Trends, composition, and sources of carbonaceous aerosol at the Birkenes Observatory, northern Europe, 2001–2018
Authors: Yttri Karl Espen , Canonaco Francesco , Eckhardt Sabine , Evangeliou Nikolaos , Fiebig Markus , Gundersen Hans , Hjellbrekke Anne-Gunn , Lund Myhre Cathrine , Platt Stephen Matthew , Prévôt André S. H. , Simpson David , Solberg Sverre , Surratt Jason , Tørseth Kjetil , Uggerud Hilde , Vadset Marit , Wan Xin , Aas Wenche 
Published: May 2021

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 PM10, PM2.5 (fine), and PM10–2.5 (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
(BSOASRT), primary biological aerosol particles (PBAP), biomass burning aerosol
(BB), ammonium-nitrate-dominated aerosol (NH4NO3), and (one low carbon
fraction) sea salt aerosol (SS). We observed significant (p<0.05), large decreases in EC in
PM10 (−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, 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
PM2.5 (−3.9 % yr−1) and PM10 (−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 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 of both elevated eBCBB and eBCFF. Dominating biogenic sources explain why there was no downward trend for OC. A relative increase in the OC fraction in PM2.5 (+3.2 % yr−1)
and PM10 (+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 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 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.