Abstract
Abstract. This paper presents the first quantitative metric for aerosol population mixing state, defined as the distribution of per-particle chemical species composition. This new metric, the mixing state index χ, is an affine ratio of the average per-particle species diversity Dα and the bulk population species diversity Dγ, both of which are based on information-theoretic entropy measures. The mixing state index χ enables the first rigorous definition of the spectrum of mixing states from so-called external mixture to internal mixture, which is significant for aerosol climate impacts, including aerosol optical properties and cloud condensation nuclei activity. We illustrate the usefulness of this new mixing state framework with model results from the stochastic particle-resolved model PartMC-MOSAIC. These results demonstrate how the mixing state metrics evolve with time for several archetypal cases, each of which isolates a specific process such as coagulation, emission, or condensation. Further, we present an analysis of the mixing state evolution for a complex urban plume case, for which these processes occur simultaneously. We additionally derive theoretical properties of the mixing state index and present a family of generalized mixing state indexes that vary in the importance assigned to low-mass-fraction species.
Highlights
Our quantitative understanding of the aerosol impact on climate still has large gaps and introduces large uncertainties in climate predictions (IPCC, 2007)
An important quantity in this context is the so-called mixing state of the aerosol population, which we define as the distribution of the per-particle chemical species compositions
Recent observations made in the laboratory and in the field using single-particle measurement techniques have revealed that the mixing states of ambient aerosol populations are complex
Summary
Our quantitative understanding of the aerosol impact on climate still has large gaps and introduces large uncertainties in climate predictions (IPCC, 2007). In the context of aerosols, we regard alpha diversity as measuring the average species diversity within a single particle, beta diversity as quantifying diversity between particles, and gamma diversity as describing the overall diversity in bulk population (see Section 2 for details). From these measures we construct the mixing state index χ as an affine ratio of alpha and gamma diversity. These simulations show how the diversity and mixing state measures evolve under common atmospheric processes, including emissions, dilution, coagulation, and gas-to-particle conversion. Note that from the information on per-particle composition, it is straightforward to calculate per-particle properties, such as hygroscopicity (Riemer et al, 2010; Zaveri et al, 2010; Ching et al, 2012; Tian et al, 2013), optical properties (Zaveri et al, 2010), or particle reactivity (Kaiser et al, 2011)
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