Characterization and source apportionment of atmospheric organic and elemental carbon during fall and winter of 2003 in Xi'an, China

Abstract. Continuous observation of atmospheric organic and elemental carbon (OC, EC) were conducted at Xi'an during high pollution seasons from September 2003 to February 2004. PM2.5 samples were collected on pre-fired quartz-fiber filters with battery-powered mini-volume samplers every day and PM10 samples were collected every third days. Three types of source samples (i.e., coal-combustion, motor vehicle exhaust, and biomass burning) were also collected during ambient sampling period. Ambient and source samples were analyzed for OC and EC by thermal/optical reflectance (TOR) following the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. The average PM2.5 OC concentrations in fall and winter were 34.1±18.0 µg m-3 and 61.9±33.2 µg m-3, respectively, while EC were 11.3±6.9 µg m-3 and 12.3±5.3 µg m-3, respectively. Most of OC and EC were associated with fine particle (PM2.5) mode. The OC and EC levels at Xi'an are higher than most urban cities in Asia. The OC and EC in fall were found to be strongly correlated (R2>0.9), with moderate correlation in winter (R2=0.66). The carbonaceous aerosol accounted for 48.8±10.1% of the PM2.5 during fall and 45.9±7.5% during winter. Average OC/EC ratio was 3.3 in fall and 5.1 in winter with individual OC/EC ratios constantly exceeding 2.0. Elevated OC/EC ratios were found during heating seasons with increased coal combustion. The contribution of secondary organic carbon was not significant during winter. The time series of OC and EC showed periodic variability. Traffic contributes 5 and 7 day peaks in the spectrum, precipitation appears as a 10 day periodicity and biomass burning can be identified as a 24 day periodicity. Total carbon (TC) was apportioned by absolute principal component analysis (APCA) using the 8 carbon fraction data (OC1, OC2, OC3, OC4, EC1, EC2, EC3, and OP [a pyrolyzed carbon fraction]). TC attributes 73% to gasoline exhaust, 23% to diesel exhaust, and 4% to biomass burning during fall. However, TC attributes 44% each to gasoline exhaust and coal burning, 9% to biomass burning, and 3% to diesel exhaust during winter.

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Characterization and source apportionment of atmospheric organic and elemental carbon during fall and winter of 2003 in Xi'an, China

Atmospheric Chemistry and Physics ◽

10.5194/acp-5-3127-2005 ◽

2005 ◽

Vol 5 (11) ◽

pp. 3127-3137 ◽

Cited By ~ 346

Author(s):

J. J. Cao ◽

F. Wu ◽

J. C. Chow ◽

S. C. Lee ◽

Y. Li ◽

...

Keyword(s):

Biomass Burning ◽

Coal Combustion ◽

Elemental Carbon ◽

Gasoline Engine ◽

Motor Vehicle ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Shaanxi Province ◽

Engine Exhaust ◽

Residential Coal

Abstract. Continuous measurements of atmospheric organic and elemental carbon (OC and EC) were taken during the high-pollution fall and winter seasons at Xi'an, Shaanxi Province, China from September 2003 through February 2004. Battery-powered mini-volume samplers collected PM2.5 samples daily and PM10 samples every third day. Samples were also obtained from the plumes of residential coal combustion, motor-vehicle exhaust, and biomass burning sources. These samples were analyzed for OC/EC by thermal/optical reflectance (TOR) following the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol. OC and EC levels at Xi'an are higher than most urban cities in Asia. Average PM2.5 OC concentrations in fall and winter were 34.1±18.0 μg m−3 and 61.9±;33.2 μg m−3, respectively; while EC concentrations were 11.3±6.9 μg m−3 and 12.3±5.3 μg m−3, respectively. Most of the OC and EC were in the PM2.5 fraction. OC was strongly correlated (R>0.95) with EC in the autumn and moderately correlated (R=0.81) with EC during winter. Carbonaceous aerosol (OC×1.6+EC) accounted for 48.8%±10.1% of the PM2.5 mass during fall and 45.9±7.5% during winter. The average OC/EC ratio was 3.3 in fall and 5.1 in winter, with individual OC/EC ratios nearly always exceeding 2.0. The higher wintertime OC/EC corresponded to increased residential coal combustion for heating. Total carbon (TC) was associated with source contributions using absolute principal component analysis (APCA) with eight thermally-derived carbon fractions. During fall, 73% of TC was attributed to gasoline engine exhaust, 23% to diesel exhaust, and 4% to biomass burning. During winter, 44% of TC was attributed to gasoline engine exhaust, 44% to coal burning, 9% to biomass burning, and 3% to diesel engine exhaust.

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Characteristics and sources of carbonaceous aerosols from Shanghai, China

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-16811-2012 ◽

2012 ◽

Vol 12 (7) ◽

pp. 16811-16849 ◽

Cited By ~ 8

Author(s):

J.-J. Cao ◽

C.-S. Zhu ◽

X.-X. Tie ◽

F.-H. Geng ◽

H.-M. Xu ◽

...

Keyword(s):

Biomass Burning ◽

Coal Combustion ◽

Stable Carbon Isotopes ◽

Motor Vehicle ◽

Large Fraction ◽

Total Carbon ◽

Motor Vehicles ◽

Diagnostic Ratios ◽

Vehicle Exhaust ◽

Carbonaceous Particles

Abstract. An intensive investigation of carbonaceous PM2.5 and TSP from Pudong (China) was conducted as part of the MIRAGE-Shanghai Experiment in 2009. Data for organic and elemental carbon (OC and EC), organic species, including C17 to C40 n-alkanes and 17 polycyclic aromatic hydrocarbons (PAHs), and stable C isotopes OC (δ13COC) and EC (δ13CEC) were used to evaluate the aerosols' temporal variations and identify presumptive sources. High OC/EC ratios indicated a large fraction of secondary organic aerosol (SOA); high char/soot ratios indicated stronger contributions to EC from motor vehicles and coal combustion than biomass burning. Diagnostic ratios of PAHs indicated that much of the SOA was produced via coal combustion. Isotope abundances (δ13COC = −24.5 &pm; 0.8‰ and δ13CEC = −25.1 &pm; 0.6‰) indicated that fossil fuels were the most important source for carbonaceous PM2.5, with lesser impacts from biomass burning and natural sources. An EC tracer system and isotope mass balance calculations showed that the relative contributions to total carbon from coal combustion, motor vehicle exhaust, and SOA were 41%, 21%, and 31%: other primary sources such as marine, soil and biogenic emissions contributed 7%. Combined analyses of OC and EC, n-alkanes and PAHs, and stable carbon isotopes provide a new way to apportion the sources of carbonaceous particles.

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Better constraints on sources of carbonaceous aerosols using a combined <sup>14</sup>C – macro tracer analysis in a European rural background site

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-5685-2011 ◽

2011 ◽

Vol 11 (12) ◽

pp. 5685-5700 ◽

Cited By ~ 100

Author(s):

S. Gilardoni ◽

E. Vignati ◽

F. Cavalli ◽

J. P. Putaud ◽

B. R. Larsen ◽

...

Keyword(s):

Organic Carbon ◽

Biomass Burning ◽

Elemental Carbon ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Back Trajectory ◽

Po Valley ◽

Background Site ◽

Potential Source ◽

Source Contributions

Abstract. The source contributions to carbonaceous PM2.5 aerosol were investigated at a European background site at the edge of the Po Valley, in Northern Italy, during the period January–December 2007. Carbonaceous aerosol was described as the sum of 8 source components: primary (1) and secondary (2) biomass burning organic carbon, biomass burning elemental carbon (3), primary (4) and secondary (5) fossil organic carbon, fossil fuel burning elemental carbon (6), primary (7) and secondary (8) biogenic organic carbon. The mass concentration of each component was quantified using a set of macro tracers (organic carbon OC, elemental carbon EC, and levoglucosan), micro tracers (arabitol and mannitol), and 14C measurements. This was the first time that 14C measurements covered a full annual cycle with daily resolution. This set of 6 tracers, together with assumed uncertainty ranges of the ratios of OC-to-EC, and the reference fraction of modern carbon in the 8 source categories, provides strong constraints to the source contributions to carbonaceous aerosol. The uncertainty of contributions was assessed with a Quasi-Monte Carlo (QMC) method accounting for the variability of OC and EC emission factors, the uncertainty of reference fractions of modern carbon, and the measurement uncertainty. During winter, biomass burning composed 64 % (±15 %) of the total carbon (TC) concentration, while in summer secondary biogenic OC accounted for 50 % (±16 %) of TC. The contribution of primary biogenic aerosol particles was negligible during the entire year. Moreover, aerosol associated with fossil sources represented 27 % (±16 %) and 41 % (±26 %) of TC in winter and summer, respectively. The contribution of secondary organic aerosol (SOA) to the organic mass (OM) was significant during the entire year. SOA accounted for 30 % (±16 %) and 85 % (±12 %) of OM during winter and summer, respectively. While the summer SOA was dominated by biogenic sources, winter SOA was mainly due to biomass burning and fossil sources. This indicates that the oxidation of semi-volatile and intermediate volatility organic compounds co-emitted with primary organics is a significant source of SOA, as suggested by recent model results and Aerosol Mass Spectrometer measurements. Comparison with previous global model simulations, indicates a strong underestimate of wintertime primary aerosol emissions in this region. The comparison of source apportionment results in different urban and rural areas showed that the sampling site was mainly affected by local aerosol sources during winter and regional air masses from the nearby Po Valley in summer. This observation was further confirmed by back-trajectory analysis applying the Potential Source Contribution Function method to identify potential source regions.

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Variability of organic and elemental carbon, water soluble organic carbon, and isotopes in Hong Kong

Atmospheric Chemistry and Physics ◽

10.5194/acp-6-4569-2006 ◽

2006 ◽

Vol 6 (12) ◽

pp. 4569-4576 ◽

Cited By ~ 96

Author(s):

K. F. Ho ◽

S. C. Lee ◽

J. J. Cao ◽

Y. S. Li ◽

J. C. Chow ◽

...

Keyword(s):

Hong Kong ◽

Organic Carbon ◽

Elemental Carbon ◽

Motor Vehicle ◽

Regional Scale ◽

Water Soluble ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Water Soluble Organic Carbon ◽

Soluble Organic Carbon

Abstract. To determine the levels and variations of carbonaceous aerosol in Hong Kong, PM2.5 and PM10 samples were collected by high volume (Hi-vol) samplers at three monitoring stations (representing middle-scale roadside, urban-, and regional-scale environments) during winter (November 2000 to February 2001) and summer (June 2001 to August 2001) periods. The highest concentrations of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were found at the middle-scale roadside site with the lowest at the regional-scale site. The percentages of WSOC in total carbon at these sites were inversely correlated with their concentrations (i.e., the highest percentages of WSOC were observed at the regional-scale site). A high WSOC fraction may be associated with aged aerosol because of the secondary formation by photochemical oxidation of organic precursors of anthropogenic pollutants during transport. The annual average of isotope abundances (δ13C) of OC and EC were –26.9±0.5‰ and –25.6±0.1‰, respectively. There were no notable differences for seasonal distributions of carbon isotopic composition, consistent with motor vehicle emissions being the main source contributors of carbonaceous aerosol in Hong Kong. OC 13C abundances at the regional-scale site were higher than those at the middle-scale roadside and urban sites, consistent with secondary organic aerosols of biogenic origin.

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Variability of organic and elemental carbon, water soluble organic carbon, and isotopes in Hong Kong

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-6-4579-2006 ◽

2006 ◽

Vol 6 (3) ◽

pp. 4579-4600 ◽

Cited By ~ 8

Author(s):

K. F. Ho ◽

S. C. Lee ◽

J. J. Cao ◽

Y. S. Li ◽

J. C. Chow ◽

...

Keyword(s):

Hong Kong ◽

Organic Carbon ◽

Elemental Carbon ◽

Motor Vehicle ◽

Regional Scale ◽

Water Soluble ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Water Soluble Organic Carbon ◽

Soluble Organic Carbon

Abstract. To determine the levels and variations of carbonaceous aerosol in Hong Kong, PM2.5 and PM10 samples were collected by high volume (Hi-vol) samplers at three monitoring stations (representing middle-scale roadside, urban-, and regional-scale environments) during winter (November 2000 to February 2001) and summer (June 2001 to August 2001) periods. The highest concentrations of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were found at the middle-scale roadside site with the lowest at the regional-scale site. The percentages of WSOC in total carbon at these sites were inversely correlated with their concentrations (i.e., the highest percentages of WSOC were observed at the regional-scale site). A high WSOC fraction may be associated with aged aerosol because of the secondary formation by photochemical oxidation of organic precursors of anthropogenic pollutants during transport. The annual average of isotope abundances (δ13C) of OC and EC were –26.9±0.5 and –25.6±0.1, respectively. There were no notable differences for seasonal distributions of carbon isotopic composition, consistent with motor vehicle emissions being the main source contributors of carbonaceous aerosol in Hong Kong. OC 13C abundances at the regional-scale site were higher than those at the middle-scale roadside and urban sites, consistent with secondary organic aerosols of biogenic origin.

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Characteristics and sources of carbonaceous aerosols from Shanghai, China

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-803-2013 ◽

2013 ◽

Vol 13 (2) ◽

pp. 803-817 ◽

Cited By ~ 87

Author(s):

J.-J. Cao ◽

C.-S. Zhu ◽

X.-X. Tie ◽

F.-H. Geng ◽

H.-M. Xu ◽

...

Keyword(s):

Carbon Isotopes ◽

Biomass Burning ◽

Coal Combustion ◽

Stable Carbon Isotopes ◽

Motor Vehicle ◽

Large Fraction ◽

Total Carbon ◽

Diagnostic Ratios ◽

Stable Carbon ◽

Carbonaceous Particles

Abstract. An intensive investigation of carbonaceous PM2.5 and TSP (total suspended particles) from Pudong (China) was conducted as part of the MIRAGE-Shanghai (Megacities Impact on Regional and Global Environment) experiment in 2009. Data for organic and elemental carbon (OC and EC), organic species, including C17 to C40 n-alkanes and 17 polycyclic aromatic hydrocarbons (PAHs), and stable carbon isotopes OC (δ13COC) and EC (δ13CEC) were used to evaluate the aerosols' temporal variations and identify presumptive sources. High OC/EC ratios indicated a large fraction of secondary organic aerosol (SOA); high char/soot ratios indicated stronger contributions to EC from motor vehicles and coal combustion than biomass burning. Diagnostic ratios of PAHs indicated that much of the SOA was produced via coal combustion. Isotope abundances (δ13COC = −24.5 ± 0.8‰ and δ13CEC = −25.1 ± 0.6‰) indicated that fossil fuels were the most important source for carbonaceous PM2.5 (particulate matter less than 2.5 micrometers in diameter), with lesser impacts from biomass burning and natural sources. An EC tracer system and isotope mass balance calculations showed that the relative contributions to total carbon from coal combustion, motor vehicle exhaust, and SOA were 41%, 21%, and 31%; other primary sources such as marine, soil and biogenic emissions contributed 7%. Combined analyses of OC and EC, n-alkanes and PAHs, and stable carbon isotopes provide a new way to apportion the sources of carbonaceous particles.

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Fossil fuel combustion, biomass burning and biogenic sources of fine carbonaceous aerosol in the Carpathian Basin

10.5194/acp-2019-792 ◽

2019 ◽

Author(s):

Imre Salma ◽

Anikó Vasanits-Zsigrai ◽

Attila Machon ◽

Tamás Varga ◽

István Major ◽

...

Keyword(s):

Biomass Burning ◽

Fossil Fuel ◽

Fuel Combustion ◽

Carpathian Basin ◽

Water Soluble ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

City Centre ◽

Fossil Fuel Combustion ◽

Regional Background

Abstract. Fine-fraction aerosol samples were collected, air pollutants and meteorological properties were measured in-situ in regional background environment of the Carpathian Basin, a suburban area and central part of its largest city, Budapest in each season for 1 year-long time interval. The samples were analysed for PM2.5 mass, organic carbon (OC), elemental carbon (EC), water-soluble OC (WSOC), radiocarbon, levoglucosan (LVG) and its stereoisomers, and some chemical elements. Carbonaceous aerosol species made up 36 % of the PM2.5 mass with a modest seasonal variation and with a slightly increasing tendency from the regional background to the city centre (from 32 to 39 %). Coupled radiocarbon-LVG marker method was applied to apportion the total carbon (TC = OC + EC) into contributions of EC and OC from fossil fuel (FF) combustion (ECFF and OCFF, respectively), EC and OC from biomass burning (BB) (ECBB and OCBB, respectively) and OC from biogenic sources (OCBIO). Fossil fuel combustion showed rather constant daily or seasonal mean contributions (of 35 %) to the TC in the whole year in all atmospheric environments, while the daily contributions of BB and biogenic sources changed radically (from

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Source apportionment of carbonaceous aerosol in southern Sweden

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-11387-2011 ◽

2011 ◽

Vol 11 (22) ◽

pp. 11387-11400 ◽

Cited By ~ 54

Author(s):

J. Genberg ◽

M. Hyder ◽

K. Stenström ◽

R. Bergström ◽

D. Simpson ◽

...

Keyword(s):

Organic Carbon ◽

Source Apportionment ◽

Fossil Fuels ◽

Elemental Carbon ◽

Transport Model ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Anthropogenic Sources ◽

Biomass Combustion ◽

Southern Sweden

Abstract. A one-year study was performed at the Vavihill background station in southern Sweden to estimate the anthropogenic contribution to the carbonaceous aerosol. Weekly samples of the particulate matter PM10 were collected on quartz filters, and the amounts of organic carbon, elemental carbon, radiocarbon (14C) and levoglucosan were measured. This approach enabled source apportionment of the total carbon in the PM10 fraction using the concentration ratios of the sources. The sources considered in this study were emissions from the combustion of fossil fuels and biomass, as well as biogenic sources. During the summer, the carbonaceous aerosol mass was dominated by compounds of biogenic origin (80%), which are associated with biogenic primary and secondary organic aerosols. During the winter months, biomass combustion (32%) and fossil fuel combustion (28%) were the main contributors to the carbonaceous aerosol. Elemental carbon concentrations in winter were about twice as large as during summer, and can be attributed to biomass combustion, probably from domestic wood burning. The contribution of fossil fuels to elemental carbon was stable throughout the year, although the fossil contribution to organic carbon increased during the winter. Thus, the organic aerosol originated mainly from natural sources during the summer and from anthropogenic sources during the winter. The result of this source apportionment was compared with results from the EMEP MSC-W chemical transport model. The model and measurements were generally consistent for total atmospheric organic carbon, however, the contribution of the sources varied substantially. E.g. the biomass burning contributions of OC were underestimated by the model by a factor of 2.2 compared to the measurements.

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Toward a standardised thermal-optical protocol for measuring atmospheric organic and elemental carbon: the EUSAAR protocol

Atmospheric Measurement Techniques ◽

10.5194/amt-3-79-2010 ◽

2010 ◽

Vol 3 (1) ◽

pp. 79-89 ◽

Cited By ~ 509

Author(s):

F. Cavalli ◽

M. Viana ◽

K. E. Yttri ◽

J. Genberg ◽

J.-P. Putaud

Keyword(s):

Elemental Carbon ◽

Thermal Evolution ◽

Standard Procedure ◽

Pyrolytic Carbon ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Optical Analysis ◽

Split Point ◽

Aerosol Fraction ◽

Eu Project

Abstract. Thermal-optical analysis is a conventional method for determining the carbonaceous aerosol fraction and for classifying it into organic carbon, OC, and elemental carbon, EC. Unfortunately, the different thermal evolution protocols in use can result in a wide elemental carbon-to-total carbon variation by up to a factor of five. In Europe, there is currently no standard procedure for determining the carbonaceous aerosol fraction which implies that data from different laboratories at various sites are of unknown accuracy and cannot be considered comparable. In the framework of the EU-project EUSAAR (European Supersites for Atmospheric Aerosol Research), a comprehensive study has been carried out to identify the causes of differences in the EC measured using different thermal evolution protocols; thereby the major positive and negative biases affecting thermal-optical analysis have been isolated and minimised to define an optimised protocol suitable for European aerosols. Our approach to improve the accuracy of the discrimination between OC and EC was essentially based on four goals. Firstly, charring corrections rely on faulty assumptions – e.g. pyrolytic carbon is considered to evolve completely before native EC throughout the analysis –, thus we have reduced pyrolysis to a minimum by favoring volatilisation of OC. Secondly, we have minimised the potential negative bias in EC determination due to early evolution of light absorbing carbon species at higher temperatures in the He-mode, including both native EC and combinations of native EC and pyrolytic carbon potentially with different specific attenuation cross section values. Thirdly, we have minimised the potential positive bias in EC determination resulting from the incomplete evolution of OC during the He-mode which then evolves during the He/O2-mode, potentially after the split point. Finally, we have minimised the uncertainty due to the position of the OC/EC split point on the FID response profile by introducing multiple desorption steps in the He/O2-mode. Based on different types of carbonaceous PM encountered across Europe, we have defined an optimised thermal evolution protocol, the EUSAAR_2 protocol, as follows: step 1 in He, 200 °C for 120 s; step 2 in He 300 °C for 150 s; step 3 in He 450 °C for 180 s; step 4 in He 650 °C for 180 s. For steps 1–4 in He/O2, the conditions are 500 °C for 120 s, 550 °C for 120 s, 700 ° C for 70 s, and 850 °C for 80 s, respectively.

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Sources and formation mechanisms of carbonaceous aerosol at a regional background site in the Netherlands: Insights from a year-long radiocarbon study

10.5194/acp-2016-624 ◽

2016 ◽

Author(s):

Ulrike Dusek ◽

Regina Hitzenberger ◽

Anne Kasper-Giebl ◽

Magdalena Kistler ◽

Harro A. J. Meijer ◽

...

Keyword(s):

Organic Carbon ◽

The Netherlands ◽

Biomass Burning ◽

Water Soluble ◽

Total Carbon ◽

Carbonaceous Aerosol ◽

Formation Mechanisms ◽

Moderate Increase ◽

Air Masses ◽

Regional Sources

Abstract. We measured the radioactive carbon isotope 14C (radiocarbon) in various fractions of the carbonaceous aerosol sampled between February 2011 and March 2012 at the Cesar observatory in the Netherlands. Based on the radiocarbon content in total carbon (TC), organic carbon (OC), water insoluble organic carbon (WIOC), and elemental carbon (EC), we estimated the contribution of major sources to the carbonaceous aerosol. The main source categories were fossil fuel combustion, biomass burning and other contemporary carbon, which is mainly biogenic secondary organic aerosol material (SOA). A clear seasonal variation is seen in EC from biomass burning (ECBB), with lowest values in summer and highest values in winter, but ECBB is a minor fraction of EC in all seasons. WIOC from contemporary sources is highly correlated with ECBB, indicating that biomass burning is the dominant source of contemporary WIOC. This suggests that most biogenic SOA is water-soluble and that water insoluble carbon stems mainly from primary sources. Seasonal variations in other carbon fractions are less clear and hardly distinguishable from variations related to air mass history. Air masses originating from the ocean sector presumably contain little carbonaceous aerosol from outside the Netherlands, and during these conditions measured carbon concentrations reflect regional sources. In these situations absolute TC concentrations are usually rather low, around 1.5 μg m−3 and ECBB is always very low (~ 0.05 μg m−3), even in winter, indicating that biomass burning is not a strong source of carbonaceous aerosol in the Netherlands. In continental air masses, which usually arrive from the East or South and have spent several days over land, TC concentrations are on average by a factor of 3 higher. ECBB increases more strongly than TC to 0.2 μg m−3. Fossil EC and fossil WIOC, which are indicative of primary emissions, show a more moderate increase by a factor of 2.5 on average. An interesting case is fossil water soluble organic carbon (WSOC, calculated as OC-WIOC), which can be regarded as a proxy for SOA from fossil precursors. Fossil WSOC has low concentrations when regional sources are sampled and increases by more than a factor of 5 in continental air masses. A longer residence time of air masses over land seems to result in increased SOA concentrations from fossil origin.

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