Frequent new particle formation events in the Indo-Gangetic Plain occur under reduced condensation sink but are obscured by air mass heterogeneity

Abstract. 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 sub–10 nm size range.

Here, we investigated the occurrence and microphysical evolution of NPF at a suburban IGP site (CAS-AO, Sonipat, Haryana), during May – December 2023 using ion and particle measurements together with meteorological data. NPF events are frequent during summer and less common in winter. The weak seasonal variability of ion concentrations relative to nucleation-mode particles suggests that neutral pathways dominate NPF in this high-sink environment. The median condensation sinks on event-days (0.024 s⁻¹) are approximately half those on non-event days (0.046 s⁻¹). Particle growth rates (maximum concentration method) increase with size, from 14.7 nm h⁻¹ (3–7 nm) to 19.0 nm h⁻¹ (7–20 nm), indicating size-dependent condensational 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.