Nucleation and growth mechanisms of daytime and nighttime new particle formation

Description

Observations suggest qualitative differences between daytime and nighttime nucleation events. The objective of the present study is to test the simple hypothesis that sulfuric acid-limited new particle formation (NPF) produces the classical banana-shaped particle size distributions (psds) suggestive of sharp nucleation pulses and seen in numerous daytime NPF studies for over a decade; while organics-limited NPF produces the more recently observed “tomato”-like psd features, suggestive of sustained nucleation events seen in nighttime NPF (Lee et. al 2008). Laboratory measurements on the ternary organic acid/sulfuric acid/ water system show that these species strongly interact to enhance nucleation rate. A multi-component nucleation theory has been developed that successfully parameterizes the measurements and provides critical nucleus composition at the molecular level (McGraw and Zhang 2008). In this theory each condensable vapor is found to contribute a power-law factor to the overall nucleation rate. For sulfuric acid the power-law exponent ranges between 4 and 8, whereas for organics the exponent is considerably reduced, ranging between 1 and 2. These exponents quantify the sensitivity of nucleation rate to vapor phase species concentration. We show that narrow nucleation pulses are a natural consequence of the higher exponents, as is consistent with sulfuric acid limited nucleation and daytime NPF. By contrast, smaller exponents are found to result in sustained periods of nucleation characteristic of nucleation processes limited by organics and nighttime NPF. Modeled processes include a continuous source of condensable vapor; vapor and size-dependent scavenging of clusters by the surface of typical background aerosols and new particles; self-coagulation; and particle growth. Simulations were performed using the quadrature method of moments (QMOM) with tracking of vapor and four radial psd moments. Lee, S-H, LH Young, DR Benson, T Suni, M Kulmala, H Junninen, T L Campos, DC Rogers, and J Jenson. 2008. “Observations of nighttime new particle formation in the troposphere.” Journal of Geophysical Research 113: D10210. McGraw, R and R Zhang. 2008. “Multivariate analysis of homogeneous nucleation rate measurements: Nucleation in the p-toluic acid/sulfuric acid/water system.” Journal of Chemical Physics 128: 064508.