Diffusion‐controlled reactions: Upper bounds on the effective rate constant

1991 ◽  
Vol 94 (12) ◽  
pp. 7967-7971 ◽  
Author(s):  
J. Blawzdziewicz ◽  
G. Szamel ◽  
H. Van Beijeren
Keyword(s):  
Rate Constant ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Upper Bounds ◽  
Diffusion Controlled

Residence-time of muonium at micelles: Effect of added micelles on the reactivity of muonium towards ionic solutes in water

1988 ◽  
Vol 66 (8) ◽  
pp. 1979-1983 ◽  
Author(s):  
Krishnan Venkateswaran ◽  
Mary V. Barnabas ◽  
Bill W. Ng ◽  
David C. Walker
Keyword(s):  
Residence Time ◽  
Rate Constant ◽  
Mean Residence Time ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Diffusion Time ◽  
Water Penetration ◽  
Micellar Structure ◽  
Micelle Core ◽  
Ionic Solutes

The effective rate constant for the reaction of muonium with NO3−, S2O32−, and Tl+ ions in water is altered by the addition of micelles. There is a decrease when the charge on the micelle is the same as that of the solute and an increase when their charges are opposite. From the magnitude of the effect a mean residence-time for muonium of 2 ns has been deduced for dodecyl sulphate micelles. This suggests there is barely any preferred localization, because 2 ns is smaller, even, than the expected diffusion time if the micelle core is as viscous as reported. This use of muonium atoms to probe the dynamics of micelles seems to support the view that there are regions of low microviscosity and considerable water penetration within the micellar structure.

Effective rate constant of ferriin reduction in the Belousov–Zhabotinsky reaction

1997 ◽  
Vol 93 (1) ◽  
pp. 69-71 ◽  
Author(s):  
J. Ungvarai ◽  
Z. Nagy-Ungvarai ◽  
J. Enderlein ◽  
S. C. Müller
Keyword(s):  
Rate Constant ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Belousov Zhabotinsky Reaction

scholarly journals The influence of small-scale variations in isoprene concentrations on atmospheric chemistry over a tropical rainforest

2011 ◽  
Vol 11 (9) ◽  
pp. 4121-4134 ◽  
Author(s):  
T. A. M. Pugh ◽  
A. R. MacKenzie ◽  
B. Langford ◽  
E. Nemitz ◽  
P. K. Misztal ◽  
...  
Keyword(s):  
Rate Constant ◽  
Atmospheric Chemistry ◽  
Tropical Rainforest ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Ozone Production ◽  
Small Scale ◽  
Aerosol Formation ◽  
Upper Limit ◽  
Model Measurement

Abstract. Biogenic volatile organic compounds (BVOCs) such as isoprene constitute a large proportion of the global atmospheric oxidant sink. Their reactions in the atmosphere contribute to processes such as ozone production and secondary organic aerosol formation. However, over the tropical rainforest, where 50 % of the global emissions of BVOCs are believed to occur, atmospheric chemistry models have been unable to simulate concurrently the measured daytime concentration of isoprene and that of its principal oxidant, hydroxyl (OH). One reason for this model-measurement discrepancy may be incomplete mixing of isoprene within the convective boundary layer, leading to patchiness or segregation in isoprene and OH mixing ratios and average concentrations that appear to be incompatible with each other. One way of capturing this effect in models of atmospheric chemistry is to use a reduced effective rate constant for their reaction. Recent studies comparing atmospheric chemistry global/box models with field measurements have suggested that this effective rate reduction may be as large as 50 %; which is at the upper limit of that calculated using large eddy simulation models. To date there has only been one field campaign worldwide that has reported co-located measurements of isoprene and OH at the necessary temporal resolution to calculate the segregation of these compounds. However many campaigns have recorded sufficiently high resolution isoprene measurements to capture the small-scale fluctuations in its concentration. Assuming uniform distributions of other OH production and loss processes, we use a box model of atmospheric chemistry, constrained by the spectrum of isoprene concentrations measured, as a virtual instrument, to estimate the variability in OH at a point and hence, to estimate the segregation intensity of isoprene and OH from high-frequency isoprene time series. The method successfully reproduces the only directly observed segregation, using measurements made in a deciduous forest in Germany. The effective rate constant reduction for the reaction of isoprene and OH over a South-East Asian rainforest is calculated to be typically <15 %. Although there are many unconstrained uncertainties, the likely nature of those processes suggests that this value represents an upper limit. The estimate is not sensitive to heterogeneities in NO at this remote site, unless they are correlated with those of isoprene, or to OH-recycling schemes in the isoprene oxidation mechanism, unless the recycling happens in the first reaction step. Segregation alone is therefore unlikely to be the sole cause of model-measurement discrepancies for isoprene and OH above a rainforest.

scholarly journals Improved simulation of isoprene oxidation chemistry with the ECHAM5/MESSy chemistry-climate model: lessons from the GABRIEL airborne field campaign

2008 ◽  
Vol 8 (2) ◽  
pp. 6273-6312 ◽  
Author(s):  
T. M. Butler ◽  
D. Taraborrelli ◽  
C. Brühl ◽  
H. Fischer ◽  
H. Harder ◽  
...  
Keyword(s):  
Rate Constant ◽  
Atmospheric Chemistry ◽  
Climate Model ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Current Generation ◽  
Mixing Ratios ◽  
Oxidation Chain ◽  
Isoprene Oxidation ◽  
Oxidation Chemistry

Abstract. The GABRIEL airborne field measurement campaign, conducted over the Guyanas in October 2005, produced measurements of hydroxyl radical (OH) concentration which are significantly higher than can be simulated using current generation models of atmospheric chemistry. Based on the hypothesis that this "missing OH" is due to an as-yet undiscovered mechanism for recycling OH during the oxidation chain of isoprene, we determine that an OH recycling of about 40–50% (compared with 5–10% in current generation isoprene oxidation mechanisms) is necessary in order for our modelled OH to approach the lower error bounds of the OH observed during GABRIEL. Such a large amount of OH in our model leads to unrealistically low mixing ratios of isoprene. In order for our modelled isoprene mixing ratios to match those observed during the campaign, we also require that the effective rate constant for the reaction of isoprene with OH be reduced by about 50% compared with the lower bound of the range recommended by IUPAC. We show that a reasonable explanation for this lower effective rate constant could be the segregation of isoprene and OH in the mixed layer. Our modelling results are consistent with a global, annual isoprene source of about 500 Tg(C) yr−1, allowing experimentally derived and established isoprene flux rates to be reconciled with global models.

The effect of parent substance genesis on the reactivity of active sodium carbonate towards sulphur dioxide

1979 ◽  
Vol 44 (4) ◽  
pp. 1201-1208 ◽  
Author(s):  
Karel Mocek ◽  
Erich Lippert ◽  
Dušan Husek ◽  
Emerich Erdös
Keyword(s):  
Experimental Evidence ◽  
Bulk Density ◽  
Rate Constant ◽  
Sodium Carbonate ◽  
Sulphur Dioxide ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Parent Substance ◽  
Active Sodium ◽  
Order Of Magnitude

The reactivity of sodium carbonate towards sulphur dioxide was studied in dependence on the effect of genesis of sodium hydrocarbonate as the parent substance of the reacting solid. The reactivity expressed by means of the effective rate constant differed approximately one half of order of magnitude among five various samples. A qualitative correlation has been found between the bulk density of the parent substance and the reactivity of active sodium carbonate prepared thereof. Experimental evidence is given that the effect of genesis of the parent substance is not decisive for the reactivity of the active sodium carbonate towards sulphur dioxide. Henceforth, as the decisive factor for the reactivity of the sodium carbonate the way of its preparation from the parent substance must be considered.

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