Toward Reconciling Bottom‐Up and Top‐Down Estimates of N ₂ O and CH ₄ Emissions in Rotterdam

Abstract Cities are key areas for implementing measures to reduce greenhouse gas emissions; however, large uncertainties exist in both “bottom‐up” inventories and “top‐down” estimates of urban N 2 O and CH 4 emissions. We combine top‐down and bottom‐up approaches to evaluate the N 2 O and CH 4 emission inventory for Rotterdam based on high‐precision measurements made using an aircraft and a mobile vehicle. We divided the Rotterdam area into two sub‐areas: (a) west industrial area (WIA) and (b) Rotterdam urban area (RUA). For the top‐down estimate, the total emissions from Rotterdam are calculated as the sum of quantified emissions from both the WIA and the RUA based on aircraft observations. We further quantified emissions from key source categories including Wastewater Treatment Plants, Traffic and Transportation, Chemical Industry, Energy, and Refineries based on mobile vehicle and aircraft measurements. These measurements revealed that N 2 O and CH 4 emissions were 2–4 and 4–16 times higher than inventory estimates, respectively. We revised the inventory after which the discrepancy between top‐down estimates and Rotterdam's emission inventory decreased from approximately a factor of 4 to about 2 for N 2 O, whereas CH 4 emissions between top‐down and inventory showed no significant difference within the uncertainty. The persistent gap between top‐down and bottom‐up estimates implies the underestimation of N 2 O emissions from other sources (such as sewerage and landfills) and/or the existence of missing sources that are not fully accounted for in the inventory. Our findings highlight the need for further comprehensive quantification across additional source categories and temporal scales.