Trends in primary and secondary particle number concentrations in urban and regional environments in NE Spain

We analysed long-term datasets of ambient air concentrations of gaseous pollutants, black carbon (BC) and particle size distributions at an urban background air quality station (Barcelona, Spain; 2013–2019) and a regional background station (Montseny, Spain; 2014–2019). Using BC as a tracer of primary emissions, we segregated the aerosol load in modes and analysed their trends, considering all-year data and seasons separately. We also applied the positive matrix factorisation (PMF) receptor model to the particle size distributions to identify different sources that contribute to the particle number concentration. Our results show that the number concentration of primary particles decreased at the urban and regional background stations (−4.1 and −4.7% per year, respectively) along the period of study. This decrease was also observed for primary particles in the nucleation, Aitken and accumulation modes separately. This change leads to a decrease in condensation sinks (CS, −3.1 and −5.6% per year), that, together with an increase in temperatures (+1.5% per year in the urban background; +1.4% per year in the regional background), results in a markedly increasing trend in the contribution of new particle formation to the total number particle concentration (+17.7% and +11.9% per year). The total particle number concentration decreased in the urban background (−2.5% per year), whereas the trend increased for the regional background (+3.0% per year). The latter result is in contrast to most literature reports and is most likely due to higher insolation and increased biogenic VOC (BVOC) emissions that would enhance photonucleation, along with a marked decrease in the CS. An increasing trend in H₂SO₄ concentrations driven by increasing SO₂ concentrations (+11.1% per year) could also be favouring the formation of new particles in the regional background, but it is not clear that the increasing trend in SO₂ is real or it is related to a faulty maintenance of the instrumentation.Overall, we conclude that decreasing primary emissions results in a decrease in the total number concentrations of urban aerosols, despite the increase in the formation of new particles by photonucleation. Moreover, the total particle number concentration of regions downwind of the urban areas increases because new particle formation is more favoured and compensates for the primary particles’ number concentration decrease.