Citation:

Cao, H., D. Henze, L. Zhu, M. Shephard, K. Cady-Pereira, E. Dammers, M. Sitewell, N. Heath, C. Lonsdale, J. Bash, K. Miyazaki, C. Flechard, Y. Fauvel, R. Wichink-Kruit, S. Feigenspan, C. Brümmer, F. Schrader, M. Twigg, S. Leeson, Y. Tang, A. Stephens, C. Braban, K. Vincent, M. Meier, E. Seitler, C. Geels, T. Ellermann, A. Sanocka, AND S. Capps. 4D-Var Inversion of European NH3 Emissions Using CrIS NH3 Measurements and GEOS-Chem Adjoint With Bi-Directional and Uni-Directional Flux Schemes. JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES. American Geophysical Union, Washington, DC, 127(9):e2021JD035687, (2022). https://doi.org/10.1029/2021JD035687

Impact/Purpose:

This study presents the first 4D-Var inversion of NH3 sources using a bidirectional NH3 flux scheme and CrIS NH3 measurements. The posterior annual anthropogenic NH3 emissions have a similar spatial distribution as the prior emissions, but are generally smaller over central Europe and larger over most of the rest of Europe compared to the prior emissions. The posterior monthly emissions generally have a more pronounced springtime peak than the prior. The posterior bidirectional regional and national total anthropogenic NH3 emissions are generally less than than the posterior unidirectional emissions by 10% to 20% for EU25, the UK, the Netherlands, and Switzerland at an annual scale, while up to -34% difference is found at a monthly scale. These differences can provide a rough estimate of the uncertainty associated with NH3 flux estimates in previous inverse modeling studies using unidirectional schemes only.

Description:

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 NH3 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 NH3 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 NH3 emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH3 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 NH3 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 NH3 emissions estimates associated with treatment of NH3 surface exchange.

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