Understanding the effects of boundary layer and synoptic meteorology on new particle formation based on WRF simulations and measurements in Southern Indiana

The purpose of this study is to investigate the relationship between the occurrence and intensity of new particle formation at the local and regional scale and i) the evolution of boundary layer properties and ii) the presence of specific synoptic conditions. Although progress has been made in understanding new particle formation dynamics, the role of boundary layer conditions and synoptic systems as limiting variables for nucleation has not been clearly quantified. Our analysis couples simulation results from the Weather Research and Forecasting Model (WRF) at 3 and 9 km resolution with measurements of vertical profiles of ultra-fine particles, gas phase concentrations and meteorological parameters collected along an 80 km transect in Southern Indiana, during the Nucleation In ForesTs (NIFTy) experiment, at three locations representative of forest, small town and urban environment. The analysis of daily vertical atmospheric profiles of Turbulence Kinetic Energy (TKE), simulated by WRF, allows us to assess the role of mixing in initiating particle nucleation as a consequence of the enhanced probability of cluster collision and the entrainment of air masses rich of nucleation precursors (e.g. sulfuric acid plumes). Further insights to identify the limiting factors for the occurrence of an event day and explain the local differences in terms of nucleation intensity among the sites are provided by the analysis of simulated profiles of the planetary boundary layer height, temperature, relative humidity and stability. In order to identify the sources of the measured nucleation precursors (sulfuric acid, volatile organic compounds and ammonia), isobaric back trajectories at multiple levels are computed and coupled with emission inventory data provided by EPA. Work is continuing to examine the influence of the synoptic meteorological context (e.g. cold front passage, atmospheric stagnation, etc.) in dictating nucleation occurrence using multi-scale WRF simulations.