Progress in quantifying urban greenhouse gas fluxes using atmospheric measurements

Accurate and precise quantification of GHG emissions from cities has emerged as a priority for climate management. Atmospheric measurements have great potential to complement the more widespread approach of quantifying GHG emissions using accounting-based methods. We report on progress in quantifying GHG emissions from the cities of Indianapolis, Indiana, and Baltimore/Washington, both NIST Greenhouse Gas Measurements Test-bed sites, using airborne GHG mole fraction measurements, tower-based GHG mole fraction measurements, and tower-based eddy covariance flux measurements. Strong progress has been made in quantifying CO₂ emissions from Indianapolis during the dormant season. Airborne- and tower-based emissions estimates, and inventory-based methods have been shown to agree to within approximately 10%, a promising result for the application of these methods to climate management problems. Methane emission estimates from Indianapolis initially appeared to diverge significantly, but integration of airborne measurements with mesoscale atmospheric modeling of the regional atmospheric boundary layer and regional methane emissions environment, and careful consideration of the areas represented by various approaches has eliminated a great deal of the apparent discrepancy. The potential for using aircraft data in a more comprehensive urban atmospheric inversion system has been demonstrated in an estimate of GHG emissions from multiple flights around the Baltimore/Washington region. Eddy-covariance flux measurements have been used to evaluate the fine-scale temporal structure of anthropogenic GHG emissions, and generally show impressive consistency with the Hestia inventory estimates that have been developed specifically for Indianapolis. Finally, a multi-year, year-round inversion has been completed and demonstrates the ability to detect long-term trends in Indianapolis CO₂ emissions using our tower-based observing network. Ongoing analyses show considerable complexity in the biogenic CO₂ fluxes in the landscapes both in and around Indianapolis. We will report on progress in characterizing these fluxes in order to improve our ability to solve for anthropogenic emissions. In summary, this collection of research shows great promise for atmospheric monitoring, merged with varying levels of inventory data, to quantify urban GHG emissions at levels of accuracy and precision that are sufficient for climate management applications.