Unveiling Light-Absorbing Carbonaceous Aerosols at a Regional Background Site in Southern Balkans

Abstract

This study examines the seasonality of Black Carbon (BC) and Brown Carbon (BrC) spectral absorption characteristics at a continental background site (Kozani) in southern Balkans (NW Greece). It aims to assess the seasonality and impact of different sources on light absorption properties, BC concentrations, and the fraction of BrC absorption. Moderate-to-low BC concentrations were observed, ranging from 0.05 µg m⁻³ to 2.44 µg m⁻³ on an hourly basis (annual mean: 0.44 ± 0.27 µg m⁻³; median: 0.39 µg m⁻³) with higher levels during winter (0.53 ± 0.33), reflecting enhanced emissions from residential wood burning (RWB) for heating purposes. Atmospheric conditions are mostly clean during spring (MAM) (BC: 0.34 µg m⁻³), associated with increased rainfall. BC components associated with fossil fuel combustion (BC_ff) and biomass burning (BC_bb), maximize in summer (0.36 µg m⁻³) and winter (0.28 µg m⁻³), respectively, while the absorption Ångstrôm exponent (AAE_370–880) values ranged from 1.09 to 1.93 on daily basis. The annual mean total absorption coefficient (b_abs,520) inferred by aethalometer (AE33) was 4.09 ± 2.65 Mm⁻¹ (median: 3.51 Mm⁻¹), peaking in winter (5.30 ± 3.35 Mm⁻¹). Furthermore, the contribution of BrC absorption at 370 nm, was also high in winter (36.7%), and lower during the rest of the year (17.3–29.8%). The measuring station is located at a rural background site 4 km outside Kozani City and is not directly affected by traffic and urban heating emissions. Therefore, the regional background atmosphere is composed of a significant fraction of carbonaceous aerosols from RWB in nearby villages, a characteristic feature of the Balkan’s rural environment. Emissions from the lignin-fired power plants, still operating in the region, have decreased during the last years and moderately affect the atmospheric conditions.

1. Introduction

Carbonaceous aerosols constitute a major fraction of fine particulate matter (PM_2.5) in both urban and rural environments, exerting multiple effects on atmospheric chemical reactions, air quality, visibility degradation, solar radiation, cloud microphysics, and human health [1,2,3,4,5]. These aerosols primarily originate from combustion processes and secondary organic aerosol formation, with additional contributions from plant debris, pollen, and biogenic emissions [6,7,8,9]. They are classified into Black Carbon (BC), a chemically inert light-absorbing component, and Brown Carbon (BrC), the absorbing fraction of organic matter. Both play significant roles in radiation absorption, atmospheric warming, and climate change from local to global scales [10,11,12,13]. While the radiative effects of BC are well-documented, those of BrC remain complex and highly uncertain, depending on source characteristics, aging and mixing processes, photolysis, and meteorological conditions [14,15]. Therefore, unravelling the amount, morphology, optical properties, chemical composition, transformation, and life cycle of carbonaceous aerosols is a complex task that needs extensive research and contributes to reduce the uncertainties in climate model predictions [16,17,18,19,20].

Phenomenological studies of BC (or Elemental Carbon, EC), organic matter (OM) and spectral light absorption inferred from multiple instruments (aethalometers; multi-angle absorption photometer (MAAP), particle soot absorption photometers (PSAP), aerosol chemical speciation monitors (ACSM)) have provided valuable insights into the properties, sources, atmospheric mixing, and climate implications of carbonaceous aerosols across different European environments (e.g., traffic, urban, suburban, continental, marine, background sites) [21,22,23]. In Greece, BC and light absorption studies have been conducted mainly in Athens [24,25,26,27,28], in Ioannina, NW Greece [29,30], and under chamber lab experiments simulating the biomass burning (BB) burdened atmosphere and nighttime chemistry in winter [31,32].

BB aerosols constitute a significant fraction of atmospheric particulate matter during winter, even in southern European cities [33,34,35,36], and the broader Mediterranean Basin during summer due to extensive seasonal forest fires [37,38,39,40]. These studies consistently highlight high carbonaceous aerosol concentrations and significant air quality degradation from urban to regional scales, with notable implications for atmospheric chemistry [41,42,43].

In Western Macedonia, NW Greece, domestic heating demands are high due to cold winter conditions. Major cities such as Kozani and Ptolemaida rely on district heating from cogeneration units in lignite power plants. However, smaller towns and villages in the region depend on fossil fuel combustion, particularly residential wood burning (RWB) in fireplaces and woodstoves. As a result, RWB emissions significantly contribute to air pollution, similar to other cities in northwestern Greece (e.g., Ioannina) and the Balkans, where climatic conditions drive increased wood consumption for heating [29,44,45,46]. Over the last decade, RWB has been recognized as a major wintertime pollution source in Greek cities, though measurements have been primarily focused on major urban centers like Athens, Patras, Heraklion, and Thessaloniki [47,48,49,50,51].

Apart from the adverse health effects related to heavy metals and polycyclic aromatic hydrocarbons (PAHs) from mining activities and emissions from the lignin-based power plants in the region of Western Macedonia [52,53,54], exposure to enhanced BC levels may contribute to cardiovascular diseases, asthma, and respiratory symptoms [2,55,56]. Despite the significant emissions from power plants—particularly in past decades—and the widespread use of RWB in rural areas, key characteristics of carbonaceous aerosols, including BC concentrations, BrC absorption, seasonal variability, and source apportionment, remained unexplored in this region. Furthermore, examining BC at a continental, regional-background site in northern Greece is important for climate issues and evaluating the impact of light-absorbing carbonaceous aerosols on radiative effects and climate change. In addition, current results may also serve as background levels for assessing urban effects on BC and BrC emissions from cities in northern Greece (i.e., Thessaloniki, Ioannina) and in Balkans.

This study provides the first comprehensive analysis of BC concentration and its source apportionment from fossil fuel combustion (BC_ff) and biomass burning (BC_bb) on an annual basis, aiming to explore the seasonality and contrasting spectral absorption properties due to BC and BrC at a continental background site in western Macedonia, Greece. The background character of the site is ideal for examining the regional atmospheric conditions far away from the source and the effects of lignin-fired power plants, transboundary Balkan pollution, and RWB on light absorption. Current results will form the basis for future studies at sites with similar meteorological and climatological characteristics in southeastern Europe. Additionally, as a continental background station, the site provides a valuable reference for comparison with urban locations that experience severe RWB emissions during winter (e.g., Ioannina). The seasonal characterization of BC and BrC absorption at this site will contribute to refining numerical simulation models of carbonaceous aerosol properties, ultimately reducing uncertainties in BC and BrC radiative effects on regional climate.

2. Study Region

Western Macedonia is a mainland region of northwestern Greece characterized by a complex mountainous topography (Figure 1). The region holds particular significance as it has historically served as the energy hub of Greece, contributing approximately 70% to the national energy mix for several decades [57,58]. Extensive industrial and mining activities have played a crucial role in this energy production but have also significantly contributed to elevated air pollutant emissions, often leading to adverse environmental conditions [59,60,61]. In recent years, however, the government’s energy transition policies have resulted in a sharp decline in industrial activity, leading to measurable improvements in air quality across the region [62,63].

The regional climate is classified as continental Mediterranean, with cold winters (mean January temperature of 2.8 °C) and hot summers (mean July temperature of 23.7 °C). Annual precipitation averages around 580 mm, distributed relatively evenly throughout the year (2017–2023 mean; https://meteo.gr/). Prevailing winds are generally weak to moderate and predominantly follow a northwest-to-southeast direction, aligning with the geographical axis of the Eordea Basin, which spans the major cities of Florina, Ptolemaida, and Kozani [64,65]. Specifically for the year 2023, the daily mean temperature ranged from −4.9 °C to 30.5 °C (annual mean of 13.4 °C), while the annual precipitation was above the 7-year mean (837.8 mm). The annual mean wind speed was 3.5 m s⁻¹, predominantly from northern directions, with higher speeds observed during summer and autumn. Overall, the meteorological conditions in 2023 were close to regional climatology but characterized by abnormally high precipitation (data available from https://meteo.gr/).

The focus area of this study was the city of Kozani, the capital of Western Macedonia, located at the southern part of the Eordea Basin and surrounded by the Vermio, Bourino, and Pieria Mountain ranges (Figure 1). Kozani is the most populous city in the region, with approximately 60,000 inhabitants. The population temporarily increases due to the presence of the University of Western Macedonia (UoWM), which attracts students from across the country. Air quality measurements for this study were conducted at the UoWM campus, situated on a hilltop at an elevation of 768 m, approximately 4 km southwest of the city center. The site is well-distanced from major roads, industrial facilities, and densely populated areas, minimizing local anthropogenic influences. A small residential settlement comprising around 300 detached houses lies in proximity to the campus, characterized by low traffic levels and minor domestic emissions.

3. Dataset and Methodology

Spectral light-absorption (b_abs,λ) and BC measurements were performed at the continental background station throughout 2023 via a 7-λ aethalometer (AE33, Magee Scientific [66]), operating at a flow rate of 5 Lmin⁻¹. The absorption coefficient (b_abs,λ) was computed at the seven wavelengths using the manufacturer-specified mass absorption efficiency (MAE), while the loading (“shadowing”) effect was internally corrected in AE33 measurements due to its dual spot technology [66]. Furthermore, BC was separated into two components related to fossil fuel combustion (BC_ff) and biomass burning (BC_bb) using the aethalometer model approach [67], with Ångström exponent (AAE) values of 1 for BC_ff (AAE_ff = 1) and 2 for BC_bb (AAE_bb = 2) [26,28]. The consideration of AAE_ff and AAE_bb is a subject of uncertainty for BC_ff and BC_bb computations, and are mostly site-specific as they might be affected by mixing processes and other potential sources (such as secondary organic aerosols) [68,69,70]. The current AAE values align closely with those used by Ivančič et al. [71] (1.15 and 2.05, respectively) and those utilized in Athens (1.18 and 2.0, respectively [28]). The AAE values are associated with changes in spectral absorption caused by carbonaceous aerosol between short and long wavelengths [72,73], while changes in AAE_ff and AAE_bb slightly modulate the relative fractions between BC_ff and BC_bb, as analyzed elsewhere [68].

On the other hand, the use of the multiple-scattering coefficient (C = 1.39) in AE33 results in significant b_abs,λ overestimation, as recent studies revealed, and therefore, a new correction factor is recommended by Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) [74,75,76]. In this study, a corrected C factor of 2.45 was applied for the estimations of b_abs via a harmonization factor (H) of 1.76, according to ACTRIS recommendations. The use of the new C factor just influences the b_abs,λ values, by reducing them by 1.76, and does not affect the spectral dependence. The AAE values for the total absorption were estimated at 4 spectral bands (370–880 nm, 470–950 nm, 370–520 nm, 520–880 nm) for quality control of the spectral datasets (non-existence of negative values) [77]. AAE values were also obtained for BrC absorption in the 370–660 nm spectral band.

AE33 measurements were interrupted after December 22, 2023, due to the instrument’s transfer for a field campaign in Athens [78] or technical issues (August 2023). The raw 1-min data underwent quality control to exclude zero values, perturbations from internal calibration and audit tests, and outliers (short-term BC spikes < 10 min). The filtered data were then averaged on an hourly basis. The total uncertainty in b_abs, BC, BC_ff, and BC_bb was considered in the order of 15–20%, mostly influenced by the MAE values, C correction factor and aethalometer model (online retrievals of BB%) [28,71].

BrC absorption (b_absBrC,λ) was determined by estimating b_abs,λ for BC using an AAE_BC = 1 and assuming that the BrC contribution at 880 nm is negligible. Then, the b_absBrC,λ was estimated by subtracting the BC from the total absorption in the spectrum 370–660 nm [27,78]. The consideration of AAE_BC = 1 is also subject to uncertainty, since coating with other species enhances the BC absorption mostly at shorter wavelengths (lensing effect) and increases the AAE_BC (e.g., [76,79]). In our case, sensitivity analysis with varying AAE_BC from 0.9 to 1.2 resulted in small changes in b_absBrC,370 (0.2–0.4 Mm⁻¹), while the percentage uncertainty was higher in summer due to very low b_absBrC,λ values. In addition, meteorological measurements (temperature, rainfall, wind speed, and direction) from the National Observatory of Athens (NOA) station in Kozani, as well as NO_X measurements at the same site were also used as supporting data.

4. Results and Discussion

4.1. BC Concentrations

The concentrations, seasonality, and source apportionment of BC at the regional background site are examined here, and compared with results from similar sites in the Mediterranean and across Europe. Figure 2 shows the daily-averaged values for BC, BC_ff, BC_bb, total, and BrC absorptions, as well as AAE_370–880, throughout 2023 at the Kozani station. Key statistical indicators from the hourly time series (mean, median, and standard deviation (StDev)) are summarized in

Table 1. Descriptive statistics (mean/median/StDev) of AE33 measurements at Kozani station, NW Greece.

	Annual	Winter	Spring	Summer	Autumn
	Μean/Median/ StDev	Μean/Median/ StDev	Μean/Median/ StDev	Μean/Median/ StDev	Μean/Median/ StDev
BC  (µg m−3)	0.44/0.39/0.27	0.53/0.48/0.33	0.34/0.30/0.20	0.44/0.41/0.18	0.48/0.45/0.30
BCff (µg m−3)	0.30/0.27/0.19	0.28/0.23/0.20	0.23/0.15/0.36	0.36/0.33/0.15	0.34/0.32/0.22
BBbb (µg m−3)	0.15/0.10/0.14	0.26/0.18/0.24	0.10/0.08/0.07	0.08/0.06/0.06	0.14/0.11/0.13
BB%	34.8/33.1/15.9	49.1/49.1/13.8	34.6/33.1/15.9	20.5/18.4/9.2	31.5/29.1/13.5
Abs,370 (Mm−1)	7.12/5.74/5.20	10.35/9.43/6.81	5.17/4.41/3.30	5.64/5.11/2.50	7.14/6.31/5.00
Abs,520 (Mm−1)	4.09/3.51/2.65	5.30/4.85/3.35	3.07/2.69/1.87	3.73/3.45/1.54	4.34/4.02/2.76
AbsBrC,370  (Mm−1)	2.26/1.29/2.82	3.99/2.81/4.14	1.63/1.16/1.48	1.04/0.73/1.05	2.18/1.81/2.45
AAE370–880	1.47/1.43/0.25	1.69/1.70/0.21	1.46/1.43/0.20	1.24/1.21/0.10	1.42/1.39/0.22
AAEBrC	3.60/3.61/1.11	3.86/4.14/1.32	3.56/3.58/0.72	3.06/2.94/0.86	3.81/3.77/1.28

on both an annual and seasonal basis. Note that for non-normally distributed values like BC, the median value is more representative, while the mean is also provided for comparisons with other studies.

Daily mean BC concentrations present a limited variability, between 0.1 and 1.36 μg m⁻³ (Figure 2), with occasional peaks only during the cold period (late autumn and winter). This pattern is characteristic of continental background stations, with an annual mean BC concentration of 0.44 ± 0.29 μg m⁻³ (median: 0.39 µg m⁻³), peaking in winter (0.53 μg m⁻³) and autumn (0.48 μg m⁻³) (Table 1), while summer and spring exhibited lower levels (0.34–0.44 μg m⁻³) and small variability. The daily BC evolution indicates standard atmospheric conditions and minimal effect from transboundary pollution events or local sources such as industries, mining, and power plants.

Most BC (65.2% of the annual mean) originated from fossil fuel combustion (i.e., thermal power plants, vehicular emissions within the University Campus, and nearby residential area), although the BC_ff concentration remained low (0.30 ± 0.19; median: 0.27 μg m⁻³, Table 1). Contrary to BC, BC_ff maximized in summer (0.36 μg m⁻³), followed by autumn (0.34 μg m⁻³) and winter (0.28 μg m⁻³), with spring showing the lowest concentrations across all BC components. The BC_ff daily variation does not reveal any strong pollution event (Figure 2) that could contribute to the summer mean; however, the enhanced summer BC_ff—compared to the rest of the year—is a significant finding that is rarely reported in the Balkans and the Mediterranean. Therefore, it needs further investigation with additional measurements to determine whether this tendency is repeated in other summer periods and represents a consistent seasonal trend in NW Greece. By analyzing the seasonality in traffic-related pollutants like NO_X, it was observed that the summer NO_X values were higher than those in spring and similar to those in winter and autumn; so increased emissions of traffic or other fossil fuel combustion sources may justify the summer BC_ff levels.

On the other hand, BC_bb exhibited the most distinct seasonality, with an annual mean of 0.15 ± 0.14 μg m⁻³ (median: 0.10 μg m⁻³) and maximum values in winter (0.26 μg m⁻³), nearly double than the autumn levels (0.14 μg m⁻³) and almost triple those measured in spring and summer (0.08 μg m⁻³). This seasonal trend clearly reflects the impact of RWB in rural areas of NW Greece, a common practice for heating during the harsh winter, which leaves a noticeable—albeit not dominant—fingerprint on atmospheric pollution [80]. Biomass burning (BB) emissions contribute equally (49.1%) to winter BC levels, while their influence during spring and summer was lower (20–34%), though still notable given the reduced vehicular emissions compared to urban areas [76,81,82].

Even at this regional background site, the BC concentrations are significantly affected by the meteorological conditions, since during the rainy days, BC, BC_ff, and BC_bb were reduced to 0.37, 0.24, and 0.12 μg m⁻³, respectively. Similarly, the lower BC levels in spring 2023 compared to other seasons are associated with higher accumulated rainfall (311.6 mm) compared to other seasons (i.e., 153 in winter, 238.2 in autumn, and 134.2 mm in summer) and a larger fraction (51%) of rainy days (Figure 3). Therefore, the cleaner atmospheric conditions in spring are mainly attributed to aerosol rainy washout, while the seasonal mean wind speed was very low (0.72 ms⁻¹) compared to summer and autumn (5.2–5.3 ms⁻¹). Seasonal wind rose diagrams for the BC_ff, BC_bb, and BrC absorption values show that the highest BC concentrations are mostly associated with northern directions, while during winter, the southern sector also contributes to enhanced BC_bb and b_absBrC values, likely related to local–regional emissions from nearby villages (Figure 3). The higher BC_ff values during summer are related to NNW winds, likely associated with polluted air masses from the Balkans or with regional ones from the lignin power plants located north of Kozani (Figure 3).

A comparison with previous BC measurements at other sites in Greece, Mediterranean, and Europe highlights the continental background character of the site and the urban effect on increased BC levels in big cities and industrialized areas. At Finokalia, a regional background station in Crete, the monthly BC levels ranged from 0.25 μg m⁻³ (January) to 0.75 μg m⁻³ (August) (year 2017), exhibiting comparable values with Kozani, but with a different seasonality of winter low (0.31 ± 0.23 μg m⁻³) and summer high (0.57 ± 0.24 μg m⁻³) [83]. Comparable to Kozani, EC concentrations at regional background sites in Europe were reported for Melpitz, Germany (0.48 μg m⁻³); Harwell, UK (0.28 μg m⁻³); while lower EC levels were found at Finokalia, Crete (0.22 μg m⁻³), Montseny, Spain (0.26 μg m⁻³); Aspvreten, Sweden (0.15 μg m⁻³); and Birkenes, Norway (0.08 μg m⁻³), characterizing marine and mountainous background conditions [21]. In addition, the annual BC of 0.88 ± 1.5 μg m⁻³ was reported at a suburban site in Helsinki [82]. In Nicosia, Cyprus, BC levels during the cold and warm periods were 2.01 ± 2.31 µg m⁻³ and 1.01 ± 1.46 µg m⁻³ with estimated regional background levels of ~0.7 µg m⁻³ and 0.22 µg m⁻³, respectively, aligning with current concentrations [84].

Monthly BC concentrations in Kozani ranged from 0.30 µg m⁻³ in April to 0.61 µg m⁻³ in February, significantly lower than those in Athens, where monthly BC levels varied from 1.16 µg m⁻³ (August) to 3.47 µg m⁻³ (December) [26]. This represents a 4–5 times increase, highlighting the urban effect (enhanced traffic, industrial, and RWB emissions). Due to missing data during 21–23 August 2023, the Evros wildfire’s impact on BC levels cannot be assessed in Kozani [39,40]. Furthermore, BC_ff in Kozani exhibited the highest values in October (mean of 0.38 µg m⁻³) and lowest in April (0.20 µg m⁻³), while the monthly variability of BC_bb was different with the lowest in June (0.05 µg m⁻³) and the highest in February (0.28 µg m⁻³). The summer mean BC concentration in Kozani (0.44 µg m⁻³) is comparable to EC measurements at regional background sites in the Mediterranean region [21,85,86,87,88,89], while higher summer means (almost triple) were reported in urban Athens (1.3 ± 1.1 µg m⁻³ [26]) and Ioannina (1.04 ± 0.67 µg m⁻³ [29]).

Greek cities presented significantly higher (3–5 times) annual BC concentrations compared to the current regional background site, like Athens (BC: 1.89 ± 2.49 μg m⁻³ [26]). Furthermore, average BC concentrations of 2.4 ± 1.0 µg m⁻³ and 1.6 ± 0.6 µg m⁻³ were reported during the cold and warm periods, respectively, at a suburban site in Athens [24]. Kaskaoutis et al. [28] found a mean BC of 2.49 ± 3.43 μg m⁻³ from December 2016 to February 2017 in Athens, in consistency with the 4 years (2015–2019) winter mean of 2.81 μg m⁻³ [26]. The current winter mean (0.53 μg m⁻³) is about 5 times below the mean concentrations in Athens, highlighting a large urban effect on BC levels, which however, are escalated by the shallow mixing layer. Contrary to the regional background conditions for northern Greece presented here, almost 10 times higher BC levels (5.2 ± 3.4 μg m⁻³) were recorded during winter in Ioannina, located in a closed basin at approximately 150 km SW of Kozani [29]. Due to the absence of a district thermal system, as in the city of Kozani, residents in Ioannina (~120,000 people) mostly use firewood for domestic heating, which resulted in a serious degradation of air quality during winter (PM_2.5: 57.5 ± 35.3 μg m⁻³; BC_bb: 4.5 μg m⁻³) [29]. However, it should be noted that the Kozani station represents a background site, in contrast to the urban measurement site in Ioannina, where the enhanced humidity from Pamvotis Lake and the closed basin topography hinder the dispersion of pollutants. In addition, current annual BC_ff levels are significantly lower than those reported in Athens, i.e., 1.42 ± 1.63 μg m⁻³ [26]; the same holds for the winter BC_ff in Kozani (0.28 μg m⁻³) with respect to those in Athens, i.e., 2.7–2.9 µg m⁻³ [47,48]. Due to enhanced RWB emissions during wintertime in Greek cities, the seasonal mean BC_bb levels can reach up to 1.07 μg m⁻³ in Athens, peaking at 18 μg m⁻³ [26], against 0.26 μg m⁻³ in Kozani. High levels of BC, BC_ff, and BC_bb concentrations (3.72, 2.43, and 1.28 µg m⁻³, respectively) were also reported at Piraeus port in winter, reflecting the effects of traffic, port activities, and RWB [90]. In synopsis, the current levels of BC_ff and BC_bb are comparable to the EU and US standards for background environments [91,92], rendering the site as an ideal place for measurements of regional carbonaceous aerosols.

Seasonally, BB% fraction peaked in winter (~49.1%), with daily maxima in the evening (20:00–23:00 LST), reflecting a balance between RWB and fossil fuel combustion in Western Macedonia (Table 1). A much higher BB% of 79.5% during winter was recorded in Ioannina due to extensive RWB for domestic heating [29]. The 4-years annual mean BB% in Athens was 22.3 ± 12.3% [26], indicating a stronger influence of fossil fuel emissions compared to continental background levels, while during winter, the BB% increased to 25–35% [24,26,48,93]. Generally, continental background or mountainous sites in Europe exhibit significantly higher BB% fractions than urban centers due to lower traffic emissions and greater reliance on wood burning for domestic heating. In this context, comparable and higher BB% fractions than those in Kozani were observed in Alpine villages, (51% to 88% [67,94,95]) and rural sites in northern Europe (56% [96], 68% [97]). A recent global emission inventory indicated that RWB is the primary source of BC (35%), followed by diesel vehicle emissions (26%) [98]. Furthermore, coatings on BC particles, particularly when originating from BB sources, enhance its absorption efficiency at shorter wavelengths (lensing effect) leading to higher BB% and enhanced AAE values [99,100,101].

Figure 4 shows the seasonal averaged diurnal patterns of BC_ff and BC_bb, with the shaded areas representing 1 StDev from the hourly mean. Key findings from this figure are (i) a morning BC_ff peak linked to traffic emissions, (ii) the balanced contribution of BC_ff and BC_bb during the night hours in winter, (iii) the higher BC_ff in summer compared to other seasons throughout the day, and (iv) the flat BC_bb diurnal pattern in all seasons except of winter. The morning BC_ff peak could be attributed to enhanced traffic emissions in the campus and in nearby residential areas and is associated with a concurrent peak in NO_x concentrations (not analyzed here). The slight morning peak in BC_bb during winter may be also related to activation of fireplaces in the nearby villages for domestic heating. On the other hand, the lower concentrations during noon-to-early afternoon hours could be ascribed to enhanced dilution within a deeper mixing layer height (MLH); however, its effect is minimal for regional background sites with low aerosol loading compared to urban environments [26,68].

Despite the background character of the site, BC components exhibited the highest diurnal variation in winter. On the other hand, the morning traffic effect on BC_ff component was apparent in all seasons. Based on these findings, the weekly variation of BC components was analyzed in Figure 5 during wintertime. Weekly patterns reveal minimal variability from day to day, as expected, while the traffic effect in the morning hours is apparent across weekdays (mostly Monday to Thursday). Saturday recorded the lowest BC levels, with equal contributions from BC_ff and BC_bb. The overall bimodal daily pattern—morning and nighttime peaks—reflects both traffic and RWB emissions, while the respective weekly patterns in summer and spring showed reduced daily and weekly variability consistent with regional background characteristics.

4.2. Absorption Characteristics

In this section the spectral light-absorption properties of BC and BrC, their seasonality, diurnal variation, and contribution of BrC to total absorption were analyzed. Such analysis is crucial for understanding the regional climate through quantifying the radiative effects of carbonaceous aerosols and their influence on radiation budget and atmospheric chemistry. A recent study revealed very high b_abs levels by both BC and BrC at Ioannina City during winter [29], while the current background levels aim to constitute the basis for comparisons and quantification of the urban effect.

The total absorption at 370 nm (b_abs,370) in Kozani exhibited an annual mean of 7.12 ± 5.20 Mm⁻¹ (median: 5.74 Mm⁻¹), with higher values in winter (10.35 Mm⁻¹), almost double the absorption coefficients in spring and summer (5.17 Mm⁻¹, 5.64 Mm⁻¹) (Table 1). In the green spectrum, b_abs,520 followed the same seasonal pattern, with lower annual and winter means of 4.09 ± 2.65 Mm⁻¹ and 5.30 ± 3.35 Mm⁻¹, respectively.

Significantly higher winter b_abs,370 values of 262 Mm⁻¹ (not corrected for the harmonization factor H = 1.76) were found in Ioannina [29] compared to the current background levels (10.35 Mm⁻¹). The 14-fold higher b_abs in Ioannina (after applying the H factor) underscores the major impact of urban wood-burning emissions for domestic heating, particularly in a city without district heating or a natural gas network. Similarly, high b_abs,370 of 123 Mm⁻¹ was reported at a rural site with valley-like characteristics in Slovenia, highlighting the large effect by BB emissions for domestic heating in the Balkans during winter [80]. In contrast, the summer b_abs,370 levels in Ioannina (12.8 Mm⁻¹ after applying the correction factor) were closer to the regional background of Kozani (5.64 Mm⁻¹), reflecting the effect of urban traffic and industrial emissions. In general, current b_abs levels are significantly lower than those reported in other urban sites in the Mediterranean [74,102,103,104], highlighting the continental background character of the site.

In this context, b_abs values in Kozani are comparable to those observed at regional background sites across Europe, mostly ranging between 5 and 10 Mm⁻¹ [23], while in contrast, mean b_abs,370 values in European cities (i.e., Paris, Milan, Athens, Rome, Granada) were considerably higher, ranging from ~18 to 30 Mm⁻¹ [23]. At Finokalia, Crete, the annual mean b_abs,625 values were found to be 2.73 ± 1.45 Mm⁻¹, ranging slightly between the seasons from 2.41 Mm⁻¹ (winter) and 3.00 Mm⁻¹ (summer) [21], while a b_abs,535 value of 5.4 ± 3.7 Mm⁻¹ (annual mean) was reported for the same site during 2000–2006 [105]. These values are comparable to those observed for b_abs,590 in Kozani i.e., 3.33 Mm⁻¹ (annual mean), 4.20 Mm⁻¹ (winter) and 3.12 Mm⁻¹ (summer). Comparable annual mean b_abs,625 values were observed at other regional background stations in Europe, such as Montseny, Spain (2.32 ± 1.53 Mm⁻¹); Harwell, UK (3.82 ± 1.56 Mm⁻¹); and Melpitz, Germany (4.39 ± 1.73 Mm⁻¹), while significantly lower values were reported at Scandinavian and Alpine remote sites (0.66 to 1.31 Mm⁻¹) [21]. On the contrary, annual (4-year) average b_abs520 values in Athens, were found to be much higher, i.e., 45.8 Mm⁻¹ (in winter), 22.3 Mm⁻¹ (in spring), 19.1 Mm⁻¹ (in summer), and 27.6 Mm⁻¹ (in autumn) [27], similar to those reported by Katsanos et al. [25] for a 1-year period, though neither study applied the harmonization factor (1.76). Compared to the background levels in Kozani, absorption at mid-visible wavelengths in Athens was 2.9 and 5.5 times higher in summer and winter, respectively, clearly reflecting the urban influence on total absorption at mid-visible wavelengths.

The optical properties of light-absorbing aerosols (BC and BrC) may be significantly different, and the temporal evolution of BC may not coincide with that of BrC. This discrepancy arises from differences in emission rates between fossil fuel combustion and biomass burning, as well as atmospheric processes such as absorption by biogenic and secondary organic aerosols (SOAs) and degradation of BrC chromophores via photobleaching and volatilization [106,107].

In this respect, Figure 6 presents the seasonal mean diurnal variations of BC concentrations, color-coded with the b_absBrC,370, aiming to explore the consistency in diurnal patterns between BC and BrC and potential variations during the day. In all seasons, BC levels exhibit low variability, characteristic of a regional background site, while the effect of wood burning is notable during night hours in winter, spring, and partly in autumn (Figure 6). Additionally, BC peaks between 08:00 and 10:00 LST in all seasons, reflecting a slight influence from local traffic near the university campus and surrounding areas, as discussed in the previous section.

Although BrC absorption does not exhibit a significant diurnal variability, some notable trends emerge. Specifically, b_absBrC,370 increases during winter and spring nights (20–02:00 LST), highlighting the radiative impact of RWB and the substantially greater light-absorbing efficiency of BrC from wood combustion compared to fossil fuels [72,108]. The ratio of BC to BrC emissions also varies between daytime and nighttime. A similar feature is observed in autumn, but with a narrower time frame of increased BrC absorption, possibly because the heating period started by the end of October.

Another important finding is the small increase in b_absBrC,370 during the morning hours in summer and spring, likely attributed to SOA formation that contributes slightly to BrC light absorption [109,110,111]. Nighttime chemistry may also produce SOA through reactions between BB-related volatile organic compounds (VOCs) and nitrate radicals, forming organonitrates and nitro-phenols that enhance BrC absorption [112,113,114]. During early afternoon hours, BrC absorption reaches its lowest values, evidencing photo-dissociation effects and bleaching of BrC chromophores due to intense solar radiation [18,30,115,116].

At this regional background site, the correlation between BC and b_absBrC,370 was found to be weak (R² = 0.22) when considering hourly annual data. However, this correlation improved to R² = 0.38 in winter, indicating that BC and BrC originate from various and often contrasting sources. Rural activities can contribute to organic aerosol emissions from non-combustion sources, while secondary and distant emissions may weaken the relationship between BC and BrC. Additionally, processes such as volatilization, atmospheric aging, and long-range transport can alter the absorbing efficiency of BrC, further decoupling its concentration from BC levels [117,118]. On the other hand, during winter, BC and BrC appear to share more common sources, mainly combustion processes in the rural landscapes surrounding the measurement site. Conversely, in Athens, a strong correlation (R² = 0.89) was found between BC and b_absBrC,370, suggesting that local and common combustion sources dominate carbonaceous aerosol emissions in the urban environment [27,78].

Figure 7 shows the seasonal averaged diurnal patterns of b_abs,370 (BC and BrC), as well as those for the BrC contribution to total b_abs,370. Notable seasonal differences were observed in both absolute absorption values and the relative contribution of BrC. In winter, the hourly-averaged b_absBC ranged from 5.4 Mm⁻¹ to 8.0 Mm⁻¹, while that of BrC ranged from 2.8 to 5.1 Mm⁻¹ (Figure 7a). In spring, summer, and autumn, the b_abs components decreased significantly, as well as the BrC contribution. Note that the diurnal pattern of BrC contribution flattens out in summer and autumn, while in winter and spring it exhibits an increasing trend during night hours, reflecting the wood burning effect (Figure 7). As mentioned above, the morning increase in BC absorption may be partially attributed to transportation near and inside the university campus, while the lower b_abs,BrC during noon-to-afternoon hours is attributed to weaker absorbing efficiency of BrC chromophores due to volatilization and photo-bleaching (discoloration or whitening of BrC) effects through chemical reactions with O₃ and OH [119,120]. Although coal-based power plants and vehicular emissions may contribute to BrC absorption [121,122], their effects on the regional background site were low, as seen from the diurnal pattern of BrC contribution. Future chemical speciation measurements would be especially informative for unravelling the chemical processes affecting carbonaceous aerosols across different seasons.

The spectral absorption characteristics of BC and BrC may be highly time variable [26,28] due to the complexity of BrC absorption, depending on the source, atmospheric mixing, photo-dissociation, and photo-bleaching processes, as well as potential meteorological effects like boundary layer dynamics, rainfall, wind speed, and direction [17,118,123,124,125,126].

4.3. Brown Carbon Absorption Properties

Regarding the estimated b_absBrC,370, an annual mean of 2.26 ± 2.82 Mm⁻¹ was found, with a maximum in winter (3.99 Mm⁻¹) and very low values in summer (1.04 Mm⁻¹) (Table 1), highlighting the strong combustion effect on BrC absorption in the UV [6,30]. On the contrary, very low b_abs,BrC values were estimated at 660 nm (annual mean: 0.23 Mm⁻¹; winter: 0.37 Mm⁻¹), indicating a negligible contribution from BB sources in the far-visible and NIR spectrum. However, a part of this small absorption may be also related with BC lensing effect, as previous studies have shown [127,128,129]. The annual mean b_absBrC,370 in Athens was estimated at 15.6 Mm⁻¹, doubling in winter (37.2 Mm⁻¹), and becoming three times lower in summer (5.3 Mm⁻¹) [27], indicating that the urban effect was almost four times above background levels (when the correction factor H was applied).

The BrC contribution to total light absorption in the UV and near-visible spectrum can be important when organic matter originates from combustion sources [130,131,132]. In this study, the annual mean BrC contribution to total absorption at 370 nm was 32.6%, maximizing in winter (38.0%), followed by spring (29.8%) and autumn (26.7%), while in summer, the Abs_BrC370/Abs₃₇₀ was 17.3%. BrC related to combustion sources exhibits much stronger absorbing efficiency than organic matter originating from biogenic sources or secondary formation [133,134,135]. This justifies the significantly higher (more than double) BrC₃₇₀ fraction in winter compared to summer, even at this background station without direct influence from RWB, traffic, and industrial emissions. The annual mean b_absBrC,370 fraction is higher than that computed in Athens (23.7 ± 11.6%) [27], and falls within the mid-range of BrC₃₇₀ contributions observed worldwide, ranging mostly between 15% and 50% [17,136]. A literature overview in Europe showed that the BrC₃₇₀ contribution ranged mostly between 15% and 30% at regional background sites, while comparable contributions were found at the urban and suburban sites [23]. High BrC₃₇₀ contributions in the order of 40–60% in winter were reported at cold, mountainous, and alpine sites in Europe [67,80,96], while the winter average b_absBrC,370 fraction in Athens was 33.5% [27] and in Ioannina, it was 65% [29]. Furthermore, BrC₃₇₀ contributions in the order of 18 to 42% were reported at several cities in France [137], well within the current estimates, while Mbengue et al. [129] estimated a BrC contribution of 19% in winter at a regional background station in the Czech Republic. In summer, the mean BrC₃₇₀ contribution to total absorption in Kozani (17.3%) was comparable to values in Athens (18.5% [27]) and Ioannina (14.5% [29]), revealing a regional background contribution of light-absorbing organics of 15–20% in the Mediterranean atmosphere during summer.

Figure 8a–c show the monthly-averaged spectral dependence of b_abs, b_absBrC, and BrC fraction, respectively, aiming to visualize the seasonality in spectral absorptions attributed to BC and BrC according to their source and atmospheric processing. The results highlight the significantly higher b_abs values in winter months (peaking in February for the year 2023), the lower b_abs values in spring and summer, especially at longer wavelengths, and the very low b_absBrC at even shorter wavelengths in spring and summer, suggesting an absence of combustion emissions. The increased photo-oxidation, photo-bleaching, and enhanced SOA formation in summer reduce the light-absorbing efficiency [110,138,139].

4.4. AAE Values for Total and BrC Absorption

The AAE quantifies the spectral dependence of the light absorption, and is affected by the emission source, aging processes, type of burned material, phase of combustion, mixing with other aerosols, and meteorological conditions [104,126,140,141]. In this respect, the seasonality, diurnal patterns, and relationships between AAE_370–880 and AAE_{BrC,370–660} were analyzed here and compared with values at other sites of different land uses over the globe.

The hourly AAE_370–880 ranged from 0.68 to 2.57 (mean of 1.47 ± 0.25), while AAE_470–950 exhibited a wider range (0.56–3.52) in a similar mean value (1.47 ± 0.27) (Table 1). On the other hand, AAE_370–520 was found to be higher (1.53) than that at longer wavelengths (AAE_520–880 = 1.43), suggesting considerable influence from light-absorbing organics with higher absorbing capability in the UV range [77,141,142].

On a seasonal basis, the mean AAE_370–880 maximized in winter (1.69), followed by spring (1.46), autumn (1.42), and summer (1.24) (Figure 2, Table 1). The annual AAE_370–880 in Kozani was higher than that found in Athens (1.31 [27]) due to a much lesser effect from fossil fuels at rural/background areas, while the same trend was followed in winter (AAE_370–880 of 1.69 in Kozani against 1.45 [27] and 1.38 [28] in Athens).

The month–hour contour plot of AAE_370–880 reveals contrasting values between the months and hours of the day, emphasizing the impact of biomass combustion during winter nighttime and the enhanced presence of UV-absorbing organics (Figure 9). Conversely, the low AAE_370–880 (~1.1–1.3) in summer indicates the absence of absorbing BrC and clear dominance of fossil fuel combustion, as discussed earlier.

In addition, hourly AAE_BrC (370–660 nm) values ranged from about 0.5 to 10.2, with an annual mean of 3.60 ± 1.11 (Table 1). On a seasonal basis, the highest mean AAE_BrC was found in winter (3.86), followed by autumn (3.81), spring (3.56), and summer (3.06), indicating relatively small seasonal variations in the spectral dependence of BrC (Figure 9b). Higher nighttime AAE_BrC values are associated with an enhanced presence of aromatic organic compounds of larger molecular weights from RWB emissions [18,30,143,144]. On the contrary, the lower values around the noon hours are likely related to photo-bleaching processes or SOA formation that degrade the BrC absorption and weaken its wavelength dependence, while a similar behavior was detected at other sites [115,145,146]. Similar AAE_BrC values were reported in Athens (3.6 ± 0.9 on an annual basis and 3.9 ± 0.9 in winter) [27], while previous studies in Mediterranean cities exhibited AAE_BrC in the same order, like 4.0 ± 0.2 in Genoa [147], 3.8 ± 0.11 in Milan [148], and 3.2 ± 0.9 in Bologna, Italy [34]. The mean AAE and AAE_BrC values at regional background sites across Europe were mostly in the range of 1.2–1.4 and 3–5, respectively [23], within the range of the current results.

The relationship between the absorption coefficient and AAE enables determination of the different aerosol types and mixing states in the atmosphere [103,149]. A seasonal analysis of b_abs,370 vs. AAE_370–880 (Figure 10) revealed that higher b_abs,370 values are associated with AAE above 1.6, suggesting enhanced presence of BrC mixed with BC from BB sources [142,149], as shown in winter and, to a lesser extent, in autumn (Figure 10). On the contrary, the spring and summer data are associated with low b_abs and variable (~1 to 2) AAE values, corresponding to clean atmospheric conditions, slightly affected by various sources. A more detailed analysis of aerosol types and source regions at this site could be achieved through additional measurements of scattering coefficients to provide more robust results.

The climate implications arising from light absorption by carbonaceous aerosols are a critical issue for regional and global models as they contribute to reducing the uncertainties of aerosol on atmospheric heating and climate change [16,119,150,151,152]. The current results on BC concentrations, and BC and BrC absorptions are characteristic for a continental regional-background atmosphere in the southern Balkans and could be used as inputs in climate models or for comparisons to assess the urban effect. BC measurements and systematic monitoring of air pollution in medium- and small-sized cities in the Balkans are scarce, despite the fact that air quality may be highly degraded in view of RWB emissions and favorable meteorological conditions like temperature inversions, calms, and fog formation [80,149]. Future studies in Western Macedonia should focus on the radiative impacts of continental background aerosols, on the identification of aerosol types and atmospheric chemistry, on the evaluation of transboundary pollution and contribution of various sources in a region undergoing decarbonization, and pursuing sustainable development.

5. Conclusions

This study examined the concentrations and spectral absorption characteristics of carbonaceous aerosols (BC and BrC) on an annual basis at a regional background site in the southern Balkans. Measurements were performed using an aethalometer (AE33) instrument at the University of Western Macedonia (UoWM) campus, about 4 km outside the city of Kozani (NW Greece). Nowadays, the region of Western Macedonia is in the phase of delignification and energy transition to more environmentally friendly energy sources (e.g., renewables), closing lignite power plants. BC and BrC absorption properties were analyzed for the first time in this area and the results were extensively compared with those from regional background sites in Greece and Europe. In addition, the inter-comparison with BC levels at urban regions revealed the urban effect on carbonaceous emissions.

During 2023, the annual mean BC concentration at the site was found to be 0.44 ± 0.27 µg m⁻³ (median: 0.39 µg m⁻³), with maximum levels in winter (0.53 µg m⁻³) and lowest in spring (0.34 µg m⁻³), comparable to those observed at several background sites in the Mediterranean and Europe. The fossil fuel-derived BC component (BC_ff) dominated, with an annual mean concentration of 0.30 ± 0.19 µg m⁻³, though it exhibited a different seasonality, with a summer maximum (0.36 µg m⁻³), associated with enhanced fossil combustion processes and favorable meteorological conditions (i.e., less rainfall). In winter, residential wood burning (RWB) in rural areas of Western Macedonia constitutes a major source of carbonaceous aerosols, resulting in an equal (50-50%) contribution of BC_ff and biomass-burning-derived BC (BC_bb). In contrast, the biomass burning (BB) contribution was lower during the rest of the year (20–30%), while the lowest BC concentrations in spring were associated with increased rainfall. The generally low temporal variability of BC components supports the background character of the site, while the levels were comparable to those observed at other regional background sites in Europe. On the other hand, BC and total absorption presented maximums during the morning hours, reflecting traffic emissions within the campus, while lower absorption values for both BC and BrC were observed at noon to early afternoon, associated with dilution processes and photo-bleaching or volatilization of BrC chromophores.

The absorption coefficient at 370 nm peaked in winter (10.35 Mm⁻¹), with an annual mean of 7.12 ± 5.20 Mm⁻¹, characteristic of continental background sites influenced by winter BB emissions, while summer levels were significantly lower (5.64 Mm⁻¹). BrC absorption exhibited a similar seasonality with maximum values in winter (3.99 Mm⁻¹ at 370 nm) and an annual mean of 2.26 ± 2.82 Mm⁻¹, contributing, on average, 32.6% to total absorption (38.0% in winter and 17.3% in summer). The seasonal mean diurnal patterns of BrC contribution did not exhibit a pronounced variability, except in spring—and partly in winter—when a notable daytime decrease was observed, which likely reflects photo-degradation processes. A moderate-to-weak correlation between BC and b_absBrC,370 (R² = 0.22) suggests multiple and often contrasting sources of BC and BrC. Furthermore, the diurnal patterns of AAE_370–880 and AAE_BrC supported the background character of the site with limited influence from local combustion sources. Both AAE_370–880 and AAE_BrC maximized during winter (1.69 ± 0.21 and 3.86 ± 1.32, respectively), while the slight nighttime increase is characteristic of the RWB effect on spectral absorption at shorter wavelengths. Photo-degradation processes, lack of BB emissions, and absorption by secondary organic aerosols during summer lead to weaker light absorptions and lower AAE values.

In synopsis, despite the regional character of the site, current results detected notable differences between sources, atmospheric processing, and photochemistry between winter and summer. Therefore, the site can be considered as representative of continental pollution conditions in northern Greece, and the findings provide key carbonaceous aerosol properties that could serve as a basis for numerical simulations of their climatic effects in the Balkans, addressing a critical gap in regional aerosol measurements. A comparison of the current levels of carbonaceous aerosols and absorption coefficients between Kozani and Ioannina (a city of similar weather conditions in NW Greece), revealed significantly lower wood burning emissions for domestic heating and much better winter air quality in the region of Kozani, because of the district heating system and lower demand for individual heating devices. Therefore, introducing new district heating networks could be an important measure that governments should consider implementing also in smaller towns across Europe.