ACCLAIM: an Airborne Science Mission to Study UTLS Aerosol Processes, Properties and Impacts

Aerosol particles in the upper troposphere and lower stratosphere (UTLS) strongly influence climate - both directly through scattering and absorption of radiation, and indirectly through modification of high altitude cirrus clouds. The importance of aerosols in this region is highlighted by model results that suggest UTLS aerosol changes since pre-industrial times contribute as much as 20% to the total anthropogenic aerosol radiative forcing. The Aerosols, Clouds, and CLimAte Interactions Mission (ACCLAIM) will address these important issues by collecting comprehensive in situ measurements of aerosol, meteorological, tracer, and cloud parameters with an airborne platform in targeted UTLS regions. ACCLAIM observations will be used to enhance the quality of retrievals of aerosol parameters from space-borne remote-sensing instruments and the combination of selected in situ and remote-sensing data will allow the critical evaluation of important aerosol parameters and associated UTLS processes controlling those parameters.

Recent work has altered our understanding of the sources and composition of UTLS aerosol, including contributions from the Asian Summer Monsoon and small to moderate volcanic eruptions, as well as significant global contributions from organics and nitrates. The magnitudes of UTLS aerosol direct and indirect radiative forcing, as well as the contributions from the ASM and from organic precursors remain highly uncertain because existing measurements of UTLS aerosol properties are sparse and typically lack the composition measurements needed to identify aerosol precursors and constrain aerosol budgets. As a result of these uncertainties, global models exhibit significant discrepancies in simulated UTLS aerosol abundance and composition. ACCLAIM will provide aerosol size distribution and composition measurements in multiple locations allowing quantification of UTLS aerosol direct and indirect radiative effects and discovery of currently unknown natural and anthropogenic influences on stratospheric aerosol.