Exploring the influence of physical and chemical factors on new particle formation in a polluted megacity†

Abstract

Delhi is one of the most polluted regions in the world, yet studies focusing simultaneously on atmospheric aerosol particle size distribution (PSD) and chemical composition, as well as their inter-relationship, are still lacking. Additionally, the high condensation sink (CS) in Delhi has drawn less attention to new particle formation (NPF) and the role of chemical composition. This study explored the intricate interplay among particle size distribution, meteorology, and chemical composition within the atmospheric environment of Delhi. Our findings reveal pronounced seasonal variations in the particle number and mass concentration levels following variations in atmospheric conditions and emission sources across different seasons. Furthermore, we identified condensation sink as a primary factor governing the NPF, with no NPF event observed when daytime CS was above 0.06 s⁻¹. While precursors such as H₂SO₄ and NH₃ were abundant, they did not appear to be limiting factors for NPF. However, due to the lack of direct measurements of sub-10 nm particles and precursor gases such as H₂SO₄, amines, and organic vapours, the conclusions regarding the role of chemical precursors remain speculative. Furthermore, on days with comparable condensation sinks, the chemical composition exhibits no significant variation between NPF and non-NPF days, with organics contributing to about 50% of the PM_2.5, emphasizing the dominance of physical processes. Our observations highlight the critical influence of relative humidity on particle formation, with higher atmospheric liquid water content inhibiting NPF. Additionally, we investigated the simultaneous time variations in PSD and mass composition of PM_2.5, revealing significant mass composition variations during the first (daytime) and second (night-time) growth. Notably, during the daytime growth of nucleated particles, increases in sulphate and low volatile oxygenated organics suggest the involvement of sulphuric acid and oxidized vapours in early particle growth. However, the unclear relationship between the growth rate and chemical composition reveals the complexity of new particle formation in polluted environments such as Delhi. While PM_2.5 composition offers insights into growth processes, its relevance to nucleation-mode particles is limited. Thus, this study further emphasizes the need for sub-10 nm PSD and precursor gaseous measurements to seek a better understanding of NPF in a high CS environment in the Global South.