Multiscale atmospheric modeling suggests ammonia is necessary but not sufficient to explain new particle formation in the Colorado boundary layer

Abstract. New particle formation (NPF) is an important source of cloud condensation nuclei (CCN) in the atmosphere, and CCN affect Earth's radiative balance via aerosol-cloud interactions. Numerous chemical species are involved, but most climate models still represent NPF only from sulfuric acid and water. However, the roles of ammonia and ions in NPF alongside sulfuric acid are also well-quantified compared to other species. Here, we tested a parameterization of ternary NPF from sulfuric acid, ammonia, ions and water in the UK Met Office Unified Model using surface and aircraft measurements from the 2014 FRAPPÉ and DISCOVER-AQ field campaigns. We used a nested convection-permitting regional model setup with a grid spacing of 3 km, which allowed us to represent the inhomogeneous sources of emissions in the area. The aircraft simultaneously measured sulfuric acid and ammonia vapor concentrations and aerosol size distributions, so we can test whether NPF from these species can explain observed aerosol number concentration. We also compared particle number concentrations in a lower resolution global simulation to surface observations. In our model, errors in the NPF mechanism are compensated by errors in simulated concentrations of gas-phase precursors. We devised a method to disentangle these errors, but only qualitative results were obtained with the datasets we used. While our results suggest ammonia and sulfuric acid are likely important to NPF in Colorado and elsewhere, other species must also make important contributions. Overall, however, the ternary NPF mechanism gives a substantial improvement on the Unified Model's existing representation of aerosol number concentrations.