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Ambient aerosol particles can undergo dynamic mixing processes as they coagulate with particles from other air masses and emission sources. Therefore, aerosols exist in a spectrum, from externally mixed to internally mixed. The mixing state of aerosols can affect their ability to uptake water (hygroscopicity) and their cloud condensation nuclei (CCN) activity, modifying their contribution to the planet’s total radiative budget. However, current water-uptake measurement methods may not be able to capture the complex mixing state. In this research, the dynamic mixing process was simulated by the particle-resolved aerosol model PartMC and also created by experiments in a laminar flow mixing tube. The mixing evolution of ammonium sulfate and sucrose binary mixtures were observed along with the changes in their water uptake properties expressed as the single hygroscopicity parameter, κ. The use of a mixing simulation in conjunction with experiments allow for better identification of the particle mixing state and the particle water uptake and show that no one kappa value can capture the complexity of mixing across the mixed particle size distribution. In other words, the PartMC simulations can be used as a guiding tool to interpret a system’s mixing state based on its experimental droplet activation spectra. This work demonstrates the importance of the integration and use of mixing models to aid in mixing state determination and hygroscopicity measurements of mixed systems.
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External and internal cloud condensation nuclei (CCN) mixtures: controlled laboratory studies of varying mixing states
Abstract. Changes in aerosol chemical mixtures modify cloud condensation nuclei (CCN)activity. Previous studies have developed CCN models and validated changesin external and internal mixing state with ambient field data. Here, wedevelop an experimental method to test and validate the CCN activation ofknown aerosol chemical composition with multicomponent mixtures and varyingmixing states. CCN activation curves consisting of one or more activationpoints are presented. Specifically, simplified two-component systems ofvarying hygroscopicity were generated under internal, external, andtransitional mixing conditions. κ-Köhler theory predictions werecalculated for different organic and inorganic mixtures and compared toexperimentally derived kappa values and respective mixing states. This workemploys novel experimental methods to provide information on the shifts inCCN activation data due to external to internal particle mixing fromcontrolled laboratory sources. Results show that activation curvesconsisting of single and double activation points are consistent withinternal and external mixtures, respectively. In addition, the height of theplateau at the activation points is reflective of the externally mixedconcentration in the mixture. The presence of a plateau indicates that CCNactivation curves consisting of multiple inflection points are externallymixed aerosols of varying water-uptake properties. The plateau disappearswhen mixing is promoted in the flow tube. At the end of the flow tubeexperiment, the aerosols are internally mixed and the CCN activated fractiondata can be fit with a single-sigmoid curve. The technique to mimicexternally to internally mixed aerosol is applied to non-hygroscopiccarbonaceous aerosol with organic and inorganic components. To ourknowledge, this work is the first to show controlled CCN activation of mixednon-hygroscopic soot with hygroscopic material as the aerosol populationtransitions from externally to internally mixed states in laboratoryconditions. Results confirm that CCN activation analysis methods used hereand in ambient data sets are robust and may be used to infer the mixingstate of complex aerosol compositions of unknown origin.
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- Award ID(s):
- 1744216
- PAR ID:
- 10108772
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 12
- Issue:
- 8
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 4277 to 4289
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-12-4277-2019
