Atmospheric new particle formation: real and apparent growth of neutral and charged particles

J Leppä,T Anttila,K E J Lehtinen,M Kulmala,V.-M Kerminen

doi:10.5194/acp-11-4939-2011

Abstract

Abstract. In this study we have provided simple analytical formulae to estimate the growth rate of a nucleation mode due to self-coagulation and the apparent growth rate due to coagulation scavenging by larger particles. These formulae were used on a set of simulations covering a wide range of atmospheric conditions. The modal growth rates were determined from the simulation results by summing the contribution of each process, by calculating the increase rate in the count mean diameter of the mode and by following the peak concentration of the mode. The results of these three methods were compared with each other and the means used to estimate the growth rate due to self-coagulation and coagulation scavenging were found to give accurate values. We also investigated the role of charged particles and electric interactions in the growth of a nucleation mode. Charged particles were found to increase the growth rate due to both self-coagulation and coagulation scavenging by a factor of ~1.5 to 2. In case of increased condensation onto charged particles, the total condensational growth rate of a nucleation mode may increase significantly in the very early steps of the growth. The analytical formulae provided by this paper were designed to provide the growth rates due to different processes from aerosol dynamic simulations, but the same principles can be used to determine the growth rates from measurement data.

Highlights

Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere (Spracklen et al, 2006; Kulmala and Kerminen, 2008; Yu et al, 2010), and a sig-nificant contributor to particles capable of acting as cloud condensation nuclei (Spracklen et al, 2008; Merikanto et al, 2009; Pierce and Adams, 2009)
In this study we have provided simple analytical formulae to estimate the growth rate of a nucleation mode due to self-coagulation and the apparent growth rate due to coagulation scavenging by larger particles
We aim to address the following specific questions: (1) How accurately can we estimate the nuclei growth rates due to individual processes by using simple analytical formulae, such as the ones derived here?, (2) What are the main numerical problems in simulating nuclei growth with a sectional model, and how can these problems be dealt with?, (3) What is the relative importance of the considered real and apparent growth processes in different atmospheric situations and for different nuclei sizes? and (4) How is nuclei growth affected by the presence of charges?

Summary

Introduction

Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere (Spracklen et al, 2006; Kulmala and Kerminen, 2008; Yu et al, 2010), and a sig-nificant contributor to particles capable of acting as cloud condensation nuclei (Spracklen et al, 2008; Merikanto et al, 2009; Pierce and Adams, 2009). The efficacy by which nucleated particles reach climatically-relevant sizes depends essentially on two competing factors: the nuclei growth rate and the scavenging of nuclei by various removal processes (Kerminen et al, 2001; Lehtinen et al, 2007; Pierce and Adams, 2007; Kuang et al, 2009; Gong et al, 2010). After being formed at about 1.5 to 2 nm (Kulmala et al, 2007), nucleated particles grow in size by condensation of low-volatile vapors, nuclei self-coagulation, and possibly by heterogeneous reactions (e.g., Stolzenburg et al, 2005). The main removal mechanism in this regard is coagulation scavenging, dry deposition being usually of minor importance (Kerminen et al, 2004). From modeling point of view, nuclei self-coagulation and coagulation scavenging can be treated quite accurately, whereas condensation and heterogeneous processes are problematic because of our incomplete knowledge of the vapors participating into these processes and the general lack of knowledge of vapor properties (e.g., Nieminen et al, 2010; Wang et al, 2010)

Objectives

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Conclusion