HALO provides new prospects for future research in the area of atmospheric aerosol particles. The ceiling of the aircraft allows studying aerosol in the TTL, in the upper free troposphere and in the lower stratosphere. Aerosol layers, volcanic ash plumes, desert dust aerosols or biomass burning plumes can be investigated in-situ or remotely with HALO. The long endurance of HALO enables to follow aerosol plumes around the globe and to study transformations during the transport of aerosol particles and their homogeneous and cloud related chemical processing.

HALO is capable of carrying a comprehensive scientific and highly sophisticated instrumental payload, which will facilitate a far more complete characterization of the atmospheric aerosol system by combining state–of–the–art in–situ and remote sensing instrumentation simultaneously on a single airborne platform.
There are many outstanding scientific problems with regard to atmospheric aerosol particles (e.g., climate effects, aerosol-cloud interaction, heterogeneous chemistry, and many more) that require the new opportunities offered by HALO.

A brief selection of critical issues in aerosol research that can be addressed by HALO–based observations:

Desert Dust Aerosol Particles: Deserts emit significant amounts of dust particles into the atmosphere and, thus, constitute one of the major sources of aerosol particles on the globe. The Sahara alone as one of the largest sources for soil–derived dust particles emits in the range of 600–1000Mt per year into the atmosphere. These huge dust loads influence directly and indirectly the atmospheric radiation budget. Desert dust constitutes the most important source of IN in the atmosphere. The emitted dust particles regionally impact surface temperature, atmospheric stability and, as a consequence, they may influence the development of tropical and extra–tropical cyclones, which may alter the hydrological cycle and the regional and global climate. Airborne soil dust particles from deserts supply nutrients to oceanic and terrestrial ecosystems; they may contribute to neutralization of acid rain and influence heterogeneous atmospheric chemistry.

Vertical Transport of Aerosol Particles: Most of the transport of aerosol particles and precursor gases into the stratosphere takes place in the tropics. Occasionally, deep convective Cumulonimbus (Cb) clouds may lift particles, together with water vapor and other trace gases into the TTL. However, usually the tops of the tropical deep convective Cb clouds do not reach the TTL region and their anvils spread below it. The TTL ranges between 13 and 18 km height, even with the high ceiling altitude of HALO the upper parts of the TTL cannot be reached. However, the lower parts are accessible with HALO and, thus, the tropical TTL will be an important and challenging area for studies of aerosol and aerosol transport in future HALO missions.

Radiative and Resulting Dynamic Effects of Aerosol: The impact of aerosol particles on the solar and terrestrial radiative energy budget has extensively been studied on a regional basis. However, due to the high variability of the particles in space and time, global and long–term estimates of their radiative impact are only possible with space–borne sensors. Satellites measure radiances and not directly irradiances which would be required to derive the radiative forcing of aerosol particles. The respective retrieval algorithms are uncertain and often not sufficiently tested by dedicated aircraft experiments. Thus, satellites cannot directly measure the aerosol direct radiative impact. Furthermore, related effects on atmospheric dynamics and stability (due to backscattering and absorption of solar radiation, and emission of terrestrial radiation) cannot be addressed by satellite observations. HALO may serve as a unique tool to validate satellite retrievals of aerosol radiative forcing and to improve the accuracy of satellite derived estimates of the direct solar radiative forcing of aerosol particles. It also offers the measurement capabilities to correlate the radiative with dynamic and stability effects. To reach these objectives new instrumentation and in particular also revised measurement strategies have to be developed.

Chemical Composition of Tropospheric Aerosol Particles: There are general characteristic features in the chemical composition of aerosol particles. Most of the sub–micrometer particles consist of sulfate, nitrate, organic and black carbon compounds, and ammonium. The chemical composition of larger particles is dominated by dust or sea salt. However, the specific particle composition is variable and depends on geographical location, anthropogenic and natural sources as well as altitude. The chemical composition of the particles has important effects on, e.g., the ability of the particles to serve as Cloud Condensation Nuclei (CCN) or IN. The chemical composition also determines the refractive index of the particles and, thus, also influences their radiative impact. Sophisticated instrumentation (in particular mass spectrometers) has recently been developed to study the chemical composition and physical properties of the particles.

Interaction of Aerosol Particles with Ice and Mixed–Phase Clouds: The processes involved in the formation of ice clouds are still not well understood. One open question is how many and which of the available aerosol particles may serve as nuclei for ice particle formation. In this regard the chemical properties, the density of surface activation sites, and the size of the IN are of special importance. Also it is unclear to which extent homogeneous or heterogeneous ice nucleation takes place during cirrus cloud formation in the upper troposphere.

New Particle Formation: The formation of new particles occurs frequently throughout the atmosphere. Sulfuric acid plays a major role as the key nucleating agent in the atmosphere. Numerous scientific questions exist with respect to the understanding of the fundamental nucleation and early growth processes, particularly in the free troposphere and in the TTL region because observations are scarce, instrumentation in the past was very limited (for example, to the detection of particles > 3 nm in diameter) and hardly any observations exist in which the fundamental parameters were all measured simultaneously. Such an investigation would require at least the measurement of gaseous H2SO4, water vapor, ultrafine particle concentration in the 1–5 nm size range, and of the condensational sink by preexisting particles. It is not understood under which circumstances efficient new particle formation takes place. It is suspected that especially in the outflow of deep convection, in and above clouds nucleation could be taking place. A thorough assessment of the role of deep convection and clouds for aerosol nucleation is needed.           Another major question concerns the role of ions for nucleation processes. To determine the role of ion-induced aerosol nucleation on upper tropospheric and TTL aerosol constitutes a scientific challenge that can be tackled and solved with HALO and the innovative instrumentation that has just become available for field measurements. Furthermore, besides the influence of ions, also the role of competing nucleation mechanisms, such as ternary nucleation with either ammonia or certain organic compounds as the third nucleating agent besides sulfuric acid and water has to be assessed by direct measurement.