Reactive uptake of N2O5 to internally mixed inorganic and organic particles: The role of organic carbon oxidation state and inferred organic phase separations

Cassandra Gaston, J. A. Thornton, N. L. Ng

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Abstract

We measured N2O5 reactive uptake onto mixed organic/inorganic submicron particles using organic compounds with a variety of oxidation states (using mainly atomic O: C ratios as a proxy) and molecular weights. The organic mass fraction, organic molecular composition, and relative humidity (RH) were varied to assess their effects separately on the N 2O5 uptake coefficient, γ (N2O 5). At a constant RH, mixtures of organic components having an O:C < 0.5 with ammonium bisulfate significantly suppressed the uptake of N 2O5(g) compared to pure ammonium bisulfate, even at small organic mass fractions (e.g., ≤15 %). The effect of the organic component became less pronounced at higher RH. In general, highly oxygenated organic components (O : C > 0.8) had a smaller or even negligible impact on N 2O5(g) uptake at all RHs probed; however, a few exceptions were observed. Notably, γ (N2O5) for mixtures of ammonium bisulfate with polyethylene glycol (PEG), PEG-300 (O :C=0.56), decreased nearly linearly as the PEG mass fraction increased at constant RH until leveling off at the value measured for pure PEG. The response of γ (N2O5) to increasing PEG mass fraction was similar to that measured on ambient atmospheric particles as a function of organic mass fraction. The effects of the organic mass fraction on γ (N 2O5), for mixtures having an O: C<~0.8, were best described using a standard resistor model of reactive uptake assuming the particles had an RH-dependent inorganic core- organic shell morphology. This model suggests that the N2O5 diffusivity and/or solubility in the organic layer is up to a factor of 20 lower compared to aqueous solution particles, and that the diffusivity, solubility, and reactivity of N 2O5 within organic coatings and particles depend upon both RH and the molecular composition of the organic medium. We use these dependencies and ambient measurements of organic aerosol from the global aerosol mass spectrometry (AMS) database to show that the typical impact of organic aerosol components is to both uniformly decrease γ (N2O 5), by up to an order of magnitude depending on the RH, organic mass fraction, and O: C ratio, and to induce a stronger dependence of γ (N 2O5) upon RH compared to purely inorganic aqueous solutions.

Original languageEnglish (US)
Pages (from-to)5693-5707
Number of pages15
JournalAtmospheric Chemistry and Physics
Volume14
Issue number11
DOIs
StatePublished - Jun 10 2014
Externally publishedYes

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relative humidity
organic carbon
oxidation
aerosol
diffusivity
solubility
aqueous solution
atmospheric particle
leveling
particle
organic compound
coating
ammonium
mass spectrometry
shell

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

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title = "Reactive uptake of N2O5 to internally mixed inorganic and organic particles: The role of organic carbon oxidation state and inferred organic phase separations",
abstract = "We measured N2O5 reactive uptake onto mixed organic/inorganic submicron particles using organic compounds with a variety of oxidation states (using mainly atomic O: C ratios as a proxy) and molecular weights. The organic mass fraction, organic molecular composition, and relative humidity (RH) were varied to assess their effects separately on the N 2O5 uptake coefficient, γ (N2O 5). At a constant RH, mixtures of organic components having an O:C < 0.5 with ammonium bisulfate significantly suppressed the uptake of N 2O5(g) compared to pure ammonium bisulfate, even at small organic mass fractions (e.g., ≤15 {\%}). The effect of the organic component became less pronounced at higher RH. In general, highly oxygenated organic components (O : C > 0.8) had a smaller or even negligible impact on N 2O5(g) uptake at all RHs probed; however, a few exceptions were observed. Notably, γ (N2O5) for mixtures of ammonium bisulfate with polyethylene glycol (PEG), PEG-300 (O :C=0.56), decreased nearly linearly as the PEG mass fraction increased at constant RH until leveling off at the value measured for pure PEG. The response of γ (N2O5) to increasing PEG mass fraction was similar to that measured on ambient atmospheric particles as a function of organic mass fraction. The effects of the organic mass fraction on γ (N 2O5), for mixtures having an O: C<~0.8, were best described using a standard resistor model of reactive uptake assuming the particles had an RH-dependent inorganic core- organic shell morphology. This model suggests that the N2O5 diffusivity and/or solubility in the organic layer is up to a factor of 20 lower compared to aqueous solution particles, and that the diffusivity, solubility, and reactivity of N 2O5 within organic coatings and particles depend upon both RH and the molecular composition of the organic medium. We use these dependencies and ambient measurements of organic aerosol from the global aerosol mass spectrometry (AMS) database to show that the typical impact of organic aerosol components is to both uniformly decrease γ (N2O 5), by up to an order of magnitude depending on the RH, organic mass fraction, and O: C ratio, and to induce a stronger dependence of γ (N 2O5) upon RH compared to purely inorganic aqueous solutions.",
author = "Cassandra Gaston and Thornton, {J. A.} and Ng, {N. L.}",
year = "2014",
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T1 - Reactive uptake of N2O5 to internally mixed inorganic and organic particles

T2 - The role of organic carbon oxidation state and inferred organic phase separations

AU - Gaston, Cassandra

AU - Thornton, J. A.

AU - Ng, N. L.

PY - 2014/6/10

Y1 - 2014/6/10

N2 - We measured N2O5 reactive uptake onto mixed organic/inorganic submicron particles using organic compounds with a variety of oxidation states (using mainly atomic O: C ratios as a proxy) and molecular weights. The organic mass fraction, organic molecular composition, and relative humidity (RH) were varied to assess their effects separately on the N 2O5 uptake coefficient, γ (N2O 5). At a constant RH, mixtures of organic components having an O:C < 0.5 with ammonium bisulfate significantly suppressed the uptake of N 2O5(g) compared to pure ammonium bisulfate, even at small organic mass fractions (e.g., ≤15 %). The effect of the organic component became less pronounced at higher RH. In general, highly oxygenated organic components (O : C > 0.8) had a smaller or even negligible impact on N 2O5(g) uptake at all RHs probed; however, a few exceptions were observed. Notably, γ (N2O5) for mixtures of ammonium bisulfate with polyethylene glycol (PEG), PEG-300 (O :C=0.56), decreased nearly linearly as the PEG mass fraction increased at constant RH until leveling off at the value measured for pure PEG. The response of γ (N2O5) to increasing PEG mass fraction was similar to that measured on ambient atmospheric particles as a function of organic mass fraction. The effects of the organic mass fraction on γ (N 2O5), for mixtures having an O: C<~0.8, were best described using a standard resistor model of reactive uptake assuming the particles had an RH-dependent inorganic core- organic shell morphology. This model suggests that the N2O5 diffusivity and/or solubility in the organic layer is up to a factor of 20 lower compared to aqueous solution particles, and that the diffusivity, solubility, and reactivity of N 2O5 within organic coatings and particles depend upon both RH and the molecular composition of the organic medium. We use these dependencies and ambient measurements of organic aerosol from the global aerosol mass spectrometry (AMS) database to show that the typical impact of organic aerosol components is to both uniformly decrease γ (N2O 5), by up to an order of magnitude depending on the RH, organic mass fraction, and O: C ratio, and to induce a stronger dependence of γ (N 2O5) upon RH compared to purely inorganic aqueous solutions.

AB - We measured N2O5 reactive uptake onto mixed organic/inorganic submicron particles using organic compounds with a variety of oxidation states (using mainly atomic O: C ratios as a proxy) and molecular weights. The organic mass fraction, organic molecular composition, and relative humidity (RH) were varied to assess their effects separately on the N 2O5 uptake coefficient, γ (N2O 5). At a constant RH, mixtures of organic components having an O:C < 0.5 with ammonium bisulfate significantly suppressed the uptake of N 2O5(g) compared to pure ammonium bisulfate, even at small organic mass fractions (e.g., ≤15 %). The effect of the organic component became less pronounced at higher RH. In general, highly oxygenated organic components (O : C > 0.8) had a smaller or even negligible impact on N 2O5(g) uptake at all RHs probed; however, a few exceptions were observed. Notably, γ (N2O5) for mixtures of ammonium bisulfate with polyethylene glycol (PEG), PEG-300 (O :C=0.56), decreased nearly linearly as the PEG mass fraction increased at constant RH until leveling off at the value measured for pure PEG. The response of γ (N2O5) to increasing PEG mass fraction was similar to that measured on ambient atmospheric particles as a function of organic mass fraction. The effects of the organic mass fraction on γ (N 2O5), for mixtures having an O: C<~0.8, were best described using a standard resistor model of reactive uptake assuming the particles had an RH-dependent inorganic core- organic shell morphology. This model suggests that the N2O5 diffusivity and/or solubility in the organic layer is up to a factor of 20 lower compared to aqueous solution particles, and that the diffusivity, solubility, and reactivity of N 2O5 within organic coatings and particles depend upon both RH and the molecular composition of the organic medium. We use these dependencies and ambient measurements of organic aerosol from the global aerosol mass spectrometry (AMS) database to show that the typical impact of organic aerosol components is to both uniformly decrease γ (N2O 5), by up to an order of magnitude depending on the RH, organic mass fraction, and O: C ratio, and to induce a stronger dependence of γ (N 2O5) upon RH compared to purely inorganic aqueous solutions.

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