Simulation of in situ ultrafine particle formation in the eastern United States using PMCAMx-UF

A three-dimensional chemical transport model has been developed incorporating the Dynamic Model for Aerosol Nucleation for the simulation of aerosol dynamics into the regional model PMCAMx. Using a scaled version of the ternary H2SO4-NH3-H2O nucleation theory and the Two Moment Aerosol Sectional algorithm, the new model (PMCAMx-UF) is used to simulate a summertime period in the eastern United States. The model predicts, in agreement with observations, frequent nucleation events that take place over hundreds to thousands of kilometers, especially in the northeastern United States. Detailed comparison with the observations of the Pittsburgh Air Quality Study suggests that the model reproduces reasonably well the details of the events in this sulfur rich area but has a tendency to overpredict the frequency of the events. Regional nucleation is predicted to increase the total number concentrations by roughly a factor of 2.5 over the whole domain. The corresponding increases for particles larger than 10 nm (N-10) and 100 nm (N-100) were 75% and 15%, respectively. In the Ohio River Valley the increases are as much as a factor of 10 for total particle number and 40% for N-100. Contrary to the total particle concentration, increases of N-100 take place often in areas different than those of the nucleation events. Nucleation is predicted to decrease the N-100 in some areas even if it increases the total number concentration. The sensitivity of the model to the nucleation rate scaling parameter and the ammonia levels is discussed.