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. In the framework of the EURODELTA-Trends (EDT) modelingexperiment, several chemical transport models (CTMs) were applied for the1990–2010 period to investigate air quality changes in Europe as well asthe capability of the models to reproduce observed long-term air qualitytrends. Five CTMs have provided modeled air quality data for 21 continuous years in Europe using emission scenarios prepared by the International Institute for Applied Systems Analysis/Greenhouse Gas – Air Pollution Interactions and Synergies (IIASA/GAINS)and corresponding year-by-year meteorology derived from ERA-Interim globalreanalysis. For this study, long-term observations of particle sulfate( SO 4 2 - ), total nitrate ( TNO3 ), total ammonium ( TNHx ) as well as sulfur dioxide ( SO2 ) and nitrogen dioxide ( NO2 ) for multiple sites in Europe were used to evaluate the model results. The trend analysis was performed for the full 21 years (referred to as PT) but also for two 11-year subperiods: 1990–2000 (referred to as P1) and 2000–2010 (referred to as P2). The experiment revealed that the models were able to reproduce the fasterdecline in observed SO2 concentrations during the first decade, i.e., 1990–2000, with a 64 %–76 % mean relative reduction in SO2 concentrations indicated by the EDT experiment (range of all the models) versus an 82 % mean relative reduction in observed concentrations. During the second decade (P2), the models estimated a mean relative reduction in SO2 concentrations of about 34 %–54 %, which was also in line with thatobserved (47 %). Comparisons of observed and modeled NO2 trendsrevealed a mean relative decrease of 25 % and between 19 % and 23 % (range ofall the models) during the P1 period, and 12 % and between 22 % and 26 %(range of all the models) during the P2 period, respectively. Comparisons of observed and modeled trends in SO 4 2 - concentrationsduring the P1 period indicated that the models were able to reproduce theobserved trends at most of the sites, with a 42 %–54 % mean relativereduction indicated by the EDT experiment (range of all models) versus a57 % mean relative reduction in observed concentrations and with goodperformance also during the P2 and PT periods, even though all the modelsoverpredicted the number of statistically significant decreasing trendsduring the P2 period. Moreover, especially during the P1 period, bothmodeled and observational data indicated smaller reductions in SO 4 2 - concentrations compared with their gas-phase precursor (i.e., SO2 ), which could be mainly attributed to increased oxidant levels and pH-dependent cloud chemistry. An analysis of the trends in TNO3 concentrations indicated a 28 %–39 % and 29 % mean relative reduction in TNO3 concentrations for the full period for model data (range of all the models) and observations,respectively. Further analysis of the trends in modeled HNO3 andparticle nitrate ( NO 3 - ) concentrations revealed that the relativereduction in HNO3 was larger than that for NO 3 - during the P1 period, which was mainly attributed to an increased availability of“free ammonia”. By contrast, trends in modeled HNO3 and NO 3 - concentrations were more comparable during the P2 period.Also, trends of TNHx concentrations were, in general, underpredicted by all models, with worse performance for the P1 period than for P2. Trends in modeled anthropogenic and biogenic secondary organic aerosol (ASOA and BSOA) concentrations together with the trends in available emissions ofbiogenic volatile organic compounds (BVOCs) were also investigated. A strongdecrease in ASOA was indicated by all the models, following the reduction inanthropogenic non-methane VOC (NMVOC) precursors. Biogenic emission data provided by themodeling teams indicated a few areas with statistically significant increasein isoprene emissions and monoterpene emissions during the 1990–2010 periodover Fennoscandia and eastern European regions (i.e., around 14 %–27 %),which was mainly attributed to the increase of surface temperature. However,the modeled BSOA concentrations did not linearly follow the increase inbiogenic emissions. Finally, a comprehensive evaluation against positivematrix factorization (PMF) data, available during the second period (P2) atvarious European sites, revealed a systematic underestimation of themodeled SOA fractions of a factor of 3 to 11, on average, mostlikely because of missing SOA precursors and formation pathways, withreduced biases for the models that accounted for chemical aging ofsemi-volatile SOA components in the atmosphere.

Trends of inorganic and organic aerosols and precursor gases in Europe: insights from the EURODELTA multi-model experiment over the 1990–2010 period