Effects Of Heterogeneous Reactions Research Articles

Numerical simulation on high-temperature supercritical CO2/gaseous H2O mixture flowing over a single reactive spherical char particle

Based on PR-DNS (Particle Resolve-Direct Numerical Simulation) method, the gasification process of a single spherical char particle moving in high-temperature supercritical CO2/gaseous H2O atmosphere at 9 MPa is conducted, and related mechanism on the reaction-affected drag of the sphere, flow field and interphase thermal as well as mass transfer near sphere surface is presented. The reaction temperature and Re (Reynolds number) of the fluid flowing over the sphere are variable within the range of 1273.15–1673.15 K and 10–200, respectively. The mass decomposition source on the sphere surface (Stefan flow), reaction heat, heat radiation between fluid species and char, porous inert ash layer and variations on physical properties of fluid mixture near the sphere surface are taken into account, while the Pseudo-Steady-State approach is employed for handling the particle shrinking. The results indicate that the higher reaction temperature leads the smaller drag coefficient, while enhances the performance of heat transfer compared with that of non-reactive cases. Due to the relative strength between convective heat transfer and endothermic effect of heterogeneous reaction, variation laws of temperature on the sphere surface vs. Re differs from each other under different reaction temperatures. Higher temperature and lower Re leads the higher surface Damköhler numbers, which indicates that the heterogeneous reactions in gasification process have normal characteristic of reactions in the kinetically-controlled regime. Porous inert ash layer increases resistance to flow as well as diffusion of species, and the variations in relative concentration of fluid reactants can also result in varied influences on above variables.

Effects of heterogeneous reaction with NO2 on ice nucleation activities of feldspar and Arizona Test Dust

Mineral dust is an important type of ice nucleating particles in the troposphere; however, the effects of heterogeneous reactions on ice nucleation (IN) activities of mineral dust remain to be elucidated. A droplet-freezing apparatus (Guangzhou Institute of Geochemistry Ice Nucleation Apparatus, GIGINA) was developed in this work to measure IN activities of atmospheric particles in the immersion freezing mode, and its performance was validated by a series of experimental characterizations. This apparatus was then employed to measure IN activities of feldspar and Arizona Test Dust (ATD) particles before and after heterogeneous reaction with NO2 (10±0.5 ppmv) at 40% relative humidity. The surface coverage of nitrate, θ(NO3−), increased to 3.1±0.2 for feldspar after reaction with NO2 for 6 hr, and meanwhile the active site density per unit surface area (ns) at -20°C was reduced from 92±5 to <1.0 cm−2 by about two orders of magnitude; however, no changes in nitrate content or IN activities were observed for further increase in reaction time (up to 24 hr). Both nitrate content and IN activities changed continuously with reaction time (up to 24 hr) for ATD particles; after reaction with NO2 for 24 hr, θ(NO3−) increased to 1.4±0.1 and ns at -20°C was reduced from 20±4 to 9.7±1.9 cm−2 by a factor of ∼2. Our work suggests that heterogeneous reaction with NO2, an abundant reactive nitrogen species in the troposphere, may significantly reduce IN activities of mineral dust in the immersion freezing mode.

The convective flow of Carreau fluid over a curved stretching surface with homogeneous-heterogeneous reactions and viscous dissipation

This article aims to develop the modeling and simulation of Carreau fluid over a curved stretching surface along with convective boundary conditions in homogeneous-heterogeneous reactions and viscous dissipation. The modeled boundary layer equations are reduced from nonlinear partial differential equations to a structure of ordinary differential equations via relevant transformations. The developed equations are solved numerically by the shooting method. The decrease in the velocity and temperature of the fluid for the growing values of curvature parameter is because the larger values of curvature decline the drag friction. The concentration profile decreases for increasing values of homogeneous reaction, but the effect of heterogeneous reaction on concentration profile away from the boundary is conserved. The Weissenberg number raises the velocity and temperature of the fluid. The impact of various flow parameters on velocity, temperature, and concentration distribution for shear thickening cases is explored graphically and deliberated in detail. Skin-friction and local Nusselt numbers are computed numerically for various parameters through tables.

Effects of heterogeneous reactions on tropospheric chemistry: a global simulation with the chemistry–climate model CHASER V4.0

Abstract. This study uses a chemistry–climate model CHASER (MIROC) to explore the roles of heterogeneous reactions (HRs) in global tropospheric chemistry. Three distinct HRs of N2O5, HO2, and RO2 are considered for surfaces of aerosols and cloud particles. The model simulation is verified with EANET and EMEP stationary observations; R/V Mirai ship-based data; ATom1 aircraft measurements; satellite observations by OMI, ISCCP, and CALIPSO-GOCCP; and reanalysis data JRA55. The heterogeneous chemistry facilitates improvement of model performance with respect to observations for NO2, OH, CO, and O3, especially in the lower troposphere. The calculated effects of heterogeneous reactions cause marked changes in global abundances of O3 (−2.96 %), NOx (−2.19 %), CO (+3.28 %), and global mean CH4 lifetime (+5.91 %). These global effects were contributed mostly by N2O5 uptake onto aerosols in the middle troposphere. At the surface, HO2 uptake gives the largest contributions, with a particularly significant effect in the North Pacific region (−24 % O3, +68 % NOx, +8 % CO, and −70 % OH), mainly attributable to its uptake onto clouds. The RO2 reaction has a small contribution, but its global mean negative effects on O3 and CO are not negligible. In general, the uptakes onto ice crystals and cloud droplets that occur mainly by HO2 and RO2 radicals cause smaller global effects than the aerosol-uptake effects by N2O5 radicals (+1.34 % CH4 lifetime, +1.71 % NOx, −0.56 % O3, +0.63 % CO abundances). Nonlinear responses of tropospheric O3, NOx, and OH to the N2O5 and HO2 uptakes are found in the same modeling framework of this study (R&gt;0.93). Although all HRs showed negative tendencies for OH and O3 levels, the effects of HR(HO2) on the tropospheric abundance of O3 showed a small increment with an increasing loss rate. However, this positive tendency turns to reduction at higher rates (&gt;5 times). Our results demonstrate that the HRs affect not only polluted areas but also remote areas such as the mid-latitude sea boundary layer and upper troposphere. Furthermore, HR(HO2) can bring challenges to pollution reduction efforts because it causes opposite effects between NOx (increase) and surface O3 (decrease).

Particle Triggered Reactions as an Important Mechanism of Alkalinity and Inorganic Carbon Removal in River Plumes

AbstractThe effects of heterogeneous reactions between river‐borne particles and the carbonate system were studied in the plumes of the Mississippi and Brazos rivers. Measurements within these plumes revealed significant removal of dissolved inorganic carbon (DIC) and total alkalinity (TA). After accounting for all known DIC and TA sinks and sources, heterogeneous reactions (i.e., heterogeneous CaCO3 precipitation and cation exchange between adsorbed and dissolved ions) were found to be responsible for a significant fraction of DIC and TA removal, exceeding 10% and 90%, respectively, in the Mississippi and Brazos plume waters. This finding was corroborated by laboratory experiments, in which the seeding of seawater with the riverine particles induced the removal of the DIC and TA. The combined results demonstrate that heterogeneous reactions may represent an important controlling mechanism of the seawater carbonate system in particle‐rich coastal areas and may significantly impact the coastal carbon cycle.

Heterogeneous reaction of NO2 with hematite, goethite and magnetite: Implications for nitrate formation and iron solubility enhancement.

Atmospheric processing may significantly increase solubility of iron in mineral dust, but the effects of heterogeneous reactions on iron solubility have been poorly understood. In this work, we investigated heterogeneous reaction of NO2 (15 ± 1 and 2.5 ± 0.1 ppmv, equal to ∼3.7 × 1014 and ∼6.2 × 1013 molecule cm-3) with hematite, magnetite and goethite at different relative humidities (RH, 0-90%), and changes in particulate nitrate and soluble iron due to heterogeneous reaction with NO2 were quantified as a function of time (up to 24 h). After reaction with 2.5 ± 0.1 ppmv NO2 for 24 h (or less time), hematite and magnetite were fully saturated, while goethite was only partly deactivated. Nitrate yield was largest for goethite, and the mass ratio of formed nitrate to unreacted mineral only reached ∼1% or less after 24 h reaction. All the three minerals showed low reactivities towards NO2, and the average reactive uptake coefficients of NO2 in the first 3 h were found to be < 5 × 10-8. In addition, the increase in iron solubility was found to be small and in some cases even insignificant for the three minerals after heterogeneous reaction with NO2 for 24 h. Overall, the impacts of heterogeneous reaction of NO2 with hematite, magnetite and goethite on nitrate aerosol formation and iron solubility could be very limited.

Effect of a catalytic segment on flame stability in a micro combustor with controlled wall temperature profile

In the present study, the stability of methane/air flames in a micro cylindrical channel with a prescribed wall temperature profile and a platinum (Pt) segment was investigated numerically. Two dimensional simulation was performed by the commonly used computational fluid dynamics software Fluent 6.3, incorporated with detailed gas combustion chemistry GRI 3.0 and catalytic reaction mechanism of methane/air on platinum. The results demonstrate that all the unstable flames become stationary in the presence of a 3-mm Pt segment located at x = 49–52 mm (total length of 80 mm) on the wall surface, and the response of flame position to inlet velocity exhibits an S-shaped curve. Detailed analysis was conducted to reveal the underlying mechanisms responsible for the existences of two turning points, which showed that the catalytic segment exerts a negative influence on homogeneous combustion at high and medium velocities, whereas it can enhance the homogeneous combustion at low velocities. In summary, the present study revealed the complicated effects of heterogeneous reaction on homogeneous combustion and suggested an effective and economic method to suppress flame instabilities in micro combustors.

Heterogeneous reactions of mineral dust aerosol: implications for tropospheric oxidation capacity

Abstract. Heterogeneous reactions of mineral dust aerosol with trace gases in the atmosphere could directly and indirectly affect tropospheric oxidation capacity, in addition to aerosol composition and physicochemical properties. In this article we provide a comprehensive and critical review of laboratory studies of heterogeneous uptake of OH, NO3, O3, and their directly related species as well (including HO2, H2O2, HCHO, HONO, and N2O5) by mineral dust particles. The atmospheric importance of heterogeneous uptake as sinks for these species is assessed (i) by comparing their lifetimes with respect to heterogeneous reactions with mineral dust to lifetimes with respect to other major loss processes and (ii) by discussing relevant field and modeling studies. We have also outlined major open questions and challenges in laboratory studies of heterogeneous uptake by mineral dust and discussed research strategies to address them in order to better understand the effects of heterogeneous reactions with mineral dust on tropospheric oxidation capacity.

Hetero-/homogeneous combustion characteristics of premixed hydrogen-air mixture in a planar micro-reactor with catalyst segmentation

Numerical simulation of stoichiometric hydrogen-air premixed flame in a platinum-coated micro channel was performed to investigate hetero-/homogeneous combustion characteristics. An elliptical two-dimensional (2D) Computational Fluid Dynamics (CFD) model with detailed reaction mechanisms for homogeneous (gas phase) and heterogeneous (catalytic) reactions was adopted. The hetero-/homogeneous reaction characteristics with different inlet flow velocities and flame speeds were used to evaluate the effect of heterogeneous reaction on homogeneous reaction in the combustor with 2mm length of catalyst segmentation. The effect of heat generation of the heterogeneous reaction was also analyzed. The numerical results indicated that the highest temperature in the catalytic combustor was lower than that in the combustor without catalyst. In the presence of heterogeneous reaction, the species (OH) concentration near the catalytic surface decreased significantly due to the dominated heterogeneous reaction at that region. The homogeneous ignition distance increased with increasing inlet flow velocity. The heat transfer from the catalytic surface to the gaseous mixture in the catalytic zone exhibited a high intensity under the large flow velocity, and the preheated incoming flow improved the heterogeneous reaction to inhibit the homogeneous reaction. The decreased flame speed in the catalytic combustor further demonstrated that the homogeneous reaction was suppressed by the heterogeneous reaction.

Coal and biomass char reactivities in gasification and combustion environments

Mass loss data obtained in gasification and combustion tests in a thermogravimetric analyzer were used to adjust parameters in an intrinsic chemical reactivity model developed to predict char conversion rates and off-gas compositions when pulverized coal and biomass char particles are exposed to reactive gases. Char reactivity tests were performed in H2O/H2/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to H2O, in CO2/CO/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to CO2, and in O2/N2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to O2. Wyodak coal and corn stover chars produced at high heating rates were used in the reactivity tests, which were performed under kinetics-controlled conditions. Values determined for the heats of formation and absolute entropies of species adsorbed onto the carbonaceous surfaces as well as the values determined for activation energies of reaction rate coefficients are in the expected ranges for the type reaction considered. The reaction mechanism correctly describes the impact of temperature, H2O, CO2 and O2 mole fractions, and total pressure on the conversion rates of coal and biomass chars and it captures the inhibiting effects of H2 and CO on char reactivities to H2O and CO2 over the ranges of temperature, pressure and gas composition relevant to coal and biomass gasifiers and combustors. The heterogeneous reaction mechanism developed accurately predicts the effects of heterogeneous reaction in gasification and combustion environments, including oxy-combustion environments.

Current progress towards the heterogeneous reactions on mineral dust and soot

Haze is a phenomenon of visibility degradation caused by the extinction effects of atmospheric particulate matter (APM); these extinction effects are related to physicochemical properties of the aerosol particles. Therefore, the study of heterogeneous reactions on APM is relevant to establishing the formation mechanisms of haze. We review the latest research on heterogeneous reactions on mineral dust and soot particles. We discuss the effects of heterogeneous reactions on sources and sinks of trace gases, the formation of secondary aerosol, and the hygroscopic behavior of particles. We also summarize reactions relevant to haze formation. A synergistic effect occurs for coexisting gases involved in interface reactions. O2 plays a critical role in surface oxidation, while adsorbed organic species in soot support photo-oxidation. A number of important chemical reactions occur during humidification–dehumidification processes in mixed particles. Another relevant reaction mechanism is that weak acids replace strong acids. Finally, there is an enhancement effect of secondary aerosol on the hygroscopicity of APM. These results are not only useful for understanding heterogeneous reactions on APM, but also reveal some of the underlying mechanisms of haze formation in the coal producing areas of China.

The Effects of Platinum on Propane/Air Stagnation Flow Flames

Platinum and quartz stagnation surfaces were used to quantify the effects of heterogeneous reaction on premixed atmospheric pressure propane/air flames. The flame extinction limits were determined for a range of flame stretch conditions (30–90 cm−1) and exhaust gas composition (CO2, CO, O2, and NO) was determined for a range of equivalence ratios between the fuel lean and fuel rich extinction limits and dilution levels (N2 to O2 ratios from 3.76 to 5). The stagnation surface was maintained at high temperature, isothermal conditions of 900°C for all experiments. The results show that the extinction limits and the products of combustion are insensitive to the presence of the platinum catalyst under the high surface temperature conditions considered in this work. The results are discussed in the context of the physical and chemical mechanisms important during extinction.

The role of organic haze in Titan's atmospheric chemistry: II. Effect of heterogeneous reaction to the hydrogen budget and chemical composition of the atmosphere

One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186–200] in a one-dimensional photochemical model. Our results indicate that 60–75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C 4H 2 and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH 4 due to an episodic CH 4 release into Titan's atmosphere.

Box Model Investigation of the Effect of Soot Particles on Ozone Downwind from an Urban Area through Heterogeneous Reactions

Soot can provide additional surface area where heterogeneous reactions can take place in the atmosphere. These reactions are dependent on the number of reactive sites on the soot surface rather than the soot surface area per se. A box model, MOCCA, is used to investigate the effects of introducing heterogeneous reactions on soot into air parcel passing over an urban area and traveling downwind. The model was run at two soot mass concentrations of 2 microg/m3 and 20 microg/m3 with a surface density of n-hexane and decane. Signifcant change in gasphase concentration was only observed for the higher soot concentration. Due to the noncatalytic nature of the heterogeneous reactions, soot sites are rapidly consumed, and soot site concentrations are greatly reduced shortly after emissions are turned off. Notable changes in gaseous concentrations due to the introduction of heterogeneous reactions are not observed in the urban setting. The impact of heterogeneous reactions is more evident after emissions are turned off (i.e. downwind from the urban center). These changes are minimal for the condition that used n-hexane surface density. For conditions that used decane soot, NOx concentrations showed a slight increase, with NO being higher in the daytime and NO2 at night. The maximum O3 reduction observed when using the higher soot concentration is 7 ppb, downwind of the urban center. Change in O3 concentration was less than 1 ppb when using the lower soot loading. The observed effects of heterogeneous reactions on soot decrease with time.

Heterogeneous reactions in aircraft gas turbine engines

[1] One-dimensional flow models and unity probability heterogeneous rate parameters are used to estimate the maximum effect of heterogeneous reactions on trace species evolution in aircraft gas turbines. The analysis includes reactions on soot particulates and turbine/nozzle material surfaces. Results for a representative advanced subsonic engine indicate the net change in reactant mixing ratios due to heterogeneous reactions is <10−6 for O2, CO2, and H2O, and <10−10 for minor combustion products such as SO2 and NO2. The change in the mixing ratios relative to the initial values is <0.01%. Since these estimates are based on heterogeneous reaction probabilities of unity, the actual changes will be even lower. Thus, heterogeneous chemistry within the engine cannot explain the high conversion of SO2 to SO3 which some wake models require to explain the observed levels of volatile aerosols. Furthermore, turbine heterogeneous processes will not effect exhaust NOx or NOy levels.

Modeling of ozone reactions on aircraft-related soot in the upper troposphere and lower stratosphere

Several studies in modeling atmospheric processes have suggested that heterogeneous chemistry on soot emitted from high altitude aircraft could affect stratospheric ozone depletion. However, these modeling studies were limited because they did not adequately consider the decrease in reaction probability with time as the surface of the soot becomes “poisoned” by its interactions with various gases. Here we extend UIUC's two-dimensional chemical-transport model to investigate possible effects of heterogeneous reactions of ozone on aircraft-generated carbon particles, including a treatment of soot poisoning in the model. We generally follow literature recommendations for ozone uptake probabilities and determine the available active sites on soot given partial pressures of the reactants, temperature, and time since soot emission in order to investigate ozone decrease. The regeneration of soot active sites is also taken into account in this study. We find that, even if active sites on soot surfaces are regenerated, upper troposphere and lower stratosphere ozone losses on aircraft emitted soot occurring through heterogeneous reactions are insignificant once poisoning effects are considered.

Effect of Heterogeneous Reactions of Coal Char on Nitrous Oxide Formation and Reduction in a Circulating Fluidized Bed

Fluidized-bed combustion has been recognized as a clean coal combustion technology. However, it has been discovered that the nitrous oxide (N2O) emission from fluidized-bed coal combustion is much higher than that from pulverized coal combustion. N2O emission from circulating fluidized-bed coal combustion is even higher. Heterogeneous reactions of char play an important role in N2O formation and reduction in circulating fluidized-bed coal combustion but there exist many unknowns. In this work the effects of heterogeneous reactions of char on N2O formation and reduction were examined in a bench-scale circulating fluidized bed. The experiments revealed that during circulating fluidized-bed coal combustion, N2O formation from oxidization of char-N has the same order as N2O formation from NO reduction by char, and neither of them is negligible (inlet O2 concentration: 21%, temperature: 1135−1225 K). Absorption of oxygen on the surface of char is not an indispensable condition for N2O formation from NO reduction, i.e., NO can also be reduced into N2O on the surface of char in absence of O2. N2O formation from NO reduction in the presence of O2 contributes about 70% and is the main path of N2O formation from NO reduction. N2O formation from NO reduction in the absence of O2 contributes about 30%, and the relative importance of these two mechanisms of N2O formation does not change significantly with NO concentration. Reduction of N2O by char is a first-order reaction with an Arrhenius dependence on temperature. The kinetics of N2O reduction by various chars were obtained (974−1223 K). Different chars have different effects on N2O reduction, suggesting that N2O reduction ability of char has a dependence on the property of the char itself. These results may partially account for the diversities in N2O emissions during combustion of different coals. On the basis of the results of this work, char-related reaction paths of N2O formation and reduction were proposed.

Nucleation with Simultaneous Chemical Reaction in the Vapor-Phase Synthesis of AlN Ultrafine Powders

Using the classical (one-component) homogeneous nucleation theory approach of a constrained equilibrium distribution of sub- critical particles, a nucleation equation that considers the effect of heterogeneous reaction is derived for the AlN system. In the absence of more detailed information about the aluminum nitride (AlN) reaction kinetics, a simplified surface reaction model is formulated. Clusters, which can be formed by AlN and pure Al, are assumed to grow from the condensation of Al vapor followed by surface reaction between condensed Al and absorbed NH3. The composition of any subcritical particle is assumed to be determined by the rates at which Al and AlN species are added to their surface and thus made entirely dependent upon local gas concentrations. This assumption resulted in important simplifications that allowed this development to be cast in a form resembling the one used by the classical theory for one-species nucleation. An analysis intended to evaluate the dependence of this nucleation...

Absorption accompanied with chemical reaction in trickle-bed reactors

A new development in the field of internals in packed columns is the use of structured packing types. Recently, a new structured packing type coated with a thin alumina layer (KATAPAKTM) has been developed. In this report, the results of an experimental and theoretical study concerning the possible applicability of this new packing material for hydrogenation processes in a trickle-bed reactor is presented. The palladium catalyzed hydrogenation of α-methylstryrene is used as a model reaction to study hydrodynamics and mass transfer characteristics in a trickle-bed reactor under reactive conditions. Converstions at several process conditions are measured in a pilot plant in which 3 mm spheres as well KATAPAKTM is applied as packing materials. A comparison of the results of some physical absorption experiments with the results of hydrogenation experiments showed that the resistance in series model—in which the total resistance against mass transfer is calculated from the separate resistances—is not valid in systems where heterogeneous reactions at the solid surface can enhance the mass transfer-rate at the gas-liquid interphase. With the aid of a developed trickle-bed reactor model, based on liquid diffusion, simultaneous reaction at the solid surface and zero volume mixing points, the mass transfer phenomena in trickle-bed reactors in conditions where the resistance in series model fails can be explained and described. The numerically solved model calculates the hydrogen profiles in the liquid films of the reactor and over all single pass conversions at several process conditions. These conclusions are confirmed by the results of the simulation of a model reactor, i.e. the laminar film reactor with a catalytically active wall. From the results of the measurements it could be concluded that in trickle-flow conditions, the application of KATAPAKTM does not significantly improved on the overall performance of trickle-bed reactors. The increase of the physical absorption rate due to better mass transfer characteristics of structured packings compared to dumped packing types—as reported in literature—will be eliminated to a certain extent in reactive systems due to the enhancement effect of heterogeneous reactions in trickle-flow operation.

Heterogeneous chlorine chemistry in the tropopause region

Satellite observations of cloud optical depths and occurrence frequencies are used as input to a two‐dimensional numerical model of the chemistry and dynamics of the atmosphere to study the effects of heterogeneous reactions on cloud surfaces upon chemical composition and ozone depletion in the tropopause region. Efficient reactions of ClONO2 with HCl and H2O, and of HOCl with HCl, are likely to take place on the surfaces of cirrus clouds [Borrmann et al., 1996] and perturb chlorine chemistry, much as they do on polar stratospheric clouds present at higher altitudes and colder temperatures. Because of the very low predicted background abundances of ClO near the tropopause, such reactions could enhance the local ClO mixing ratios by up to 30‐fold at midlatitudes. Substantial perturbations are also predicted for related chemical species (e.g., HCl, HOCl, ClONO2, NO2, HO2) in the midlatitude and tropical tropopause regions due to these heterogeneous reactions. If cirrus clouds occur with sufficient frequency and spatial extent, they could influence not only the chemical composition but also the ozone depletion in the region near the tropopause. Because of variations in observed cloud occurrence frequency and in photochemical and dynamical timescales, the presence of cirrus clouds likely has its largest effect on ozone near the midlatitude tropopause of the northern hemisphere in summer.