Vertical Distribution of Boundary Layer New Particle Formation and Implications on Nanoparticle Growth Mechanisms

Principal Investigator(s):
James Smith, University of California Irvine

Jeffrey Pierce, Colorado State University

New particles can form high above ground in the atmosphere before such formation events are observed by ground-based instruments. Once formed, these particles can take up significant amounts of water vapor and become more liquid-like, which has implications on the mechanisms by which nanoparticles grow and, ultimately, impact cloud formation processes. These observations motivate the two hypotheses that we shall test in this project using a combination of modeling, laboratory experiments, and the analysis of field observations. Our first hypothesis is that ground-based measurements do not always accurately represent the location and timing of new particle formation events, nor do they adequately characterize the dominant physical and chemical processes that are responsible for the subsequent growth of newly-formed particles. Our second hypothesis is that freshly nucleated nanoparticles take on considerable amounts of water in the atmosphere, which results in unique and previously unexplored chemical pathways for nanoparticle growth and also are important for assessing aerosol-cloud interactions.

This project will have two main objectives, each of which directly addresses our two hypotheses. Our first is to investigate the timing, distribution, meteorological conditions, and nanoparticle properties associated with boundary layer new particle formation through the analysis of ARM field campaigns and long-term observations. This activity will provide a comprehensive description, both in time and in space, of the gases and nanoparticles that are responsible for these events. We will also perform modeling studies to explore the vertical profile and timing of new particle formation events from these campaigns. Our second objective is to perform laboratory and process-level modeling studies to investigate the role that low ambient temperature and high relative humidity (as would be typical at the top of the boundary layer) play in determining unique chemical pathways for nanoparticle growth. We shall explore the validity of common representations of the interactions between gases and particles for predicting the uptake of representative gases and possible surface and volume reactions on or within growing nanoparticles. We will also revisit the representation of nanoparticle growth in global models and update assumptions of formation and growth based on the findings of the field, laboratory, and process-level-model work.

Our exploration of the vertical extent of new particle formation will provide insight into the most important region of the atmospheric boundary layer for future observational and modeling studies. It will also identify the most relevant temperatures and relative humidities in which nanoparticle growth occurs, which will aid in the design of laboratory studies and process-level model experiments. The potential role of trace water in nanoparticle growth represents a new perspective that has not been adequately studied previously. The unique pathways that we discover can be incorporated into regional and global models, thereby improving predictions and attribution of the role of new particle formation in, e.g., air pollution formation and cloud properties.