The HALO aircraft represents a major improvement in the airborne research capability for research institutes and universities to study atmospheric phenomena and their interactions from local to global scales. Aircraft measurements are particularly valuable to describe processes at the scales of transport and photochemistry. The observed spatial variability in clouds, aerosols, water vapour and ozone, for example, ranges from less than 100 m (turbulence) to more than 1000 km (synoptic weather systems). Since oxidation processes in the atmosphere proceed through radical reaction chains, chemical measurements must typically be performed at a time resolution of seconds to minutes. Because of its relatively large size, HALO will facilitate the deployment of comprehensive sets of instrumentation, as developed within Germany or elsewhere, to simultaneously measure physical and chemical parameters to characterise transport, radiation and chemical processes. Furthermore, the long range and high altitude performance greatly increases the fraction of the global atmosphere in which fundamental physical and chemical processes can be directly observed.

Intensive field measurement campaigns that involve a large number of institutions and universities are conducted. The aim is to reduce uncertainties in the understanding of crucial atmospheric processes, in particular: