| Popis: |
We present 18 years (2001–2018) of aerosol measurements: organic- and elemental carbon (OC and EC), organic tracers (levoglucosan, arabitol, mannitol, trehalose, glucose, 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 PM10, PM2.5 (fine) and PM10-2.5 (coarse) size fractions, providing the opportunity for a nearly two-decade long assessment. Using positive matrix factorisation (PMF) we identify six carbonaceous aerosol sources at Birkenes: Mineral dust dominated (MIN), traffic/industry-like (TRA/IND), short range transported biogenic secondary organic aerosol (BSOASRT), primary biological aerosol particles (PBAP), biomass burning (BB), and ammonium nitrate dominated (NH4NO3), and one low carbon fraction, sea salt (SS). We observed significant (p 10 (−3.9 % yr−1) and PM2.5 (−4.2 % yr−1), and a smaller decline in levoglucosan (−2.8 % yr−1), suggesting that OC / EC from traffic and industry is decreasing, while abatement of OC / EC from biomass burning has been slightly less successful. EC abatement of anthropogenic sources is further supported by decreasing EC fractions in PM2.5 (−4.0 % yr−1) and PM10 (−4.7 % yr−1). PMF apportioned 72 % of EC to fossil fuel sources, further supported by PMF applied to absorption photometer data, which yielded a two-factor solution with a low aerosol Ångstrøm exponent (AAE = 0.93) fraction assumed to be equivalent black carbon from fossil fuel combustion (eBCff), contributing 78 % to eBC mass. The higher AAE fraction (AAE = 2.04) is likely eBC from BB (eBCbb). Source receptor model calculations (FLEXPART) showed that Continental Europe and western Russia were the main source regions both of elevated eBCbb and eBCff. A relative increase in the OC fraction in PM2.5 (+3.2 % yr−1) and PM10 (+2.3 % 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 PM2.5, PM10-2.5 and PM10 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 aerosol composition at Birkenes and thus also in the relative source contributions. The need for diverse off-line and on-line carbonaceous aerosol speciation to understand carbonaceous aerosol sources, including their seasonal, annual, and long-term variability has been demonstrated. |
