Long-term changes in Aitken-mode particle number concentration in Brisbane: roles of meteorology, traffic, and biomass burning.

Particle number concentration (PNC) is a critical component of urban air pollution with important implications for human health. This study investigates decadal-scale changes in Aitken-mode PNC (25-100 nm) in Brisbane, Australia, using intermittent monitoring across four periods between 1998 and 2025. Phase-based non-parametric statistical analyses were applied to monthly median PNC to evaluate temporal differences across overall, seasonal, and diurnal subsets. Long-term datasets of meteorology, traffic activity, and biomass burning episodes were analysed to support the interpretation of observed changes. Median concentrations decreased by 46% from 3,700 cm⁻3 in 1998-2000 to 2,000 cm⁻3 in 2021-2023. Reductions were most pronounced in autumn and winter and during morning hours, with significant differences observed across all diurnal periods. However, the discontinuous monitoring record does not demonstrate a continuous decline. Concentrations measured during 2025 were higher than those in 2021-2023, indicating variability among recent years. Meteorological analyses showed no consistent long-term improvement in dispersion-related conditions. Traffic activity declined markedly during the COVID-19 period and subsequently recovered, suggesting that changes in commuting, mobility, and urban activity may have contributed to the low PNC observed in 2021-2023. However, traffic and PNC patterns were not consistently aligned, while meteorological and biomass-burning records did not clearly explain the observed differences, indicating that multiple factors likely contributed. This study provides rare long-term evidence from a subtropical Southern Hemisphere city and highlights the need for continuous PNC monitoring together with vehicle fleet composition, emissions, and meteorological data.