Properties of forest fire aerosols after transformation processes during long-range transport across the Atlantic Ocean

Abstract

As part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study in summer 2004, a field experiment on the Intercontinental Transport of Ozone and Precursors (ITOP) took place in July/August 2004. In the course of the ITOP study, measurements on the intercontinental transport of aerosol from biogenic and anthropogenic origins across the Atlantic Ocean were performed. The DLR Falcon 20 research aircraft operated from an airport north of Paris, probing the air at the European west coast. The aircraft was equipped with extensive in-situ aerosol and trace gas instrumentation. Additionally, a ground-based aerosol lidar at Palaiseau south of Paris monitored the vertical structure of the atmosphere during the entire observation period. During the ITOP observation period, biomass-burning plumes from Canada and Alaska were transported across the Northern Atlantic very frequently and probed by the Falcon and observed by the lidar. Trajectory analyses permitted the identification of the plume origins for several events. The biomass-burning aerosol layers showed several characteristic properties. The aerosol light absorption coefficient inside the probed biomass-burning layers increased the background value by more than two orders of magnitude. The nucleation mode was completely absent in strong, undiluted biomass-burning layers. Furthermore, the biomass-burning layers had an almost depleted Aitken mode and an enhanced accumulation mode compared to the undisturbed background aerosol. Volatility analyses of Aitken and accumulation mode particles indicate that the dilution process takes place in the Aitken mode. It turned out that the accumulation mode aerosol inside these biomass-burning plumes was completely internally mixed with non-volatile cores, while in the undisturbed background aerosol a considerable fraction of entirely volatile particles was found even in the accumulation mode. The biomass-burning plumes also showed enhanced CO mixing ratios. An overview over the aerosol properties after transformation and mixing processes during long-range transport across the Atlantic Ocean will be given on the basis of a large set of investigated case studies.