A summer field campaign was conducted at the forested background site of K-puszta in Hungary. The main aim was to assess the contribution of terpene-derived particulate organic compounds to the PM_2.5 organic carbon (OC) and of the secondary organic carbon (SOC) from α-pinene to the OC. The study lasted from 24 May to 29 June 2006; the first half the weather was cold, while the second half was warm. Separate daytime and night-time PM_2.5 samples were collected with a high-volume sampler and the samples were analysed by several analytical techniques, including ion chromatography (IC) and liquid chromatography–mass spectrometry (LC/MS). The latter technique was used for measuring the terpene-derived species. Ancillary high time resolution measurements of volatile organic compounds (VOCs) were made with proton-transfer reaction–mass spectrometry. The temporal and diurnal variability of the particulate compounds and VOCs and interrelationships were examined. It was found that the monoterpenes and a number of terpene-derived particulate compounds, such as cis-pinic and cis-caric acid, exhibited a strong day/night difference during the warm period, with about 10 times higher levels during the night-time. During the warm period, the IC compounds and LC/MS compounds accounted, on average, for 3.1% and 2.0%, respectively, of the OC, whereas the contribution of SOC from α-pinene to the OC was estimated at a minimum of 7.1%.

This study presents a comparative analysis of Cr₂O₃ nanoparticles synthesized by two different methods: a green route using Cordyline fruticosa extract (Cr₂O₃-G) and a chemical route via a solvent-controlled sol–gel process with ethanol (Cr₂O₃-C). Their photocatalytic performance in degrading methylene blue (MB) was evaluated under UV irradiation. Comprehensive characterization was performed using FTIR, XRD, UV–vis spectroscopy, particle size analysis, zeta potential measurements, SEM, and EDX to examine structural, optical, and morphological features. Crystallite sizes estimated by the Scherrer equation were 38 nm (Cr₂O₃-G) and 44 nm (Cr₂O₃-C), while Williamson–Hall analysis gave slightly larger values of 48 nm and 58 nm, respectively. Specific surface area (SSA) calculations from XRD indicated a higher SSA for Cr₂O₃-G (30 m²/g) compared to Cr₂O₃-C (26 m²/g). The optical band gap was slightly lower for Cr₂O₃-G (3.05 eV) than for Cr₂O₃-C (3.07 eV). Zeta potential values showed better stability for Cr₂O₃-G (–44.3 mV) than Cr₂O₃-C (–21.8 mV). SEM analysis revealed hexagonal structures in Cr₂O₃-G and cubic structures in Cr₂O₃-C. Both catalysts demonstrated excellent photocatalytic activity, achieving nearly complete MB degradation within 20 minutes (97% for Cr₂O₃-G and 96% for Cr₂O₃-C). Although their efficiencies were comparable, notable differences in stability, morphology, surface area, and band gap highlight the strong influence of synthesis methods on the physicochemical properties of Cr₂O₃ nanoparticles. Graphical Abstract