Atmospheric aerosols influence climate and climate change on local to global scales by affecting the atmospheric radiation balance directly through scattering and absorbing incoming solar radiation and indirectly as cloud condensation nuclei (CCN). Atmospheric measurement and modeling studies have shown that new particle formation (NPF), through photochemical reactions of gas-phase precursors, greatly increases the number concentration of atmospheric aerosols, and is often followed by rapid growth of the nucleated aerosol to a CCN-active size, significantly increasing the CCN population. This rapid growth, often many times faster than that of the growth assuming the condensation of sulfuric acid alone, is neither well understood nor represented in regional and global models. This lack of understanding limits the ability to realistically assess the impact of NPF on the global surface CCN population and its contribution to the aerosol indirect effect.

This study presents the first measurements of size-dependent particle diameter growth rates for freshly nucleated particles down to 1 nm geometric diameter. Data analysis methods were developed, de-coupling the size and time-dependence of particle growth rates by fitting the aerosol general dynamic equation to size distributions obtained at an instant in time. Size distributions of freshly nucleated particles were measured during two intensive NPF measurement campaigns in different environments (Atlanta, GA and Boulder, CO) using a recently developed electrical mobility spectrometer with a diethylene glycol-based ultrafine condensation particle counter as the detector. Size and time-dependent growth rates were obtained directly from measured size distributions and were found to increase approximately linearly with size from ~1 to 3 nm geometric diameter. The resulting growth rate enhancement, defined as the ratio of the observed growth rate to the growth rate due to the condensation of sulfuric acid only, was also found to increase approximately linearly with size from ~ 1 to 3 nm geometric diameter. Survival probability calculations comparing constant and size-dependent growth indicate that neglecting the strong growth rate size dependence from 1 to 3 nm observed in this study could lead to a significant overestimation of CCN survival probability.