Ambient ultrafine particles: classification, chemical characterization, and quantification of ubiquitous PAHs via DTD-GC×GC-TOFMS

Importance of the Topic

Ultrafine particles (UFPs) are atmospheric aerosols smaller than 100 nm that pose significant analytical and health‐related challenges. Their high surface‐to‐mass ratio enables adsorption of polycyclic aromatic hydrocarbons (PAHs) and other semi‐volatile organic compounds, potentially increasing toxicity. Despite updated exposure limits for PM2.5 and PM10, UFPs lack regulatory thresholds, underlining the need for advanced chemical characterization methods in environmental monitoring and risk assessment.

Objectives and Study Overview

This study aimed to develop and apply a robust workflow for the classification, chemical profiling, and quantification of particle‐bound PAHs in ambient UFP samples. Field campaigns were conducted in Augsburg, Germany, during March and September 2023 to capture seasonal variability. Key goals included source attribution through hopane analysis, method validation for trace PAH quantification, and evaluation of correlations between chemical markers and physico‐chemical parameters.

Methodology and Instrumentation

Sampling employed a three‐stage cascade impactor operating at 30 L/min over 24 h to segregate PM10, PM1 and PM0.1 fractions. Collected UFPs on 7 mm filter punches were spiked with a suite of deuterated PAH internal standards (fluoranthene-d10, pyrene-d10, benz[a]anthracene-d12, etc.). Analysis used direct thermal desorption combined with comprehensive two‐dimensional gas chromatography and time‐of‐flight mass spectrometry (DTD-GC×GC-TOFMS) on a LECO Pegasus BT4D. Chromatographic separation employed a nonpolar primary column (BPX5) and a midpolar secondary column (BPX50). External calibration curves, corrected by internal standards, were validated against HPLC-FLD and NIST standard materials.

Key Findings and Discussion

PAH concentrations in the UFP fraction were relatively stable in March but increased sharply in late September. Benz[a]pyrene reached up to 37 pg/m3, chrysene 54 pg/m3 and pyrene 28 pg/m3 during peak days. No significant correlation was found between PAH levels and PM2.5, PM10, black carbon or ozone. However, slight correlations emerged with total particle number concentration and NOx, suggesting traffic‐related influences. Chemical classification revealed a dominant contribution of 4- and 5-ring PAHs and their alkylated homologs. Hopane fingerprinting indicated mineral-oil derived emissions in September (high hopane-index values) and potential coal combustion inputs in March, based on Ts/Tm and homohopane ratios.

Benefits and Practical Applications

• The DTD-GC×GC-TOFMS workflow enables sensitive and comprehensive profiling of UFP‐bound PAHs, supporting source apportionment and exposure assessment.
• Validated quantification with deuterated standards ensures data reliability for laboratory quality control and regulatory monitoring.
• Hopane pattern analysis provides a practical tool to distinguish between petroleum and coal combustion sources in ambient air.

Future Trends and Opportunities

Advances in real‐time or near‐real‐time UFP analysis, integration of high‐resolution mass spectrometry with online sampling, and expansion to broader semi‐volatile compound classes will enhance source resolution. Coupling chemical data with meteorological and traffic flow information can refine exposure models. Standardized UFP monitoring guidelines and regulatory frameworks are needed to translate these analytical capabilities into public health protection.

Conclusion

The combined sampling and DTD-GC×GC-TOFMS approach delivers high‐fidelity classification and quantification of ubiquitous PAHs in ultrafine particles. Seasonal variations in PAH loads and hopane source markers underscore the complexity of UFP emissions and highlight the importance of targeted analytical methods for environmental assessment and policy development.

Reference

• Kwon HS, Ryu MH, Carlsten C. Ultrafine particles: unique physicochemical properties relevant to health and disease. Exp Mol Med. 2020;52:318–328.
• Das A, Pantzke J, Jeong S, Zimmermann R. Generation, characterization, and toxicological assessment of reference ultrafine soot particles with different organic content for inhalation studies. Sci Total Environ. 2024.
• Eckenberger E, Mittereder A, Gawlitta N, et al. Performance evaluation of four cascade impactors for airborne ultrafine‐particle collection. Aerosol Research. 2025;3:45–56.
• Hartner E, Paul A, Käfer U, et al. Integrating GC×GC‐TOFMS for aerosol SVOC analysis. ACS Earth Space Chem. 2022;6(5):1358–1374.
• Schnelle-Kreis J, Sklorz M, Orasche J, et al. Semi‐volatile organic compounds in ambient PM2.5: seasonal trends and source contributions. Environ Sci Technol. 2007;41(11):3821–3828.