A Platform for Atmospheric Studies in the Planetary Boundary Layer
In order to understand the influential role of natural and anthropogenic emissions on air quality, climate and ecosystems, formation and transformation processes in the PBL have to be known. The following types of relevant processes are of particular interest:
- gas-phase oxidation processes involving free radicals that directly affect the abundances of reactive gases in the atmosphere and convert part of them into condensable vapours,
- processes that form aerosols either directly or indirectly from gaseous precursors,
- heterogeneous chemical processes that establish a direct feed back of aerosols to the atmospheric gas-phase composition, and result in significant modification of aerosol properties,
- transport processes that distribute primary emissions and secondary oxidation products by advection and turbulent diffusion within the PBL, ventilate trace substances into the free troposphere, or remove material by wet and dry deposition.
The investigation of the complex interactions of gases and aerosols requires a large set of instrumentation for the simultaneous measurement of free radicals, trace gases, aerosol size and composition, solar radiation and meteorological parameters. In the past, field studies with such comprehensive equipment were mostly performed at ground or in the free troposphere on large aircraft. Only very few missions including radical measurements were carried out in-flight in the planetary boundary layer. The main reason is the difficulty to find an appropriate carrier for the complete instrumentation needed to quantify the oxidation chemistry of trace gases and the physico-chemical processing of aerosols. Large airplanes capable of carrying the necessary equipment are not permitted to fly for extended time periods at low altitudes in the PBL, at least not over densely populated regions in Europe. Moreover they are generally moving too fast to allow a reasonable resolution of small-scale spatial patterns as encountered in the PBL within the instrumental response times. Helicopters are not suitable for instruments requiring contamination free air-sampling due to the turbulent airflow around the cabin caused by the rotors. Blimps, which have no internal rigid structure, are not well manoeuvrable and suffer from vibrations since the propellers are directly connected to the cabin. As a novel approach, a large rigid-frame Zeppelin NT has been tested in 2007 by our institute as a platform for chemical and physical observations in the planetary boundary layer and since then used for several flight missions within the project ZEPTER and PEGASOS.
The airship was developed by Zeppelin Luftschifftechnik GmbH & Co. KG (ZLT) in Friedrichshafen, Germany, for multi-mission purposes (passenger transport, advertisement, surveillance, scientific data gathering etc.). It has a length of 75 m, a diameter of about 14 m and a maximum payload of 1.8 ton. With financial support by BMBF, the Zeppelin NT was modified to be used as an airborne platform for atmospheric research.
For field experiments in the lower troposphere the airship offers a unique combination of capabilities, which is not available when employing other aircraft. Important features of the Zeppelin NT are:
- a high scientific payload (instruments + operators): ~ 1 ton
- high manoeuvrability in all directions due to a vectored thrust propulsion system
- flight speed: 0 - 115 km per hour
- horizontal reach: 1111 km
- operating altitude: 20 - 3000 m
- maximum flight endurance: 20 hours
Beside the possibility to install measurement instruments in the gondola beneath the Zeppelin, the rigid framework of the Zeppelin can support additional measuring platforms mountable on the airship. Unlike on airplanes, specially designed aerodynamic sampling inlets for reactive gases and aerosols are not necessary, due to the relative low travelling speed of the Zeppelin. Accordingly, measurement instruments can be operated under similar conditions like at ground.
Based on these features the airship is especially suited to the following applications:
- Measurements of locally limited phenomena with high spatial resolution: the capability of the airship to permanently stay at a fixed position allows for the observation of the time development of events like biomass burning or industrial emission, as well as of the biogenically relevant CO2 and H2O fluxes. Furthermore, flying at low speed allows to resolve small scale patterns over highly structured landscapes and source regions, inside air plumes or between clouds.
- Fast measurements of vertical profiles: since the Zeppelin has a maximum climb and sink rate of 5 - 6 m/s, it will be possible to probe the vertical development of the planetary boundary layer and its transition into the free troposphere with a high time resolution. The possibility to operate at low altitudes allows to measure vertical profiles of trace compounds even in the lowest hundred meters above ground, where the largest gradients are expected. For example, a platform equipped with instruments can be lowered from the cabin, while the Zeppelin is drifting in a stable position (e.g. near ground or above a cloud) to allow for measurements without any disturbance from the body of the airship.
- Measurements along Lagrange trajectories: the airship is able to drift with the surrounding air mass to monitor in-situ the formation and transformation of gases and aerosols. The long operation time allows to determine complete day/night cycles of chemistry and meteorology in these air mass packages above ocean or land regions.
- Long-term measurements: field experiments of about 20 hours endurance are possible at reduced speed and correspondingly low fuel consumption. This enables the repetition of flight patterns with combined vertical and horizontal measurements.