Fig 1.Airborne Laboratory for Atmospheric Research (ALAR). An outfitted twin- engine Beechcraft Duchess jointly owned by the Aviation Technology and the Department of Chemistry at Purdue University.
We have been funded by NSF’s Biocomplexity in the Environment program, as well as the Showalter Trust, to develop the capability to conduct measurements of fluxes of volatile organic compounds (VOCs) from a light aircraft (Figure 1). Our objectives include developing the capability to conduct fluxes of biogenic VOCs, isoprene, and CO2, using a conditional sampling technique known as disjunct eddy accumulation (Rinne et al., 2000).
Real-time (50Hz) vertical wind data is measured using a nine-holed pressure port probe, also known as the “Best Air Turbulence” (BAT) probe (Crawford et al., 1992; Figure 2). The vertical wind data is complemented by aircraft attitude 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.
Fig 2.Best Air Turbulence” (BAT) wind turbulence probe and the fast ultra- sensitive temperature (FUST) probe on ALAR.
A more detailed explanation of the wind data characterization can be found in Garman et al., 2006. The wind system coupled with a fast temperature measurements are used to characterize the turbulence and stability of the boundary layer and to calculate and interpret fluxes (Figure 3).
Cloud water was collected during summer 2005 over the forested region of the northern part of the Lower Peninsula of Michigan, using the ALAR aircraft and a modified Mohnen Slotted Rod collector (Figure 4). These experiments were used to test the hypothesis that processing of atmospheric nitrogen by clouds may alter the type and amount of nitrogen that can be deposited by wet and dry deposition to a forest. It was shown that cloud type and air mass origin affect observed cloud water concentrations of inorganic ionic nitrogen but has no influence on dissolved organic nitrogen concentrations (Figure 5).
Fig 3.Vertical profile of temperature during the ascent (light red) and descent (dark red) and CO concentration (black).
Collaborative efforts between Atmospheric Observing Systems (AOS), inc. and ALAR have resulted in reliable CO2 flux measurements over the Ameriflux and PROPHET towers in Pellston, MI. A non-dispersive infrared CO2 instrument developed and built by AOS, inc. that is relatively insensitive to motion was installed on the Beechcraft Duchess and data were collected at 2Hz over a relatively homogenous deciduous northern hardwood forest. These data represent the first flux measurements from the airborne flux platform. Figure 6 shows a vertical profile of CO2 fluxes on July 21, 2006. Aircraft fluxes were measured at 150, 210, and 275m above the ground and were compared to CO2 fluxes measured on top of the 46m Ameriflux tower. As expected (e.g. Gioli et al., 2004), the magnitudes of the CO2 fluxes decrease with increasing height since CO2 uptake by the vegetation is the dominant process driving the flux. A more detailed analysis of the flux uncertainties and flux profiles is on-going.
Fig 4.A modified Mohnen Slotted Rod cloud water collector.
The ALAR research team, in collaboration with NOAA and AOS, is taking part in the Mid-Continent Intensive by providing high-precision CO2 concentration measurements along horizontal and vertical transects within the boundary layer across a ring of stationary tower CO2 concentration platforms in the upper Midwest (Figure 7). These measurements will be used to evaluate the horizontal and vertical heterogeneity of CO2 concentration across the sample domain. A Lagrangian flux experiment will be conducted to estimate the CO2 fluxes across the sampling domain to validate and compare “top-down” atmospheric approaches to “bottom-up” budgets and inventories. Test flights have already been conducted with the CO2 installed on the aircraft (Figure 3) and the flights will be conducted in June 2007.
Fig 5.Concentration of each measured ion and estimated liquid water content in the sampled clouds reported as a function of altitude above cloud base. All species except pH and liquid water content are reported in units of nmol m 3 of cloudy air. Cloudy air concentrations are calculated using the estimated liquid water content of the cloud and the measured cloud water concentration. Open symbols denote convective cumulus clouds, and solid symbols indicate clouds with little convection (stratus and fair weather cumulus (FWC)). A typical error bar is shown in each plot, as the errors are quite large due to the estimated liquid water content.
A Disjunct Eddy Accumulation (DEA) sampler was built in collaboration with NCAR and is currently being characterized in the lab (Figure 8). It will be installed on the aircraft during the summer of 2007.
The sampler will be evaluated by comparing CO2 fluxes measured using the DEA with eddy covariance measurements. Eddy covariance CO2 fluxes will be measured using a modified Licor 7000 non-dispersive infrared gas analyzer. This instrument is pressure and temperature controlled to minimize response of the instrument to changing ambient conditions of the aircraft.
Fig 6.Vertical profile of CO2 fluxes during the summer of 2006 over the Ameriflux tower in Pellston, MI. The aircraft fluxes were averaged over 3 laps (~24km) above the tower at each altitude. The tower fluxes (height = 46m) are 30min averages of 10Hz data that overlap the time at which the aircraft fluxes were averaged.