Ion and particle size distribution measurements in the Indo-Gangetic Plain: seasonal insights and challenges in new particle formation event characterization

New particle formation (NPF) is a major source of atmospheric aerosols and cloud condensation nuclei, influencing climate and air quality. In highly polluted regions such as the Indo-Gangetic Plain (IGP), where precursor concentrations and condensation sinks are among the highest globally, NPF remains poorly constrained due to limited observations of particles and ions in the sub–10 nm size range. Here, we investigated the occurrence and growth of NPF at a suburban IGP site (CAS-AO, Sonipat, Haryana), during May–December 2023 using ion (0.8–42 nm) and particle (2.5–42 nm) measurements together with meteorological data. NPF events were frequent during summer, occurring on 30 of 35 valid days in May–June (86 %), and were much less frequent during the winter/post-monsoon period, with 20 out of 73 valid days (28 %). The weak seasonal variability of ion concentrations relative to nucleation-mode particles suggests that particle survival and growth were strongly modulated by condensation sink, meteorology, and air-mass heterogeneity. The median daily condensation sink during 06:00–12:00 LT was lower on event days than on non-event days (0.0095 vs. 0.0292 s ⁻¹ ). Particle growth rates increased with size, from 14.7 nm h ⁻¹ in the 3–7 nm range to 19.0 nm h ⁻¹ in the 7–20 nm range, indicating size-dependent growth. In this high-background setting, pollution plumes and meteorological variability intermittently mask or distort NPF signals, limiting the direct applicability of both visual and automated classification methods and growth rate estimation methods. Our observations highlight that careful application and improvement of data analysis methods, along with precursor gas measurements, are required to better constrain nucleation processes by avoiding methodological sensitivity due to the complexity of aerosol dynamic processes in multi-source, high-condensation-sink environments, such as IGP.