Abstract
Abstract We report the CH4 emission flux from the city of Indianapolis, IN, the site of the Indianapolis Flux Experiment (INFLUX) project for developing, assessing, and improving top-down and bottom-up approaches for quantifying urban greenhouse gas emissions. Using an aircraft-based mass balance approach, we find that the average CH4 emission rate from five flight experiments in 2011 is 135 ± 58 (1σ) moles s-1 (7800 ± 3300 kg hr-1). The effective per capita CH4 emission rate for Indianapolis is 77 kg CH4 person-1 yr-1, a figure that is less than the national anthropogenic CH4 emission (∼91 kg CH4 person-1 yr-1) but considerably larger than the global figure (∼48 kg CH4 person-1 yr-1). We consistently observed elevated CH4 concentrations at specific coordinates along our flight transects downwind of the city. Inflight investigations as well as back trajectories using measured wind directions showed that the elevated concentrations originated from the southwest side of the city where a landfill and a natural gas transmission regulating station (TRS) are located. Street level mobile measurements downwind of the landfill and the TRS supported the results of aircraft-based data, and were used to quantify the relative contributions from the two sources. We find that the CH4 emission from the TRS was negligible relative to the landfill, which was responsible for 33 ± 10% of the citywide emission flux. A regression of propane versus methane from aircraft flask samples suggests that the remaining citywide CH4 emissions (∼67%) derive from the natural gas distribution system. We discuss the combination of surface mobile observations and aircraft city-wide flux measurements to determine the total flux and apportionment to important sources.
Highlights
We find that for small area and point sources, the variability in the estimated emissions ranged from 10% to 50%, with an average precision of 30% (Cambaliza et al, 2014), which is at the low end of reported uncertainties of previous studies that made use of an aircraft-based mass balance approach to quantify the emissions from urban environments or small area sources
In a separate manuscript (Cambaliza et al, 2014), we examined the uncertainties in our aircraft-based mass balance approach, and estimated the uncertainty of the citywide emissions to be ± 50% with the largest uncertainties contributed by the variability in the background concentrations and boundary layer depth
We report the Hestia fossil fuel CO2 flux in Table 3 corresponding to the flight dates and the mass balance CH4/CO2 emission flux ratios
Summary
Methane (CH4) is the second most important anthropogenic greenhouse gas, contributing ∼0.48 W m–2 (∼17%) to the total direct radiative forcing by long-lived greenhouse gases Most sources and sinks of methane are identified but their contributions to the total global emission remain uncertain (Kirschke et al, 2013). Global inverse modeling efforts (Mikaloff Fletcher, et al, 2004; Chen and Prinn, 2005; Neef et al, 2010) show that the total emission from anthropogenic sources currently accounts for ∼60% of the source budget. Because of its relatively short lifetime (9.1 – 11.2 years, Myhre et al, 2013) and its large global warming potential, reduction in its emission may have a significant effect on the climate system in the near term (Montzka et al, 2011)
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