Quantification of anthropogenic emissions from an urban region: Early results from the Indianapolis Flux Project (INFLUX)

The Indianapolis Flux Project (INFLUX) is a NIST funded project with the goal of developing and assessing methods to quantify greenhouse gas emissions at the urban scale from top-down and bottom-up approaches. Indianapolis was chosen as an ideal test case, since it has relatively straightforward meteorology; a contained, isolated, urban region; and substantial and well-known fossil fuel CO₂ emissions. INFLUX incorporates atmospheric measurements of greenhouse and other trace gases from light aircraft (providing high spatial resolution) and from a network of cell phone towers (providing high temporal coverage) surrounding the Indianapolis urban area. Both platforms make in situ measurements of CO₂, CH₄ and CO are made using cavity ring down spectrometers, and flasks are collected and analyzed for ~55 trace gases and isotopes including CO₂, CH₄, CO, and 14CO₂ (as a proxy for fossil fuel CO₂). Bottom-up inventory estimates from Vulcan and Hestia provide perhaps the best-known fossil fuel CO₂ emissions of any urban region. Modeling efforts span the range of simple plume models to a high-resolution regional inversion using the WRF and LPDM models. The observations and models are used to estimate the urban greenhouse gas emissions, primarily fossil fuel CO₂ and CH₄. The top-down results are compared with the bottom-up inventory data, allowing realistic estimates of overall uncertainties in the top-down approach, as well as improvements in the bottom-up inventory data and methods. The latter part of this presentation will focus on experimental design and flask measurements from the towers. The towers were selected to obtain samples both upwind and downwind of the urban region, so that background mixing ratios can be accurately quantified. A newly developed time-integrated flask sampling system is used to provide hourly averaged flask samples, taken in mid-afternoon only on days when the appropriate wind conditions occur. Trace species associated with urban emissions are consistently enhanced in the downwind samples relative to the upwind site. Trace gases associated with combustion sources, such as CO, fossil fuel CO₂ and hydrocarbons are well correlated. Total CO₂ is also consistently enhanced in the downwind samples, even in summer. In winter, total CO₂ enhancement is slightly higher than the fossil fuel CO₂ enhancement, in agreement with Indiana's requirement for 10% bioethanol use in gasoline.