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.