Seasonal analysis of organic aerosol composition resolves anthropogenic and biogenic sources at a rural background station in central Europe

Introduction

Fine particulate matter (PM_2.5) has a major impact on the climate¹ and can affect human health.² Though the major fraction of submicron PM is from organic compounds,³ their sources or organic precursor vapours, their atmospheric oxidation mechanisms and cross-reactions with inorganic trace gases remain unknown and are the focus of ongoing research. Volatile organic compounds of biogenic and anthropogenic origin can be oxidised in the atmosphere.⁴ The oxidation leads to a higher functionality, which reduces the volatility of the products, hence gas-to-particle conversion contributes to the formation of secondary organic aerosol (SOA) particles.²

 

Methods

From August 2021 to August 2022, we sampled PM_2.5 glass fiber filters with a high-volume sampler for 12 hours at a rural background monitoring station. We measured the sample extracts in full scan MS with data dependent tandem mass spectrometry on a high-resolution hybrid quadrupole-Orbitrap mass spectrometer (Q Exactive Focus). Analytes were ionized with a heated electrospray ionisation source. For separation we used a ultra-high-performance liquid chromatography (Vanquish Flex) on a reversed phase column. To identify known and unknown compounds we used non-target analysis (Compound Discoverer 3.3), implementing fragmentation spectra search with mzCloud and the aerosolomics database. Hierarchical cluster analysis (HCA) and concentration-weighted trajectories (CWT) supports the interpretation of the results.   

 

Results

The HCA groups the 6,080 detected compounds into two main clusters. Based on the chemical composition we interpret the compounds therein as of biogenic and anthropogenic origin. Sample clustering shows a clear seasonal cycle of the SOA mass and its chemical composition. During summer the SOA is dominated by biogenic compounds indicating a strong local influence of the vegetation. Anthropogenic compounds are relatively enriched during colder conditions with strong transport of air pollution during singular events. CWT show the air mass origins of these pollution events and allow for an interpretation of potential sources such as coal-fired power plants in eastern Germany and eastern Europe during stable, warm and dry weather conditions in Europe.

Our top-down approach could be valuable for understanding the variability and complexity of SOA processes and origins, helping to estimate anthropogenic influences on SOA formation, and thus for validating the anthropogenic aerosol forcing in Earth system models.

 

Literature

• 1. Shrivastava, M. et al. Recent advances in understanding secondary organic aerosol: Implications for global climate forcing. Rev. Geophys. 55, 509–559 (2017).
• 2. Fan, W. et al. A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates. Journal of Hazardous Materials 430, 128406 (2022).
• 3. Jimenez, J. L. et al. Evolution of Organic Aerosols in the Atmosphere. Science 326, 1525–1529 (2009).
• 4. Atkinson, R. & Arey, J. Atmospheric Degradation of Volatile Organic Compounds. Chem. Rev. 103, 4605–4638 (2003).