Assessing the combined roles of ammonia, sulfuric acid, and biogenic volatile organic compounds in urban new particle formation.

Abstract

Atmospheric new particle formation (NPF) contributes to aerosol number concentrations and their contribution to cloud condensation nuclei. However, the relative roles of species such as sulfuric acid (H2SO4), ammonia (NH3), and biogenic volatile organic compounds (BVOCs) in regulating particle formation and growth are not well understood in urban environments. To address this gap, continuous in-situ measurements of NH3, BVOCs, and particle number size distributions were conducted between June 2023 and January 2024 at the Thissio urban background site in Athens, Greece. H2SO4 concentrations were also estimated with an advanced proxy method. The period-mean NH3 concentration was 2.3 ± 0.7 μg m-3, with warm-season variability consistent with air-mass transport from surrounding agricultural areas. NPF occurred on 19% of measurement days and exhibited the highest frequencies (29‒33%) in autumn, when atmospheric ventilation was reduced and the condensation sink was lower, while enhanced growth rates were observed in summer. Limited variability in H2SO4, NH3, and BVOCs was observed between event and non-event days, indicating that single-precursor abundance alone cannot explain NPF occurrence. The highest formation rates (J at 14 nm) were calculated under intermediate NH3 and H2SO4 concentrations together with relatively low BVOC levels, highlighting the combined influence of precursor availability, condensation sink (CS), and cluster survival. In contrast, gaseous precursors exhibited clearer relationships with particle growth, with higher growth rates occurring under enhanced NH3 and BVOC (isoprene and monoterpene) availability. Our findings emphasize that NPF occurrence under urban Eastern Mediterranean conditions is not exclusively determined by precursor abundance, condensational losses, and particle survivability, but rather by a combination of these factors, while BVOCs show a clear association with subsequent particle growth.