Trends of inorganic and organic aerosols and precursor gases in Europe: insights from the EURODELTA multi-model experiment over the 1990–2010 period
Authors: Ciarelli Giancarlo , Theobald Mark R. , Vivanco Marta G. , Beekmann Matthias , Aas Wenche , Andersson Camilla , Bergström Robert , Manders-Groot Astrid , Couvidat Florian , Mircea Mihaela , Tsyro Svetlana , Fagerli Hilde , Mar Kathleen , Raffort Valentin , Roustan Yelva , Pay Maria-Teresa , Schaap Martijn , Kranenburg Richard , Adani Mario , Briganti Gino , Cappelletti Andrea , D'Isidoro Massimo , Cuvelier Cornelis , Cholakian Arineh , Bessagnet Bertrand , Wind Peter , Colette Augustin 
Published: November 2019

Abstract. 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
(SO42-), 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 faster
decline 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 that
observed (47 %). Comparisons of observed and modeled NO2 trends
revealed a mean relative decrease of 25 % and between 19 % and 23 % (range of
all 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 SO42- concentrations
during the P1 period indicated that the models were able to reproduce the
observed trends at most of the sites, with a 42 %–54 % mean relative
reduction indicated by the EDT experiment (range of all models) versus a
57 % 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
SO42- 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 and
particle nitrate (NO3-) concentrations revealed that the relative
reduction in HNO3 was larger than that for NO3- during the P1 period, which was mainly attributed to an increased availability of
“free ammonia”. By contrast, trends in modeled HNO3 and
NO3- 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 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 %–27 %),
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.