europe and theMediterranean Sea, using two twin simulations over the period 1979–2016 withand without A&N aerosols. This evaluation is performed at local andregional scales, using surface stations and satellite measurements. Despitean overestimate of the surface nitrate concentration, the model is able toreproduce its spatial pattern including local maxima (Benelux, Po Valley).Concerning the simulated aerosol optical depth (AOD), the inclusion of A&Naerosols significantly reduces the model bias compared to both AERONETstations and satellite data. Our results indicate that A&N aerosols cancontribute up to 40 % of the total AOD 550 over Europe, with anaverage of 0.07 (550 nm) over the period 2001–2016. Sensitivity studiessuggest that biases still present are related to uncertainties associatedwith the annual cycle of A&N aerosol precursors (ammonia and nitric acid).The decrease in sulfate aerosol production over Europe since 1980 producesmore free ammonia in the atmosphere leading to an increase in A&Nconcentrations over the studied period. Analyses of the different aerosoltrends have shown for the first time to our knowledge that, since 2005 overEurope, A&N AOD 550 and A&N shortwave (SW) direct radiative forcing(DRF) are found to be higher than sulfate and organics, making these thespecies with the highest AOD and the highest DRF. On average over the period1979–2016, the A&N DRF is found to be about −1.7  W m −2 at thesurface and −1.4  W m −2 at the top of the atmosphere (TOA) in all skyconditions over Europe, with regional maxima located at the surface over thePo Valley ( −5  W m −2 ). Finally, the dimming effect of A&N aerosolsis responsible for a cooling of about −0.2 ∘  C over Europe(summer), with a maximum of −0.4 ∘ C over the Po Valley. Concerningprecipitation, no significant impact of A&N aerosols has been found.

atmospheric chemistry and physics

Model simulation of ammonium and nitrate aerosols distribution in the Euro-Mediterranean region and their radiative and climatic effects over 1979–2016