Insights into the sources of ultrafine particle numbers at six European urban sites obtained by investigating COVID–19 lockdowns

Abstract. Lockdown restrictions in response to the COVID–19 pandemic led to the curtailment of many activities and reduced emissions of primary air pollutants. Here, we applied Positive Matrix Factorization to particle size distribution (PSD) data from six monitoring sites (three urban background and three roadside) between four European cities (Helsinki, Leipzig, Budapest, and London) to evaluate how particle number concentrations (PNCs) and their sources changed during the respective 2020 lockdown periods compared to the reference years 2014–2019. A number of common factors were resolved between sites, including nucleation, road traffic semi–volatile fraction (road traffic_svf), road traffic solid fraction (road traffic_sf), diffuse urban (woodsmoke + aged traffic), ozone–associated secondary aerosol (O₃–associated SA), and secondary inorganic aerosol (SIA). Nucleation, road traffic, and diffuse urban factors were the largest contributors to mean PNCs during the reference years and respective lockdown periods. However, SIA factors were the largest contributors to particle mass concentrations, irrespective of environment type. Total mean PNCs were lower at two of the urban background and all roadside sites during lockdown. Nucleation and road traffic_svf factors response to lockdown restrictions were highly variable, although road traffic_sf factors were consistently lower at roadside sites. The responses of diffuse urban factors were largely consistent and were mostly lower at urban background sites. Secondary aerosols (O₃-associated SA and SIA) exhibited extensive reductions to their mean PNCs at all sites. These variegated responses to lockdowns across Europe point to a complex network of sources and aerosol sinks contributing to PSDs.