On the derivation of particle nucleation rates from experimental formation rates

A. Kürten,J. Almeida,C. Williamson,J. Kirkby,J. Curtius

doi:10.5194/acp-15-4063-2015

Abstract

Abstract. Particle formation rates are usually measured at sizes larger than the critical size at which nucleation occurs. Due to loss of particles during their growth to the detection threshold, the measured formation rate is often substantially lower than the nucleation rate. For this reason a correction needs to be applied in order to determine the nucleation rate from the measured formation rate. Analytical formulae for the correction factor are provided in the literature. However, these methods were derived for atmospheric nucleation measurements and therefore need to be adjusted in order to be applied to chamber nucleation studies. Here we propose an alternative, numerical method that allows precise nucleation rates to be determined in arbitrary experimental environments. The method requires knowledge of the particle size distribution above detection threshold, the particle growth rate, and the particle loss rates as a function of particle size. The effect of self-coagulation, i.e., cluster–cluster collisions, is taken into account in the method.

Highlights

Aerosol nucleation, or new particle formation (NPF), is an important phenomenon taking place throughout the Earth’s atmosphere (Kulmala et al, 2004)
We present here a new method that yields accurate results for any environment – be they chamber or atmospheric data – provided the particle size distribution above a certain threshold size is known, as well as the particle growth rate, and where all loss processes are quantified as a function of size
In order to derive their analytical formulae, Kerminen and Kulmala (2002) as well as Lehtinen et al (2007) made the following assumptions: 1. the only important sink for new particles is their coagulation with larger pre-existing particles, 2. the new particles grow at a constant rate, 3. the population of pre-existing particles remains unchanged during the new particle growth

Summary

Introduction

New particle formation (NPF), is an important phenomenon taking place throughout the Earth’s atmosphere (Kulmala et al, 2004). The key parameter of interest is the nucleation rate, which is defined as the formation rate (cm−3 s−1) of new particles at the critical size. The critical size is the smallest size at which the growth rate of a particle is on average faster than its evaporation rate. This size depends mainly on the concentrations and other properties of the nucleating vapors, as well as on temperature. 2 nm mobility diameter under atmospheric conditions (Kulmala et al, 2013) It can be as small as two molecules in the case of barrierless, kinetically limited particle formation, where the dimer is already stable against evaporation (McMurry, 1980; Kürten et al, 2014)

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Discussion

Conclusion