PAHs are a family of organic compounds built from two or more fused benzene rings. Their high thermal stability, low water solubility, and lipophilic chemistry allow them to persist in the environment and accumulate in food webs. In the atmosphere, they split between the gas phase and airborne particles depending on molecular weight: lighter two- and three-ring compounds such as phenanthrene and anthracene stay largely in the gas phase, four-ring species partition between both, and heavier molecules with five or more rings ride almost exclusively on soot particles. The United States Environmental Protection Agency lists sixteen PAHs as priority pollutants, and the International Agency for Research on Cancer classifies benzo[a]pyrene as a Group 1 human carcinogen, directly linked to lung, skin, and bladder cancers. Chronic exposure is associated with asthma, chronic obstructive pulmonary disease, cardiovascular harm, reproductive toxicity, and neurodevelopmental deficits in children.

Urban PAHs arrive through two fundamental pathways. Petrogenic inputs involve uncombusted petroleum products released by fuel evaporation, storage leaks, solvents, and the wear of asphalt and lubricating oils. Pyrogenic sources, which dominate the urban burden, arise when organic matter burns incompletely or thermally cracks under oxygen-starved, high-temperature conditions, whether in industrial furnaces, residential stoves, or vehicle engines. Vehicle exhaust carries a distinctive fingerprint: gasoline engines, running hot, tend to emit lighter PAHs, while diesel engines, through heterogeneous compression-ignition, release heavier four- to six-ring compounds bound to fine soot. Congested stop-and-go driving and aging fleets without modern catalytic converters or particulate filters amplify these emissions, a pattern especially pronounced in Latin American cities where fleet turnover lags behind that of high-income regions.

For decades, researchers have traced these fingerprints using diagnostic ratios, simple quotients of the concentrations of specific PAH pairs. Ratios such as fluoranthene divided by fluoranthene plus pyrene, or indeno[1,2,3-cd]pyrene divided by itself plus benzo[ghi]perylene, are compared against threshold values drawn from the literature to label a sample as petrogenic, liquid fossil fuel combustion, or biomass burning. The trouble is that these thresholds overlap. Fuel composition, engine technology, air-to-fuel ratios, atmospheric reactivity, and photochemical degradation all blur the boundaries, and different studies assign the same ratio interval to different sources. Interpreted one at a time, diagnostic ratios can yield contradictory verdicts about the very same air sample.

The research team, led by Carla Sofía Stadler of the Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires and colleagues at CONICET institutes in Argentina, set out to close this gap. Their decision-support matrix takes five established diagnostic ratios, harmonizes their classification thresholds from published literature, and converts each ratio into a discrete score of zero, one, or two points for each of three source categories. A ratio earns two points when it points unambiguously to a single source, one point when it falls in an overlapping transitional zone, and zero when it provides no attribution. The scores are then normalized so that every sample receives relative contributions from all three categories rather than a single verdict. Where the literature disagreed, for example on the upper boundary of the indeno-pyrene ratio, the matrix defined a transitional interval in which both combustion sources share the evidence equally, and a sensitivity analysis shifting those boundaries by five and ten percent confirmed the framework’s robustness.

To test the tool, the researchers monitored PM2.5 and PM10 at three sites in Tandil, a rapidly growing city of roughly 145,000 inhabitants in Buenos Aires province, from June 2025 to May 2026. The sites spanned a traffic gradient: a high-traffic location in the commercial center frequented by passenger cars, buses, and delivery trucks; a medium-traffic site dominated by passenger vehicles; and a low-traffic residential neighborhood. Particulate matter was collected on glass fiber filters over 24-hour periods using low-volume samplers, weighed gravimetrically, and analyzed for eighteen priority PAHs by gas chromatography coupled with mass spectrometry, with calibration linearity exceeding 0.994 and detection limits in the sub-nanogram range.

The measurements revealed a clear combustion-dominated atmosphere. High-molecular-weight PAHs made up 98 percent of the total PAH burden in both particle fractions. Principal component analysis found that a single combustion-related component, loaded heavily with benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[a]pyrene, indeno-pyrene, and their heavy-weight relatives, explained more than 76 percent of the variance in PM2.5 and 82 percent in PM10. A second, much weaker component carried the lighter phenanthrene and anthracene, hinting at lower-temperature combustion and petrogenic traces. Total PAH concentrations peaked at the medium-traffic site and were lowest in the residential area, and they rose during the colder months, a seasonal pattern consistent with heating activity.

When the decision matrix was applied, it assigned biomass combustion the highest relative diagnostic score at every site, ranging from 54.3 to 73.9 percent of the attribution. Liquid fossil fuel combustion followed at 20.0 to 38.3 percent, while petrogenic inputs remained minor, below 12.3 percent. Crucially, even the high-traffic city center showed a biomass-dominated signature, suggesting that a widespread urban combustion background, driven by wood stoves and open fireplaces, blankets the entire city and dilutes the contrast between neighborhoods of different traffic intensity. The finding echoes earlier studies across Latin America, from Lima and Medellín to São Paulo and Mexican cities, which likewise identified biomass burning alongside vehicular activity as a leading PAH source.

The matrix and the multivariate statistics told a mutually reinforcing story. Spearman correlations, adjusted for multiple comparisons using the Benjamini-Hochberg procedure, showed that the indeno-pyrene ratio was the only diagnostic indicator consistently aligned with the principal component structure in both particle fractions, with a strong negative correlation of minus 0.81 for PM2.5. Ratios built from lighter PAHs, by contrast, lost their statistical footing, likely because those compounds are more vulnerable to gas-particle partitioning and photochemical aging during atmospheric transport. This divergence among individual ratios is precisely the weakness the matrix was designed to absorb: by pooling evidence, it reduces the uncertainty of any single molecular indicator.

The authors are careful about what the tool is and is not. The matrix yields relative diagnostic evidence, not absolute mass fractions, and its thresholds, drawn from literature values, may need refinement in regions with different fuels or combustion technologies. The 24-hour sampling window also smooths over short traffic peaks. Still, its virtues are practical: it requires only routinely measured PAH concentrations, simple statistics, and no specialized modeling expertise, making it a realistic screening instrument for medium-sized cities that lack the resources to run full receptor models such as Positive Matrix Factorization continuously. The researchers envision the framework eventually feeding into smart-city platforms, digital twins, and AI-driven transport models, giving urban planners a fast, reproducible way to decide which emission sources deserve regulatory attention first. For Tandil, the immediate message is clear: cleaning the air will take more than electrifying the bus fleet, because the smoke rising from the city’s chimneys matters just as much as what pours from its tailpipes.

Subject of Research: Source apportionment of particle-bound polycyclic aromatic hydrocarbons in urban air using a semi-quantitative diagnostic ratio decision matrix

Article Title: A semi-quantitative decision matrix for Polycyclic aromatic hydrocarbons source apportionment to support sustainable urban transport and air quality management

Article References: A semi-quantitative decision matrix for Polycyclic aromatic hydrocarbons source apportionment to support sustainable urban transport and air quality management. (n.d.). https://doi.org/10.3389/ffutr.2026.1964066

Image Credits: AI Generated

DOI: 10.3389/ffutr.2026.1964066

Keywords: polycyclic aromatic hydrocarbons, air quality, source apportionment, diagnostic ratios, biomass combustion, vehicular emissions, PM2.5, PM10, principal component analysis, urban transport, Argentina, decision-support matrix