4D‐Var Inversion of European NH 3 Emissions Using CrIS NH 3 Measurements and GEOS‐Chem Adjoint With Bi‐Directional and Uni‐Directional Flux Schemes
Résumé
We conduct the first 4D-Var inversion of NH3 accounting for NH3 bi-directional flux, using CrIS
satellite NH3 observations over Europe in 2016. We find posterior NH3 emissions peak more in springtime
than prior emissions at continental to national scales, and annually they are generally smaller than the prior
emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH 3
emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close
(<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the
Netherlands, and Switzerland are generally 10%–20% smaller than when treating NH 3 fluxes as uni-directional
emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly
mean in-situ observations, our posterior NH 3 emissions from both schemes generally improve the magnitude
and seasonality of simulated surface NH 3 and bulk NHx wet deposition throughout most of Europe, whereas
evaluation against hourly measurements at a background site shows the bi-directional scheme better captures
observed diurnal variability of surface NH3. This contrast highlights the need for accurately simulating
diurnal variability of NH 3 in assimilation of sun-synchronous observations and also the potential value of
future geostationary satellite observations. Overall, our top-down ammonia emissions can help to examine the
effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping
us understand the uncertainty in top-down NH 3 emissions estimates associated with treatment of NH3 surface
exchange