Chemical and isotopic genesis of fine and ultra-fine aerosols in the coastal urban atmosphere

Particulate matter (PM) is one of the largest contributors to air pollution and is associated with several adverse health effects. In this study, we collected PM₁ and PM_2.5 samples in Gdynia (northern Poland) during five campaigns in January-March and September-October 2022. Analysis of the chemical composition of these aerosols (organic carbon [OC], elemental carbon [EC] and water-soluble ions) supplemented with their carbon isotope signatures (δ¹³C_TC), meteorological parameters, HYSPLIT air masses backward trajectories and PCA-MLRA receptor modelling allowed us to recognize the significant differences in seasonal concentrations of both aerosol fractions and to identify the potential sources/processes that influenced their chemical composition. In general, the influence of both the anthropogenic sources, mainly related to fuel combustion and agricultural/fertilization activity, as well as natural sources, particular terrigenous was found. PCA-MLRA revealed that fuel burning contributed on average, with 77.9 % and 83.7 % to total PM₁ and PM_2.5 mass, respectively, while agricultural activities represented, 9.5 % and 9.1 % of PM₁ and PM_2.5 mass, respectively. Despite the location of the measurement station in the vicinity of the sea coastal zone, during the analysed period it was not found that the sea water was a dominant source of aerosols in both size fractions, contributed with 9.2 % and 4.6 % to total PM₁ and PM_2.5 mass, respectively. Regardless of the season several indicators - OC/EC, EC/TC, K⁺/TC and TC/SO₄^2- - supported the important influence of coal burning, with a less important contribution from biomass burning. The maturation processes in the atmosphere were also important. For PM₁ aerosols the clear ¹³C enrichment during the heating period was noticed, what was confirmed by isotope mass balance (Keeling plot approach, -25.05 ‰ ± 0.21 ‰ and - 25.90 ‰ ± 0.28 ‰ in the heating and vegetative seasons, respectively). It was associated with high coal usage for heating purposes. For PM_2.5 aerosols such a clear tendency was already not observed (-25.37 ‰ ± 0.22 ‰ for the heating season and - 25.32 ‰ ± 0.40 ‰ for the vegetative season). During the heating season a substantial contribution from a mobile source was also found (NO₃^-/non-sea-salt SO₄^2- ratio, equal to 1.14). In turn, during the vegetative season the importance of the stationary sources (NO₃^-/non-sea-salt SO₄^2- ratio, equal to 0.34) and agriculture (about 2 times higher K⁺ and Cl^- concentrations) had increased. The PM_2.5 concentrations exceeded the WHO guidelines 34 times, 18 times during the heating season and 16 times during the vegetative season. Based on the chemical characteristics and meteorological patterns it was concluded that high concentrations episodes of PM₁ and PM_2.5 were likely caused by the overlapping of local emission sources (mainly for heating purposes, agriculture and transport) and long-distance transport of polluted air masses from southern Poland.