ALAR: Shepson Atmospheric Chemistry Group: Purdue University

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 accommodate 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


The Northeast Corridor

We have been working with the National Institute of Standards and Technology (NIST) and the University of Maryland (UMD) to also focus on the northeast corridor urban areas, primarily Washington, D.C., Baltimore, and more recently New York City.  This has included many mass balance flights, many of which make use of both UMD and Purdue aircraft to more thoroughly understand emissions.  Most recently our work has involved using modeling to better understand emissions in New York City and how our measurements compare to inventory estimates, as shown below for one of the inventories tested.

Power Plant Pseudo-Controlled Release

ALAR has completed many mass balance experiments downwind of power plants to investigate the potential CH4 emissions.  However, power plants are required to measure their CO2 emissions using Continuous Emissions Monitoring Systems (CEMS) and report them to the air market program data.  This provides us with the opportunity to test our GHG quantification methodology.  We use this dataset of over 50 mass balance experiments downwind of power plants to compare directly to these reported emissions and assess our technique's bias and uncertainties.

Emission rates compared to reportedModeling Turbulence in NYC

TKE scaled NY flight trackPart of quantifying GHG emissions from cities requires accurate model depictions of the airflow in and around those cities. Cities present a rough surface, often rougher than the natural environment, that can create eddies in the atmosphere which affect mixing and vertical motion, and thus GHG dispersion. Turbulent kinetic energy (TKE) is a measure of this turbulence in the atmosphere.

Our group takes high-frequency wind data from ALAR's BAT probe to quantify the TKE around New York City, (example flight on left,) then simulates the atmosphere on those days in the Weather Research and Forecasting (WRF) model to see how well the model simulates the observed TKE. We find that WRF generally underestimates TKE directly downwind of the major skyscrapers in Manhattan (curtain plots below) using several of the different schemes available in WRF for modeling the atmospheric boundary layer.

TKE curtains

Highlights from past ALAR projects: