New particle formation (NPF) and ozone (O3) episodes are products of photochemically-driven processes. However, O3 formation occurs under the presence of precursors (Monks et al., 2015), whereas NPF usually requires clean atmospheres (Boy and Kulmala, 2002). Clustering studies on particle number size distribution (PNSD) time series in high insolation urban areas revealed the frequent occurrence of photochemical NPF in spring and summer (Brines et al., 2015). Averaged O3 concentrations reached the highest concentrations for the NPF-cluster in most of the studied cities. We intend here to elucidate if this simultaneous peaking of NPF and O3 is due to the similar seasonal patterns of both pollutants or because NPF episodes coincide with high O3 episodes. To this end we analyzed 2014-2018 spring and summer time series of PNSD and O3 concentrations at Montseny, a regional background station 40 km NE of Barcelona (Spain) included in the ACTRIS and GAW networks.We categorized NPF events with the method proposed by (Dal Maso et al., 2005). K-means clustering was applied to the O3 daily series. The number of clusters was chosen so that only one clusters contained all the days registering an exceedance of the information value (180 μg m-3 h-1), using the minimum number of clusters. This was achieved by using 4 clusters. For each of them we studied the occurrence of NPF and the average daily cycles of other variables: PNSD, concentration of NO2, SO2, and PM1, and meteorological variables. The results suggest that, for spring and summer, the highest regional background UFPs concentrations coincide with the highest O3 episodes, but the probability of NPF during these days is the lowest, even though the nucleation-mode number concentration is the highest. These days also present the maximum temperature, solar radiation, and wind speed, and the lowest relative humidity. The condensation sink and concentration of other pollutants are also the highest and peak simultaneously with the wind speed. This suggests that during these days pollutants are transported from the Barcelona metropolitan area to the regional background station. Conversely, when the plume does not reach the station, UFPs and O3 concentrations are the lowest, and the probability of NPF is the highest
