High-Time-Resolution Measurements of Equivalent Black Carbon in an Urban Background Site of Lecce, Italy

Abstract

Carbonaceous aerosols represent a significant component of atmospheric aerosol, with implications for climate and human health. The recent EU Directive 2024/2881 highlights the need to monitor emerging pollutants like black carbon more effectively. This study presents an brief field campaign at an urban background site aimed at characterizing carbonaceous aerosols. Daily samples of PM₁₀ and PM_2.5 were analyzed using a Sunset thermal-optical analyzer to determine organic and elemental carbon (OC, EC), while real-time equivalent black carbon (eBC) was measured with three independent instruments: MAAP, AE33, and Giano BC1. Total carbon (TC) was monitored using an online TCA08 thermo-catalytic analyzer. The average concentration of PM₁₀ was 17.1 µg/m³ and 10.4 µg/m³ for PM_2.5. On average, OC and EC represented 16.5% and 3.6% of PM₁₀ mass, and 22.6% and 5.5% of PM_2.5. SOC accounted for 36% of OC. The in situ Mass Absorption Cross-section (MAC), recalculated for the ECO site, was between 8.0 and 12.2 m²/g. eBC concentrations were modulated by the daily evolution of the planetary boundary-layer height and combustion sources. The apportionment of eBC was 65% from fossil fuel and 35% from biomass burning. Biomass-burning emissions were further confirmed by optical measurements, with BrC contributing 35% of absorption at 370 nm.

1. Introduction

Carbonaceous aerosols (CAs) are an atmospheric pollutant of major relevance at local, regional, and global scales due to their involvement in several atmospheric processes, including radiative forcing [1,2], heterogeneous chemical reactions, cloud development, and the reduction in regional visibility, as well as their potential harmful effects on human health [3,4,5]. The carbonaceous fraction constitutes a significant portion of PM, typically accounting for 20–50% of its mass [6,7], and is predominantly derived from anthropogenic sources [8] and can be described by numerous fractions of different origin and composition. According to the literature, these fractions are commonly indicated by acronyms [9]: TC (total carbon), being the total mass of carbon in an aerosol sample, frequently measured by thermal evolution or thermo-optical analysis; EC (elemental carbon), defined as the fraction of TC which does not volatilize at low temperature during a thermo-optical analysis, usually below 550°; and OC (organic carbon), being the fraction of TC contained in organic molecules, including thousands of different organic compounds with widely varying chemical and physical properties. Organic carbon (OC) may be directly released as Primary OC (POC) from various sources, such as combustion processes, industrial activities, geological inputs, and natural emissions. It can also be formed in the atmosphere as Secondary OC (SOC) through the chemical transformation of volatile and semi-volatile organic compounds, and the products undergo condensation or nucleation. Another commonly used acronym is the eBC (equivalent black carbon) defined as the TC fraction which shows a high absorption across a wide spectrum of visible and infrared wavelengths. The term eBC refers to the outcomes of optical methods used to determine the carbon content in PM, whereas EC denotes the results obtained through thermo-optical analysis [9]. Because of its detrimental impact on human health, the emission of eBC aerosols has become a serious concern: it can immediately enter the human respiratory system, reaching alveoli because of its small particle size and suspension capabilities, causing several health problems and posing a serious risk for public health [10]. In addition to health concerns, eBC is regarded as the second most major contributor to global warming after CO₂ [11,12,13], highlighting the need to deeply characterize the optical properties of eBC aerosols [14]. In addition to the eBC, a fraction of organic aerosols, collectively referred to as brown carbon (BrC), contribute to aerosol radiative effects due to their absorption in the visible and near-ultraviolet (UV) spectral regions [15]. BrC is predominantly emitted from low-temperature combustion processes, such as residential wood combustion and other forms of biomass and biofuel burning identified as its major atmospheric sources [16,17,18,19,20]. Although BrC plays a dominant role in radiation absorption, a widespread lack of characterization and quantification of its emissions persists.

In April 2024, the European Union adopted a new directive (Directive 2024/2881/EU) aiming to improve air quality across Europe and better understand the health and environmental impacts caused by emerging pollutants including eBC. eBC can be considered as a valuable indicator of human impact on the atmosphere.

With the aim to provide a comprehensive characterization of carbonaceous aerosol at an urban background site in South Italy, an intensive observatory aerosol campaign has been performed between March and April 2023, collecting in parallel daily samples of PM₁₀ and PM_2.5, determining their content in OC and EC concentrations and integrating multiple measurement techniques to evaluate primary and secondary contribution to carbonaceous aerosol as well as optical properties related to black and brown carbon.

The specific objectives of this study are (i) to combine daily and high-time-resolution measurements to provide an in-depth characterization of carbonaceous aerosol and its sources; (ii) to perform an intercomparison of different instruments measuring eBC and TC; and (iii) to determine a new in situ MAC value (an important parameter to describe the optical properties of EC) for the ECO site, recalculated from collocated optical and thermo-optical measurements, and to compare it with the default manufacturer value. This last objective, in particular, represents one of the novel aspects of the study, as previous research has not reported an in situ MAC value specifically determined for this site. Despite extensive research on carbonaceous aerosols, high-time-resolution, multi-instrument measurements at Mediterranean urban background sites remain scarce. By addressing this gap, the present study provides a detailed characterization of carbonaceous aerosol composition, sources, and optical properties, while also offering a site-specific in situ MAC value to improve the accuracy of optical measurements and source apportionment.

2. Materials and Methods

2.1. Measurement Site, Set-Up and Carbon Analysis

The sampling site is on the roof of the Italian national facility of the ACTRIS network, which hosts the Environmental Climate Observatory (named in the following ECO) (SE Italy, 40°20′8″ N–18°07′28″ E, 37 m a.s.l.) (Figure 1a). This is an urban background site [21], often impacted by long-range transport processes from East Europe and coarse dust advection events from Africa, as well as sea spray contribution from the close coastline (far about 20 km) and influenced by secondary aerosol formation [22].

In the period 22 March 2023–19 April 2023, 86 aerosol samples (57 PM₁₀ and 29 PM_2.5) were collected, using quartz fiber filters (Whatmann, Maidstone, UK, QM-A quartz filters, 47 mm in diameter). All filters were thermally pre-treated (for 2 h at 700 °C) for removing, before sampling, any residual carbon contamination on blank membranes [22]. The collected daily PM samples were stored at 4 °C until the EC/OC analysis. The aerosol sampling set-up used was composed of two automatic dual-channel low-volume (2.3 m³/h) samplers: the former, located in the ECO shelter (Figure 1b), simultaneously providing PM₁₀ and PM_2.5 mass concentrations according to the ß-attenuation method (SWAM 5A, FAI Instruments srl, Rome, Itay); the latter, installed close to the ECO shelter (Figure 1b), equipped with a PM_2.5 size selective inlet and an optical module for eBC online measurements (Giano BC1 Dado Lab srl, Milan, Itay). For all data analyses performed in this study, the PM₁₀ and PM_2.5 concentrations measured with the SWAM sampler were used as reference. Typical average uncertainties are 2.5% for PM_2.5 and 3.2% for PM₁₀ [23]. In Figure 1c, a schematic diagram of the sampling set-up used is reported.

Regarding the eBC online measurements in ECO site, three instruments have been used: a Multiangle Absorption Photometer (MAAP, Thermo Scientific, Waltham, MA, USA, mod. 5012, operating at 1 m³/h sampling flow rate, hourly LOD 0.01 µg/m³ of black carbon), located inside the ECO shelter, equipped with a PM₁₀ head, operating at a wavelength of 670 nm [24]; an aethalometer (AE33, Magee Scientific, Ljubljana, Slovenia, operating at 5 L/min, hourly LOD 0.005 µg/m³ of black carbon), located inside the ECO shelter, equipped with a PM₁₀ head, operating at the measurement wavelength of 880 nm; the black carbon analyzer Giano BC1 (Dadolab Srl, Milan, Italy, operating at 2.3 m³/h, hourly LOD 0.05 µg/m³ of black carbon) equipped with a PM₁₀ head and operating at the measurement wavelength of 635 nm. The aethalometers used dried inlets while the inlet of Giano BC1 was located outdoors at environmental relative humidity. The reported uncertainty for the absorption of MAAP at 637 nm is 15%, as stated by Müller et al. [25] and by Zanatta et al. [26]. Regarding the AE33, uncertainties of around 15% have been reported for absorption measurements [27]. Comparison of the Dadolab and MAAP absorption measurements showed differences within 20% in Caponi et al. [28].

As already stated, in measurements of eBC, the Mass Absorption Cross-section (MAC) is an important parameter to describe the optical properties of EC [29]. Bond et al. [30] introduced this parameter as the light absorption cross-section normalized to the mass of a specific aerosol component (e.g., BC), expressed in m²/g, and it is used to convert the light absorption coefficient into eBC mass concentration. Commercial instruments generally rely on predefined constant MAC values; in this work, the values adopted were 6.6 m²/g for MAAP, 7.77 m²/g for AE33, and 10 m²/g for Giano BC1. Nevertheless, the fixed MAC often does not correspond to the actual MAC of the sampled aerosol, leading to inconsistencies between eBC and EC determinations. According to Zanatta et al. [26], MAC values within the European ACTRIS network ranged from 4.3 to 22.7 m²/g at 637 nm. More recent studies have stressed the importance of harmonizing methodologies and instruments employed in large-scale intercomparison campaigns to achieve a more reliable assessment of MAC or eBC [31].

The AE33 instrument differs from MAAP and Giano BC1 aethalometers by measuring aerosol light absorption at seven different wavelengths (λ): 370 nm, 470 nm, 520 nm, 590 nm, 660 nm, 880 nm and 950 nm. This feature allows us to gather more information about the optical properties and the type of absorbing aerosol present at the sampling site, such as the presence of brown carbon (BrC) together with the eBC, also providing the possibility to distinguish between fossil fuel and biomass-burning contributions to the eBC absorption. This is made possible through the application of the aethalometer model, a well-established approach that has been widely used for apportioning eBC absorption between fossil fuel (eBC_ff) and biomass burning (eBC_bb) sources. In the approximation of the aethalometer model, fossil fuel and wood burning are considered the only sources of light-absorbing species in PM [9].

Further information regarding carbonaceous aerosol were gained by a Real Time Total Carbon Aerosol Analyzer (TCA08, Aerosol Magee Scientific, Ljubljana, Slovenia, operating at 1 m³/h, LOD 0.3 µg/m³ of carbon) located in the ECO observatory. This instrument, equipped with a PM_2.5 head, uses a thermal method for TC determination. Further, it is possible to obtain the concentration of organic carbon (OC) by subtracting black carbon concentration measured by the AE33, from the total carbon concentration measured by the TCA08. The use of aethalometer AE33 data with those obtained from TCA08 also allowed us to derive the secondary and the primary organic carbon (SOC and POC) for the ECO site. The comparison of TCA08 online measurements of TC showed a maximum uncertainty, when compared with offline laboratory measurements of 12% in a previous study [32].

The analysis of carbonaceous species (i.e., EC and OC) in the daily PM samples were performed using a Sunset OC/EC analyzer (Sunset Laboratory Inc., Tigard, OR, USA, Figure 1b) operating with the EUSAAR2 protocol (thermo-optical method). A multipoint calibration, using as external standard a sucrose solution (2.198 g/L in water, CPAchem Ltd., Bogomilovo, Bulgaria), was performed to correct OC and EC measured concentrations. Linear calibration had a slope of 0.97, a negligible intercept, and a determination coefficient R² = 1. The average uncertainties of OC and EC concentrations were 5% (OC) and 10% (EC) as discussed in a previous work [23]. LOD was 0.1 µg/cm² of carbon for both OC and EC.

2.2. Determination of Specific MAC and BrC Absorption Coefficients in ECO Site

In this study, the in situ MAC for the ECO site was calculated [1,26,33], knowing that the MAC used in each instrument at a defined measurement wavelength (λ), is a predefined constant value to gather eBC_raw mass concentration (µg/m³) from measurements of light absorption coefficient (σ_abs). For example, in MAAP where MAC is 6.6 m²/g at the measurement wavelength of 670 nm, we have:

eBC_raw = σ_abs,_670nm/6.6

(1)

MAC = σ_abs,670nm × (1/CF)/(EC × 10⁶) = σ_abs,637nm/(EC × 10⁶)

(2)

eBC = σ_abs,637nm/MAC × 10⁶

(3)

where CF = 0.952 serves as a conversion factor used on the firmware output of the MAAP, to derive the absorption coefficient at 637 nm [26]. This approach has also been applied to calculate the new MAC of AE33 and of Giano BC1 at their specific wavelength. AE33 data have been used according to the ACTRIS recommendations [34].

To calculate the BrC absorption coefficients, the absorption Angstrom exponent (AAE) for the aerosol collected in ECO site should be firstly determined. The AAE is a widely used parameter that describes the wavelength (λ) dependence of aerosol light absorption according the following equation:

$$\mathrm{A}\mathrm{A}\mathrm{E}=-{\displaystyle \frac{\mathrm{l}\mathrm{n}(\frac{{\mathrm{b}}_{\mathrm{a}\mathrm{b}\mathrm{s}}\left(\mathsf{\lambda }\right)}{{\mathrm{b}}_{\mathrm{a}\mathrm{b}\mathrm{s}\left({\mathsf{\lambda }}_{\mathrm{r}\mathrm{e}\mathrm{f}}\right)}})}{\mathrm{l}\mathrm{n}(\frac{\mathsf{\lambda }}{{\mathsf{\lambda }}_{\mathrm{r}\mathrm{e}\mathrm{f}}})}}$$

(4)

where b_abs(λ) is the absorption coefficient, and λ is the wavelength of interest, 470 nm, and λ_ref is the reference wavelength, 880 nm. The reference wavelength of 880 nm was chosen because the black carbon at this wavelength is expected to fully dominate light absorption [35,36]. It is generally recognized that black carbon exhibits an AAE in the range of 0.9 to 1.1. [1,15,29]; conversely, BrC shows stronger absorption at shorter wavelengths and therefore presents a larger AAE [37,38]. Then, an increase in AAE of the total aerosol is associated with an increased contribution of BrC to the total absorption. The wavelength dependence of BC absorption (b_BC,λ) can be determined from the measured total aerosol absorption coefficients (b_total,λ) by applying the following equation [39]:

$${\mathrm{b}}_{\mathrm{B}\mathrm{C},\mathsf{\lambda }}={\mathrm{b}}_{\mathrm{t}\mathrm{o}\mathrm{t},880\mathrm{n}\mathrm{m}}\times ({{\displaystyle \frac{880\mathrm{n}\mathrm{m}}{\mathsf{\lambda }}})}^{\mathrm{A}\mathrm{A}\mathrm{E}}$$

(5)

In this study, an AAE of 1 was applied for the estimation of b_BC,λ. Then, the spectral dependence of the aerosol absorption (b_tot,λ), collected in ECO site, has been estimated using the AAE ECO site. Finally, the b_BrC,λ at 370, 470, 520, 590, and 660, 880 and 950 nm could be calculated following the equation [40]:

b_BrC,λ = b_tot,λ − b_BC,λ

(6)

2.3. Determination of POC and SOC from TCA Data

Data collected with the TCA08 analyzer allowed for the determination of the secondary organic carbon (SOC) and of the primary organic carbon (POC), following the MRS method described in Ivančič et al. [32] and assuming that POC and eBC are co-emitted by combustion sources. The equations are as follows:

POC = (OC/eBC)_prim × eBC

(7)

SOC = OC − POC

(8)

The MRS method was introduced by Wu and Yu (2016) [41]; it assumes source-independent EC and SOC and a hypothetical (OC/EC)_prim ratio corresponding to the minimum correlation between the estimated SOC and measured EC. In the MRS method, the squared-correlation (R²) between measured EC and estimated SOC is examined as a function of a series of hypothetical (OC/EC)_prim ratios varying between 1 and 8. If variations in EC and SOC are independent, the (OC/EC)_prim corresponding to the minimum R² represents the actual (OC/EC)_prim ratio. This method was chosen because it performs better compared to other methods, especially when the contribution of biomass burning is relevant, as is the case at this site [42].

3. Results and Discussion

3.1. OC and EC in Aerosol Samples

In Figure 2, a summary of daily PM₁₀ and PM_2.5 measurements together with OC/EC thermo-optic determination is reported for the studied period.

The average PM concentrations obtained were 17.1 (±1.1) µg/m³ for PM₁₀ and 10.4 (±1.0) µg/m³ for PM_2.5. During the whole measurement period, OC and EC accounted for, on average, 16.5% and 3.6% of PM₁₀ in mass, and 22.6% and 5.5% of PM_2.5. These values agree with those reported in a recent study of Merico et al. [42].

3.2. Evaluation of In-Situ MAC and EC vs. eBC

The complete dataset of eBC raw data, collected with MAAP, AE33 and Giano BC1, and EC concentration, obtained with the Sunset (EUSAAR2 protocol), were used to estimate the in situ MAC, as described in Section 2.2. In

Table 1. Re-calculated MAC values for instruments MAAP, AE33, and Giano BC1. Standard errors are reported in parentheses.

Instrument	Wavelength (λ)	MAC (m2/g)
MAAP	670 nm	12 (±0.3)
AE33	880 nm	8.1 (±0.3)
Giano BC1	635 nm	12.2 (±0.5)

, the calculated values are reported.

These values are in agreement with those obtained in the work of Zanatta et al. [26], where the MAC for numerous European sites ranged between 8.9 (±1.7) m²/g and 17 (±1.7) m²/g, with a geometric mean value of 10 (±1.3) m²/g. Additionally, Savadkoohi et al. [31] explored MAC values at different site types in Europe, including urban background, rural, and traffic locations. The comparison between the MAC values from this study and those reported for urban background sites showed that the ECO results aligned with the range of 10–14 m²/g observed by Savadkoohi et al. [31]. Other investigations carried out at 637 nm using the MAAP instrument also reported similar MAC estimates of 10.4 (±0.2), 10.9 (±3.5), and 10.2 (±3.2) m²/g for regional backgrounds [34], remote mountains [43] and rural high alpine sites [44,45], respectively. This confirmed the limited spatial variability of MAC, in contrast to the physical and chemical transformations of eBC in the atmosphere, that could influence its light absorption [26,46,47]. The comparison of the eBC daily concentrations from MAAP, AE33, and Giano BC1 and the EC data concentrations resulted in good agreement (Figure 3a,b), with observed R² greater than 0.97 for all instruments.

3.3. Daily Pattern of Carbonaceous Aerosol

In Figure 4, the daily pattern of eBC, measured with MAAP, AE33, and GIANO BC1, is reported.

The diurnal variation highlights several notable features. The minimum concentrations are recorded around midday and in the early afternoon, clearly influenced by the daily growth of the planetary boundary layer (PBL). A distinct peak appears in the early morning, around 6 a.m., most likely linked to traffic emissions. In the evening and nighttime, a wider eBC peak emerges, which can be attributed to the combined effects of PBL behavior together with traffic and residential heating emissions.

In Figure 5a, a linear fit between the TC (μg/m³) measured with TCA08 and the sum “OC + EC” (μg/m³) measured with Sunset (EUSAAR2 protocol) is shown: the scatter plot indicates a strong correlation between TC and “EC + OC” data (R² = 0.97), with a slope of 1.08 indicating that the measured TC values are about 8% higher than the sum of EC and OC measured by the Sunset analyzer. In Figure 5b, the OC daily concentration obtained by TCA08 measurements is reported together with secondary organic carbon and primary organic carbon concentrations. SOC and POC concentrations are not directly measured but calculated according to the formula reported in Section 2.3. The (OC/EC)_prim obtained for the considered period was 2.9 (with a R² minimum of 0.06) and is close to the values determined by Merico et al. [42] reporting a (OC/EC)_prim of 2.4 and 2.1 for PM₁₀ and PM_2.5 during the cold period. The scatter plot of SOC, calculated using this ratio, and eBC is reported in Figure 5c. On average, the SOC, representing 36% of the measured OC, has a concentration close to 1 μg/m³ the whole day, with a clear daily pattern. Furthermore, the primary organic carbon is characterized by the same daily pattern observed for eBC, confirming the primary origin of the eBC. Figure 5d shows the daily patterns for the ratios OC/eBC, SOC/OC, and POC/OC, obtained combining TCA08 and AE33 measurements. In particular, the lowest OC/eBC values are observed during the night and early morning; during this period, the ECO site is likely dominated by primary combustion emissions, such as traffic and biomass burning (domestic heating). This observation is confirmed by the pattern of POC/OC that shows the highest values during the night and early morning, reflecting then the dominance of fresh emissions from combustion sources. The highest OC/eBC values are observed from late morning to early afternoon (10:00–16:00), indicating increased contributions from secondary organic aerosols (i.e., SOC), due to a stronger photochemical activity; this observation is confirmed by SOC/OC pattern that shows higher SOC contributions in the midday hours, even if a certain activity of SOC production is observed in the nighttime.

These kinds of patterns have been already observed in other studies performed at the ECO site. For instance, Cesari et al. [21] reported an average daily cycle of hourly eBC concentrations characterized by two peaks: the first occurring in the early morning and the second in the late evening, both linked to local combustion sources such as traffic and residential heating. A comparable trend was also identified for fine and coarse-particle number concentrations at the same location [22], attributed to the diurnal variation in emissions influenced by the evolution of the planetary boundary layer.

In Figure 6a, the contribution of biomass burning (eBC_bb) to eBC measured with AE33 is reported together with the contribution of fossil fuel (eBC_ff) as a daily pattern. In Figure 6b, the ratio eBC_bb/eBC_ff is also reported.

As outlined in Section 2.1, data from the multi-wavelength aethalometer can be applied to estimate the contributions of fossil fuel and biomass burning to eBC (denoted as eBCff and eBCbb, respectively). This method, generally known as the “Aethalometer model,” is based on the assumption that all light-absorbing particles derive solely from traffic and biomass-burning sources, with Ångström exponents of 1.0 and 2.0, respectively [48,49]. Results showed that, on average, eBC_ff accounted for about 64% (0.71 mg/m³) of the measured eBC, while the eBC_bb was only 36% (0.39 mg/m³), indicating that the ECO site was, on average, more influenced by fossil fuel combustion. The analysis of the daily pattern showed two pronounced peaks for eBC_ff: the former in the early morning at 6 a.m., the second in the evening at 10 p.m., clearly associated with human activity (traffic emissions). However, the ratio eBC_bb/eBC_ff also highlights the relevant role of biomass-burning emissions (for domestic heating) especially during the early hours of the day and in the late afternoon/night (after the 4 p.m.), as the ratio during these hours ranges between 0.6 and 0.9. As previously stated, the nighttime peaks are also due to a combined effect of combustion emissions and the PBL height.

3.4. Absorption by BrC

The average AAE calculated for aerosols collected at the ECO site was 1.46, suggesting the presence of brown carbon (BrC). It is important to highlight that the reported AAE values are average estimates that may vary under real atmospheric conditions, being site-dependent, and can vary significantly due to factors such as PM source contributions, atmospheric processing, and the evolving properties of carbonaceous aerosol, including morphology, size distribution, aging, transport, and microphysical characteristics [1]. As stated in Helin et al. [50], distinguishing between black carbon and brown carbon solely based on AAE may not be quantitatively precise, as auxiliary measurements providing information on particle size, coating thickness, chemical composition, and particle morphology are necessary to support accurate aerosol characterization. However, despite these uncertainties, determining the AAE remains crucial for understanding the optical properties of different aerosol types. Accordingly, the results presented in this study should be interpreted as indicative of the presence and relative contribution of BrC, rather than as exact quantitative values. In Figure 7, the absorption coefficients (b_abs) are reported as a function of wavelength in the investigated range 370 nm–950 nm. The red line represents the total absorption given by AE33 for the aerosol collected in ECO site (AAE = 1.46), while the black line represents absorption by eBC (AAE = 1), with the gray-shaded area indicating the eBC absolute contribution. The orange-shaded area represents the BrC contribution, calculated as the difference between the total AE33 absorption and the estimated eBC absorption.

This contribution is especially significant at shorter wavelengths, especially for 370 nm and 470 nm wavelengths, where the relative contribution of BrC to the absorption has been, on average, 35% and 26%, respectively. This observation is in agreement with other studies: although BrC exhibits notable absorption over a wide spectral range, its absorption is commonly evaluated at selected wavelengths, typically 370 or 470 nm [40,51]. For example, in Fang et al.’s study [52], a BrC absorption ranging from 33% to 90% at 370 nm has been observed during different biomass experiments, while Zhang et al. [40] showed BrC contributions ranging between 18% and 42% to total absorption at 370 nm in different sampling sites across France. In this study, the b_abs of BrC and eBC at a wavelength of 370 nm were considered for further observations. In Figure 8, a daily pattern for absorption coefficients b_abs for eBC and BrC at a wavelength of 370 nm is reported.

The graph displays a typical diurnal cycle for both eBC and BrC, with two maxima observed: the former in the morning, at around 6 a.m., and the latter in the late evening, after 22 p.m., with a pronounced minimum during midday. The figure clearly shows that the patterns of BC and BrC are modulated by a combined effect due to both the local micrometeorology, and to traffic and biomass-burning emissions acting in ECO site, with a traffic emission pronounced at 6 a.m. (typical rush hour) and a biomass burning emission higher in the late afternoon and evening, according to domestic heating emissions [40]. However, it is worth noting that, in addition to primary emissions (i.e., traffic and biomass-burning emissions), secondary formation and atmospheric aging processes likely contribute to BrC levels. Photochemical reactions can generate secondary BrC from volatile organic compounds. This secondary contribution, combined with photochemical aging, may partly explain the observed diurnal minimum in BrC absorption. Therefore, the BrC diurnal pattern at the ECO site reflects a dynamic interplay of primary biomass burning, potential secondary formation, local meteorology, and chemical processing in the atmosphere.

4. Conclusions

This study provided a characterization of carbonaceous aerosol at the ECO site, integrating multiple measurement techniques, with different time resolutions, to evaluate primary and secondary contribution to carbonaceous aerosol as well as optical properties related to black and brown carbon.

The average concentrations of PM obtained were 17.1 (±1.1) µg/m³ for PM₁₀ and 10.4 (±1.0) µg/m³ for PM_2.5. Considering the entire measurement period, OC and EC represented, on average, 16.5% and 3.6% of PM₁₀ in mass, and 22.6% and 5.5% of PM_2.5.

The comparison between eBC and EC concentrations demonstrated good agreement (with R² > 0.97), especially after using in situ MAC values. In particular, the value of in situ MAC was estimated for the three instruments and ranged between 8.0 (±0.33) m²/g (at 880 nm) and 12.22 (±0.47) m²/g (at 635 nm), in agreement with values found for other different typologies of sites in Europe.

The diurnal variations in eBC and POC concentrations highlighted the strong influence of local sources at ECO site, i.e., traffic (contributing to the eBC for 64%) and biomass burning for residential heating (contributing to the eBC for 36%), modulated by local meteorological dynamics. The application of the aethalometer model showed that fossil fuel combustion was the dominant source of eBC at ECO site, though biomass burning also played a relevant role, especially during evening and early morning hours. SOC contributions, accounting for 36% of the measured OC, were more prominent during midday hours, reflecting the enhanced photochemical activity in this part of the day.

The analysis of the aerosol optical properties performed with AE33 indicated the presence of a certain amount of BrC in the collected aerosol, particularly at shorter wavelengths (up to 35% of absorption at 370 nm). The BrC_abs diurnal pattern at 370 nm was higher during the nighttime and decreased in the diurnal hours, suggesting that this parameter was mainly influenced by the biomass-burning source (or domestic heating) at ECO site.

The study showed that the integration of data coming from different instruments and data analysis (Giano BC1, MAAP, AE33, TCA, and Sunset analyzer) could result in a robust characterization of carbonaceous aerosols, inferring indication on the relative sources and processes (for example, photochemical activity), for a better understanding of local air-quality dynamics. This study offers several important implications for air-quality monitoring and policy in the context of the new EU Directive 2024/2881/EU. The successful intercomparison of multiple instruments underscores the value of deploying complementary measurement techniques to ensure robust and comparable datasets across monitoring networks. Further, the determination of in situ MAC values provides a site-specific reference that can improve the accuracy of optical measurements and reduce uncertainties in routine monitoring. These findings can guide the design of targeted monitoring strategies, helping regulatory agencies implement more effective mitigation measures. In particular, integrating real-time optical measurements of eBC and BrC with routine chemical analyses can enhance the detection of emerging pollutants, support compliance with EU air-quality standards, and inform local and regional policy decisions aimed at reducing human exposure to carbonaceous aerosols. Ultimately, this work demonstrates how site-specific, high-resolution aerosol characterization can directly contribute to evidence-based policymaking and the optimization of air-quality monitoring strategies across Europe. However, a main limitation of this study is the relatively short observation period (two months), which restricts the ability to capture seasonal variability and to perform robust comparisons with other European sites. For future work, we propose to extend the monitoring period across multiple seasons to better characterize seasonal variability and long-term trends. Performing detailed chemical characterization of collected filters, including the determination of specific tracers such as levoglucosano, to more accurately quantify biomass-burning contributions. Expanding similar measurement campaigns to other urban and semi-rural sites in the Mediterranean region to improve understanding of regional carbonaceous aerosol dynamics and support EU-wide air-quality management strategies.