Optical properties and chemical composition of stratified aerosol layers and aerosol spatial/temporal variability in springtime Arctic

Description

The objective of this work is to investigate the variability in the particle number concentration (Na) as well as to study the optical and physicochemical properties of stratified aerosol layers that may affect climate change assessment for Arctic regions. The Indirect and Semi-Direct Aerosol Campaign (ISDAC) was conducted in April 2008, in the vicinities of Fairbanks and Barrow, Alaska. Variability in Na for cloud-free conditions was observed for both vertical profiles and constant altitude flight legs. Observations from four flights during ISDAC were used to examine properties of aerosol particles in stratified Arctic layers. Flight 31 on April 26, with the lowest overall number concentrations (compared to the cases studied here, with Na<250 cm-3), had particle size distribution mode diameters of 0.15–0.2 μm, and the chemical compositions were varied mixes of organics and sulfates with some indications of biomass burning (BB) influence. On April 8 (Flight 15), the aerosol properties resembled more typical Arctic Haze conditions with relatively high levels of soot and sulfate and lower levels of organics. On April 19¬–20, two flights (Flight 25 and 26) demonstrated strong BB influences, with Na exceeding 2000 cm-3, and larger modal diameters within the BB plumes were observed, reaching 0.3 μm. It is possible that the larger BB plumes were influenced by significant contributions from secondary organic aerosol production during transport from the source region leading to larger particle sizes within the BB plume. During the BB plumes, the absorption had increased significantly, but the single-scattering albedo was 0.94–0.95, indicating that despite the higher absorption, these plumes still contained significant amounts of materials that scatter efficiently. In the lower Na cases, such as Flight 31, the single-scattering albedo was 0.88–0.91, suggesting overall more absorption than the polluted cases. This may be explained by deposition of the higher scattering material (which may have been more soluble), and thus the higher absorption material may have remained. It is concluded that the variation in the particle light-scattering coefficient among these transported aerosol layers indicates the importance of accurately representing the evolution of the particle-size distributions for cloud-climate studies.