Molecular-resolution simulations of new particle formation: Evaluation of common assumptions made in describing nucleation in aerosol dynamics models

Abstract

Aerosol dynamics models that describe the evolution of a particle distribution incorporate nucleation as a particle formation rate at a small size around a few nanometers in diameter. This rate is commonly obtained from molecular models that cover the distribution below the given formation size -although in reality the distribution of nanometer-sized particles cannot be unambiguously divided into separate sections of particle formation and growth. When incorporating nucleation, the distribution below the formation size is omitted, and the formation rate is assumed to be in a steady state. In addition, to reduce the modeled size range, the formation rate is often scaled to a larger size based on estimated growth and scavenging rates and the assumption that also the larger size is in a steady state. This work evaluates these assumptions by simulating sub-10 nm particle distributions in typical atmospheric conditions with an explicit molecular-resolution model. Particle formation is included either (1) dynamically, that is, the whole size range starting from single vapor molecules is modeled explicitly or (2) implicitly by using an input formation rate as is done in aerosol models. The results suggest that while each assumption can affect the outcome of new particle formation modeling, the most significant source of uncertainty affecting the formation rates and resulting nanoparticle concentrations is the steady-state assumption, which may lead to an overprediction of the concentrations by factors of approximately from two to even orders of magnitude. This can have implications for modeling and predicting atmospheric particle formation. EDITORNicole Riemer

New Particle Formation in the Atmosphere: From Molecular Clusters to Global Climate

“…In areas where CoagS is low and GR is slow, self-coagulation of newly formed particles can be important (Anttila et al, 2010). For certain combinations of J and CoagS, changes in vapor concentrations due to the diurnal cycle may lead to very large biases in the formation rates of particles parameterized in this way (Olenius & Riipinen, 2017).…”

Resolving nanoparticle growth mechanisms from size- and time-dependent growth rate analysis

“…2.2 both methods of the developed growth rate analysis do not take into account the interaction with particles smaller than the size detected experimentally (i.e., below 3 nm in diameter). This effect is known to cause difficulties in aerosol dynamics simulations (Olenius and Riipinen, 2017). While it is rather unlikely that those small particles affect the deposition of larger particles to the chamber walls significantly, they might cause additional particle growth due to coagulation.…”

“…This can be explained by a multi-component Kelvin effect, where some of the α-pinene reaction products can only participate in growth when particles have grown large enough to overcome the Kelvin barrier, as shown in Tröstl et al (2016) for the α-pinene system. For the peak at 5 nm we can exclude the contribution of particle coagulation below the measurement size range (Olenius and Riipinen, 2017) as shown in Appendix D.…”

Excess thermal energy and latent heat in nanocluster collisional growth

“…Though there are notable exceptions, 13,14,15,16,17,18 these assumptions are utilized in classical nucleation theory 19,20,21 and modifications of it, 22,23 and are almost universally invoked in population balance models of nanocluster and nanoparticle growth. 24,25 Combined, such assumptions enable prediction of nanocluster population distribution dynamics based upon knowledge of collision rate coefficients and of the equilibrium coefficients for the elementary reactions considered. However, as nucleation and growth models often fail to reflect accurately experimental measurements (e.g., inferred nucleation rates deviate from measurement by a large amount 22,26,27,28,29 ), both of these assumptions merit further scrutiny.…”