Atmospheric aerosols play critical roles in air quality, public health, and visibility. In addition, they strongly influence climate by scattering solar radiation and by changing the reflectivity and lifetime of clouds. One major but still poorly understood source of atmospheric aerosol is new particle formation, which consists of the formation of ther-5 15 described as heterogeneous nucleation of organic molecules onto these clusters. We find that the strong gradient in cluster population with

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... respect to its size lead to positive cluster number flux, and therefore driving the growth of clusters substantially smaller than the Kelvin diameter, conventionally considered as the minimum particle size that can be grown through condensation. The conventional approach neglects this con-20 tribution from the cluster concentration gradient, and underestimates the rate of new particle formation by a factor of up to 60.