Atmospheric new particle formation (NPF) leads to the formation of around half of the cloud condensation nuclei needed to form the seeds of cloud droplets. Therefore, the participation of primarily anthropogenic species such as ammonia in NPF may influence the radiative forcing of climate. However, most climate models do not account for how ammonia helps sulfuric acid to form new aerosols. Here we incorporate a ternary NPF parameterization into the UK Met Office Unified Model (UM) using results from the CERN CLOUD chamber. The parameterization also includes the influence of ions produced from radon and cosmic rays. We test the parameterization in a one-way nested UM configuration, where a regional simulation with a 3km horizontal grid resolution over the Colorado Front Range region (centered at 40.0, -105) runs inside a global simulation(N96). The model simulates the 2014 Summer period (July 20th, 2014 to August 10th, 2014) when we are able to evaluate both aerosol and precursor concentrations using measurement data from the DISCOVER-AQ and FRAPPE field campaigns.
The updated NPF simulation significantly enhances the predicted aerosol number concentration in the nucleation mode at low altitudes, producing much better agreement with observations. However, within the regional model, the UM overestimates sulfur dioxide (SO2) and sulfuric acid (H2SO4) gas concentrations, particularly at lower altitudes. Conversely, ammonia (NH3) is underestimated in the same model configuration. At high altitudes, above 5 km, the model overestimates aerosol number concentration both with and without ternary nucleation.
An offline box model is employed to separate and analyze the role of biases in simulated precursor concentrations. In this analysis, we assume that the mass balance of nucleation mode particles (d=3-10nm) is governed by the NPF process contributing to this mode, along with coagulation loss and condensation growth contributing to particles exiting this mode. The results indicate that even when employing ’ground-truth’ precursor values, our model falls short in predicting the observed nucleation mode particle number concentrations. This disparity might stem from the lack of specific parameterizations within the UM, for example for the role of HOMs in NPF, and inadequate representation of vapors that grow newly formed aerosols (e.g. organics) within our GLOMAP aerosol microphysics submodule.
The NPF scheme development has the potential for significant effects on simulated clouds, with implications for climate. On average, the ternary nucleation scheme increases the total number concentration of aerosols(d >100nm) by 25% over the entire Colorado region, with its most pronounced impact observed at altitudes ranging from 500m to 1400m. We also present global simulations incorporating this updated ternary NPF scheme. The distribution of the increase in 100nm aerosol number concentration due to the new NPF parameterization on a global scale displays heterogeneity, with high values over Mid-USA (80%), west-Europe (120%), and south-east Australia (200%). On average, the global increase stands at approximately 65%. Notably, during the summer season (June), the increase peaks, particularly in regions like the Pacific Northwest (250%) and Midwest USA (150%).