Abstract
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.
Highlights
Carbonaceous aerosol has been studied intensively over the last 20 years due to its influence on radiative forcing (Bond et al, 2013; Myhre and Samset, 2015; Lund et al, 2018), both directly by scattering and absorption of sunlight, and semi-directly and indirectly by influencing cloud properties (Boucher et al, 2013; Hodnebrog et al, 2014; Myhre et al, 2013)
European Monitoring and Evaluation Programme (EMEP) included OC/EC measurements in 2004 after a pioneering measurement campaign at 12 European sites from 2002 to 2003 (Yttri et al, 2007a; Tørseth et al, 2012), showing that carbonaceous aerosol was a major constituent of the ambient aerosol in the European rural background environment, accounting for 9 %–37 % (OM, organic matter) and 1 %–5 % (EC) of PM10, and that OM was more abundant than sulfate (SO24−) at sites reporting both variables (Yttri et al, 2007a)
The present study focuses on carbonaceous aerosol, using OC, EC, and highly source-specific organic tracers as input in addition to inorganic species and elements used by Aamundsen et al (1992) and Maenhaut (2018)
Summary
Carbonaceous aerosol has been studied intensively over the last 20 years due to its influence on radiative forcing (Bond et al, 2013; Myhre and Samset, 2015; Lund et al, 2018), both directly by scattering and absorption of sunlight, and semi-directly and indirectly by influencing cloud properties (Boucher et al, 2013; Hodnebrog et al, 2014; Myhre et al, 2013). Carbonaceous aerosol (here elemental carbon, EC, and organic carbon, OC) is measured regularly in major air monitoring networks such as the European Monitoring and Evaluation Programme (EMEP) and Interagency Monitoring of Protected Visual Environments (IMPROVE) EMEP included OC/EC measurements in 2004 after a pioneering measurement campaign at 12 European sites from 2002 to 2003 (Yttri et al, 2007a; Tørseth et al, 2012), showing that carbonaceous aerosol was a major constituent of the ambient aerosol in the European rural background environment, accounting for 9 %–37 % (OM, organic matter) and 1 %–5 % (EC) of PM10, and that OM was more abundant than sulfate (SO24−) at sites reporting both variables (Yttri et al, 2007a). Similar conclusions were reached by another long-term campaign, CARBOSOL (Gelencsér et al, 2007; Pio et al, 2007), which monitored atmospheric aerosol and its components for 2 years at six sites along a west–east transect extending from the Azores, in the mid-Atlantic Ocean, to K-Puszta (Hungary), in central Europe
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