Vertical distribution of organic tracer compounds in atmospheric aerosols: analysis of low-volume samples from meteorological balloon flights

The evolution and intensity of air pollution episodes are significantly influenced by atmospheric conditions. In particular, stagnant conditions and thermal inversions often involve precautionary health warnings in populated areas. These conditions restrict the dispersion of pollutants leading to exacerbated air quality in the lower layer of the troposphere. Additionally, photochemical reactions contribute to qualitative and quantitative changes in aerosols, with secondary organic aerosols (SOA) formation processes that are not fully understood. This study aims to investigate the vertical distribution of particle-bound organic compounds and Black Carbon (BC) under both temperature inversions and standard ambient lapse rates.

Total suspended particles were simultaneously collected on filters at two different heights (1 m and 400 m) with tethered balloons equipped with low-volume air pumps and BC monitors for periods of 3-hours. Sampling campaigns were performed in industrial, sub-urban and rural background sites in different seasons during 2021 and 2022. Furthermore, sampling was conducted at two different elevations within the city of Barcelona.

Particle-bound organic molecular compounds were analyzed in the low-volume samples (<1 m³) including polycyclic aromatic hydrocarbons (PAHs) and their oxy and methyl derivatives, hopanes, biomass burning anydrosaccharides, fungal tracers, dicarboxylic acids, and secondary products of isoprene or α-pinene oxidation. After sample processing, polar compounds were analyzed with GC-MS while a high-resolution Q Exactive GC Orbitrap MS was used for the rest of compounds.

The high sensitivity and selectivity of the methodology allowed to identify and quantify a large number of organic compounds that were used as tracers to identify the contribution of the emission sources and secondary formation processes to PM. The data describes an increase of toxic exposure under temperature inversion episodes, particularly associated to primary combustion sources in the industrial and suburban sites in wintertime samples with steep vertical concentration gradients, especially before sunrise.

Conversely, vertical distribution of SOA products was not so evident. In general, these compounds were found at similar concentrations at both heights indicating a homogeneous distribution. However, in some occasions SOA was more abundant at height indicating a formation and/or an accumulation of secondary products in the residual layers.

With this work, we contribute to a better understanding of the changes in PM composition at molecular level at different altitudes to help to define effective strategies to mitigate health risks associated with high pollution episodes.