Fig. 4: Conditions for rapid growth.
From: Rapid growth of new atmospheric particles by nitric acid and ammonia condensation
Persistent supersaturations of ammonia and nitric acid with respect to ammonium nitrate will be sustained by inhomogeneity in urban conditions with high source strength. This will be sufficient to accelerate particle growth in the range 1–10 nm, where survival is threatened by the high coagulation sink of pre-existing particles from pollution. a, Conceptual image of urban conditions, where inhomogeneities in the concentrations of ammonia and nitric acid vapour and in temperatures are caused by non-uniform sources and large-scale eddies. b, Particles nucleate and grow slowly as (base-stabilized) sulfate (red). The activation size (shown with dp on the x-axis) correlates inversely with the ammonium nitrate saturation ratio (shown qualitatively on the y-axis), as indicated by the dashed curve. Available concentrations of gas-phase nitric acid can exceed those of sulfuric acid by a factor of 1,000, so modest supersaturation drives rapid growth (blue) above an activation diameter determined by particle curvature (the Kelvin term). c, d, Monodisperse thermodynamic growth calculations (from MABNAG simulations) for high (c) and low (d) saturation ratios of ammonium nitrate, corresponding to b and to the closed and open diamonds towards the upper right in Fig. 3a. For a saturation ratio near 4, activation is predicted to occur near 4 nm, consistent with our observations.
