An indicator for sulfuric acid–amine nucleation in atmospheric environments

New particle formation (NPF) occurs frequently in various atmospheric environments and it is a major source of ultrafine particles. This study proposes an indicator, I, for the occurrence of NPF in the atmosphere based on the mechanism of H2SO4-amine nucleation. It characterizes the synergistic effects of the governing factors for H2SO4-amine nucleation, including H2SO4 concentration, amine concentrations, the stability of H2SO4-amine clusters, and aerosol surface area concentration. Long-term measurements in urban Beijing were used to validate this indicator. Good consistency was found between this indicator and the occurrence of NPF. NPF was usually observed with I > 1 for typical conditions in urban Beijing. The derivation and expressions of I also indicate a good positive association between the H2SO4 dimer concentration and NPF, as also verified by measurements. I was shown to be also applicable in urban Shanghai. Copyright (c) 2021 American Association for Aerosol Research