Aerosol and trace gas measurements are strongly affected by the ship exhaust and other human activities (e.g., helicopter flights). Therefore, it is important to discriminate between clean and polluted periods. This mask provides an indication of clean periods that were unaffected by human activities and are therefore representative of background Arctic conditions. This mask was specifically developed for the new particle formation inlet used during the campaign, other inlets located in different places may be affected by pollution in slightly different ways. This mask refers to measurements that were performed on the 4th deck of icebreaker Oden during August and September 2018 along the track of the Arctic Ocean 2018 expedition.