Understanding the sources, scales and fate of ultrafine particles

The biggest source of uncertainty in understanding anthropogenic forcing of the climate system derives from the role of atmospheric particles. The ability of particles to influence climate both through direct effects (i.e. increasing albedo) and indirect effects (i.e. changing cloud properties) are non-linear functions of the number concentration, size distribution and chemical composition of in situ particles. These are, in turn, determined by particle dynamics processes ¡V nucleation (new particle formation (NPF)), direct emission of particles, condensation, coagulation, and deposition. Each of these processes is incompletely understood. My research has focused on understanding controls on NPF and initial particle growth from initial detectable sizes of 3 nm diameter to climate relevant sizes (∼100 nm). NPF events have been observed in numerous locations, but questions remain as to the scale of these events and their importance to regional/global particle number concentrations, size distributions and climate forcing. In my research I improved the understanding of mechanisms responsible for NPF and growth, the spatial scales of, and vertical and horizontal variability in, NPF and growth over eastern North America. Results from my analyses show that NPF events and high concentrations of ultrafine particles (UFP) extend for several hundreds of kilometers as a result of the major role played by the regional background in providing chemical precursors for particle nucleation. Further, the appearance of freshly nucleated particles appears to be strongly linked to boundary layer dynamics and that the nucleation mechanism may be initiated by an increase in turbulence from aloft. Thus, events observed near the surface often result from the entrainment of either freshly nucleated particles or nucleation precursors formed aloft. Given the difficulty of directly measuring continuous particle size distributions (PSDs) at many locations, the use of observations from space would represent a unique tool for predicting NPF occurrence. In this regard I developed and evaluated an innovative algorithm to estimate UFP number concentrations based on satellite retrieved variables which shows good skill for predicting UFP concentrations and relative to global model output.