A top-down/Bottom-up greenhouse gas quantification experiment in the city of Indianapolis
Paul Shepson (PI), Kevin Gurney, Obie Cambaliza, Purdue University.
Colm Sweeney, Jocelyn Turnbull, NOAA/ESRL, Boulder, CO.
Natasha Miles, Scott Richardson, Thomas Lauvaux, Penn State University.
The Hestia Project has shown that fossil fuel CO2 emissions can be quantified down to the building level utilizing a bottom-up approach (Zhou and Gurney, 2010). Accomplished using the city of Indianapolis as a test case, the approach leverages a variety of data sources and model constructs to optimally estimate emissions in a comprehensive, methodologically consistent manner. While high quality data (fuel use, transportation data, etc.) is also available for most other industrialized nations from which to generate high quality emissions inventories, the data quality is much less certain for most developing nations. Furthermore, even in industrialized contexts, the ability to conform bottom-up inventories with top-down atmospheric measurements remains limited and in the research, rather than operational, realm.
As developing nations increase their total fossil fuel use, this also increases the uncertainty in global estimates of fossil fuel use. For example, inventory data indicates that China surpassed the United States as the largest national emitter of fossil fuel CO2 in 2006 (NEAA, 2007; Gregg et al., 2008), but uncertainties on the Chinese data are ~20% for the annual national totals (vs 3-5% for the US), and substantially higher for finer scale data (e.g. Gregg et al., 2008). Top-down estimates of the emissions can potentially circumvent these issues, not only assessing the reliability of reported emissions, but reducing the uncertainties on the emissions. However, methods to evaluate/verify the emissions estimates are extremely limited. Top-down atmospheric measurements have the potential to constrain fossil fuel CO2 emissions, and to date, such measurements have been made for other trace gases (with less certain emissions), with uncertainties likely not better than ±50% (Trainer et al., 1995), primarily due to uncertainties in wind speed measurements. Fossil fuel derived CO2 is entirely devoid of the rare radioactive isotope 14C, due to its extreme age, whereas all other sources (oceans and terrestrial biosphere) have 14C contents close to that of the atmosphere. Thus the addition of 14C-free fossil fuel CO2 lowers the 14C content of atmospheric CO2 (reported as 14CO2), and these additions can be precisely quantified in atmospheric samples (Riley et al., 2008, Turnbull et al., 2006; Levin et al., 2003; Meijer et al., 1996). In order to translate these measurements into quantified fossil fuel CO2 emissions, they must be convolved with atmospheric transport; to date, little research has been done to achieve this.
The primary goal of this research effort is to use 14CO2 and CO measurements to develop methods to obtain top-down estimates of fossil fuel emissions, and compare these to the high quality Hestia data product, thus quantifying the methodological uncertainties and biases.
The overall research objective for this project is stated as follows:
The uncertainty in area/regional greenhouse gas flux measurements can be ±20% or better, and this uncertainty can be constrained and defined through improved measurement techniques, comparison to inventory data, and use of carbon isotope ratio data.
To accomplish this objective, we are engaged in a three-year effort aimed at improvement of the CRDS/mass balance-based measurement of CO2 and CH4 fluxes, through an extensive improvement of the instrumentation and calibration methods, sampling approach, and wind measurement methods. Evaluation of the measurements will make use of the Hestia inventory for Indianapolis, and use of 14C measurements for whole-air samples obtained during the experiments from both the aircraft (see figure 1) and the surface (see figure 2) around the city of study. Here we propose a new approach to area-wide flux measurements that combines highly accurate aircraft-based boundary layer CO2 and CH4 concentration and wind speed measurements, with 14CO2 and CO measurements to quantify the absolute fossil fuel emissions. These will be compared with emission model results to improve and define the uncertainty of measurements and models. The field measurements will be conducted in Indianapolis, IN, over years two and three of the study. The results of this effort will define a new protocol for greenhouse gas emissions flux measurements that can act as a standard against which our understanding of regional greenhouse gas emissions can be assessed. Such a capability is critically important to the process of defining defensible approaches to emissions reduction strategy development and an international template for monitoring, reporting and verification (MRV).
Figure 1: ALAR aircaft over an instrumented tower
Figure 2: Proposed instrumented tower locations