Nocturnal long-range transport of Alternaria spores in Central Europe: synoptic and boundary-layer drivers

Airborne Alternaria spores represent a major aeroallergen and phytopathogen in Central Europe, yet the boundary-layer mechanisms and synoptic conditions controlling their regional-scale atmospheric transport remain poorly characterised. This study identifies and characterises nocturnal long-distance transport (LDT) episodes of Alternaria spores reaching Bratislava (southwestern Slovakia) from agricultural source regions of the Pannonian Plain. Spore concentrations were monitored at two sites 38 km apart during the main sporulation seasons of 2022 and 2023 using Hirst-type volumetric spore traps. Ten potential LDT episodes were identified based on elevated nocturnal concentration peaks recorded simultaneously at both sites, deviating from the typical daytime-dominated pattern. Four representative episodes were investigated in detail using 24-hour backward trajectory and particle dispersion modelling (HYSPLIT), ERA5 reanalysis data, and spore records from monitoring stations in Austria and Hungary. Principal source areas were identified using the CORINE Land Cover 2018 dataset. Three LDT episodes were predominantly associated with stable anticyclonic circulation, weak pressure gradients, and persistent southerly to south-easterly airflow from western Hungary and Lower Austria, with transport distances of approximately 100–300 km, whereas one episode was characterised by local-to-regional recirculation under stagnant conditions. Daytime convective mixing likely supported spore uplift and regional transport, while evening boundary layer stabilisation may have contributed to nocturnal persistence and gradual sedimentation over Bratislava. These findings suggest that elevated nocturnal Alternaria concentrations outside the typical diurnal cycle can serve as a useful indicator of potential regional atmospheric transport, with implications for allergenic exposure assessment and agricultural disease management.