Sources of ultrafine particles at a rural Midland site in Switzerland

Abstract. Ultrafine Particles (UFPs, i.e. atmospheric aerosol particles smaller than 100 nm in diameter) are known to be responsible for a series of adverse health effects as they can deposit in humans' bodies. So far, most field campaigns studying the sources of UFPs focused on urban environments. This study investigates the outdoor sources of UFPs at the atmospheric monitoring station in Payerne, which represents a typical rural location in Switzerland. We aim to quantify the primary and secondary fractions of UFPs based on specific measurements between July 2020 and July 2021 complementing a series of operational meteorological, trace gas and in-situ aerosol observations. To distinguish between primary and secondary contributions, we use a method that relies on measuring the non-volatile particles fraction as a proxy for primary particles. We further compare our measurement results to previously established methods. We find that primary particles resulting from traffic and residential wood burning (direct emissions), contribute less than 40 % to the total number of UFPs, mostly in the Aitken mode. On the other hand, we observe local NPF events (observed from ~1 nm) evident from the increase of cluster ions and nucleation mode particles concentrations, especially in spring and summer. These events, mediated by sulfuric acid, contribute to increasing the UFPs number concentration, especially in the nucleation mode. Besides NPF, chemical processing of particles emitted from multiple sources (including traffic and residential wood burning) contribute substantially to the nucleation mode particle concentration. Under the present conditions investigated here, we find that secondary processes mediate the increase of UFP concentration to levels equivalent to those in urban locations, affecting both air quality and human health.