Purdue University: Shepson Atmospheric Chemistry Group: ALAR

Airborne Laboratory for Atmospheric Research


We have developed the capability to routinely measure the fluxes of greenhouse gases using ALAR, an instrumented Beechcraft BE76 Duchess aircraft (top left picture). We focus on quantifying emissions of carbon dioxide (CO2) and methane (CH4) from urban areas and point sources, such as power plants, landfills, and natural gas drilling and holding sites. A Picarro cavity ring-down spectrometer (CRDS; bottom left picture) has permanent residence behind the ALAR's pilot seat for measurements of CO2, CH4, and H2O. The rear two seats of the aircraft have been removed to accomodate the installation of other instruments which we rotate depending on our science questions. 


Real-time (50Hz) vertical wind data is measured using the BAT probe developed by Crawford et al., Bound. Lay. Meteor, 1992.  The vertical wind data is complemented by aircraft altitude measurements using an inertial navigation system and Global Positioning System.  A set of wind tunnel and in-flight experiments were used to calibrate and characterize the vertical wind system to minimize systematic errors caused by airflow measurements that depart from a commonly used theoretical potential flow model.  The results of these vertical wind studies are published in Garman et al., J. Atmos. Ocean. Tech., 2006.

Current Projects:

The INdianapolis FLUX Experiment (INFLUX)

Quantifying greenhouse gas (GHG) emissions from Indianapolis is an ongoing project in our group, part of the Indianapolis Flux Experiment (INFLUX), that we created. INFLUX is a multi-institutional collaborative effort aimed at developing, improving and evaluating techniques to quantify GHG emissions. Because cities are major GHG sources, future attempts at carbon mitigation strategies will likely rely on GHG reduction from cities.  This will require the ability to quantify and verify emission reductions.

As air flows across urban areas, it picks up CO2 and CH4 emissions from point sources such as power plants, cars, and natural gas leaks, as depicted in the schematic below (left).  Flying downwind of the city at multiple altitudes and perpendicular to the prevailing wind direction produces a picture of the urban CO2 and CH4 plume. Using this airborne mass-balance approach (urban enhancement = downwind-background) we can calculate the emission rate of GHG's from urban areas. We show an example mass balance flight path below on the right.

MBE_Schematic  MBE_FlightPath

Quantifying CH4/CO2 Ratios from Natural Gas-Fired Power Plants

CH4 is a short-lived gas, but has a global warming potential much greater than CO2. It has ~85 times the global warming potential of CO2 based on mass over a 20 year-period. Given that CH4 is the primary component of natural gas, many have discussed that a large enough loss of CH4 to the atmosphere from well production to end use could negate the climate benefits of natural gas. Alvarez et al. (2012) estimated that a loss of 3.2% would offset the climate benefits of natural gas.


Natural gas-fired power plants remain under-studied as a potential source of methane loss along this chain of well to user. We measured CH4 and CO2 emissions directly from the stacks and downwind of 15+ natural gas-fired power plants in multiple states using an aircraft-based mass balance technique. An example of the a flight path colored by CO2 conducted downwind of a natural gas-fired power plant is shown above. Lavoie et al. (2017) saw large scale facility-wide leaks of CH4 at three natural gas plants in Utah and Illinois. In more recent work, we have seen only stack-based emissions of CH4 from power plants, and in some cases even saw small losses of CH4 in stack plumes relative to background air. The CH4/CO2 ratio (plot shown below) of stack emissions we’ve measured were approximately 3x larger than the EPA estimates. This work is still ongoing.



Highlights from past ALAR projects:

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