In case of the low ozone test concentration, number concentration increased from around 4 × 103 to 1 × 105 #/cm3 within the first 30 min after the application, and with a delayed increase of the particle mass (Fig. 2a). The mass/time profile showed a low background of 1.2 μg/m3. Mass concentration increased with the number concentration and reached a maximum of 4 μg/m3 after 116 min.