UK Ammonia Emissions Estimated With Satellite Observations and GEOS-Chem

Agricultural emissions of ammonia (NH₃) impact air quality, human health, and the vitality of aquatic and terrestrial ecosystems. In the UK, there are few direct policies regulating anthropogenic NH₃ emissions and development of sustainable mitigation measures necessitates reliable emissions estimates. Here, we use observations of column densities of NH₃ from two space-based sensors (IASI and CrIS) with the GEOS-Chem model to derive top-down NH₃ emissions for the UK at fine spatial (∼10 km) and time (monthly) scales. We focus on March-September when there is adequate spectral signal to reliably retrieve NH₃. We estimate total emissions of 272 Gg from IASI and 389 Gg from CrIS. Bottom-up emissions are 27% less than IASI and 49% less than CrIS. There are also differences in seasonality. Top-down and bottom-up emissions agree on a spring April peak due to fertilizer and manure application, but there is also a comparable summer July peak in the top-down emissions that is not in the bottom-up emissions and appears to be associated with dairy cattle farming. We estimate relative errors in the top-down emissions of 11%–36% for IASI and 9%–27% for CrIS, dominated by column density retrieval errors. The bottom-up versus top-down emissions discrepancies estimated in this work impact model predictions of the environmental damage caused by NH₃ emissions and warrant further investigation.