Combining Top‐Down and Bottom‐Up Approaches to Evaluate Recent Trends and Seasonal Patterns in UK N₂O Emissions

Atmospheric trace gas measurements can be used to independently assess national greenhouse gas inventories through inverse modeling. Atmospheric nitrous oxide (N 2 O) measurements made in the United Kingdom (UK) and Republic of Ireland are used to derive monthly N 2 O emissions for 2013–2022 using two different inverse methods. We find mean UK emissions of 90.5 ± 23.0 (1 σ ) and 111.7 ± 32.1 (1 σ ) Gg N 2 O yr −1 for 2013–2022, and corresponding trends of −0.68 ± 0.48 (1 σ ) Gg N 2 O yr −2 and −2.10 ± 0.72 (1 σ ) Gg N 2 O yr −2 , respectively, for the two inverse methods. The UK National Atmospheric Emissions Inventory (NAEI) reported mean N 2 O emissions of 73.9 ± 1.7 (1 σ ) Gg N 2 O yr −1 across this period, which is 22%–51% smaller than the emissions derived from atmospheric data. We infer a pronounced seasonal cycle in N 2 O emissions, with a peak occurring in the spring and a second smaller peak in the late summer for certain years. The springtime peak has a long seasonal decline that contrasts with the sharp rise and fall of N 2 O emissions estimated from the bottom‐up UK Emissions Model (UKEM). Bayesian inference is used to minimize the seasonal cycle mismatch between the average top‐down (atmospheric data‐based) and bottom‐up (process model and inventory‐based) seasonal emissions at a sub‐sector level. Increasing agricultural manure management and decreasing synthetic fertilizer N 2 O emissions reduces some of the discrepancy between the average top‐down and bottom‐up seasonal cycles. Other possibilities could also explain these discrepancies, such as missing emissions from NH 3 deposition, but these require further investigation.