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•Pronounced seasonal variation in aerosol absorption in Athens over a 4-year period.•Significant BrC contribution (23.7%) to the total aerosol absorption at 370 nm.•Strong winter-time ...correlations between BrC and BB-related organic aerosols.•The BrCsec absorption is related to residential wood burning during winter nights.
This study analyses 4-years of continuous 7-λ Aethalometer (AE-33) measurements in an urban-background environment of Athens, to resolve the spectral absorption coefficients (babs) for black carbon (BC) and brown carbon (BrC). An important BrC contribution (23.7 ± 11.6%) to the total babs at 370 nm is estimated for the period May 2015–April 2019, characterized by a remarkable seasonality with winter maximum (33.5 ± 13.6%) and summer minimum (18.5 ± 8.1%), while at longer wavelengths the BrC contribution is significantly reduced (6.8 ± 3.6% at 660 nm). The wavelength dependence of the total babs gives an annual-mean AAE370-880 of 1.31, with higher values in winter night-time. The BrC absorption and its contribution to babs presents a large increase reaching up to 39.1 ± 13.6% during winter nights (370 nm), suggesting residential wood burning (RWB) emissions as a dominant source for BrC. This is supported by strong correlations of the BrC absorption with OC, EC, the fragment ion m/z 60 derived from ACSM and PMF-analyzed organic fractions related to biomass burning (e.g. BBOA). In contrast, BrC absorption decreases significantly during daytime as well as in the warm period, reaching to a minimum during the early-afternoon hours in all seasons due to photo-chemical degradation. Estimated secondary BrC absorption is practically evident only during winter night-time, implying the fast oxidation of BrC species from RWB emissions. Changes in mixing-layer height do not significantly affect the BrC absorption in winter, while they play a major role in summer.
Biomass burning is a major source of Brown Carbon (BrC), strongly contributing to radiative forcing. In urban areas of the climate-sensitive Southeastern European region, where strong emissions from ...residential wood burning (RWB) are reported, radiative impacts of carbonaceous aerosols remain largely unknown. This study examines the absorption properties of water- and methanol-soluble organic carbon (WSOC, MeS_OC) in a city (Ioannina, Greece) heavily impacted by RWB. Measurements were performed during winter (December 2019 – February 2020) and summer (July – August 2019) periods, characterized by RWB and photochemical processing of organic aerosol (OA), respectively. PM2.5 filter extracts were analyzed spectrophotometrically for water- and methanol-soluble BrC (WS_BrC, MeS_BrC) absorption. WSOC concentrations were quantified using TOC analysis, while those of MeS_OC were determined using a newly developed direct quantification protocol, applied for the first time to an extended series of ambient samples. The direct method led to a mean MeS_OC/OC of 0.68 and a more accurate subsequent estimation of absorption efficiencies. The mean winter WS_BrC and MeS_BrC absorptions at 365 nm were 13.9 Mm−1 and 21.9 Mm−1, respectively, suggesting an important fraction of water-insoluble OA. Mean winter WS_BrC and MeS_BrC absorptions were over 10 times those observed in summer. MeS_OC was more absorptive than WSOC in winter (mean mass absorption efficiencies – MAE365: 1.81 vs 1.15 m2 gC−1) and especially in summer (MAE: 1.12 vs 0.27 m2 gC−1) due to photo-dissociation and volatilization of BrC chromophores. The winter radiative forcing (RF) of WS_BrC and MeS_BrC relative to elemental carbon (EC) was estimated at 8.7 % and 16.7 %, respectively, in the 300–2500 nm band. However, those values increased to 48.5 % and 60.2 % at 300–400 nm, indicating that, under intense RWB, BrC forcing becomes comparable to that of soot. The results highlight the consideration of urban BrC emissions in radiative transfer models, as a considerable climate forcing factor.
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•Water/methanol-soluble BrC absorptions under intense residential wood burning conditions•New protocol for measurement of methanol-soluble organic carbon provides reliable MAE.•Methanol-soluble BrC absorption 1.6 times higher than water-soluble BrC in winter•Wood burning linked to higher MAE and lower AAE for MeS_BrC compared to WS_BrC•Near-UV radiative forcing by BrC relative to EC of 48.5 % (WS_BrC) and 60.2 % (MeS_BrC)
The current research provides a newly developed method to quantify methanol-soluble organic carbon (MeS_OC) in aerosol samples. This analytical procedure allows an accurate separation of MeS-OC ...component, which is critical for the calculation of mass absorption efficiency (MAE) of ambient Brown Carbon (BrC) and consequently its climate relevant potential. The method includes extraction, filtering and condensation stages, leading to the preparation of a highly concentrated product in which MeS-OC can be precisely quantified by a Sunset Carbon Analyzer in a single analysis step. This method can be applied on aerosol collected using either high or low volume samplers, since a relatively small filter area is required for the determination. Furthermore, it eliminates any misestimation of the MeS-OC mass that may appear in other reported techniques that don't seem to include the precise separation of methanol-soluble fraction in their quantification process.
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The mass quantification of methanol-soluble organic carbon is essential, contributing up to 50% to the absorptivity of organic aerosol (BrC) at shorter wavelengths.
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The method provides a direct measurement of methanol-soluble aerosol components, resolving any potential uncertainties of previously applied methods.
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The adoption of this direct quantification approach leads to a rationalization of past MAE estimates for BrC with implications for radiative transfer models.
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Biomass burning is a major source of atmospheric brown
carbon (BrC), and through its absorption of UV/VIS radiation,
it can play an important role in the planetary radiative balance and
atmospheric ...photochemistry. The considerable uncertainty of BrC impacts is
associated with its poorly constrained sources, transformations, and
atmospheric lifetime. Here we report laboratory experiments that examined
changes in the optical properties of the water-soluble (WS) BrC fraction of
laboratory-generated biomass burning particles from hardwood
pyrolysis. Effects of direct UVB photolysis and OH oxidation in the aqueous
phase on molecular-weight-separated BrC were studied. Results indicated that the majority of
low-molecular-weight (MW)
BrC (<400 Da) was rapidly photobleached by both direct photolysis and OH
oxidation on an atmospheric timescale of approximately 1 h. High MW BrC
(≥400 Da) underwent initial photoenhancement up to ∼15 h,
followed by slow photobleaching over ∼10 h. The laboratory experiments
were supported by observations from ambient BrC samples that were collected
during the fire seasons in Greece. These samples, containing freshly emitted
to aged biomass burning aerosol, were analyzed for both water- and
methanol-soluble BrC. Consistent with the laboratory experiments, high-MW BrC
dominated the total light absorption at 365 nm for both methanol and
water-soluble fractions of ambient samples with atmospheric transport times
of 1 to 68 h. These ambient observations indicate that overall,
biomass burning BrC across all molecular weights has an atmospheric lifetime
of 15 to 28 h, consistent with estimates from previous field studies –
although the BrC associated with the high-MW fraction remains relatively
stable and is responsible for light absorption properties of BrC throughout most of its atmospheric lifetime. For
ambient samples of aged (>10 h) biomass burning emissions, poor linear
correlations were found between light absorptivity and levoglucosan,
consistent with other studies suggesting a short atmospheric lifetime for
levoglucosan. However, a much stronger correlation between light absorptivity
and total hydrous sugars was observed, suggesting that they may serve as more
robust tracers for aged biomass burning emissions. Overall, the results from
this study suggest that robust model estimates of BrC radiative impacts
require consideration of the atmospheric aging of BrC and the stability of
high-MW BrC.
Submicron aerosol chemical composition was studied during a year-long period (26 July 2016-31 July 2017) and two wintertime intensive campaigns (18 December 2013-21 February 2014 and 23 December ...2015-17 February 2016), at a central site in Athens, Greece, using an Aerosol Chemical Speciation Monitor (ACSM). Concurrent measurements included a particle-into-liquid sampler (PILS-IC), a scanning mobility particle sizer (SMPS), an AE-33 Aethalometer, and ion chromatography analysis on 24 or 12 h filter samples. The aim of the study was to characterize the seasonal variability of the main submicron aerosol constituents and decipher the sources of organic aerosol (OA). Organics were found to contribute almost half of the submicron mass, with 30 min resolution concentrations during wintertime reaching up to 200 µg m.sup.-3 . During winter (all three campaigns combined), primary sources contributed about 33 % of the organic fraction, and comprised biomass burning (10 %), fossil fuel combustion (13 %), and cooking (10 %), while the remaining 67 % was attributed to secondary aerosol. The semi-volatile component of the oxidized organic aerosol (SV-OOA; 22 %) was found to be clearly linked to combustion sources, in particular biomass burning; part of the very oxidized, low-volatility component (LV-OOA; 44 %) could also be attributed to the oxidation of emissions from these primary combustion sources.
In an attempt to take effective action towards mitigating pollution episodes in Athens, precise knowledge of PM.sub.2.5 composition and its sources is a prerequisite. Thus, a 2-year chemical ...composition dataset from aerosol samples collected in an urban background site in central Athens from December 2013 to March 2016 has been obtained and a positive matrix factorization (PMF) was applied in order to identify and apportion fine aerosols to their sources. A total of 850 aerosol samples were collected on a 12 to 24 h basis and analyzed for major ions, trace elements, and organic and elemental carbon, allowing us to further assess the impact of residential heating as a source of air pollution over Athens.
This study aims to delineate the characteristics of Black Carbon (BC) in the atmosphere over Athens, Greece, using 4-year (May 2015–April 2019) Aethalometer (AE-33) measurements. The average BC ...concentration is 1.9 ± 2.5 μg m−3 (ranging from 0.1 to 32.7 μg m−3; hourly values), with a well-defined seasonality from 1.3 ± 1.1 μg m−3 in summer to 3.0 ± 4.0 μg m−3 in winter. Pronounced morning and evening/night peaks are found in the BC concentrations in winter, while during the rest of the seasons, this diurnal cycle appears to flatten out, with the exception of the morning traffic peak. On an annual basis, the biomass-burning fraction (BB%) of BC accounts for 22 ± 12%, while the fossil-fuel combustion (BCff) component (traffic emissions and domestic heating) dominates during summer (83%) and in the morning hours. BCwb exhibits higher contribution in winter (32%), especially during the night hours (39%). BC levels are effectively reduced by precipitation, while they significantly build-up for wind speeds <3 m s−1 and mixing-layer height (MLH) < 500 m. Normalizing the BC diurnal course by the MLH variations on a seasonal basis reveals that the residential wood-burning emissions are mostly responsible for the large BC increase during winter nights, whereas the low BC levels during daytime in the warm season are mainly attributed to dilution into a deeper MLH. BCwb is highly correlated with other BB tracers during winter nights (e.g. levoglucosan, non-sea-salt-K+, m/z 60 fragment), as well as with the fine fraction (PM2.5) OC and EC. The Delta-C, which represents the spectral dependence of BC as the absorption difference between 370 and 880 nm, is analyzed for the first time in Athens. It exhibits a pronounced seasonality with maximum values in winter night-time, and it appears as a valid qualitative marker for wood combustion.
•4-years assessment of BC concentrations and emission sources in Athens.•Biomass-burning tracers justify the high wood-burning emissions in winter.•The winter-time residential wood-burning persists during the study period.•Remarkable effects of low wind speed and mixing-layer height in BC levels.•Equal BC contribution from fossil fuel and wood burning during winter nights.
Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants in fine particulate matter (PM) long known to have mutagenic and carcinogenic effects, but much is unknown about the importance of local ...and remote sources for PAH levels observed in population-dense urban environments. A year-long sampling campaign in Athens, Greece, where more than 150 samples were analyzed for 31 PAHs and a wide range of chemical markers, was combined with positive matrix factorization (PMF) to constrain the temporal variability, sources, and carcinogenic risk associated with PAHs. It was found that biomass burning (BB), a source mostly present during wintertime intense pollution events (observed for 18 % of measurement days in 2017), led to wintertime PAH levels that were 7 times higher than in other seasons and was as important for annual mean PAH concentrations (31 %) as diesel and oil (33 %) and gasoline (29 %) sources. The contribution of non-local sources, although limited on an annual basis (7 %), increased during summer, becoming comparable to that of local sources combined. The fraction of PAHs (12 members that were included in the PMF analysis) that was associated with BB was also linked to increased health risk compared to the other sources, accounting for almost half the annual PAH carcinogenic potential (43 %). This can result in a large number of excess cancer cases due to BB-related high PM levels and urges immediate action to reduce residential BB emissions in urban areas facing similar issues.
This study examines the concentrations and characteristics of carbonaceous aerosols (including saccharides) and inorganic species measured by PM2.5 filter sampling and a multi-wavelength Aethalometer ...during two campaigns in a mountainous, medium-sized, Greek city (Ioannina). The first campaign was conducted in summer and used as a baseline of low concentrations, while the second took place in winter under intensive residential wood burning (RWB) emissions. Very high winter-mean OC concentrations (26.0 μg m−3) were observed, associated with an OC/EC ratio of 9.9, and mean BCwb and PM2.5 levels of 4.5 μg m−3 and 57.5 μg m−3, respectively. Simultaneously, record-high levoglucosan (Lev) concentrations (mean: 6.0 μg m−3; max: 15.9 μg m−3) were measured, revealing a severely biomass burning (BB)-laden environment. The water-soluble OC component (WSOC) accounted for 56 ± 9% of OC in winter, exhibiting high correlations (R2 = 0.93–0.97) with BB tracers (nss-K+, BCwb, Lev), nitrate and light absorption, potentially indicating the formation of water-soluble brown carbon (BrC) from fast oxidation processes. The examination of diagnostic ratios involving BB tracers indicated the prevalence of hardwood burning, while the mean Lev/OC ratio (22%) was remarkably higher than literature values. Applying a mono-tracer method based on levoglucosan, we estimated very high BB contributions to OC (∼92%), EC (∼64%) and WSOC (∼87%) during winter. On the contrary, low levels were registered during summer for all carbonaceous components, with winter/summer ratios of 4–5 for PM2.5 and BC, 10 for OC, 30 for BCwb and ∼1100 for levoglucosan. The absence of local BB sources in summer, combined with the photochemical processing and aging of regional organic aerosols, resulted in higher WSOC/OC fractions (64 ± 13%). The results indicate highly soluble fine carbonaceous aerosol fraction year-round, which when considered alongside the extreme concentration levels in winter can have important implications for short- and long-term health effects.
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•PM2.5 chemical characterization in a semi-mountainous city of Southeast Europe.•Severe PM2.5, OC and levoglusocan levels are found due to wood burning in winter.•Biomass burning contributes 92%, 87%, 64% to OC, WSOC, EC, respectively in winter.•High levoglucosan-to-OC ratio (22%) is found for local hardwood burning emissions.•Fresh wood burning aerosols were highly water-soluble with implication for toxicity.
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