Trends, composition, and sources of carbonaceous aerosol at the Birkenes Observatory, northern Europe, 2001–2018

Abstract. We present 18 years (2001–2018) of aerosol measurements, including organic and elemental carbon (OC and EC), organic tracers (levoglucosan, arabitol, mannitol, trehalose, glucose, and 2-methyltetrols), trace elements, and ions, at the Birkenes Observatory (southern Norway) – a site representative of the northern European region. The OC / EC (2001–2018) and the levoglucosan (2008–2018) time series are the longest in Europe, with OC / EC available for the PM 10 , PM 2.5 (fine), and PM 10–2.5 (coarse) size fractions, providing the opportunity for a nearly 2-decade-long assessment. Using positive matrix factorization (PMF), we identify seven carbonaceous aerosol sources at Birkenes: mineral-dust-dominated aerosol (MIN), traffic/industry-like aerosol (TRA/IND), short-range-transported biogenic secondary organic aerosol (BSOA SRT ), primary biological aerosol particles (PBAP), biomass burning aerosol (BB), ammonium-nitrate-dominated aerosol (NH 4 NO 3 ), and (one low carbon fraction) sea salt aerosol (SS). We observed significant ( p ), large decreases in EC in PM 10 ( −3.9  % yr −1 ) and PM 2.5 ( −4.2  % yr −1 ) and a smaller decline in levoglucosan ( −2.8  % yr −1 ), suggesting that OC / EC from traffic and industry is decreasing, whereas the abatement of OC / EC from biomass burning has been slightly less successful. EC abatement with respect to anthropogenic sources is further supported by decreasing EC fractions in PM 2.5 ( −3.9  % yr −1 ) and PM 10 ( −4.5  % yr −1 ). PMF apportioned 72 % of EC to fossil fuel sources; this was further supported by PMF applied to absorption photometer data, which yielded a two-factor solution with a low aerosol Angstrom exponent (AAE  =  0.93) fraction, assumed to be equivalent black carbon from fossil fuel combustion (eBC FF ), contributing 78 % to eBC mass. The higher AAE fraction (AAE  =  2.04) is likely eBC from BB (eBC BB ). Source–receptor model calculations (FLEXPART) showed that continental Europe and western Russia were the main source regions of both elevated eBC BB and eBC FF . Dominating biogenic sources explain why there was no downward trend for OC. A relative increase in the OC fraction in PM 2.5 ( +3.2  % yr −1 ) and PM 10 ( +2.4  % yr −1 ) underscores the importance of biogenic sources at Birkenes (BSOA and PBAP), which were higher in the vegetative season and dominated both fine (53 %) and coarse (78 %) OC. Furthermore, 77 %–91 % of OC in PM 2.5 , PM 10–2.5 , and PM 10 was attributed to biogenic sources in summer vs. 22 %–37 % in winter. The coarse fraction had the highest share of biogenic sources regardless of season and was dominated by PBAP, except in winter. Our results show a shift in the aerosol composition at Birkenes and, thus, also in the relative source contributions. The need for diverse offline and online carbonaceous aerosol speciation to understand carbonaceous aerosol sources, including their seasonal, annual, and long-term variability, has been demonstrated.