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<p>In the framework of the EURODELTA-Trends (EDT) modeling experiment, several chemical transport models (CTMs) were applied for the 1990–2010 period to investigate air quality changes in Europe as well as the capability of the models to reproduce observed long-term air quality trends. 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 global reanalysis. For this study, long-term observations of particle sulfate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="43c7521f547bc9d1c731092871dcc89b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00001.svg" width="29pt" height="17pt" src="gmd-12-4923-2019-ie00001.png"/></svg:svg></span></span>), total nitrate (<span class="inline-formula">TNO₃</span>), total ammonium (<span class="inline-formula">TNH_<i>x</i></span>) as well as sulfur dioxide (<span class="inline-formula">SO₂</span>) and nitrogen dioxide (<span class="inline-formula">NO₂</span>) for multiple sites in Europe were used to evaluate the model results. The trend analysis was performed for the full 21 years<span id="page4924"/> (referred to as PT) but also for two 11-year subperiods: 1990–2000 (referred to as P1) and 2000–2010 (referred to as P2).</p> <p>The experiment revealed that the models were able to reproduce the faster decline in observed <span class="inline-formula">SO₂</span> concentrations during the first decade, i.e., 1990–2000, with a 64&thinsp;%–76&thinsp;% mean relative reduction in <span class="inline-formula">SO₂</span> concentrations indicated by the EDT experiment (range of all the models) versus an 82&thinsp;% mean relative reduction in observed concentrations. During the second decade (P2), the models estimated a mean relative reduction in <span class="inline-formula">SO₂</span> concentrations of about 34&thinsp;%–54&thinsp;%, which was also in line with that observed (47&thinsp;%). Comparisons of observed and modeled <span class="inline-formula">NO₂</span> trends revealed a mean relative decrease of 25&thinsp;% and between 19&thinsp;% and 23&thinsp;% (range of all the models) during the P1 period, and 12&thinsp;% and between 22&thinsp;% and 26&thinsp;% (range of all the models) during the P2 period, respectively.</p> <p>Comparisons of observed and modeled trends in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="8c898138530c760447165fe6cdc920bb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00002.svg" width="29pt" height="17pt" src="gmd-12-4923-2019-ie00002.png"/></svg:svg></span></span> concentrations during the P1 period indicated that the models were able to reproduce the observed trends at most of the sites, with a 42&thinsp;%–54&thinsp;% mean relative reduction indicated by the EDT experiment (range of all models) versus a 57&thinsp;% mean relative reduction in observed concentrations and with good performance also during the P2 and PT periods, even though all the models overpredicted the number of statistically significant decreasing trends during the P2 period. Moreover, especially during the P1 period, both modeled and observational data indicated smaller reductions in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="815783a157bc15e547bdd7a24388d96b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00003.svg" width="29pt" height="17pt" src="gmd-12-4923-2019-ie00003.png"/></svg:svg></span></span> concentrations compared with their gas-phase precursor (i.e., <span class="inline-formula">SO₂</span>), which could be mainly attributed to increased oxidant levels and pH-dependent cloud chemistry.</p> <p>An analysis of the trends in <span class="inline-formula">TNO₃</span> concentrations indicated a 28&thinsp;%–39&thinsp;% and 29&thinsp;% mean relative reduction in <span class="inline-formula">TNO₃</span> concentrations for the full period for model data (range of all the models) and observations, respectively. Further analysis of the trends in modeled <span class="inline-formula">HNO₃</span> and particle nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="48a6d5724cc017ced9c974ab9a81c03a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00004.svg" width="25pt" height="16pt" src="gmd-12-4923-2019-ie00004.png"/></svg:svg></span></span>) concentrations revealed that the relative reduction in <span class="inline-formula">HNO₃</span> was larger than that for <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7dd3c683c0655cd2a5c1ed2d08ea01e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00005.svg" width="25pt" height="16pt" src="gmd-12-4923-2019-ie00005.png"/></svg:svg></span></span> during the P1 period, which was mainly attributed to an increased availability of “free ammonia”. By contrast, trends in modeled <span class="inline-formula">HNO₃</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="827b0fe0e97f70953101fc9e20cd0031"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-4923-2019-ie00006.svg" width="25pt" height="16pt" src="gmd-12-4923-2019-ie00006.png"/></svg:svg></span></span> concentrations were more comparable during the P2 period. Also, trends of <span class="inline-formula">TNH_<i>x</i></span> concentrations were, in general, underpredicted by all models, with worse performance for the P1 period than for P2.</p> <p>Trends in modeled anthropogenic and biogenic secondary organic aerosol (ASOA and BSOA) concentrations together with the trends in available emissions of biogenic volatile organic compounds (BVOCs) were also investigated. A strong decrease in ASOA was indicated by all the models, following the reduction in anthropogenic non-methane VOC (NMVOC) precursors. Biogenic emission data provided by the modeling teams indicated a few areas with statistically significant increase in isoprene emissions and monoterpene emissions during the 1990–2010 period over Fennoscandia and eastern European regions (i.e., around 14&thinsp;%–27&thinsp;%), which was mainly attributed to the increase of surface temperature. However, the modeled BSOA concentrations did not linearly follow the increase in biogenic emissions. Finally, a comprehensive evaluation against positive matrix factorization (PMF) data, available during the second period (P2) at various European sites, revealed a systematic underestimation of the modeled SOA fractions of a factor of 3 to 11, on average, most likely because of missing SOA precursors and formation pathways, with reduced biases for the models that accounted for chemical aging of semi-volatile SOA components in the atmosphere.</p>