Abstract
Abstract. It is commonly assumed that models are more prone to errors in predicted cloud condensation nuclei (CCN) concentrations when the aerosol populations are externally mixed. In this work we investigate this assumption by using the mixing state index (χ) proposed by Riemer and West (2013) to quantify the degree of external and internal mixing of aerosol populations. We combine this metric with particle-resolved model simulations to quantify error in CCN predictions when mixing state information is neglected, exploring a range of scenarios that cover different conditions of aerosol aging. We show that mixing state information does indeed become unimportant for more internally mixed populations, more precisely for populations with χ larger than 75 %. For more externally mixed populations (χ below 20 %) the relationship of χ and the error in CCN predictions is not unique and ranges from lower than −40 % to about 150 %, depending on the underlying aerosol population and the environmental supersaturation. We explain the reasons for this behavior with detailed process analyses.
Highlights
The mixing state of an aerosol population depends on the distribution of chemical compounds across the population (Riemer and West, 2013)
In this study we focus on cloud condensation nuclei (CCN) activity, and many CCN closure studies show that the quality of the closure depends crucially on the assumptions about aerosol mixing state (McFiggans et al, 2006; Wang et al, 2010; Bhattu and Tripathi, 2015; Ervens et al, 2010)
Closure studies show that, in areas close to emission sources, a certain degree of external mixing needs to be assumed to obtain good closure. This is confirmed by modeling studies showing that, without introducing fresh emissions, aging processes transform the mixing state of aerosol populations so that the CCN properties can be deduced from the bulk aerosol composition, and mixing state details become negligible (Zaveri et al, 2010; Ching et al, 2012; Fierce et al, 2013)
Summary
The mixing state of an aerosol population depends on the distribution of chemical compounds across the population (Riemer and West, 2013). In this study we focus on CCN activity, and many CCN closure studies show that the quality of the closure depends crucially on the assumptions about aerosol mixing state (McFiggans et al, 2006; Wang et al, 2010; Bhattu and Tripathi, 2015; Ervens et al, 2010) Based on these observational findings, it is commonly assumed that the internal mixture assumption works well for regions that are not directly influenced by fresh emission sources. Closure studies show that, in areas close to emission sources, a certain degree of external mixing needs to be assumed to obtain good closure This is confirmed by modeling studies showing that, without introducing fresh emissions, aging processes transform the mixing state of aerosol populations so that the CCN properties can be deduced from the bulk aerosol composition, and mixing state details become negligible (Zaveri et al, 2010; Ching et al, 2012; Fierce et al, 2013).
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