Comment on egusphere-2025-5777

<strong class="journal-contentHeaderColor">Abstract.</strong> The use of a diffusion charger based AQ Urban sensors to monitor particle number concentrations was investigated in Helsinki metropolitan area. The comparisons between the AQ Urban sensors and traditional butanol CPCs were made at a heavily trafficked street canyon (Traffic Supersite) and at an urban background site (UB Supersite) in 2022. The agreement with the measured particle number concentrations within different AQ Urban units was good. Comparison of the AQ Urban sensor with the two CPCs showed that AQ Urban sensors should be suitable to measure concentration of particles approx. larger than 10 nm in highly trafficked areas. The long-term agreement between AQ Urban sensors and CPCs was also investigated in the two different environments between January 1^st and August 15^th, 2022. Overall, the correlation between AQ Urban sensors and the CPCs was good at both sites (<em>r</em> being 0.93 and 0.89, respectively). The increased concentration of particles smaller than 10 nm and long-range transported pollution affected the accuracy of AQ Urban sensors. Despite this downside of the method, the correlation between the AQ Urban sensor and the CPCs was good during the whole measurement period, indicating that the sensor is well suitable for long-term particle number concentration monitoring in urban environments in Finland. However, the observed effect of bi-modal particle size distribution suggests that the performance of diffusion charger-based sensors may vary in different geographic regions depending on the regional background concentrations of accumulation mode particles which should be considered when applying the method in different locations.