Sources and atmospheric processing of size segregated aerosol particles revealed by stable carbon isotope ratios and chemical speciation

Dual-isotope ratios of carbonaceous aerosols for seasonal observation and their assessment as source indicators

Carbonaceous aerosols exhibit seasonal variations due to a complex interplay of emission sources, meteorological conditions, and chemical processes. This study presents the first year-round dual‑carbon isotopic analysis of carbonaceous aerosols in Northeastern Europe (Lithuania). The emphasis was placed on the processes affecting carbonaceous submicron particle (PM1) concentrations and their isotopic composition (δ13CTC, fc) during different seasons. Aerosol particles were collected in the two distinct sites: at an urban background site (Vilnius) and a coastal site (Preila). The concentrations of total carbon (TC) and black carbon (BC) varied both spatially and temporally. The annual average concentrations were 4 μg/m3 for TC and 2.3 μg/m3 for BC at the urban background site. They were considerably lower at the coastal site with 2.9 μg/m3 for TC and 0.74 μg/m3 for BC. The peak concentrations of TC and BC that occur during the cold season indicate a significant impact from residential heating. The δ13C in aerosols exhibited a distinct seasonal cycle with depleted δ13CTC values during the warm season (April-October). Through the integration of isotopic composition, contemporary carbon (fc), and BC source apportionment, we achieved precise predictions of isotopic parameter changes, encompassing pollution sources and the influence of meteorological parameters. To better understand the respective contributions of local and regional sources, air mass trajectories, wind patterns (speed and direction), and the polar conditional probability function (CPF) were studied in parallel. The study indicates that the isotopic composition of PM1 at both sites is primarily controlled by emission sources (local and regional), while meteorological conditions (temperature and mixing layer height) have less influence. These variations have important implications for regional air quality, climate dynamics, and public health, which are persistently subject to continuous research and monitoring.

Cell Death Pathways: The Variable Mechanisms Underlying Fine Particulate Matter-Induced Cytotoxicity

Recently, the advent of health risks due to the cytotoxicity of fine particulate matter (FPM) is concerning. Numerous studies have reported abundant data elucidating the FPM-induced cell death pathways. However, several challenges and knowledge gaps are still confronted nowadays. On one hand, the undefined components of FPM (such as heavy metals, polycyclic aromatic hydrocarbons, and pathogens) are all responsible for detrimental effects, thus rendering it difficult to delineate the specific roles of these copollutants. On the other hand, owing to the crosstalk and interplay among different cell death signaling pathways, precisely determining the threats and risks posed by FPM is difficult. Herein, we recapitulate the current knowledge gaps present in the recent studies regarding FPM-induced cell death, and propose future research directions for policy-making to prevent FPM-induced diseases and improve knowledge concerning the adverse outcome pathways and public health risks of FPM.

Vast ecosystem disturbance in a warming climate may jeopardize our climate goal of reducing CO2: a case study for megafires in the Australian 'black summer'

A warming climate is one of the most important driving forces of intensified wildfires globally. The unprecedented wildfires broke out in the Australian 'Black Summer' (November 2019-February 2020), which released massive heat, gases, and particles into the atmosphere. The total carbon dioxide (CO₂) emissions from wildfires were estimated at ∼963 million tons by using a top-down approach based on direct satellite measurements of CO₂ and fire radiative power. The fire emissions have led to an approximately 50-80 folds increase in total CO₂ emission in Australia compared with the similar seasons of 2014-2019. The excess CO₂ from wildfires has offset almost half of the global anthropogenic CO₂ emission reductions due to the Corona Virus Disease 2019 in 2020. When the wildfires were intense in December 2019, they caused a 1.48 watts per square meter additional positive radiative forcing above the monthly average in Australia and the vicinity. Our findings demonstrate that vast ecosystem disturbance in a warming climate can strongly influence the global carbon cycle and hamper our climate goal of reducing CO₂.

Tracing the predominant sources of carbon in PM2.5 using δ13C values together with OC/EC and select inorganic ions over two COALESCE locations

As part of the COALESCE (Carbonaceous Aerosol Emissions, Source apportionment and Climate Impacts) campaign, ambient PM_2.5 was collected at two regional sites (Bhopal and Mysuru) in India during 2019. We utilized organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions together with δ¹³C values, to better understand total carbon (TC) sources at these locations. The annual average δ¹³C values (-26.2 ± 0.6‰) at Mysuru and Bhopal (-26.6 ± 0.6‰) were comparable. However, at Mysuru, except during winter, day-to-day variability was much lower (narrow range of -26.8 to -26.0‰) than that at Bhopal (range: -28.1 to -24.7‰), suggesting that TC was contributed by few sources, likely dominated by vehicular emissions. Seasonal average δ¹³C values at Bhopal increased slightly (-25.8 ± 0.5‰) during the winter (Jan-Feb) and decreased (-27.0 ± 0.3‰) during the monsoon (Jun-Sep) season compared to the annual average. The decrease in δ¹³C values during the monsoon season was likely driven by enhanced secondary organic aerosol formation. Further, based on MODIS derived fire spots and back trajectories, we infered that the δ¹³C values (-27.5 to -26.0‰) in Bhopal during the post-monsoon season (Oct-Dec) were indicative of dominant biomass burning contributions. The inorganic ions/TC ratio during this season suggested that biomass burning aerosol was aged and may have been transported from crop residue burning in the Indo-Gangetic plains. At Mysuru, like the trend at Bhopal, the δ¹³C values during the monsoon season were lower than those during the winter season. Finally, δ¹³C values were input to a Bayesian model-MixSIAR to demonstrate the usefulness of such models in apportioning TC. In its simplest implementation, the model separated TC sources into fossil fuel emissions and non-fossil fuel sources . Fossil fuel combustion emissions accounted for 47 ± 19% and 62 ± 22% of the TC at Bhopal and Mysuru, respectively.

Stable C and N isotopes of PM2.5 and size-segregated particles emitted from incense stick and cigarette burning

This work measured the δ¹³C and δ¹⁵N signatures in PM_2.5 and size-segregated particles emitted from incense stick and cigarette burning in different brands or nicotine contents for pollution source identification indoors. Three popular brands of incense stick and cigarette were selected for experiments. A personal environmental monitoring sampler and a Sioutas cascade impactor were used to collect PM_2.5 and size-segregated particles, respectively, for isotopic signatures analyses. Our data showed that both δ¹³C and δ¹⁵N values were heavier from incense stick burning (δ¹³C: 27.3 ± 0.5; δ¹⁵N: 8.63 ± 1.35) than cigarette (δ¹³C: 28.5 ± 0.2; δ¹⁵N: 4.15 ± 0.69). The scatter plots of δ¹³C and TC/PM_2.5 and of δ¹⁵N and TN/PM_2.5 can be applied to distinguish particle pollution sources and assess the influence of cigarette burning to PM_2.5 according to different nicotine contents. The δ¹³C values in size-segregated particles were similar to incense stick or cigarette burning; the δ¹³C values in PM_2.5 were significantly higher than those in size-segregated particles. However, the nitrogen amount was too low in most of the size-segregated particles to analyze δ¹⁵N from incense stick and cigarette burning. These results suggest that the δ¹³C signatures on PM_2.5 cannot represent the isotopic characteristics of size-segregated particles and δ¹⁵N has limitation for pollution source identification of different particle sizes.

13C signatures of aerosol organic and elemental carbon from major combustion sources in China compared to worldwide estimates

Carbon isotope signatures are used to gain insight into sources and atmospheric processing of carbonaceous aerosols. Since elemental carbon (EC) is chemically stable, it is possible to apportion the main sources of EC (C3/C4 plant burning, coal combustion, and traffic emissions) using a dual ¹⁴C-¹³C isotope approach. The dual-isotope source apportionment crucially relies on accurate knowledge of ¹³C source signatures, which are seldom measured for EC. In this work, we present ¹³C signatures of organic carbon (OC) and EC for relevant sources in China. EC was isolated for ¹³C analysis based on the OC/EC split point of a thermal-optical method (EUSAAR_2 protocol). A series of sensitivity studies were conducted to investigate the EC separation and the relationship of the thermal-optical method to other EC isolation methods. Our results show that, first, the ¹³C signatures of raw materials and EC related to traffic emissions can be separated into three groups according to geographical location. Second, the ¹³C signature of OC emitted by the flaming combustion of C4 plants is strongly depleted in ¹³C compared to the source materials, and therefore EC is a better tracer for this source than total carbon (TC). A comprehensive literature review of ¹³C source signatures (of raw materials, of TC, and of EC isolated using a variety of thermal methods) was conducted. Accordingly, we recommend composite ¹³C source signatures of EC with uncertainties and detailed application conditions. Using these source signatures of EC in an example dual-isotope source apportionment study shows an improvement in precision. In addition, ¹³C signatures of OC were measured at three different desorption temperatures roughly corresponding to semi-volatile, low-volatile, and non-volatile OC fractions. Each source category shows a characteristic trend of ¹³C signatures with desorption temperature, which is likely related to different OC formation processes during combustion.

An automated method for thermal-optical separation of aerosol organic/elemental carbon for 13C analysis at the sub-μgC level: A comprehensive assessment

We describe and thoroughly evaluate a method for ¹³C analysis in different fractions of carbonaceous aerosols, especially elemental carbon (EC). This method combines a Sunset thermal-optical analyzer and an isotope ratio mass spectrometer (IRMS) via a custom-built automated separation, purification, and injection system. Organic carbon (OC), EC, and other specific fractions from aerosol filter samples can be separated and analyzed automatically for ¹³C based on thermal-optical protocols (EUSAAR_2 in this study) at sub-μgC levels. The main challenges in isolating EC for ¹³C analysis are the possible artifacts during OC/EC separation, including the premature loss of EC and the formation of pyrolyzed OC (pOC) that is difficult to separate from EC. Since those artifacts can be accompanied with isotope fractionation, their influence on the stable isotopic composition of EC was comprehensively investigated with various test compounds. The results show that the thermal-optical method is relatively successful in OC/EC separation for ¹³C analysis. The method was further tested on real aerosols samples. For biomass-burning source samples, (partial) inclusion of pOC into EC has negligible influence on the ¹³C signature of EC. However, for ambient samples, the influence of pOC on the ¹³C signature of EC can be significant, if it is not well separated from EC, which is true for many current methods for measuring ¹³C on EC. A case study in Xi'an, China, where pOC is enriched in ¹³C compared to EC, shows that this can lead to an overestimate of coal and an underestimate of traffic emissions in isotope-based source apportionment.

Organic aerosol formation and aging processes in Beijing constrained by size-resolved measurements of radiocarbon and stable isotopic 13C

This study investigates the sources and atmospheric processes of size-resolved carbonaceous aerosols in winter 2018 in urban Beijing, based on analysis of dual-carbon isotopes (i.e., radiocarbon and the stable isotope ¹³C). We found a size dependence of fossil source contributions to elemental carbon (EC), but no clear size dependence for organic carbon (OC). Comparable fossil source contributions to water-insoluble OC (WIOC; 55 ± 3%) and to water-soluble OC (WSOC; 54 ± 4%) highlight the importance of secondary aerosol formation, considering that fossil sources emit only small amounts of primary WSOC. OC concentrations increased during high PM_2.5 pollution events, with increased fossil and non-fossil WSOC concentrating at larger particles (0.44-2.5 µm) than WIOC (0.25-2.5 µm), highlighting the aqueous-phase chemistry as an important pathway for OC production. The ratio of ¹³C/¹²C (expressed as δ¹³C) of total carbon (-27.0‰ to -23.3‰) fell in the range of anthropogenic aerosol, reflecting small biogenic influence. δ¹³C of OC increased with desorption temperature steps (200 °C, 350 °C and 650 °C). The strongly enriched δ¹³C_OC,650 (-26.9‰ to -20.3‰) and large mass fraction of OC_650°C in total desorbed OC, both increasing with the increase of particle sizes, were caused by photochemical aging, especially during low and moderate PM_2.5 pollution events, when regional, aged aerosol played an important role. During low pollution events, higher δ¹³C_OC,650 and WSOC/OC ratios reflect a larger contribution and more extensive chemical processing of aged aerosol. In contrast, relatively low δ¹³C_OC,200 (-27.2‰ to -25.7‰) suggests the influence of secondary OC formation on the more volatile OC desorbed at 200 °C. δ¹³C_OC,200 was similar for all particle sizes and for different pollution events, pointing to an internal mixture of local and aged regional OC. Our results show that the organic aerosol in Beijing arises from a mixture of various sources and complex formation processes, spanning local to regional scales. Particle sizes < 250 nm show strong contribution from local secondary OC formation, whereas refractory OC in particles around 1 µm shows strong evidence for regional aging processes. In summary, primary emission, secondary and aqueous-phase formation, and (photo-)chemical aging all need to be considered to understand organic aerosol in this region and their importance varies with particle size.

Carbon fractionation and stable carbon isotopic fingerprint of road dusts near coal power plant with emphases on coal-related source apportionment

Road dust from coal utilization is a significant source contributing to the generation of pollutants that can affect the health of people residing within close proximity to roadways. In this study, road dust samples were collected from different directions centered around a coal-fired power plant in Huainan. Black carbon (BC), soot, char, organic carbon (OC) and total carbon (TC), as well as the δ¹³C of samples, were determined. Compared to the reference locations which were distant from the power plant, the research areas surrounding the power plant were featured by significantly higher OC/BC ratio and TC concentration. The OC/BC showed significant difference in urban vs. rural areas, and at different distances from the central power plant, which implied that the source and spread of carbonaceous species was dominantly affected by wind direction and urban/rural area differences. Surface morphology analysis showed that the road dust was mixed with spherical particles similar to fly ash. High-resolution XPS C1s spectrum revealed the existence of metal carbide, metal carbonate, and CF₃ in the road dust samples. The speciation of carbon in road dusts was found correlated with sampling directions and urban functional areas. Based on the δ¹³C and OC/BC, it could be inferred that coal-related substances might be important sources of road dusts.