Secondary inorganic aerosol simulations for Europe with special attention to nitrate
Nitrate is an important component of (secondary inorganic) fine aerosols in Europe. We present a model sim- ulation for the year 1995 in which we account for the forma- tion of secondary inorganic aerosols including ammonium sulphate and ammonium nitrate, a semi volatile component. For this purpose, the chemistry-transport model LOTOS was extended with a thermodynamic equilibrium module and ad- ditional relevant processes to account for secondary aerosol formation and deposition. During winter, fall and especially spring high nitrate levels are projected over north western, central and eastern Europe. During winter nitrate concen- trations are highest in Italy, in accordance with observed data. In winter nitric acid, the precursor for aerosol nitrate is formed through heterogeneous reactions on the surface of aerosols. Modelled and observed sulphate concentrations show little seasonal variation. Compared to sulphate lev- els, appreciable ammonium nitrate concentrations in summer are limited to those areas with high ammonia emissions, e.g. the Netherlands, since high ammonia concentrations are nec- essary to stabilise this aerosol component at high tempera- tures. As a consequence of the strong seasonal variation in nitrate levels the AOD depth of nitrate over Europe is es- pecially significant compared to that of sulphate in winter and spring when equal AOD values are calculated over large parts of Europe. Averaged over all stations the model re- produces the measured concentrations for NO3, SO4, NH4, TNO3 (HNO3+NO3), TNH4 (NH3+NH4) and SO2 within 20%. The daily variation is captured well, albeit that the model does not always represent the amplitude of single events. The model underestimates wet deposition which was attributed to the crude representation of cloud processes. Comparison of retrieved and computed aerosol optical depth (AOD) showed that the model underestimates AOD significantly, which was expected due to the lack of carbonaceous aerosols, sea salt and dust in the model. The treatment of ammonia was found to be a major source for uncertainties in the model representation of secondary aerosols. Also, inclu- sion of sea salt is necessary to properly assess the nitrate and nitric acid levels in marine areas.