Fig. 4: Major source regions of long-range transported Arctic OA factors.

Merged results from the CWT-based back-trajectory (BT) analysis with ZeFir (Methods) at different Arctic stations (Supplementary Text 5 and Supplementary Fig. 11) showing long-term pan-Arctic hot spots of transported anthropogenic-dominated (Haze and POA) and natural-dominated (MSA-OA and BSOA) OA factors. The entire time series of each factor mass concentration at the different stations (time periods shown in Supplementary Table 1 and Supplementary Fig. 10) were used to create the maps (see Supplementary Text 5 for a discussion of the potential uncertainties in the source regions). The trajectories represent 5 days back in time for MSA-OA and (up to) 10 days for the other factors (Supplementary Text 5 and Supplementary Fig. 11). Colour scales indicate the water-soluble factor concentrations linked to the major source regions (‘long-range’ probability heat maps). The individual station results shown in Supplementary Fig. 11 were merged for each factor, except for POA, for which only six stations with winter data were considered here (no GRU and TIK), to indicate specific regions with intense gas-flaring activity during winter (for example, the Komi Republic, Khanty-Mansisk and Yamalo-Nenets autonomous districts in West Siberia). PBOA is expected to reside mainly in the coarse aerosol mode, and thus has a relatively short atmospheric lifetime (and hence more local and/or regional origins), and the formation of OOA might be linked to a prior accumulation of volatile organic compounds (thus probably not directly transported in the particle phase); therefore, the merged results for these factors are shown only in the Supplementary Information (Supplementary Fig. 11). The World Maps available with ZeFir are taken from Natural Earth Data.