Ultrafine particles in Australian and New Zealand cities: characteristics, trends, and relationship with ambient air pollutants

) are well studied through extensive monitoring and modelling, UFPs-measured as particle number concentration (PNC)-remain poorly quantified worldwide, including in Australasia. Available data on UFP concentrations and size distributions are scarce, fragmented, and often based on short-term or single-site studies, largely due to monitoring challenges. This limits the ability to detect regional trends, assess urban influences, and design effective air quality policies. This study addresses this gap in knowledge by analysing one year of PNC and particle size distribution (PSD) data from four cities - Brisbane, Melbourne, Sydney, and Auckland - representing four distinct environments. Cross-city comparisons were conducted using PNC harmonised to a common size range, while temporal variability, pollutant associations, and dominant PSD regimes were examined within each city. Harmonised median PNC levels across the three Australian sites were comparable, whereas the Auckland roadside site exhibited higher PNC associated with local source proximity. Diel patterns showed persistent traffic-related peaks and intermittent midday enhancements associated with new particle formation that varied by site type and season. Seasonal peaks occurred during the colder months, likely due to reduced atmospheric mixing and combustion-related sources. Findings highlight the need for dedicated PNC monitoring networks to improve understanding, guide air quality management, and reduce health risks from particulate pollution in urban environments.