A better knowledge of the local and regional sources of the atmospheric particulate matter provides policy makers with the proper awareness when acting to improve air quality, in order to protect public health. A source apportionment study of the carbonaceous aerosol in Naples (Italy) is presented here, in order to improve this understanding in a vulnerable urban area. The aim of this study is quantifying directly fossil and non-fossil contributions to carbonaceous aerosol, by means of radiocarbon measurements. This is the first time that such an approach is implemented in this area. Fine particles with diameter ≤ 2.5 µm (PM_2.5) were collected daily on top of a building in the city center, from November 2016 until January 2017. The carbonaceous aerosol was separated into organic carbon (OC) and elemental carbon (EC), by a two-step thermal desorption method. Subsequent radiocarbon analysis enabled the partitioning of the major sources of carbonaceous aerosol into fossil and non-fossil ones by applying radiocarbon isotopic mass balance. The PM_2.5 concentration was on average 29 ± 3 µg⁄m³ (mean ± standard error; n = 18), with a maximum of 68.6 ± 0.7 µg⁄m³ on a day when air masses back-trajectories suggest a local origin and stagnant airflow conditions in the region. The carbonaceous component accounts for roughly half of the PM_2.5 mass. Fossil fuel emissions are a minor source of OC (23%), but the dominant source of EC (66%), which is directly emitted during combustion processes. However, overall only 30% of the total carbon is of fossil origin, accounting for 14% of PM_2.5 mass. Surprisingly, a comparable contribution is due to primary biomass burning carbon, which accounts in total for 15% of PM_2.5 mass. Traffic pollution, the main cause of fossil fuel emissions in urban areas, is a significant, but not the predominant source of carbonaceous particle concentration. These findings support the conclusion of a predominant contribution from non-fossil sources to the carbon in airborne particulate matter, which policy makers should take into account when planning mitigation strategies to improve urban air quality.

Radiocarbon analysis is a widely-used tool for source apportionment of aerosol particles. One of the big challenges of this method, addressed in this work, is to isolate elemental carbon (EC) for ¹⁴C analysis. In the first part of the study, we validate a two-step method (2stepCIO) to separate total carbon (TC) into organic carbon (OC) and EC against the EUSAAR_2 thermal-optical method regarding the recovered carbon concentrations. The 2stepCIO method is based on the combustion of OC in pure oxygen at two different temperature steps to isolate EC. It is normally used with a custom-built aerosol combustion system (ACS), but in this project, it was also implemented as a thermal protocol on a Sunset OC-EC analyzer. Results for the recovered EC mass concentration showed poor agreement between the 2stepCIO method on the ACS system and on the Sunset analyzer. This indicates that the EC recovery is sensitive not only to the temperature steps, but also to instrument-specific parameters, such as heating rates. We also found that the EUSAAR_2 protocol itself can underestimate the EC concentration on untreated samples compared to water-extracted samples. This is especially so for highly loaded filters, which are typical for ¹⁴C analysis. For untreated samples, the EC concentration on long-term filter samples (two to five days sampling time) was 20–45% lower than the sum of EC found on the corresponding 24-h filter samples. For water-extracted filter samples, there was no significant difference between long-term and the sum of daily filter samples. In the second part of this study, the ¹⁴C was measured on EC isolated by the 2stepCIO method and compared to methods from two other laboratories. The different methods agree well within their uncertainty estimates.

Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. ¹³C/¹²C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The ¹³C/¹²C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.

Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. ¹³C/¹²C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The ¹³C/¹²C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.

Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. ¹³C/¹²C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The ¹³C/¹²C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.

Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. ¹³C/¹²C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The ¹³C/¹²C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.