First insights into the new particle formation mechanism in Istanbul.

Atmospheric new particle formation (NPF) is a major source of ultrafine particles (UFPs) in urban environments. However, the characteristics and controlling mechanisms of NPF remain poorly understood in megacities influenced by complex emission mixtures and meteorological conditions. This study presents the first multi-site observational evidence of NPF in Istanbul, Turkiye, based on particle number size distribution (PNSD) measurements from 10 to 400 nm conducted over approximately 20 days in each season within a single year across three locations representing distinct urban characteristics; suburban background (Maslak), urban background (Alibeykoy), and traffic-influenced (Aksaray). The analysis adopted a seasonal and spatially resolved framework. NPF events were observed on 23% of days in suburban Maslak, 26% in urban Alibeykoy, and 10% in the traffic area of Aksaray, exhibiting pronounced seasonal and diurnal variability. NPF occurrence peaked in spring at Maslak, in spring and autumn at Alibeykoy, and in winter at Aksaray. Growth and formation rates, together with the condensation sink (CS) and precursor gas dependencies (e.g., sulfur dioxide, sulfuric acid), revealed that NPF was systematically favored under conditions of low pre-existing aerosol surface area, enhanced solar radiation, and air masses originating from the western marine sector and northern Europe. Polluted continental flow and desert-influenced trajectories were associated with suppressed NPF activity. This work emphasizes the importance of incorporating NPF-related observations into future air quality and exposure assessments in major Mediterranean and Middle Eastern cities.