The determination of the carbonaceous components of atmospheric particulate matter, in particular Elemental Carbon (EC) and Organic Carbon (OC), is increasingly becoming a technical and regulatory field of major importance for the assessment of air quality and emission sources. For PM_TEN, this topic is not merely an area of expertise: it is an integral part of its scientific identity. Our group has been involved for many years in the monitoring of carbonaceous aerosols, developing advanced expertise and participating in research projects with studies and results that are widely recognized in the field.
This article explores three key aspects: the role of EC and OC in atmospheric particulate matter, with particular attention to their environmental relevance and implications for air quality; the main UNI standards governing the determination of carbonaceous fractions; and the currently recognized determination methodologies, with a focus on thermo-optical methods used as reference standards. This framework makes it possible to understand not only how measurements are carried out, but also why they are fundamentally important.
Atmospheric particulate matter and air quality
Atmospheric particulate matter is a complex mixture of solid and liquid particles suspended in the air, characterized by highly variable size, chemical composition, and origin. Aerosol particles may be directly emitted into the atmosphere (primary aerosols) or formed in the atmosphere from precursor gases (secondary aerosols). The most commonly used classification distinguishes between PM10 – particles with an aerodynamic diameter smaller than 10 µm – and PM2.5, the finer fraction with a diameter smaller than 2.5 µm. This distinction is not merely metrological: particle size strongly influences atmospheric behavior, penetration into the respiratory system, and participation in physicochemical processes in the air.
Particulate matter is one of the main indicators of air quality, as it significantly contributes to urban and industrial air pollution. Its concentrations and characteristics are closely linked to public health impacts, with well-established evidence connecting PM exposure to respiratory and cardiovascular diseases as well as long-term chronic effects. For this reason, PM is central to European and national environmental policies, which set limit values, measurement methodologies, and mitigation strategies.
Beyond quantity, the chemical composition of particulate matter is a key element in understanding emission sources and their impact on public health and the environment. Analyzing the various components makes it possible to identify dominant sources, distinguish between primary and secondary contributions, and effectively support mitigation actions and pollution-reduction policies.
Elemental Carbon (EC) and Organic Carbon (OC): definition and role in PM
Within atmospheric particulate matter, the carbonaceous component represents a significant fraction of the total mass, typically ranging between 20% and 50%. This fraction includes different types of compounds with distinct properties. This article focuses in particular on two key components: Organic Carbon (OC) and Elemental Carbon (EC).
Below are the most widely used operational definitions.
- Organic Carbon (OC): OC comprises a highly heterogeneous set of organic compounds – volatile and semi-volatile -in which tetravalent carbon atoms are chemically bonded to other carbon and/or hydrogen atoms and to other elements (such as oxygen, sulfur, nitrogen, phosphorus, chlorine, etc.). From an operational standpoint, OC represents the carbonaceous fraction that evolves in an inert atmosphere at temperatures below 1,000 °C. OC may have a primary origin, when directly emitted from combustion processes, industrial activities, or biogenic sources, or a secondary origin, when it forms in the atmosphere through chemical reactions that transform precursor gases into particles (Secondary Organic Aerosol, SOA).
- Elementare Carbon (EC): EC is the most refractory and stable fraction of carbonaceous particulate matter. It does not volatilize at low temperatures (below ~550 °C) and shows a strong correlation with light absorption. It consists of condensed graphitic structures that are resistant to thermal and chemical degradation. Operationally, EC is defined as “the thermally stable carbonaceous fraction of particulate matter obtained after volatilization of the organic component in an inert atmosphere up to temperatures above 350 °C and which can be brought into the gas phase through oxidation to CO₂ at temperatures above 340 °C,” with detection by NDIR or PID (following post-reduction to CH₄), once all organic carbon has been removed. EC primarily originates from fossil fuel combustion, biomass burning, and industrial processes and is typically associated with fine and ultrafine anthropogenic particles.
Other relevant quantities include Total Carbon (TC), defined as the sum of OC and EC and representing the entire carbonaceous fraction of particulate matter, and Black Carbon (BC), which identifies carbonaceous particles capable of absorbing visible light (380–760 nm). Due to its optical properties, BC is a key indicator in climate studies, as it represents the main anthropogenic component absorbing solar radiation. Although EC and BC are both associated with the refractory carbonaceous aerosol fraction, they do not define exactly the same particulate fraction, as they are based on different measurement methodologies (optical vs. thermo-optical).
Carbonaceous aerosols have a significant impact on climate -through their role in radiative processes and cloud formation – and on human health. In 2021, the WHO identified Black Carbon and Elemental Carbon as pollutants of health concern and recommended their monitoring. The composition of these aerosols also contributes to the distinctive “fingerprint” of different particulate matter sources. Accurate determination of EC and OC is therefore essential for analyses such as source apportionment, i.e., estimating the contribution of different emission sources (traffic, biomass combustion, industry, secondary aerosol formation) to collected atmospheric particulate matter.
Reference UNI standards
Regulation of carbonaceous aerosols is becoming increasingly important within the European air quality framework. With the entry into force of EU Directive 2024/2881 (10 December 2024), the European Union introduced mandatory monitoring of certain emerging pollutants, including BC and EC, both at rural background and urban background sites. This choice is fully aligned with WHO recommendations aimed at improving understanding of their health and environmental effects.
In Italy, UNI standards represent the essential technical reference to ensure harmonized measurement methods compliant with European standards. For the determination of carbonaceous fractions in atmospheric particulate matter, the reference standard is UNI EN 16909:2017, which defines the official method for EC/OC measurement in PM2.5 using thermo-optical analysis.
These standards define:
- requirements for particulate matter sampling
- operational conditions for thermo-optical analysis
- calibration and quality control procedures
- criteria to ensure traceability and comparability of results
UNI EN 16909:2017 describes in detail the analytical procedures for quantifying EC and OC deposited on quartz fiber filters, expressed in µg/cm², and for calculating the corresponding air concentrations (µg/m³).
Regarding sampling, the standard requires that filters be collected in accordance with UNI EN 12341:2023, which defines the reference gravimetric method for determining the mass concentration of suspended particulate matter PM₁₀ or PM₂.₅.
EC/OC determination is carried out using a thermo-optical analyzer, in which quartz fiber filters are subjected to controlled heating, first in an inert atmosphere and subsequently in an oxidizing atmosphere. During these phases, carbonaceous compounds are volatilized and oxidized, and the resulting CO₂ is quantified using FID or FT-IR detection systems. Separation of the organic (OC) and elemental (EC) components is achieved through optical measurement in transmittance (TOT) or reflectance (TOR).
The analytical output includes:
- OC concentration (µg/cm² e µg/m³)
- EC concentration (µg/cm² e µg/m³)
- TC concentration (µg/cm² e µg/m³) as the sum of OC and EC
Thanks to these references, EC/OC determination can be carried out reliably and in a standardized manner, supporting monitoring, research, and emission source assessment activities.
Methodologies for EC and OC determination
Thermo-optical methods
Heating-based methods are currently the most widely used approach for quantifying total carbon and its main fractions, OC and EC. Thermo-optical methods allow an operational distinction between organic and elemental fractions through a combination of controlled heating and optical measurement.
Thermo-optical analysis is based on the volatilization and oxidation of carbon present on a quartz fiber filter, subjected to heating under two different atmospheres:
- inert atmosphere (helium), for OC removal
- oxidizing atmosphere (helium/oxygen mixture), for EC oxidation
During the process, released carbon is converted to CO₂ and subsequently quantified by NDIR detection or, after reduction to CH₄, by FID.
During the first phase, part of the OC may undergo pyrolysis (charring), leading to the formation of pyrolytic carbon (PyrC), a compound with optical properties similar to EC. In the second phase, both EC and the PyrC formed during the first phase are oxidized and quantified.
To correct for the pyrolysis effect, the thermo-optical technique uses a laser to monitor sample transmittance (TOT) or reflectance (TOR). During the inert phase, PyrC formation causes a decrease in the optical signal. During the oxidizing phase, combustion of PyrC causes the signal to rise again. The point at which transmittance/reflectance returns to its initial value is called the split point:
- all carbon released before the split point is defined as OC
- all carbon released after the split point is defined as EC
This distinction is based on the assumption that EC is the only PM species capable of absorbing light at the laser wavelength and that PyrC behaves optically in an equivalent way. In reality, other optically active species (mineral dust, brown carbon) or other compounds (alkali metals, adsorbed VOCs) may be present and influence charring and optical response.
Operational phases of the analysis
Sampling is carried out according to UNI EN 12341:2023 using thermally pretreated quartz fiber filters. Sampling membranes are subject to adsorption of organic vapors; therefore, prior to use they must be pretreated in a furnace at a temperature of at least 500 °C for approximately 3 hours to remove potential organic interferences.
Analysis is performed on a filter portion of 1–1.5 cm², and the typical detection limit is about 0.2 µg C/cm² for both fractions.
Currently, four different thermal protocols are used in the scientific community for sample heating: NIOSH-like (QUARTZ), NIOSH 5040, IMPROVE, and EUSAAR-2. These protocols mainly differ in the maximum temperature of the first phase. Since the split point strongly depends on the adopted thermal profile—i.e., the combination of temperatures and heating ramp durations—the choice of protocol directly affects the operational distinction between OC and EC.
UNI EN 16909:2017 adopts the EUSAAR-2 thermo-optical protocol, which is currently the European standard for EC/OC determination in PM₂.₅.
Importance of data quality and regulatory compliance
Although thermo-optical techniques are the reference standard for measuring carbonaceous fractions, they present intrinsic challenges, particularly related to the operational distinction between OC and EC. Determination of total carbon is generally more accurate than the separate determinations, as TC is the primary measured quantity. For this reason, it is advisable to always perform and archive TC measurements alongside OC and EC results.
Data quality depends on several technical factors. Filter substrate selection is crucial: thermo-optical analysis requires quartz fiber filters, the only ones capable of withstanding the high temperatures (over 800 °C) reached during analysis. Moreover, since analysis is performed on a portion of the filter, it is essential that particulate deposition be homogeneous across the entire surface.
Correct execution of the method also requires strict control of the thermal cycle – which is destructive and not always repeatable – and the presence of highly qualified personnel with specific experience in the use of thermo-optical analyzers and interpretation of optical signals.
In this context, compliance with UNI standards ensures uniformity in monitoring and full consistency with European directives, guaranteeing that collected data are comparable across different monitoring networks and time periods.
Practical applications and use contexts
In recent years, the study of carbonaceous aerosols has gained increasing importance in both scientific and regulatory contexts, as it is now well established that carbonaceous compounds significantly influence climate and human health. From a climatic perspective, they contribute both to direct effects (absorption and scattering of electromagnetic radiation) and indirect effects by influencing cloud and ice formation processes. From a health standpoint, since 2021 BC and EC have been recognized as pollutants associated with documented health risks, and their systematic monitoring has been recommended by the WHO. As of 2024, with the entry into force of the new European air quality directive, monitoring of these pollutants has become mandatory at both rural and urban background sites, in line with WHO guidance.
In emission source studies, EC/OC data are an essential indicator for distinguishing contributions from vehicular traffic, biomass combustion, industrial activities, and secondary aerosol formation. This information is crucial for defining air quality improvement plans, as it allows assessment of the effectiveness of implemented measures and supports emission reduction policies targeting the most relevant sources.
A particularly important application concerns monitoring occupational exposure to diesel engine exhaust. Diesel exhaust gases have been classified as carcinogenic to humans by IARC, and EU Directive 2019/130 – implemented in Italy by the DM Lavoro 11/02/2021 – has included Elemental Carbon among the carcinogenic substances to be monitored in workplaces as a marker of exposure to diesel engine exhaust. In this context, EC determination is an indispensable tool for risk assessment and protection of workers’ health.
In summary, the availability of reliable EC and OC data enables a more informed approach to addressing air pollution challenges, providing crucial information for public health protection, emission management, and the development of effective environmental policies.
