Fig. 1: Sites, OA factors and chemical characteristics.

a, Arctic political map showing the aerosol filter sampling stations (Supplementary Text 1 and Supplementary Table 1). A, Alert; B, Cape Baranova; G, Gruvebadet; P, Pallas-Matorova; T, Tiksi; U, Utqiaġvik; V, Villum Research Station; Z, Zeppelin. Adapted from Hugo Ahlenius/GRID-Arendal (https://www.grida.no/resources/8378). b, Station-specific average total OA mass concentrations (whiskers are 1 standard deviation, s.d., corresponding to sample-to-sample variability) based on a statistical analysis of the water-soluble fraction to obtain factor recoveries (Supplementary Text 4) in the polar night (winter) versus midnight sun (summer) periods (Supplementary Table 2), sorted in descending order of the station annual average, and percent contribution to the particulate mass (including non-sea salt sulfate, nitrate, ammonium, EC and estimated sea salt⁶³). The dashed blue lines connect winter and summer contributions at each station (no winter samples for G and T, and U winter samples were not analysed for ions). c, Van Krevelen plot as a tool for compositional differentiation among samples: atomic O:C ratio versus H:C ratio of the Arctic AMS-PMF-based factors (Supplementary Fig. 2), and individual PMF input bulk samples (colour coded by month: 1, January, through to 12, December). Red and blue dashed curves refer to the triangle reported by Ng et al.⁸⁵. Grey dashed lines denote two example oxidation states (OS). Error bars correspond to 1 s.d. from a bootstrap analysis (Methods and Supplementary Text 4). d, Spatial distribution and seasonal variability in the average factor percentage contributions to total OA (water-soluble (Supplementary Fig. 7); entire time-series (Supplementary Fig. 10)). Factors are sorted from bottom (Haze) to top (POA) based on their onset (see Fig. 3), starting from late winter for Haze. Primary OAs, POA + PBOA (top). Orange outline, sum of natural-dominated OAs.