Observations of nanoparticle shrinkage phenomena

Abstract. Atmospheric new particle formation (NPF) is a major source of aerosol particles in the Earth's atmosphere. However, process-level understanding of the early stages of particle formation and growth remains poorly represented in climate models, limiting accurate estimates of aerosol effective radiative forcing. Here, we use comprehensive observations from the Spring Particles in Cyprus (SPICY) field campaign conducted at a rural background site in Cyprus. We report new observations of nanoparticle shrinkage (NPS), marked by the rapid decrease in size of sub-20 nm particles occurring in the absence of preceding NPF event. Thus, the particle size distributions exhibit a mirror image of the conventional “banana-shaped” NPF pattern, forming a distinctive “reverse-NPF” pattern. We identified three NPS events during the campaign and show that this phenomenon is not primarily driven by low concentrations of condensable vapours, their scavenging by pre-existing particles, or primary nanoparticle sources. Instead, it is associated with atmospheric dilution, as indicated by air-mass trajectory analysis. Furthermore, fast-moving air masses can enhance turbulent mixing, thereby altering particle size distributions. Together with volatility-resolved analysis, these results suggest that NPS is governed by atmospheric dilution, which reduces particle-phase organic mass and shift the gas-particle equilibrium toward evaporation, with contributions dominated by organic compounds of low and moderate volatility. Our results demonstrate that NPS events provide a previously unrecognised sink for nanoparticles, which are controlled by air-mass dynamics and organic vapour volatility.