In many locations worldwide aerosol particles have been shown to be made up of organic aerosol (OA). The boreal forest is a region where aerosol particles possess a high OA mass fraction. Here, we studied OA composition using the longest time series of OA composition ever obtained from a boreal environment. For this purpose, we tested a new analysis framework and discovered that most of the OA was highly oxidized, with strong seasonal behaviour reflecting different sources in summer and winter.