Indianapolis Flux Experiment
Since the Copenhagen accord in 2009, several countries have confirmed their commitment to reduce their greenhouse gas emissions. During the recent United Nations Framework Convention on Climate Change negotiating session held in Paris 2015 (COP21/CMP11), the United States announced a goal of 26-28% reduction in emissions by 2025 compared to 2005 levels, while China committed to a carbon dioxide emissions reduction of 60-65% per unit of gross domestic product by 2030. Europe adopted a reduction target of at least 40% below 1990 levels by 2030 (30-40% below 2005 depending on activity sectors). To achieve such targets, coherent and effective strategies in mitigating atmospheric carbon emissions must be implemented in the next decades. Whether such goals are actually achieved, they require that reductions are "measurable", "reportable", and "verifiable”. Management of greenhouse gas emissions must focus on urban environments since ~74% of CO2 emissions worldwide will be from cities, while measurement approaches are highly uncertain (~50% to >100%).
The Indianapolis Flux Experiment (INFLUX) is a multi-institution, collaborative project that aims to develop and assess top-down and bottom-up approaches for quantifying urban-scale greenhouse gas emissions. Indianapolis was used as a test case because it is a compact, isolated urban center with relatively simple meteorology and a strong fossil fuel CO2 emission (~3.4 MtC yr-1) that is readily detectable in the atmosphere. Greenhouse gases (CO2, CH4) and other trace gases (hydrocarbons and halogenated compounds) are measured using an aircraft-based platform and a network of towers around Indianapolis to provide high spatial coverage and temporal resolution. Both platforms make use of cavity ring-down spectroscopy for in situ measurement of carbon dioxide and methane as well as flask packages for taking intermittent air samples that are analyzed for ~55 trace gases and isotopes including CO2, CH4, CO, and 14CO2, which is used as a proxy for fossil fuel CO2. Using a mass-balance approach, fluxes of CO2 and CH4 downwind of the city are estimated from the aircraft-based measurements. Both tower-based and aircraft-based measurements will be combined with modeling efforts to independently estimate the city-wide CO2and CH4 emissions. The top-down estimates will be compared with the high resolution bottom-up emission inventory from Vulcan and Hestia to constrain the urban-scale CO2 and CH4 budget.
To perform our flight experiments we use our research aircraft: the Purdue Airborne Laboratory for Atmospheric Research (ALAR). By flying perpendicularly to the prevailing wind direction, the mobility of the aircraft allows for multiple horizontal transect measurements (Figure 1) at various altitudes up to the top of the convective boundary layer to rigorously intercept and quantify the methane and carbon dioxide plumes. Figure 1 shows the flight path on June 1, 2011 as a function of the height above the ground. Also shown are the contour shape of Indianapolis and the locations of various CO2 and CH4 sources. From the horizontal transect measurements, the interpolated greenhouse gas (CO2 and CH4) distributions directly downwind of the city are obtained and shown (Figure 2) as functions of the heights above the ground and the horizontal distances. Also shown in Figure 2 are the CO2 and CH4 hotspots corresponding to specific sources in the city, with enhancements in concentrations of about 10 ppm and 60 ppb above the background, respectively.
While CO2 and CO have been previously suspected to come only from fossil fuel combustion during the late fall (Turnbull et al., 2015), Cambaliza et al. (2015) recently demonstrated that the remaining CH4 at Indianapolis, besides coming from the landfill, can be attributed to emissions from the natural gas distribution system only. Additionally, Heimburger et al. (2016) show that repetitive sampling can enable improvement in precision of the aircraft top-down methods through averaging. See below for our most recent publications:
Additional information about INFLUX can be found here.
Cambaliza MOL, Shepson PB, Caulton DR, Stirm B, Samarov D, et al. 2014. Assessment of uncertainties of an aircraft-based mass balance approach for quantifying urban greenhouse gas emissions. Atmos Chem Phys 14: 9029-9050. doi: 10.5194/acp-14-9029-2014. link
Cambaliza MOL, Shepson PB, Bogner J, Caulton DR, Stirm B, et al. 2015. Quantification and source apportionment of the methane emission flux from the city of Indianapolis. Elementa 3: 000037. doi: 10.12952/journal.elemneto.000037link
Heimburger AMF, Shepson PS, Stirm BH, Susdorf C, Turnbull J, et al. 2017. Improving and assessing aircraft-based greenhouse gas emission rate measurements for the city of Indianapolis, as part of the INFLUX project. Elementa, submitted.
Figure 1: Flight path on June 1, 2011 showing the horizontal flight segments perpendicular to the wind direction; also shown are the locations of CO2 and CH4 sources in the city of Indianapolis.
Figure 2: Interpolated carbon dioxide and methane distribution as a function of the height above the ground and horizontal distance along the horizontal flight segments shown in Figure 1. CO2 and CH4 hotspots are observed at specific coordinates along the curtain flight segments. The CO2 and CH4 concentration enhancements above background levels were as much as 10 ppm and 60 ppb, respectively.