Anthropogenic and natural gaseous emissions of chemical substances like VOC, nitrogen, sulfur and halogen species undergo chemical and physical transformations which eventually determine the composition of the atmosphere. Short lived species like OH and peroxy radicals are involved in most of the atmospheric oxidation reactions which control the atmospheric formation and removal of most of the atmospheric constituents (Monks, 2005). In the course of oxidation and photochemical processes, secondary products like ozone, organic nitrates, organic acids or particulate matter are formed, which can have direct and indirect influences on the biosphere, radiation and climate.

Knowledge about radicals and short lived species is therefore crucial for the determination of the oxidising capacity of the atmosphere and the related lifetime and global distribution of air pollutants and green house gases like ozone. The relevance of reactive halogens for the tropospheric oxidising capacity has lately been experimentally confirmed in a wide range of environments (Saiz-Lopez and von Glasow, 2012). Furthermore, complex free radical chemistry is associated with atmospheric oxidations of biogenic hydrocarbons. Furthermore, non photochemical production of radicals in particular in polluted areas is gaining increasing interest (Brown and Stutz, 2012). Overall, discrepancies between modeled and measured radicals in unpolluted, forested regions (Lelieveld et al., 2008) as well as in the presence of high VOC load ing (Hofzumahaus et al., 2009) have recently indicated the necessity to revise our knowledge of radical budgets and recycling mechanisms to improve the confidence of predictions of the current and the future atmospheric composition. In that context, HALO in–situ measurement of gas and photochemical species will significantly enhance the interpretation of ground–based measurements, global and regional modeling and satellite measurements. HALO missions will contribute to the investigation of gas phase processes and photochemistry by focusing on interrelated issues.

Understanding of Radical Budgets in Different Environments: Sources, Sinks and Transformations

The heavy HALO payload, high ceiling and large spatial coverage enable the use of a comprehensive suite of measurements over large and partly unexplored areas due to their difficult accessibility. Therefore HALO missions can provide essential information about:

• Horizontal and vertical distribution of ozone, free radicals like OH, HO2, RO2, NO3 and halogen atoms, and their precursors.
• Extension of oxidation processes at different tropospheric levels, and the related effect in the photochemical O3 production in different layers of the troposphere.
• Global distribution of halogen oxides and effect on the VOC oxidation, on the partitioning of HOx and NOx and on the O3 production in different environments.
• Variability of the oxidising capacity of the tropical troposphere.

Global Transport of Air Pollution

This topic is an important issue in an increasingly contaminated world. In the context of the Kyoto protocol and following international agreements it is important to get accurate information about emission sources, regional and global transport. Whilst the knowledge about gas and aerosol composition and local chemical evolution of urban plumes has improved considerably, transport patterns and scaling of potential impacts are still mainly based on modeling, which is partially supported by satellite data. Apart from the potential input in the regional validation of emission-reduction strategies, HALO missions can effectively provide important knowledge about:

• Extension of the horizontal and vertical transport and chemical transformation of emission plumes, e.g., from major population centers. This comprises the determination of transport and transformation pathways, dispersion and distribution of anthropogenic pollutants as well as natural emissions.
• Investigation of the relation between different precursors affecting photochemical ozone production.
• Effect of emission synergies in regional and global air quality, in the oxidising capacity of the atmosphere and inferred radiative forcing
• Interactions between climate-chemistry due to the influence of pollution and formation of particles on the radiative budget and aerosol microphysics and the associated climatic impact.

Global Transport of Biomass Burning Emissions of Natural and Anthro-pogenic Origin

Some world areas are marked by regular strong biomass burning emissions which may have a relevant impact at the regional and global scale. In that context, significant O3 production rates have been estimated in middle tropospheric African biomass burning plumes transported downwind over the Atlantic Ocean (Mari et al., 2011, Real et al., 2010). Thoroughly planned HALO missions can provide essential input for the characterization of transport and chemical patterns of biomass burning plumes.

Identification of Deep Convection Zones

These zones are characterized by effective injection of pollutants or biogenic emissions into the upper troposphere. Lifetime of some short lived species like reactive halogens increases drastically in the upper troposphere, and, thus, their potential effect in the oxidizing capacity. In that context, the measurement of gaseous species and aerosols can trace the atmospheric dynamics.

Sources and Sinks of Long–lived Greenhouse Gases

Since the beginning of the industrial era, the Earth’s greenhouse gases have been significantly modified by human activities. Continuous growth of the world human population and economies may further increase our impact on the environment and continued increase of Long-Lived Greenhouse Gases (LLGHG) is expected to result in a warmer climate (IPCC, 2013). As the climate, biogeochemical processes and natural ecosystems are closely interlinked, changes in any one of these may affect the others and be detrimental to humans and other organisms. Emissions of man-made gaseous species (and particulate matter) alter the energy balance of the atmosphere, which in turn has implications for the interactions in the system atmosphere – hydrosphere – biosphere. Large uncertainties in budget of the LLGHGs, however, and feedback mechanisms which are, if at all, only partly understood, limit the accuracy of climate change projections. In order to reliably predict the climate of our planet, and to help constrain political conventions on greenhouse gas avoidance, adequate knowledge of the sources and sinks of these greenhouse gases, complex interaction processes, and their feedbacks is mandatory, yet still inadequate (IPCC, 2013). Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have been recognized by the International Panel of Climate Change (IPCC) as the most important of the anthropogenic greenhouse gases (IPCC, 2013). Non-methane halocarbons and other halogenated species are also potent greenhouse gases with some also acting as ozone-depleting compounds. Ozone is also a greenhouse gas in itself, however, due to its high variability it is difficult to identify a global long-term trend of surface O3. In that context, HALO remote sensing and in–situ measurements of long-lived greenhouse gases will contribute to the understanding of processes, improve climate predictability and help validate and interpret satellite measurements.