The NPF Effect on CCN Number Concentrations: A Review and Re-Evaluation of Observations From 35 Sites Worldwide

The new particle formation (NPF) effect on cloud condensation nuclei (CCN) differs from region to region and remains highly uncertain. Here, we re-evaluate the NPF-initiated enhancements in CCN number concentrations (E_N_CCN) by compiling results from 35 field sites worldwide and five global models. We show the E_N_CCN at sites close to the anthropogenic sources are more than 2-fold as large as that at remote sites with supersaturations of 0.1%–1.0%. The enhancements anti-correlate with the time scales for the new particles to grow to CCN size, which are 5–10 and 20–30 hr at polluted and remote sites, respectively. We further reveal that under polluted environments the condensational growth of new particles may be accelerated, leading to larger E_N_CCN. The models fail to simulate the E_N_CCN in polluted regions. Our study highlights the importance of parameterizing the time scale of NPF to affect CCN differently for remote and polluted regions in models.