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 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 the rural background site in Cyprus. We report new observations of atypical nanoparticle shrinkage (NPS), characterised by rapid shrinkage of sub-20 nm particles occurring in the absence of preceding NPF that typically accompanies decreasing mode diameter events. The particle size distributions exhibit a mirror image of the conventional “banana-shaped” NPF pattern, forming a distinctive “reverse-NPF” signature. We identified three NPS events during the campaign and show that this phenomenon is not driven by limited precursor gas availability, the oxidation extent of precursor vapour, or vapour scavenging by pre-existing particles. Instead, entrainment of cleaner, relatively drier air and meteorology-driven dilution, together with volatility-resolved analysis, indicate that these events are governed by atmospheric dilution and 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.