The Impact of a Combined Organic and Sulfuric Acid Nucleation Parameterization on the Predicted Aerosol Number Size Distribution During the CARES Campaign

Atmospheric aerosols have important effects on climate and human health. A significant fraction of atmospheric aerosol particles is due to the formation of new particles through nucleation of gas phase species and their subsequent condensational growth to larger sizes. The nanometer size particles can grow into sizes where they can act as cloud condensation nuclei (CCN) and thus they can influence the clouds and climate, and it has been shown that nucleation is an important source of the total particle number concentration and CCN concentration on the global scale. In spite of recent advancements, the aerosol nucleation and growth processes are not yet completely understood, mainly due to the limited measurements of aerosol nucleation precursors (such as H2SO4, NH3, and organic acids) as well as aerosol size, composition, and concentrations of newly formed nanometers particles.

Several mechanisms involving sulfuric acid vapor have been proposed to explain the formation of new particle in the troposphere, including binary homogeneous nucleation (BHN) of H2SO4-H2O, ternary nucleation (TN) of H2SO4-H2O-NH3, ion mediated nucleation (IMN), and the empirical particle formation mechanism (activated-type - ACT). Recent studies, however, have shown that the sulfuric acid alone cannot explain the abundance of new particle in the troposphere and that organic gases likely contribute to new particle formation events. Consequently, new kinetic type combined organic and sulfuric acid (kinetic-type - ORG) mechanisms have recently been proposed.

Chemical transport models, such as the regional-scale WRF-Chem model, are an important tool that can be used to simulate the formation, transport and transformation of aerosols and gases, and improve our knowledge of primary and secondary sources of aerosol particles. The aim of this study is to evaluate the ability of WRF-Chem to adequately represent the spatially and temporally varying number concentration and size distribution of aerosol particles. For this purpose, the MOSAIC sectional aerosol model in WRF-Chem was extended to represent ultrafine particles by using 20 size bins starting at 1 nm diameter. We test a number of nucleation parameterizations including the BHN, ACT, and ORG mechanisms. Measurements collected during the CARES field campaign (Central Valley of California during June 2010) are used to assess the ability of the model to reproduce the observed size distribution of aerosol number and mass. The importance of different nucleation mechanisms on the evolution of the aerosol size distribution is discussed. We find that at the surface, the BHN mechanism is not able to reproduce neither the observed nucleation events nor the observed size distribution, while the ACT and the ORG mechanism better represent the temporal variation and size distribution of the observed events. The comparison of observed and simulated total particle number concentration and size distribution aloft along aircraft flight paths exhibit a different behavior than at the surface. Thus, the ambient conditions affect the predicted particle number concentration and size distribution in addition to the parameterization of new particle formation events. Budget diagnostics for aerosol nucleation, emission, condensational growth, coagulation, transport, and dry deposition tendencies that have been added to the model are used to illustrate when and where new particle formation events occur and their influence on the evolution of the aerosol size distribution.