Ambrosia pollen season dynamics and meteorological drivers in Zagreb (2020–2024): evidence from zero-inflated count models

Abstract Ragweed ( Ambrosia artemisiifolia L.) pollen represents a major aeroallergen in urban environments across Central and Southeastern Europe, yet quantitative assessments of its seasonal dynamics and meteorological drivers remain limited for many regions. This study examines the characteristics of the ragweed pollen season and daily meteorological drivers in Zagreb, Croatia, utilizing five consecutive years of monitoring data (2020–2024). Daily airborne pollen concentrations were analysed alongside local meteorological variables using a main pollen season (MPS) definition using the 5–95% cumulative sum method. To account for the highly skewed and zero-inflated nature of pollen data, negative binomial (NB) and zero-inflated negative binomial (ZINB) regression models were applied, complemented by nonparametric correlation analysis and multivariate ordination. Ragweed pollen seasons in Zagreb were consistently confined to a narrow late-summer window, with minimal interannual variability in timing and durations of approximately 30–36 days. In the context of a consistent phenological structure, atmospheric temperature was identified as the primary meteorological factor influencing daily fluctuations in pollen levels, showing a clear positive correlation with concentrations. In contrast, relative humidity and precipitation showed negative associations during the flowering period, whereas wind speed displayed weaker and context-dependent relationships. ZINB models enabled separate modelling of structural zeros and daily pollen concentration intensity. Multivariate analyses independently confirmed a dominant thermal–humidity axis supporting a dominant thermal–humidity association underlying pollen variability. These findings demonstrate that ragweed pollen dynamics in Zagreb are governed by a stable and highly concentrated seasonal structure, within which short-term meteorological variability, particularly temperature, controls daily exposure levels. The integration of phenologically informed MPS definitions with zero-inflated modelling provides an interpretable analytical framework for understanding urban ragweed pollen dynamics and supports evidence-based interpretation of aerobiological monitoring data.