Gas-phase Chemical Kinetic Mechanism Research Articles

Uncertainty Analysis of the Chemical Reactor Network Approach for Predicting the Pollutant Emissions in a Double-Swirl Combustor

Abstract The purpose of this study is to predict the pollutant emissions generated within an aero-engine combustor model using the computational fluid dynamics-chemical reactor network (CFD-CRN) approach by modeling combustion in highly swirled flows. The selected test case is a laboratory double swirled combustor that came with an extensive experimental database from previous works for CH4/air diffusion flames at atmospheric pressure. The CFD-CRN modeling approach is initiated by solving Reynolds-averaged Navier–Stokes (RANS) equations for a 3D computational domain. The numerically achieved time-averaged values of the velocity components are in good agreement with the experimental data for two different thermal power. The CRN is obtained by dividing the flow field into ideal chemical reactors using various filters on the CFD results. The temperature, axial velocity, CH4, and O2 mass fractions distributions are selected as the splitting criteria for constructing the CRN. An uncertainty analysis is carried out to investigate the effects of different splitting approaches for the temperature criteria since it significantly affected the pollutant emissions in the gas turbine combustor. The simulations of the pollutant emissions are performed via the detailed gas-phase chemical kinetic mechanism of GRI-Mech 3.0. The nonlinear distribution of the temperature intervals result in lower uncertainty and provide reliable results even with a small number of ideal reactors. Also, it is observed that the CRN can be used in different operating conditions and provide suitable results if it is constructed with exceptional consideration. Moreover, a parametric study is performed by varying the equivalence ratio and air inlet temperature to investigate the trends of the NO and CO emissions.

Development of a numerical model to simulate carbon black synthesis and predict the aggregate structure in flow reactors

Carbon black (CB) has diverse applications which depend on the physical structure of the particle, especially aggregate structure. CB manufacturers can benefit from a predictive tool which estimates the mean properties and the aggregate shape of the final product. The number of CB numerical models that can predict the aggregate structure is limited. However, in the soot research field, these tools have been developed and validated extensively. Therefore, this work seeks the implementation of a recently developed soot model into a plug flow reactor code to simulate the CB synthesis in flow reactors to predict the aggregate structure. The aerosol dynamics model used in this study includes reversible particle inception, reversible surface polycyclic aromatic hydrocarbon (PAH) addition, hydrogen abstraction carbon addition (HACA), oxidation, and particle aggregation. The modeling results are validated with experimental gas-phase species measurements, particle volume fraction (FV), particle size distribution (PSD), and the aggregate structure. Four gas-phase chemical kinetic mechanisms are compared which all show a reasonably good agreement with the gas-phase species measurements. Two chemical kinetic mechanisms when coupled to the aerosol dynamics model predict FV and PSD profiles fairly well. The models capability of predicting the number of primary particles per aggregate is promising. The modeling results show that PAH addition is more important than HACA growth, and aggregation significantly reduces the number density of the particles below 10 nm.

Studying the Effect of Different Gas-Phase Chemical Kinetic Mechanisms on the Formation of Oxidants, Nitrogen Compounds and Ozone in Arid Regions

CMAQ was implemented in the central region of Saudi Arabia and the effect of simulating models using various chemical mechanisms on selected oxidants, nitrogen species, and O3 was investigated. CB05TUCL predicted OH, MEPX, and NOz about 7%, 7.7%, and 8% more than CB05E51 respectively; however, there was no observable difference in the O3 predictions. The differences in variations of SAPRC07 mechanism (SAPRC07TB, SAPRC07TC, and SAPRC07TIC) for all parameters were less than 1%. RACM2 produced the highest OH and H2O2 concentrations. RACM2 enhanced OH production in the range of 24% - 32% and H2O2 by 9% over other mechanisms; these are comparatively less than the findings of other studies. Similarly, CB05 produced over 40% more PAN concentration than CB05. Moreover, PAN concentrations produced by all mechanisms were very high compared to other studies. SAPRC07 produced approximately 3% more mean surface O3 concentration than RACM2 and approximately 10% more than CB05. RACM2 O3 predictions were higher than CB05 by 7%. The predicted O3 concentrations by CB05, RACM2, and SAPRC07 were 6%, 11%, and 15% more than the average observed concentrations, which indicate that closest predictions to the observed values were by CB05. This study concludes that there is a wide variation of mechanisms with respect to the predictions of oxidants and nitrogen compounds; however, less variation is noticed in predictions of O3. For any air pollution control strategies and photochemical modeling studies in the current region or in any other arid regions, the CB05 mechanism is recommended.

Towards improved partial oxidation product yield in mixed ionic-electronic membrane reactors using CSTR and CFD modelling

Mixed ionic-electronic conducting (MIEC) membrane reactors are attractive for partial oxidation and oxidative coupling of methane (OCM) because of their ability to separate oxygen from air with a low energetic penalty, and introduce the oxygen into a reaction zone with spatial and temporal control. To facilitate design and optimization of such reactors, two numerical models for MIEC membrane reactors for methane reforming are introduced: a computationally inexpensive, coupled, two-chamber CSTR model, and a CFD model for reacting flow. The CFD model considers diffusive transport with a mixture averaged model, and was shown to agree well with previous experimental and modelling works. Both models utilize a detailed gas-phase chemical kinetic mechanism and a membrane oxygen permeation model that considers the local oxygen concentration. It is demonstrated that the CSTR model can be used to evaluate a large number of reactor parameters efficiently to identify optimal conditions for OCM. The most promising reactor operating points identified using the CSTR for a particular reactor size are further considered with spatial detail using the CFD model. It is shown that the coupling between oxygen permeation, gas phase chemical kinetics, and flow field can have a significant impact on the reactor performance. Here the flow direction in a typical button reactor was reversed, which resulted in a 40% increase in predicted C2 yield. The increase is attributed to the shorter post-reaction residence time of the C2 products in the reactor, thereby inhibiting deeper oxidation. This significant impact on the reactor performance can be utilized to improve the yield of partially oxidized products from membrane reactors.

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.

Dimethoxymethane Oxidation in a Flow Reactor

ABSTRACTThe simultaneous reduction of NOx and soot emissions from diesel engines is a major research subject and a challenge in today’s world. One prospective solution involves diesel fuel reformulation by addition of oxygenated compounds, such as dimethoxymethane (DMM). In this context, different DMM oxidation experiments have been carried out in an atmospheric pressure gas-phase installation, in the 573–1373 K temperature range, from pyrolysis to fuel-lean conditions. The results obtained have been interpreted by means of a detailed gas-phase chemical kinetic mechanism. Results indicate that the initial oxygen concentration slightly influences the consumption of DMM. However, certain effects can be observed in the profiles of the main products (CH4, CH3OH, CH3OCHO, CO, CO2, C2H2, C2H4, and C2H6). Acetylene, an important soot precursor, is only formed under pyrolysis and reducing conditions. In general, a good agreement between experimental and modeling data was observed.

High Pressure Oxidation of Dimethoxymethane

The oxidation of dimethoxymethane (DMM) has been studied under a wide range of temperatures (373–1073 K), pressures (20–60 bar) and air excess ratios (λ = 0.7, 1 and 20), from both experimental and modeling points of view. Experimental results have been interpreted and analyzed in terms of a detailed gas-phase chemical kinetic mechanism for describing the DMM oxidation. The results show that the DMM oxidation regime for 20, 40 and 60 bar is very similar for both reducing and stoichiometric conditions. For oxidizing conditions, a plateau in the DMM, CO and CO2 concentration profiles as a function of the temperature can be observed. This zone seems to be associated with the peroxy intermediate, CH3OCH2O2, whose formation and consumption reactions appear to be important for the description of DMM conversion under high pressure and high oxygen concentration conditions.

An experimental and modeling study of the influence of flue gases recirculated on ethylene conversion

The influence of those gaseous compounds that can be typically present in combustion processes with flue gas recirculation (FGR) techniques: CO2, H2O, CO, NO, NO2, N2O and SO2, on ethylene conversion was analyzed through an experimental and modeling study. Ethylene oxidation experiments in the presence of the different gaseous compounds were carried out in the 700–1400K temperature range, at atmospheric pressure, from fuel-lean to fuel-rich conditions, using N2 as bath gas. These experiments were modeled by means of a detailed gas-phase chemical kinetic mechanism, which was used to identify the implications of the different gaseous compounds recirculated for the ethylene oxidation scheme, as well as for their own conversion. Overall, good agreement was obtained between the experimental data and the modeling, and thus the proposed mechanism can be successfully used to model the ethylene oxidation in the presence of flue gases recirculated (CO2, H2O, CO, NO, NO2, N2O and SO2) in a wide range of operating conditions.

A numerical study of high pressure, laminar, sooting, ethane–air coflow diffusion flames

A numerical study is conducted of ethane–air coflow diffusion flames at pressures from 2 to 15atm. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a detailed sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with increasing pressure without any tuning of the model for different pressures. The model shows good agreement with the experimental data on both the flame wings and centerline regions. Peak wing and centerline soot volume fractions are found to scale with P2.49 and P2.02 respectively. This scaling compares well to that observed experimentally for methane–air and ethylene–air flames. As pressure is increased, the flame cross-sectional area decreases according to P−1.0 due to a constant mass flux and a thinning of the flame, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation dominates centerline soot formation. Surface growth and PAH condensation increase with increasing pressure primarily due to both of these processes being a function of surface area. This causes increases in soot volume fraction to further accelerate surface growth and PAH condensation, acting in a positive feedback manner. This positive feedback mechanism is initiated by increases in reaction rates caused by increases in gas phase density. Additionally, the significance of surface growth decreases with increasing pressure, while the role of PAH condensation increases.

Gas and soot products formed in the pyrolysis of acetylene mixed with methanol, ethanol, isopropanol or n-butanol

The pyrolysis of acetylene-methanol, acetylene-ethanol, acetylene-isopropanol and acetylene-n-butanol mixtures has been studied in a flow reactor in the 975–1475 K temperature range. The purpose of this work is to analyze the effect of each alcohol on soot and gas products coming from the pyrolysis of the mixtures compared to the results observed in the pyrolysis of pure acetylene, taken as a reference. Results show that the presence of alcohols always reduces the formation of soot and that the lower the atomic carbon/oxygen (C/O) ratio in the reacting mixture, the higher the soot reduction achieved, mainly due to the enhancement of oxidation reactions by the presence of O in the fuel mixture. The experimental evolution of gas products at the reactor outlet is interpreted through a detailed gas phase chemical kinetic mechanism, which allows insight into the causes for soot reduction by the presence of the different alcohols. This analysis reveals that including methanol in the reacting mixture favours mainly the formation of CO, preventing most of the carbon coming from the alcohol to take part in soot formation and its precursors. The rest of the alcohols not only decompose into oxidation products but they can also form species that may contribute to soot formation. In particular, ethanol promotes the formation of CO and CH4, which come from competing reactions that prevent PAH formation, but also forms C2H4 that may contribute to soot precursors growth. Isopropanol contributes to disfavour PAH formation because it decomposes into CO and CH4, but it also forms C2 and C3 hydrocarbons that play an important role in PAH formation and growth. N-butanol enhances oxidation reactions to CO and CH4 formation in a lower degree than the rest of the alcohols and tends to decompose into small hydrocarbons, able to contribute to PAH formation and growth.

Comparison of Different Gas-Phase Mechanisms and Aerosol Modules for Simulating Particulate Matter Formation

ABSTRACT The effects of two gas-phase chemical kinetic mechanisms, Regional Atmospheric Chemistry Mechanism version 2 (RACM2) and Carbon-Bond 05 (CB05), and two secondary organic aerosol (SOA) modules, the Secondary Organic Aerosoi Model (SORGAM) and AER/EPRI/Caltech model (AEC), on fine (aerodynamic diameter ≤2.5 μm) particulate matter (PM2.5) formation is studied. The major sources of uncertainty in the chemistry of SOA formation are investigated. The use of all major SOA precursors and the treatment of SOA oligomerization are found to be the most important factors for SOA formation, leading to 66% and 60% more SOA, respectively. The explicit representation of high-NOx and low-NOx gas-phase chemical regimes is also important with increases in SOA of 30–120% depending on the approach used to implement the distinct SOA yields within the gas-phase chemical kinetic mechanism; further work is needed to develop gas-phase mechanisms that are fully compatible with SOA formation algorithms. The treatment of isoprene SOA as hydrophobic or hydrophilic leads to a significant difference, with more SOA being formed in the latter case. The activity coefficients may also be a major source of uncertainty, as they may differ significantly between atmospheric particles, which contain a myriad of SOA, primary organic aerosol (POA), and inorganic aerosol species, and particles formed in a smog chamber from a single precursor under dry conditions. Significant interactions exist between the uncertainties of the gas-phase chemistry and those of the SOA module. IMPLICATIONS The current state of the science is more advanced for the gas-phase chemistry of ozone formation than for the chemistry and gas/particle partitioning of particulate matter (PM) formation. As a result, there are larger uncertainties associated with aerosol modules than with gas-phase chemical kinetic mechanisms. Nevertheless, the uncertainties associated with those modules are not additive in an air quality model and there are close interactions between the gas-phase chemical mechanism and the secondary aerosol formation. In particular, the effect of the NOx regime on SOA formation should be explicitly treated in air quality models.

Formation of secondary aerosols over Europe: comparison of two gas-phase chemical mechanisms

Abstract. The impact of two recent gas-phase chemical kinetic mechanisms (CB05 and RACM2) on the formation of secondary inorganic and organic aerosols is compared for simulations of PM2.5 over Europe between 15 July and 15 August 2001. The host chemistry transport model is Polair3D of the Polyphemus air-quality platform. Particulate matter is modeled with a sectional aerosol model (SIREAM), which is coupled to the thermodynamic model ISORROPIA for inorganic species and to a module (MAEC) that treats both hydrophobic and hydrophilic species for secondary organic aerosol (SOA). Modifications are made to the gas-phase chemical mechanisms to handle the formation of SOA. In order to isolate the effect of the original chemical mechanisms on PM formation, the addition of reactions and chemical species needed for SOA formation was harmonized to the extent possible between the two gas-phase chemical mechanisms. Model performance is satisfactory with both mechanisms for speciated PM2.5. The monthly-mean difference of the concentration of PM2.5 is less than 1 μg m−3 (6%) over the entire domain. Secondary chemical components of PM2.5 include sulfate, nitrate, ammonium and organic aerosols, and the chemical composition of PM2.5 is not significantly different between the two mechanisms. Monthly-mean concentrations of inorganic aerosol are higher with RACM2 than with CB05 (+16% for sulfate, +11% for nitrate, and +10% for ammonium), whereas the concentrations of organic aerosols are slightly higher with CB05 than with RACM2 (+22% for anthropogenic SOA and +1% for biogenic SOA). Differences in the inorganic and organic aerosols result primarily from differences in oxidant concentrations (OH, O3 and NO3). Nitrate formation tends to be HNO3-limited over land and differences in the concentrations of nitrate are due to differences in concentration of HNO3. Differences in aerosols formed from aromatic SVOC are due to different aromatic oxidation between CB05 and RACM2. The aromatic oxidation in CB05 leads to more cresol formation, which then leads to more SOA. Differences in the aromatic aerosols would be significantly reduced with the recent CB05-TU mechanism for toluene oxidation. Differences in the biogenic aerosols are due to different oxidant concentrations (monoterpenes) and different particulate organic mass concentrations affecting the gas-particle partitioning of SOA (isoprene). These results show that the formulation of a gas-phase chemical kinetic mechanism for ozone can have significant direct (e.g., cresol formation) and indirect (e.g., oxidant levels) effects on PM formation. Furthermore, the incorporation of SOA into an existing gas-phase chemical kinetic mechanism requires the addition of reactions and product species, which should be conducted carefully to preserve the original mechanism design and reflect current knowledge of SOA formation processes (e.g., NOx dependence of some SOA yields). The development of chemical kinetic mechanisms, which offer sufficient detail for both oxidant and SOA formation is recommended.

Influence of the concentration of ethanol and the interaction of compounds in the pyrolysis of acetylene and ethanol mixtures

The pyrolysis of acetylene, ethanol and acetylene–ethanol mixtures in the 975–1475 K temperature range has been studied. The purpose of this work is to analyze the variation of soot and gas products coming from the pyrolysis of acetylene when adding increasing amounts of ethanol. Gas and soot products coming from the pyrolysis of 30,000 ppm of acetylene and different concentrations of ethanol (0–20,000 ppm) have been identified and quantified, and the influence of the amount of ethanol added has been analyzed. Analyses are supported on model calculations run with Chemkin using a detailed gas phase chemical kinetic mechanism, including subsets for acetylene and ethanol reactions and PAH formation and consumption reactions. In addition, the sum of the soot obtained from the individual pyrolysis of acetylene and ethanol is compared with the results coming from the pyrolysis of the corresponding acetylene–ethanol mixtures in order to analyze the interaction when they react jointly. Experimental results highlight the importance of the relative concentration acetylene/ethanol in the mixtures. Moreover, the results show that adding very small concentrations of ethanol, i.e. up to 600 times lower than acetylene concentration, leads to a diminution on the production of soot from the pyrolysis of acetylene.

Experimental and modeling study on the molecular weight distribution and properties of carbon particles in premixed sooting flames

The evolution of the molecular weight (MW) distribution and structural properties of carbon particulate formed in methane, ethylene and benzene fuel-rich premixed flames, burning in similar conditions of maximum flame temperature, was experimentally measured and modeled.Both solubility and chromatographic separation of the carbon particulate allowed to follow the variation of molecular weight distributions of the different fractions. Structural properties of the carbon particulate in terms of H/C ratio and UV–visible absorption have been determined from particles inception to their maturation as a function of fuel identity. The experimental data were numerically predicted using a detailed gas-phase chemical kinetic mechanism coupled with a discrete sectional approach for the modeling of the molecule-to-particle process. The model was able to follow both particle concentration and structural properties for different fuels.In the benzene flame high molecular mass aromatic compounds appeared very early in the oxidation region and were no longer detected at the end of flame whereas aliphatic fuels showed both aromatic molecules and freshly-nucleated soot particles persisting downstream of the flame. Elemental analysis and UV–visible properties of the carbon particulate showed that also the graphitization process is earlier and much faster in the benzene flame whereas in aliphatic flames the final ordered structure is produced later on the flame and with a slower rate. Detailed modeling of the carbon particulate yields and properties confirms the differences in the evolution of carbon particulate and its structure in benzene and aliphatic flames, also individuating the classes of species: molecules and clusters of molecules, responsible for soot inception.

Comparison of two gas-phase chemical kinetic mechanisms of ozone formation over Europe

Two recent gas-phase chemical kinetic mechanisms for tropospheric ozone formation, one based on the lumped-structure approach (CB05) and the other based on the lumped-molecule approach (RACM2), are compared for simulations of ozone over Europe. The host air quality model is POLAIR3D of the Polyphemus modeling platform. A one-month period (15 July to 15 August 2001) is simulated. Model performance is satisfactory with both mechanisms. Overall, the two mechanisms give similar results with a domain-averaged difference of 3 ppb and a mean fractional absolute difference of 5% (values averaged over the month for the daily 8-h average maximum ozone concentrations). This difference results from different treatments in the two mechanisms for both inorganic and organic chemistry. Differences in the treatment of the inorganic chemistry are due mainly to differences in the kinetics of two reactions: NO + O3 $\longrightarrow$ NO2 + O2 and NO + HO2 $\longrightarrow$ NO2 + OH. These differences lead to a domain-averaged difference in ozone concentration of 5%, with RACM2 kinetics being more conducive to ozone formation. Differences in the treatment of organic chemistry lead to a domain-averaged difference in ozone concentration of 3%, with CB05 chemistry being more conducive to ozone formation. This average difference results in part from compensating effects among various VOC classes and some significant differences are identified at specific locations (the coastline of northern Africa and eastern Europe: 9%) and for specific organic classes (aldehydes, biogenic alkenes and aromatics). Differences in the treatment of the organic chemistry result from various aspects. For some VOC classes, such as aldehydes and biogenic alkenes, the more detailed explicit treatments using more model species in RACM2 lead to either greater or lower reactivity depending on the assumptions made for the oxidation products. For other VOC species, such as aromatics, the assumptions made about the major chemical oxidation pathways (aromatic alcohol formation in CB05 vs. ring opening in RACM2) affect the ozone formation significantly. Reconciliation of different chemical kinetic mechanisms will require experimental data to reduce current uncertainties in the kinetic (e.g., NO oxidation) and mechanistic (e.g., aromatics oxidation) representations of major chemical pathways.

Modeling Secondary Organic Aerosol Formation via Multiphase Partitioning with Molecular Data

A new model for atmospheric secondary organic aerosol (SOA) is presented for biogenic compounds. It is based to the extent possible on experimental molecular SOA data, and it is compatible with any existing gas-phase chemical kinetic mechanism. Six SOA precursors or groups of precursors are used to represent biogenic monoterpenes and sesquiterpenes. SOA formation is modeled using five SOA surrogates to represent classes of compounds with different partitioning properties, e.g., hydrophobicity, aqueous solubility, acid dissociation, and saturation vapor pressure. Model simulations are evaluated against smog chamber data for SOA yields and some adjustments are made to uncertain stoichiometric coefficients and saturation vapor pressure parameters to improve model performance. The model is applied undertypical atmospheric conditions to exemplify the effect of relative humidity on SOA formation and the relative contributions of hydrophilic and hydrophobic SOA.

Reaction Kinetics in Silicon Chemical Vapor Deposition

AbstractFor Abstract see ChemInform Abstract in Full Text.

An Investigation of Particle Dynamics in a Rotating Disk Chemical Vapor Deposition Reactor

This paper describes a numerical model for the nucleation, growth, and transport of gas-phase particles formed during the chemical vapor deposition (CVD) of epitaxial silicon from silane. These particles can lower the deposition rate by consuming precursor, and contaminate the growing film via diffusion to the surface. This model has been constructed for use with the Sandia SPIN code, which contains a solver for the reacting flow and heat transfer in a vertical, rotating disk CVD reactor. A detailed gas-phase chemical kinetic mechanism for the thermal decomposition of silane was developed to simulate formation of small silicon clusters and the depletion of reactive intermediates through condensation. The particle model uses a moment transport formulation to examine the effects of total reactor pressure, temperature, rotation rate, inlet gas composition, and rate of particle growth via condensation on the characteristics of the particle population. Numerical results are presented in terms of the integral moments of the particle distribution which correspond physically to the particle number concentration, average particle diameter, and particle light scattering intensity. In situ validation experiments have been performed in an optically accessible reactor under conditions typical of silicon CVD. The rate of particle growth via condensation, controlled numerically by a global condensation parameter (GCP), was found to control the characteristics of the particle population. The numerical results were found to compare favorably with experiment if this GCP was properly chosen. © 2003 The Electrochemical Society. All rights reserved.

Reaction kinetics in silicon chemical vapor deposition

Gas-phase reactions of simple silicon hydride species underlying many types of chemical vapor deposition processes for silicon-based thin-film growth are reviewed in this paper. Mass spectrometry and laser-based spectroscopy are applied to identify gas-phase intermediates in thermal and hot-wire chemical vapor deposition processes. The mechanism of the thermal decomposition of silanes, including reactions that lead to the formation of hydrogenated silicon clusters, is examined. The gas-phase chemical kinetic mechanism in hot-wire chemical vapor deposition is proposed to explain precursor molecules for the film growth.

A photochemical box model for urban air quality study

The photochemical box model (PBM) developed in the present study is based on the principle of mass conservation. It has a horizontal domain of the size of a typical city and a vertical dimension defined by the mixed-layer height. The concentration of any pollutant is determined by horizontal advection, vertical entrainment, source emissions and chemical reactions. A one-dimensional high resolution boundary layer model by Blackadar ( Preprints, Third Symp. on Atmospheric Turbulence, Diffusion, and Air Quality, Raleigh, Am. Met. Soc., pp. 443–447, 1976; Advances in Environmental Sciences and Engineering, Vol. 1, No. 1 (edited by Pfafflin J. and Ziegler E.), pp. 50–85. Gordon and Breach, New York, 1979) has been incorporated in the PBM and further developed to consider the effect of urban heat islands in the simulation of mixed layer height. The predicted mixed-layer heights compare very well with observations. The gas phase chemical kinetic mechanism used in the Regional Acid Deposition Model II (RADM2) and that of an earlier version of PBM have been used to calculate the contributions of chemical reactions to the changes of pollutant concentrations. Detailed analysis and comparisons of the two chemical mechanisms have been made. The simulated pollutant concentrations using both chemical mechanisms are in very good agreement with available observations for CO, NO, NO 2 and O 3. A radiative transfer model developed by Madronich ( J. geophys. Res. 92, 9740–9752, 1987) has been incorporated in the PBM for the calculation of actinic flux and photolytic rate constants. Height-averaged and radiation-corrected photolytic rate constants are used for the photochemical reactions. Budget analyses conducted for CO, NO, NO 2 and O 3 have enhanced our understanding of the relative contributions of horizontal advection, vertical entrainment, source emissions and chemical reactions to the overall rate of change of their concentrations. Model predictions are not sensitive to the large number of peroxy radical-peroxy radical reactions in the RADM2 chemical mechanism under urban conditions.