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

Abstract

Abstract. Aerosols play an important role in Europe and the Mediterranean area where different sources of natural and anthropogenic particles are present. Among them ammonium and nitrate (A&N) aerosols may have a growing impact on regional climate. In this study, their representation in coarse and fine modes has been introduced in the prognostic aerosol scheme of the ALADIN-Climate regional model. This new aerosol scheme is evaluated over Europe and the Mediterranean Sea, using two twin simulations over the period 1979–2016 with and without A&N aerosols. This evaluation is performed at local and regional scales, using surface stations and satellite measurements. Despite an overestimate of the surface nitrate concentration, the model is able to reproduce its spatial pattern including local maxima (Benelux, Po Valley). Concerning the simulated aerosol optical depth (AOD), the inclusion of A&N aerosols significantly reduces the model bias compared to both AERONET stations and satellite data. Our results indicate that A&N aerosols can contribute up to 40 % of the total AOD550 over Europe, with an average of 0.07 (550 nm) over the period 2001–2016. Sensitivity studies suggest that biases still present are related to uncertainties associated with the annual cycle of A&N aerosol precursors (ammonia and nitric acid). The decrease in sulfate aerosol production over Europe since 1980 produces more free ammonia in the atmosphere leading to an increase in A&N concentrations over the studied period. Analyses of the different aerosol trends have shown for the first time to our knowledge that, since 2005 over Europe, A&N AOD550 and A&N shortwave (SW) direct radiative forcing (DRF) are found to be higher than sulfate and organics, making these the species with the highest AOD and the highest DRF. On average over the period 1979–2016, the A&N DRF is found to be about −1.7 W m−2 at the surface and −1.4 W m−2 at the top of the atmosphere (TOA) in all sky conditions over Europe, with regional maxima located at the surface over the Po Valley (−5 W m−2). Finally, the dimming effect of A&N aerosols is responsible for a cooling of about −0.2∘ C over Europe (summer), with a maximum of −0.4 ∘C over the Po Valley. Concerning precipitation, no significant impact of A&N aerosols has been found.