Going Off Grid: A Comparative Study of the Lagrangian and Eulerian Perspectives of New Particle Formation Events

New particle formation (NPF) is the process by which new ultrafine particles are formed from gas-phase precursors. NPF is the dominant source of global aerosol number and a major contributor to global cloud condensation nuclei (CCN) concentrations. The availability of CCN affects Earth’s climate through the modulation of cloud formation, lifetime, and brightness. Most observations of NPF events are conducted at stationary sites; however, NPF observed from stationary sites may not adequately capture the isolated effects of aerosol chemistry and physics due to gradual or rapid changes in the air masses passing over the site. In this work, we compare modeled aerosol size distributions at a stationary site and along Lagrangian trajectories crossing the site during NPF events.

We ran a 3-D model of atmospheric composition (GEOS-Chem-TOMAS) that represents changes in aerosol number and component mass due to condensation/evaporation, coagulation, and nucleation across 15 size bins ranging from 3 nm to 10 𝜇m. We ran the model using MERRA-2 meteorology over the central United States for 2019 and compared aerosol size distributions with measurements at the Southern Great Plains (SGP) observatory, Oklahoma. Finally, we compared the modeled aerosol size distributions at the SGP observatory with aerosol size distributions derived along Lagrangian trajectories passing over the SGP site.

GEOS-Chem is generally able to simulate the frequency of NPF, undefined (evidence of NPF but unable to quantify nucleation and growth rates), and non-event days. Additionally, in comparing the Lagrangian and stationary perspectives, we are able to re-categorize the majority of undefined NPF events as either non-events or NPF events. We find that most of the undefined NPF events are due to isolated and inhomogeneous NPF occurring upwind of the stationary site, often in the plume outflow of urban regions. Finally, since aerosol size distributions along the Lagrangian trajectories are more continuous, we can derive particle growth and nucleation rates for many of the undefined NPF events, which has not been possible before. We will present an analysis of several case studies, highlighting the discrepancies between the Lagrangian and stationary size distributions, and we will examine some of the potential implications for our understanding of NPF and growth in the atmosphere.