Transport of mineral dust into the Arctic : Evaluation of two reanalysis datasets of atmospheric composition

Sammanfattning: The main purpose of this thesis is to examine the mineral dust aerosol transport into the Arctic. Two three-dimensional reanalysis datasets of atmospheric composition, the Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA) and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), are analyzed with regard to dust transport into the Arctic. The reanalyses agree on that the largest mass transport of dust into the Arctic occurs across western Russia during spring and early summer, although large dust transport events can occur across other geographical areas during all seasons. In several aspects, the reanalyses show substantial differences. The transport in CAMSRA is considerably smaller, more concentrated and occurs at lower altitudes. Furthermore, the transport in CAMSRA is to a larger extent than MERRA-2 driven by well-defined events of dust transport in space and time.The reanalysis data are compared with surface measurements of dust in the Arctic and dust extinction satellite retrievals from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The comparison indicates that CAMSRA underestimates the dust transport into the Arctic and that MERRA- 2 likely overestimates it. The discrepancy between CAMSRA and MERRA-2 can in part be explained by the assimilation process where too little dust is assimilated in CAMSRA while MERRA-2 overestimates the production of light particles, causing an excessive transport, and the assimilation process further increases the dust concentration in remote areas. Despite the clear differences between the reanalyses, this study provides new insights into the spatio-temporal distribution of the dust transport into the Arctic and the transported mass is estimated to be within the range 1.5–31 Tg yr-1.The thesis also briefly examines the aerosol transport of all five aerosol species carried by the reanalyses, that in addition to dust are black carbon, organic matter, sea-salt and sulfate. The annual aerosol mass transport to the Arctic in CAMSRA and MERRA-2 are 24 Tg and 50 Tg respectively. The reanalyses show substantial differences regarding the proportions of the different aerosol types — emphasizing that it is crucial that the aerosol module manages to simulate the correct aerosol mass fractions, as the assimilation of AOD alone cannot change the proportions between the aerosols.