The contribution of plume-scale nucleation to global and regional aerosol and CCN concentrations (Invited Presentation)

New-particle formation in the plumes of coal-fired power plants and other anthropogenic sulfur sources are an important source of particles in the atmosphere. These particles may grow to sizes where they can act as cloud condensation nuclei (CCN), and thereby increase cloud reflectivity and lifetime, resulting in a cooling of surface temperatures. It has been unclear how best to reproduce this plume-scale new-particle formation in global and regional aerosol models with grid-box lengths that are tens of kilometres and larger. The predictive power of these models has thus been limited by the resultant uncertainties in aerosol size distributions. We have implemented online TwO-Moment Aerosol Sectional (TOMAS) microphysics into the System for Atmospheric Modelling (SAM), a Large-Eddy Simulation / Cloud-Resolving Model (LES/CRM). This model is evaluated against aircraft observations of new-particle formation in two different power-plant plumes and reproduces the major features of the observations. We show how the downwind plume aerosols can be greatly modified by both meteorological and background aerosol conditions.

Based on the results of the SAM-TOMAS model, we develop the Predicting Particles Produced in Power-Plant Plumes (P6) parameterization: a computationally-efficient, but physically-based, parameterization that predicts the characteristics of aerosol formed within sulphur-rich plumes based on large-scale mean meteorological parameters, emissions directly from the source, and mean background pollutant concentrations.

Finally, we implement the P6 parameterization in the GEOS-Chem-TOMAS global chemical-transport model in order to evaluate the contribution of coal-fired power plants globally to particle number and CCN concentrations. Globally, use of the P6 parameterization resulted in predicted CCN concentrations smaller than or similar to previous sub-grid sulphate schemes depending on the model configuration. We find that the sub-grid scale new-particle formation predicted by P6 is most sensitive to uncertainties in the model-predicted aerosol condensation sink. For constant SO2 emissions, fewer new particles are formed in more polluted regions. This spatial heterogeneity in new-particle formation cannot be resolved by previous treatments of sub-grid sulphate but is captured by P6.