Commentary on &amp;quot;Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification&amp;quot; by Knopf et al.

Tabazadeh, A.

doi:10.5194/acp-3-863-2003

Cited by 14 publications

(18 citation statements)

References 13 publications

(10 reference statements)

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“…A reduction in the surface nucleation rates by a factor between 20 and 10 corresponds to an increase of ≈4% or ≈1 kcal mol −1 in the free energy ( G) of formation of a crystal nucleus on the droplet surface, applying the parameterization of G by Tabazadeh et al (2002). As mentioned by the authors, the empirical parameterizations of G are based on laboratory nucleation experiments on concentrated, pure aqueous nitric acid, extrapolated to stratospheric conditions with much more dilute solutions.…”

Section: Discussionmentioning

confidence: 99%

“…Assuming these hydrate particles form by homogeneous nucleation would require production rates orders of magnitude higher than derived from laboratory experiments (Knopf et al, 2002). We have applied the surface nucleation mechanism of Tabazadeh et al (2002) because this, to our knowledge, is the latest published available parameterisation of hydrate nucleation out of STS working at temperature above the ice frost point. Secondly, the surface nucleation seems more consistent with laboratory experiments (Tabazadeh et al, 2002), compared to a corresponding volume nucleation parameterisation (Tabazadeh et al, 2001;Salcedo et al, 2001).…”

Section: Discussionmentioning

confidence: 99%

“…As mentioned above the homogeneous surface nucleation rates for NAD, given by Tabazadeh et al (2002), have been reduced by a factor 20 (i.e. the nucleation rates have been multiplied by an adjustment factor 0.05).…”

Section: Discussionmentioning

confidence: 99%

“…The model calculates the time dependent changes in size distributions of liquid sulphate and STS particles, and solid NAT, ice, and sulphuric acid tetrahydrate (SAT) particles together with changes in gas phase HNO 3 and H 2 O, using the basic vapour diffusion equation and applying a full kinetic approach. Homogeneous volume dependent nucleation of ice 3-4 K below the ice frost point (Koop et al, 2000) and homogeneous surface dependent nucleation of nitric acid dihydrate (NAD) (Tabazadeh et al, 2002) above T ice are included in the model, assuming an instantaneous conversion of NAD to NAT particles. The surface nucleation rates have been reduced by a factor 20 as discussed in more detail below.…”

Section: Microphysical Simulationsmentioning

confidence: 99%

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Formation of solid particles in synoptic-scale Arctic PSCs in early winter 2002/2003

et al. 2004

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Abstract. Polar stratospheric clouds (PSC) have been observed in early winter (December 2002) during the SOLVE II/Vintersol campaign, both from balloons carrying comprehensive instrumentation for measurements of chemical composition, size distributions, and optical properties of the particles, as well as from individual backscatter soundings from Esrange and Sodankylä. The observations are unique in the sense that the PSC particles seem to have formed in the early winter under synoptic temperature conditions and not being influenced by mountain lee waves. A sequence of measurements during a 5-days period shows a gradual change between liquid and solid type PSCs with the development of a well-known sandwich structure. It appears that all PSC observations show the presence of a background population of solid particles, occasionally mixed in with more optically dominating liquid particles. The measurements have been compared with results from a detailed microphysical and optical simulation of the formation processes. Calculated extinctions are in good agreement with SAGE-III measurements from the same period. Apparently the solid particles are controlled by the synoptic temperature history while the presence of liquid particles is controlled by the local temperatures at the time of observation. The temperature histories indicate that the solid particles are nucleated above the ice frost point, and a surface freezing mechanism for this is included in the model. Reducing the calculated freezing ratesCorrespondence to: N. Larsen (nl@dmi.dk) by a factor 10-20, the model is able to simulate the observed particle size distributions and reproduce observed HNO 3 gas phase concentrations.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Microphysical Simulationsmentioning

confidence: 99%

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Formation of solid particles in synoptic-scale Arctic PSCs in early winter 2002/2003

et al. 2004

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“…Knopf et al (2002) have also shown that the linear extrapolation of laboratory data to stratospheric conditions by Tabazadeh et al (2001) gave physically unrealistic results under some conditions, and was likely to overestimate freezing rates. The general applicability of laboratory-measured homogeneous nucleation rates to the stratosphere has been called into question because of the potential role of surface contaminants (Tabazadeh et al, 2003). At present, the possibility of homogeneous nucleation of nitric acid hydrates remains an open and poorly quantified issue.…”

Section: Introductionmentioning

confidence: 99%

3-D microphysical model studies of Arctic denitrification: comparison with observations

et al. 2005

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Abstract. Simulations of Arctic denitrification using a 3-D chemistry-microphysics transport model are compared with observations for the winters 1994/95, 1996/97 and 1999/2000. The model of Denitrification by Lagrangian Particle Sedimentation (DLAPSE) couples the full chemical scheme of the 3-D chemical transport model, SLIMCAT, with a nitric acid trihydrate (NAT) growth and sedimentation scheme. We use observations from the Microwave Limb Sounder (MLS) and Improved Limb Atmospheric Sounder (ILAS) satellite instruments, the balloon-borne Michelsen Interferometer for Passive Atmospheric Sounding (MIPAS-B), and the in situ NO y instrument on-board the ER-2. As well as directly comparing model results with observations, we also assess the extent to which these observations are able to validate the modelling approach taken. For instance, in 1999/2000 the model captures the temporal development of denitrification observed by the ER-2 from late January into March. However, in this winter the vortex was already highly denitrified by late January so the observations do not provide a strong constraint on the modelled rate of denitrification. The model also reproduces the MLS observations of denitrification in early February 2000. In 1996/97 the model captures the timing and magnitude of denitrification as observed by ILAS, although the lack of observations north of ∼67 • N in the beginning of February make it difficult to constrain the actual timing of onset. The comparison for this winter does not support previous conclusions that denitrification must be caused by an ice-mediated process. In 1994/95 the model notably underestimates the magnitude of denitrification observed during a single balloon flight of the MIPAS-B instruCorrespondence to: S. Davies (stewart@env.leeds.ac.uk) ment. Agreement between model and MLS HNO 3 at 68 hPa in mid-February 1995 is significantly better. Sensitivity tests show that a 1.5 K overall decrease in vortex temperatures, or a factor 4 increase in assumed NAT nucleation rates, produce the best statistical fit to MLS observations. Both adjustments would be required to bring the model into agreement with the MIPAS-B observations. The agreement between the model and observations suggests that a NAT-only denitrification scheme (without ice), which was discounted by previous studies, must now be considered as one mechanism for the observed Arctic denitrification. The timing of onset and the rate of denitrification remain poorly constrained by the available observations.

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Do NAD and NAT Form in Liquid Stratospheric Aerosols by Pseudoheterogeneous Nucleation?

Knopf

2006

J. Phys. Chem. A

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Laboratory data of the freezing of nitric acid hydrates (NAD, NAT) from HNO(3)/H(2)O and HNO(3)/H(2)SO(4)/H(2)O solution droplets have been evaluated with respect to a "pseudoheterogeneous" (surface-induced) nucleation mechanism of NAD and NAT, which has been argued to possibly lead to the formation of polar stratospheric clouds (PSCs). In addition, a parametrization of pseudoheterogeneous nucleation of NAD and NAT suggested recently (Tabazadeh et al. J. Phys. Chem. A 2002, 106, 10238-10246) has been analyzed, showing that this parametrization should not be used in stratospheric modeling studies. The analysis of several laboratory data sets yields an upper limit of the pseudoheterogeneous nucleation rate coefficient of NAD of 2.2 x 10(-5) cm(-2) s(-1). In contrast, the upper limit of the pseudoheterogeneous nucleation rate coefficient of NAT could not be constrained satisfactorily, since formation of NAT has not been observed at stratospheric conditions in laboratory experiments applying small droplets. Maximum NAD production rates of 9.6 x 10(-9) cm(-3) (air) h(-1) in the stratosphere have been estimated assuming a pseudoheterogeneous nucleation mechanism that is constrained by the experimental observations. If maximum NAD supersaturation persisted for 4 weeks in the polar stratosphere the corresponding NAD particle number densities are estimated to be about 6 x 10(-6) cm(-3). These particle number densities are 3 orders of magnitude lower than particle number densities recently observed in the stratosphere. In conclusion, on the basis of laboratory data it is found that a pseudoheterogeneous nucleation mechanism is not sufficient to explain recent observations of large nitric acid containing particles in the polar stratosphere.

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Commentary on "Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification" by Knopf et al.

Cited by 14 publications

References 13 publications

Formation of solid particles in synoptic-scale Arctic PSCs in early winter 2002/2003

Formation of solid particles in synoptic-scale Arctic PSCs in early winter 2002/2003

3-D microphysical model studies of Arctic denitrification: comparison with observations

Do NAD and NAT Form in Liquid Stratospheric Aerosols by Pseudoheterogeneous Nucleation?

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