Journal of Geophysical Research: Atmospheres High-resolution atmospheric inversion of urban CO 2 emissions during the dormant season of the Indianapolis Flux Experiment (INFLUX)

Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of greenhouse gases (GHGs), we developed the ﬁrst comprehensive monitoring systems of CO 2 emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012–2013 dormant season showed a statistically signiﬁcant increase of about 20% (from 4.5 to 5.7 MtC ± 0.23 MtC) compared to the Hestia CO 2 emission estimate, a state-of-the-art building-level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to aﬀect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO 2 boundary inﬂow and to the diﬀerent prior emissions (i.e., ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO 2 emissions based on the ensemble mean and quartiles (5.26–5.91 MtC) were statistically diﬀerent compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the diﬀerent parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming suﬃcient data to measure the inﬂow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution. This study presents a high-resolution inversion system based on a Four-Dimensional Data Assimilation meteorological system to simulate the atmospheric dynamics at 1 km resolution over the city of Indianapolis. The inverse emissions were evaluated over 8 months (i.e., September 2012 to April 2013) using two diﬀerent a priori emissions, Hestia, a state-of-the-art building-level emission product and ODIAC, a space-based emission product. The reference inversion produced whole city inverse emissions of 5.5 MtC, about 20% higher than the prior emissions from Hestia (4.6 MtC). Total CO 2 emissions for the Indianapolis urban area based on the ensemble mean and quartiles are 5.26–5.91 MtC, i.e., a statistically signiﬁcant diﬀerence compared to the two prior total emissions of 4.1 to 4.5 MtC. Biogenic ﬂuxes due to soil respiration which are not accounted for in the prior CO 2 emissions could represent about 5% of the total city signals based on Δ 14 CO 2 measurements, or about a quarter of the corrections by the inversion. Single conﬁgurations of the inversion system produced lower posterior uncertainties than the ensemble spread, reﬂecting the uncertainties associated with the various assumptions. Transport model errors were estimated from the WRF-FDDA system and introduced in the inversion system through the use of hourly variances adjusted according to the hourly model performances. The upwind conditions were prescribed by using two towers located at about 20 to 30 km from the city, with an hourly site selection based on wind observations. However, several parameters of the inverse system remain underconstrained, at the origin of the ensemble variability. In particular, spatial structures in prior emission errors, mostly undetermined, aﬀect signiﬁcantly the spatial pattern in the inverse solution, as well as the carbon budget over the urban area. We therefore conclude that atmospheric inversions are able to constrain the carbon budget of the whole city to an absolute uncertainty of about 25%,