Combining aerosol lidar and in-situ methods for the validation of current and future satellite-borne aerosol instruments: Lessons learned from LACE 98, ITOP 2004 and SAMUM 2006

Kurzfassung

The Institute of Atmospheric Physics of DLR has participated in recent years in several field experiments on the vertical aerosol distribution, aerosol radiative effects and long-range transport of aerosol particles. For all experiments, the DLR Falcon 20 was used. The research aircraft was equipped with extensive aerosol microphysical and optical instrumentation, trace gas instrumentation and a nadir-looking multiple-wavelength aerosol lidar. Respective field experiments are the Lindenberg Aerosol Characterisation Experiment LACE in 1998, the experiment on Intercontinental Transport of Ozone and Precursors ITOP in 2004 and the recent studies MEGAPLUME and Saharan Mineral Dust Experiment SAMUM in 2006. Various aerosol types have been investigated during these experiments, such as continental boundary layer aerosol at summer conditions (LACE 98), elevated forest fire smoke plumes from Northern America (LACE 98, ITOP 2004), marine boundary layer aerosol (ITOP 2004), Saharan dust near the source and in the long-range transport regime (SAMUM 2006) urban pollution plumes from North America (ITOP 2004) and Asian pollution plumes (MEGAPLUME 2006) after long-range transport. The experience gained from these field studies covers flight patterns dedicated for remote sensing validation studies, closure studies for aerosol optical properties which use information from the aerosol lidar, a sun photometer and aerosol microphysics, and studies on the vertical distribution of aerosol particles for various aerosol types. Lessons learned concentrate on the requirements for flight patterns, approaches for the determination of aerosol optical properties like the extinction coefficient which are required for closure studies, and requirements concerning the altitude range which should be covered by a research aircraft during satellite validation experiments. The LACE 98 results demonstrated a very good agreement of aerosol optical depths from sun photometry and from aerosol in-situ size-distributions when the scattering coefficient is calculated from the measured size distributions, while the direct measurement of the scattering coefficient by a Nephelometer suffers from inlet problems. The achieved agreement for values calculated from size distributions was better than 30% for the lidar backscatter coefficient and better than 16% for the aerosol extinction coefficient or aerosol optical depth, respectively (Petzold et al. 2002). For a North American forest fire plume probed during ITOP, an aerosol optical depth of 0.13 (@550) was calculated from in-situ size distributions, while values of 0.1 -0.15 were determined from ground-based lidar data (Real, 2006; Petzold et al. 2006). Comparable data analyses for desert dust plumes from SAMUM 2006 are under way. The altitude range required for such studies can be estimated from observations of elevated aerosol layers of various origins: Dust plumes reached altitudes of 5- 6 km above sea level over Southern Morocco. North American forest fire plumes were observed over Europe up to an altitude of 9 km asl. On the other hand, marine aerosol, continental BL aerosol and urban pollution plumes were not observed above 3 km asl during our studies. These values show that in case of long—range transport events (dust, forest fire plumes), the examined altitude range should cover the entire troposphere for mid-latitude conditions. Requirements for future aircraft activities combined with satellite missions will be put up for discussion. Topics are instrumentation, flight patterns and altitude range.