A New Physical Mechanism of Rainfall Facilitation to New Particle Formation

Newly formed particles and their growth contribute significantly to cloud condensation nuclei. However, the effect of rain on new particle formation (NPF) is poorly understood. Rainfall removes the pre-existing large particles by below-cloud scavenging to reduce condensation and coagulation sinks by 4.0 x 10-2 and 1.5 x 10-4 s-1. The scavenging effect largely depends on the amount and duration of rain and the surface area concentration of the raindrops. NPF events can be facilitated by a combination of fewer condensation sinks and favorable meteorological conditions, contributing 3%-47% of PM2.5 (particulate matter smaller than 2.5 mu m). The physical mechanism was confirmed by theoretical analysis. The results help elucidate the NPF process from a physical perspective, which can improve the prediction of the occurrence and duration of haze events. Until precursors are reduced, significant reductions in particulate matter (PM) may be difficult because NPF and its growth recharge the atmosphere with PM. New particle formation (NPF) significantly contributes to the ultrafine particle number and thus impacts the cloud properties. Newly formed particles may also be inhaled into the pulmonary alveoli and blood systems, harming human health. However, compared with the chemical mechanism, the underlying physical mechanism of NPF events is poorly understood, especially the relationship between rainfall and subsequent NPF. NPF events will be more frequent owing to the combination of warmer and wetter climates and fewer coagulation sinks in response to clean air action. This study deepens our understanding of the interaction between NPF and meteorology. A reduction in precursor emissions for NPF should be imposed to further reduce particulate matter. Positive correlation between N13-25 and rainfall was showed in spring and summerNew particle formation (NPF) was facilitated by reduced condensation sinks and favorable meteorology in response to rainfallSurface area concentration of the raindrops is a key parameter for modulating NPF