The formation and growth of ultrafine particles in two contrasting environments: a case study

Abstract. Formation of ultrafine particles and their subsequent growth have been examined during new particle formation (NPF) events in two contrasting environments under varying ambient conditions, one for a tropical semi-urban coastal station, Trivandrum, and the other for a high-altitude free-tropospheric Himalayan location, Hanle. At Trivandrum, NPF bursts took place in the late evening/night hours, whereas at Hanle the burst was a daytime event. During the nucleation period, the total number concentration reached levels as high as ~ 15 900 cm⁻³ at Trivandrum, whereas at Hanle, the total number concentration was ~ 2700 cm⁻³, indicating the abundant availability of precursors at Trivandrum and the pristine nature of Hanle. A sharp decrease was associated with NPF for the geometric mean diameter of the size distribution, and a large increase in the concentration of the particles in the nucleation regime (D_p < 25 nm). Once formed, these (secondary) aerosols grew from nucleation (diameter D_p < 25 nm) to Aitken (25 ≤ D_p ≤ 100 nm) regime and beyond, to the accumulation size regimes (100 ≤ D_p ≤ 1000 nm), with varying growth rates (GR) for the different size regimes at both the locations. A more rapid growth ~ 50 nm h⁻¹ was observed at Trivandrum, in contrast to Hanle where the growth rate ranged from 0.1 to 20 nm h⁻¹ for the transformation from the nucleation to accumulation – a size regime that can potentially act as cloud condensation nuclei (CCN). The faster coagulation led to lifetimes of < 1 h for nucleation mode particles.