Comparing mass balance and adjoint methods for inverse modeling of nitrogen dioxide columns for global nitrogen oxide emissions

TLDR

Satellite observations provide data for top‑down constraints on emission inventories via inverse modeling. The study compares mass‑balance and adjoint inverse modeling methods for estimating NOx emissions from NO2 columns. Synthetic NO2 columns derived from known NOx emissions were used to apply an iterative finite‑difference mass‑balance inversion and an adjoint‑based 4D‑Var inversion, with the latter serving as a benchmark. The iterative mass‑balance approach reduces error by up to a factor of two, matches the adjoint method in resolving horizontal smearing and complex emission changes, and yields similar global inventories with a 3–4× reduction in a priori error, demonstrating comparable accuracy and the feasibility of non‑adjoint models.

Abstract

Abstract Satellite observations offer information applicable to top‐down constraints on emission inventories through inverse modeling. Here we compare two methods of inverse modeling for emissions of nitrogen oxides (NO x ) from nitrogen dioxide (NO 2 ) columns using the GEOS‐Chem chemical transport model and its adjoint. We treat the adjoint‐based 4D‐Var modeling approach for estimating top‐down emissions as a benchmark against which to evaluate variations on the mass balance method. We use synthetic NO 2 columns generated from known NO x emissions to serve as “truth.” We find that error in mass balance inversions can be reduced by up to a factor of 2 with an iterative process that uses finite difference calculations of the local sensitivity of NO 2 columns to a change in emissions. In a simplified experiment to recover local emission perturbations, horizontal smearing effects due to NO x transport are better resolved by the adjoint approach than by mass balance. For more complex emission changes, or at finer resolution, the iterative finite difference mass balance and adjoint methods produce similar global top‐down inventories when inverting hourly synthetic observations, both reducing the a priori error by factors of 3–4. Inversions of simulated satellite observations from low Earth and geostationary orbits also indicate that both the mass balance and adjoint inversions produce similar results, reducing a priori error by a factor of 3. As the iterative finite difference mass balance method provides similar accuracy as the adjoint method, it offers the prospect of accurately estimating top‐down NO x emissions using models that do not have an adjoint.