Understanding the Simulated Ammonia Increasing Trend from 2008 to 2015 over Europe with CHIMERE and Comparison with IASI Observations
Résumé
The objective of this study is to assess and understand the NH3 recent trends and to identify
the key components driving its concentrations. We have simulated the seasonal cycle, the interannual
variability, and the trends in NH3 vertical column densities (VCD) from 2008 to 2015 over Europe, with
the CHIMERE regional chemistry–transport model. We have also confronted the simulations against
the Infrared Atmospheric Sounding Interferometer (IASI) satellite observations. IASI often shows a
strong maximum in summer in addition to the spring peak, whereas CHIMERE only shows a slight
peak in summer some years. This result could point to a misrepresentation of the temporal profile of
the NH3 emissions, i.e., to missing emission sources during summertime either due to more than
expected fertilizer use or to increased volatilization under warmer conditions. The simulated NH3
VCDs present an increasing trend over continental Europe (+2.7 ± 1.0 %/yr) but also at the national
scale for Spain, Germany, UK, France, and Poland. Sensitivity tests indicate that these simulated
positive trends are mainly due to (i) the trends in NH3 emissions, found heterogeneous in the EMEP
NH3 emissions with strong disparities depending on the country, and (ii) the negative trends in NOx
and SOx emissions. The impact of reductions in NO2 and SO2 emissions on NH3 concentrations
should therefore be taken into account in future policies. This simulated NH3 VCD increase at
the European scale is confirmed by IASI-v3R satellite observations in spring and summer, when
ammonia emissions strongly contribute to the annual budget in accordance with crop requirements.
Nevertheless, there are remaining differences about the significance and magnitude between the
simulated and observed trends at the national scale, and it warrants further investigation
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