Reactive Gas Concentration Research Articles

Dynamic response characteristics and water-gas-heat synergistic transport mechanism of proton exchange membrane fuel cell during transient loading

Dynamic response characteristics and water-gas-heat synergistic transport mechanism of proton exchange membrane fuel cell during transient loading

Reliability-based thermal-fluid-structural topological optimization of three-dimensional coolant channels in proton exchange membrane fuel cells

Reliability-based thermal-fluid-structural topological optimization of three-dimensional coolant channels in proton exchange membrane fuel cells

Revisitation of reactive direct current magnetron sputtering discharge: Investigation of Mg–CF4, Mg–O2, and Ti–O2 discharges by probe measurements

The reactive direct current (DC) magnetron sputtering discharges of Mg–CF4, Mg–O2, and Ti–O2 were investigated using probe measurements as a function of reactive gas flow ratio. The emission spectroscopy, which was conducted before the probe measurements, demonstrates that all the three DC discharges transit from nonreactive to reactive discharge mode with increasing reactive gas flow ratio. The probe measurements show that the plasma potentials of the Mg–O2 and Ti–O2 DC discharges slightly increase or remain almost constant with increasing reactive gas flow ratio, whereas that of the Mg–CF4 DC discharge drastically decreases at the mode transition. For the same change in reactive gas flow ratio, the discharge voltage of the Mg–CF4 DC discharge slightly increases and that of the Mg–O2 DC discharge drastically increases at the mode transition, whereas that of the Ti–O2 DC discharge slightly decreases at the mode transition. The changes in the cathode sheath potential difference at the mode transition differ between the Mg–CF4 and Ti–O2 DC discharges and the Mg–O2 DC discharge because of the difference in the probability of secondary electron emission at the cathode surface; furthermore, the changes in the anode sheath potential difference at the mode transition differ between the Mg–CF4 DC discharge and the Mg–O2 and Ti–O2 DC discharges because of the difference in the probability of negative-ion formation in the plasma bulk. The most informative results obtained in this study were the differences in the potential differences at the cathode and anode sheaths among the Mg–CF4, Mg–O2, and Ti–O2 DC discharges. They well demonstrated the effects of the change in secondary-emitted species at the cathode surface and the change in reactive gas concentration in the plasma on the potential configuration.

Organosilicon-Based Thin Film Formation in Very High-Frequency Plasma Under Atmospheric Pressure

Owing to recent interest in the production of flexible devices, it is necessary to develop a more convenient approach in which silicon (Si) thin film transistors (TFTs) are fabricated directly onto the flexible substrates at low substrate temperatures. Unfortunately, the physical limitations of conventional plasma-enhanced chemical vapor deposition (PECVD) under low pressures becomes a critical obstacle. In this study, Si film deposition using PECVD under atmospheric pressure excited by very high-frequency electrical power was investigated to overcome this issue. Tetramethylsilane [Si(CH3)4] is used as a source gas that is much safer than silane (SiH4) gas. We investigated the effects of the reactive gas concentration and specific energy (the ratio of input power to unit volume of the reaction gas) on carbon incorporation into the resultant films. Based on the results, we discuss the possibility of forming Si films with sufficiently low carbon content, which is applicable to Si TFTs.

A novel inlet for enriching concentrations of reactive organic gases in low sampling flows

Abstract. Preconcentration of samples is often necessary to detect the low levels of volatile organic compounds present in the atmosphere. We introduce a novel inlet that uses selective permeation to continuously concentrate organic gases in small sample flows (up to several standard cubic centimeters per minute) and consequently improve the sensitivity and limits of detection of analytical instruments. We establish the dependence of enrichment on the sample flow (decreasing with increasing flow) and pressure differential across its walls (increasing with increasing pressure differential). We further show that while there is some dependence on the permeability of the target analyte, most analytes of atmospheric interest exhibit similar enrichment. Enrichments between 4640 % and 111 % were measured at flows of 0.2 to 3 sccm for major reactive atmospheric gases: isoprene (C5H8), monoterpenes (α-pinene, C10H16), and alkanes (C3-C6). The relationships between inlet design parameters, operating conditions, and inlet efficiency are modeled and validated, enabling predictable enrichment of most atmospheric gases.

Comparison of Conventional and Microcellular Injection Using New Chemical Agents

ABSTRACT This study is mainly focused to obtain stable dimensions for plastic products used in manufacturing industries. A technique of microcellular foaming injection method was chosen with added chemical blowing agents to decrease the mass, clamping force, and cycle time. Mold flow analysis was run to compare the conventional plastic injection (KP) method with chemical foaming injection molding method using added TecoCell H1 (TH) and CS (TCS). The primary process conditions are considered as temperature of the mold, velocity of injection, pressure and time of holding, and cooling time. The experiments were carried under polymer material as (polypropylene) Moplen EP548P. The new chemical blowing agents TH and TCS, having different reactive material ratios (50% and 25% endothermic) and gas concentration ratios, were utilized at a rate of 1% by weight claiming to be a novel method for producing the parts with high efficiency. The results of both the flow analysis and the injection molding experiments demonstrate that, plastic samples produced by conventional plastic injection could be made approximately 7.5% lesser weight, with 33% lesser force of clamping, and a gain of cycle time nearly 15% by adding chemical blowing agents to the process. Topsis technique is carried for determining the best fabrication method. It was observed that, the injection molding method with Tecocell H1 (TH) chemical foaming agent exhibited better results in terms of less clamping force, reduced cycle time, and weight of the product. The experimental results are validated using the Topsis method and the results are satisfied. Hence, the chemical blowing agent Tecocell H1 (TH) can be applied as a foaming agent for manufacturing the plastic products and increase the efficiency of the production.

Prediction and optimization of gas distribution quality for high-temperature PEMFC based on data-driven surrogate model

• Data-driven surrogate model for HT-PEMFC is established. • Two intelligent modeling methods with better efficiency and accuracy are compared. • Effects of operating parameters on gas distribution quality of HT-PEMFC are analyzed. • Optimal operating conditions with better gas distribution quality are obtained. Suitable operating conditions can improve the internal gas distribution of high-temperature proton exchange membrane fuel cell (HT-PEMFC), reduce the occurrence of local gas starvation, and thus prolong the lifespan. Therefore, it is important to investigate the gas distribution quality of HT-PEMFC based on quantitative indicators. In this paper, data-driven surrogate models are established based on the simulation results of a three-dimensional validated numerical model to study the gas distribution quality of HT-PEMFC via two qualitative evaluation indexes, the value of mean and standard deviation of the reactant gas concentration in catalyst layer. In order to obtain the surrogate model more efficiently and accurately, genetic algorithm optimized deep belief network (GA-DBN) and Autogluon (an automated machine learning method) are applied. Based on the surrogate models, the influence of the operating condition parameters on each evaluation index is revealed, which provides theoretical basis for the life-extension design of HT-PEMFC. Then, Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS), a Multi-Criteria Decision Aid (MCDA) method, is used to select the optimal operating conditions according to the two evaluation indexes of gas distribution quality from the extensive computational results of the surrogate models, which provides specific guidance for the life duration improvement of HT-PEMFC.

Numerical Investigation of Flow Channel Design and Tapered Slope Effects on PEM Fuel Cell Performance

High-power proton exchange membrane (PEM) fuel cell vehicles are important for the realization of carbon neutrality in transportation. However, it is difficult to maintain enough fuel supply and quick water removal capacity at a high current density where reactant gas transportation and water concentration are directly affected by flow channel configurations. This study aims to investigate the tapered slope effects of a flow channel on fuel cell performance using a 3-D CFD model. The positive, negative, zero and hybrid tapered slopes are proposed to illustrate the fuel cell voltage, reactant gas and water vapor concentration in the flow channels. Among them, the flow channel with a positive tapered slope performs better, especially at a high current density. Then, the positive tapered slope effects are discussed, including different tapered slopes, inlet depths and widths of flow channels. The results show that the larger the tapered slope, the smaller the depth and width, and the better the fuel cell performs; the corresponding current densities are increased by a maximum of 6.53%, 12.72% and 61.13%. The outcomes stated above provide a key direction for flow channel design that can particularly achieve higher fuel cell power density at high current densities.

Modeling and CFD simulation of heat transfer process coupled with Unmixed Combustion for the application of generating superheated steam

• Unmixed combustion (UMC) process is investigated for generating supherheated steam. • A design of steam superheater integrated with UMC based reactor system is proposed. • A model of the proposed system is developed and CFD simulations are performed. • Superheated steam of desired temperature (1 atm) is obtained with sustained UMC operation. • Generation of LP and MP superheated steam using UMC was also found to be feasible. Superheated steam is produced in a super-heater using the energy of hot flue gases generated in the premixed combustion and it requires a larger surface area. Recently, unmixed combustion (UMC), an environmentally better process to premixed combustion, a variant of chemical looping combustion (CLC), has been investigated successfully for transferring heat to air, with a relatively compact system. Hence to address the limitations of the size of a super-heater and environmental aspects, it is decided to investigate the application of UMC for generating superheated steam. With this objective, the existing design of UMC based heat transfer system was modified for heating saturated steam to a desired temperature on a continuous basis. The modified system with Cu-based Oxygen Storage and Release Material (OSRM) was investigated using modeling and simulation study. A 2-D computational fluid dynamics (CFD) model was developed and simulated using COMSOL TM . Oxidation and reduction cycles of UMC were studied separately and the study was restricted to laminar steam flow conditions. Results of the representative cases showed that the superheated steam of desired bulk temperature of 650 – 720 K at atmospheric pressure and heat flux of 13,000 – 15000 W/m 2 at the interface can be obtained with sustained combustion (the bed temperatures between 723 and 873 K) in both the cycles. The effect of four important operating parameters viz. inlet steam velocity, inlet reactive gas velocity and concentration, metal loading, was also investigated and optimum ranges were suggested to achieve the maximum possible heat transfer without affecting the bed operation. The simulation study was then extended to generate low and medium pressure superheated steams and obtained results have indicated that it is still possible to generate superheated steam at desired temperatures using UMC.

Assessment on Effect of Molar Ratio of Inflow Gas on Characteristics of Biogas Dry Reforming

To improve the H2 yield and thermal efficiency for the biogas dry reforming process, this study has been conducted the experimental investigation using a membrane reactor by changing reaction temperature, differential pressure between reaction chamber and sweep chamber, and molar ratio of inflow gas with and without a sweep gas. In particular, we have focused on the effect of molar ratio of inflow gas. The concentrations of reactant and product gases were measured by gas chromatography to clarify the effects of the above operation conditions on the reaction characteristics. We have evaluated the conversion of the reacted gases (CH4 and CO2), H2 yield, H2 permeability, and thermal efficiency. As a result, the largest H2 was produced at the molar ratio of 1.5:1 irrespective of reaction temperature, and the differential pressure between reaction chamber and sweep chamber with and without a sweep gas. It was observed that the concentration of CO is larger than that of H2 irrespective of reaction temperature, differential pressure between reaction chamber and sweep chamber, and molar ratio of inflow gas with and without a sweep gas. Under the experimental conditions in this study, the H2 separation rate which is enhanced by the differential pressure and sweep gas exceeded the kinetic rate of H2 production, resulting in a low concentration of produced H2. Therefore, it is necessary to clarify the optimum conditions where the H2 separation rate matches the kinetic rate of H2 production.

Amorphization of thin supermalloy films Ni79Fe17Mo4 with oxygen during magnetron sputtering

Reactive sputtering of permalloy typically reduces crystallite size in films, if the concentration of reactive gas is low. This advances magnetic properties: decreases coercivity and increases good in-plane magnetic anisotropy. But oxygen is rarely applied for this purpose. Here, peculiar deposition conditions, including the geometry of the vacuum deposition system and large polymer substrate, allowed for the deposition of supermalloy with unusually high admixture of oxygen. Evolution of both static and dynamic magnetic properties with an increase in oxygen concentration proved to be dealt with the balance between grain size, surface structure, disordering of atomic structure and internal stresses. The critical concentration when supermalloy shows indications of oxidation is 5% of O 2 . Coercivity, resistivity, roughness and ferromagnetic resonance frequency are the parameters that are most sensitive to oxidation. • Oxygen admixture during sputtering decreases grain size in Ni 78 Fe 18 Mo 4 films. • Oxidation starts at 5 vol% of O 2 in a sputtering mixture Ar + O 2 . • Lower grain size de creases stresses and in creases in-plane magnetic anisotropy.

Features of the stages of heterogeneous catalytic processes under the influence of microwave radiation

Use of microwave electromagnetic radiation as an acting factor allows to have a significant influence on such processes as heating, phase transitions (melting, evaporation, drying), some stages of heterogeneous catalytic reactions. Consider the external task of diffusion kinetics of the process when using microwave radiation. For this purpose the investigated process is carried out in a flow reactor filled with grains of a solid catalyst, on the surface of which a catalytic reaction takes place. Let us imagine that the gas stream consists of a core, in which complete mixing takes place, and immobility of diffusion film of gas with thickness, through which diffusion to external surface of catalyst grains takes place. The concentration of reactant gas in the core of the stream is kept constant. Its concentration at the catalyst surface is lower due to the reaction; This difference results in a diffusion flow of reactant to the surface.

Origin of dissolved gas (CO2, O2, N2, alkanes) in pore waters of a clay formation in the critical zone (Tégulines Clay, France)

Abstract Understanding weathering processes in clay formations is an issue of primary importance for the preservation of our natural environment. Reactive-transport modeling used to simulate weathering of clay formations has indicated that reactive gases (CO2 and O2) are major parameters in controlling weathering processes. The Lower Cretaceous Tegulines marine-clay formation outcropping in the area of Brienne-le-Chateau (north-eastern France) has been investigated in the context of a sub-surface waste repository. We developed gas monitoring (CO2, O2, N2, alkanes) of core samples from two boreholes that entirely crosscut the Tegulines Clay formation, to define the consequences of weathering and oxidation processes on gases dissolved in pore waters. We discuss amounts of gas and the carbon isotopic composition of CO2 in terms of pore water chemistry including dissolved-inorganic carbon (DIC) and alkalinity, mineral reactivity, organic-matter degradation and oxygen diffusion. Degassing of samples conditioned under He atmosphere provided evidence of very high CO2 production in the soil (0–30 cm), and high CO2 degassing associated with a high oxygen level in the first 2–10 m of the clay. The CO2 degassing increase observed in weathered clay relative to preserved clay resulted from calcite dissolution due to pyrite oxidation and organic matter degradation. The δ13C of CO2 indicates that organic matter degradation was a major source of CO2 at shallow depths and down to 10–12 m, which is the maximum depth at which we observed fossil roots. Then the CO2 degassing decreased down to a constant value in preserved clay, where the carbonate system and the mineral assemblage control dissolved carbonates in pore waters. The profile of the δ13CCO2 also provides evidence of progressive CO2 diffusion of organic origin from the underlying Greensands aquifer in the lower part of Tegulines Clay up to ~40 m in the AUB230 borehole. As a first step toward understanding interactions between Tegulines Clay and near surface waters or water at the Greensands interface, we developed a reactive-transport model to simulate in one dimension weathering processes under ambient temperature, constrained by geochemical reactions in soil (organic matter degradation) and in the clay (pyrite oxidation and calcite dissolution), exchange, DIC and pore water chemistry. The simulation was carried out for 10 kyrs, assuming that weathering and soil formation began after the last glacial maximum. The DIC profile cannot be simulated without considering evaporation processes in agreement with the isotopic data. This type of approach combining a complete field dataset (reactive-gas concentrations, δ13C of CO2, major-ion concentrations, δ18O and δD of pore waters) and reactive-transport modeling is necessary for better understanding of chemical weathering processes in the critical zone.

Ambient concentrations and total deposition of inorganic sulfur, inorganic nitrogen and base cations in the Athabasca Oil Sands Region

Trace gas, particulate matter and deposition data collected in the Athabasca Oil Sands Region (AOSR) from 2000 to 2017 were evaluated as part of a broad scientific programmatic review. Results showed significant spatial patterns and temporal trends across the region. Concentrations of reactive gases were highest near the center of surface oil sands production operations and decreased towards the edges of the monitoring domain by factors of 8, 20, 4 and 3 for SO2, NO2, HNO3 and NH3, respectively. 18 of 30 sites showed statistically significant (p<0.05) negative trends in SO2 concentrations suggesting an~40% decrease since 2000. In contrast, only 2 of 30 sites showed statistically significant temporal trends (1 positive, 1 negative) for NO2. NH3 data showed (i) intermittent wildfire impacts, and (ii) high seasonality, with low concentrations during winter and significantly higher values during the summer. PM10 measurements were more limited, but also showed significant spatio-temporal variability. Comparison of PM10 and PM2.5 data showed that>80% of SO42- was in the PM2.5 fraction, while>60% of Ca2+, Mg2+, Na+ and Cl- were in the PM10-2.5 fraction. Ion balances of both PM10 and PM2.5 contained cation excesses at near-field oil sand sites, but PM2.5 samples at forest health sites>20km from surface production locations contained anion excesses. Monthly average concentrations of PM10 ions showed peak Ca2+ during March-April to November, but peak SO42-, NH4+ and NO3- from November-March. Deposition estimates showed rapid declines as a function of distance to oil sand operations. Estimated total N and total S deposition to forest health monitoring sites ranged from 2.0 to 5.7kgha-1a-1 and 2.1-14.0kgha-1a-1, respectively. Potential acid input (PAI) ranged from -0.46 to 0.79keq ha-1a-1 and was mostly 0.1-0.2keqha-1a-1 throughout the domain, except for two clusters of sites near oil sand operations.

P3420Is there an association between Saharan dust events and acute coronary syndrome incidence?

Abstract Background Asian dust and air pollution have been recently recognized as triggers in the incidence of acute myocardial infarction. The inflow of dust from the Sahara into Spain causes an increase in particulate matter (PM) levels in the atmosphere. The proximity to the Western Coast of Morocco and the Sahara Desert promotes the seasonality arrival of natural PM on the Canary Islands (Spain), leading to high concentrations of PM with an aerodynamic diameter over 10μm (PM10). The association of dust transport from the Sahara to the Atlantic Ocean and Mediterranean areas over the incidence of acute coronary syndromes (ACS), is unknown. Purpose The aim of the present study was to elucidate whether Saharan dust events in the dust belt - Canary Islands - is associated with the incidence of ACS. Methods We retrospectively collected data of hospitalizations due to ACS in 2416 consecutive patients from a tertiary care hospital in Tenerife (Canary Islands–Spain), from December 2012 to December 2017. Taking advantage of Canary Islands location, we characterized the Saharan dust events using PMx measurements and dust modeling. Concentrations of PM10 and reactive gases are measured in the Air Quality Network of the Canary Islands by using the European reference method. We applied the time-stratified case crossover design to examine the association between Saharan dust events and the incidence of ACS. This design allows us to adjust for individual confounders, season, time trend and the day of week. Using conditional Poisson regression models, we estimated the impact of PM10 Saharan dust events on the incidence of ACS. Because the effects of Saharan dust events could persist over the course of a few days, we examined the lag effect from Day 0 to Day 5. Results The occurrence of Saharan dust events observed 0–5 days before the ACS was not significantly associated with the incidence of ACS. PM10 (μg/m3) effect was: lag 0 (IRR: 0.999, 95% CI: 0.977–1.022), lag 1 (IRR: 1.025, 95% CI: 0.988–1.063), lag 2 (IRR: 0.98, 95% CI: 0.928–1.042), lag 3 (IRR: 0.976, 95% CI: 0.944–1.011), lag 4 (IRR: 0.996, 95% CI: 0.983–1.008) and lag 5 (IRR: 0.996, 95% CI: 0.984–1.007). (Figure 1) Poisson regression model of PM10 Conclusions This negative study, the first to assess the impact of Saharan dust events as a potential trigger in the onset of ACS, shows that African dust is unlikely to be associated with the incidence of ACS.

Chemically Induced Sintering of Nanoparticles

We have observed solid-state growth of pre-existing silver nanoparticles (AgNPs) upon exposure to trace (ppb) concentrations of reactive gases at room temperature. The consequent change in localized surface plasmon resonances alters the visible absorbance of dried, printed sensor spots made from inks of 10 nm-AgNPs and provides a novel mechanism for trace detection and dosimetry of reactive gases. Colorimetric sensor arrays based on these AgNP inks offer dosimetric identification of acidic and oxidizing gases and other reactive vapors with limits of detection below ppb levels for 1 h exposures. For an array of AgNP inks with various capping agents, a unique color response pattern is observed for each specific analyte. Excellent discrimination among 11 reactive gases was demonstrated using standard chemometric methods. The chemically induced sintering of NPs paves the way for novel solid-state sensors for the ultrasensitive detection of reactive gases and their application to the monitoring of trace airborne pollutants.

Overview of HOMEChem: House Observations of Microbial and Environmental Chemistry.

The House Observations of Microbial and Environmental Chemistry (HOMEChem) study is a collaborative field investigation designed to probe how everyday activities influence the emissions, chemical transformations and removal of trace gases and particles in indoor air. Sequential and layered experiments in a research house included cooking, cleaning, variable occupancy, and window-opening. This paper describes the overall design of HOMEChem and presents preliminary case studies investigating the concentrations of reactive trace gases, aerosol particles, and surface films. Cooking was a large source of VOCs, CO2, NOx, and particles. By number, cooking particles were predominantly in the ultrafine mode. Organic aerosol dominated the submicron mass, and, while variable between meals and throughout the cooking process, was dominated by components of hydrocarbon character and low oxygen content, similar to cooking oil. Air exchange in the house ensured that cooking particles were present for only short periods. During unoccupied background intervals, particle concentrations were lower indoors than outdoors. The cooling coils of the house ventilation system induced cyclic changes in water soluble gases. Even during unoccupied periods, concentrations of many organic trace gases were higher indoors than outdoors, consistent with housing materials being potential sources of these compounds to the outdoor environment. Organic material accumulated on indoor surfaces, and exhibited chemical signatures similar to indoor organic aerosol.

Large-scale multi-phase simulation of proton exchange membrane fuel cell

Abstract Limited by the computational efficiency and stability, traditional 3D (three-dimensional) CFD (computational fluid dynamics) simulations of PEMFC (proton exchange membrane fuel cell) are always in single-channel scale, which neglect the realistic flow field structures in commercial PEMFC. In this study, a large-scale PEMFC (109.93 cm2), which is a repeated unit in commercial stacks and includes realistic anode and cathode flow fields, is investigated in detail utilizing a comprehensive 3D multi-phase model. In particular, the Eulerian-Eulerian model is chosen for the solution of gas and liquid two-phase flow in flow fields and the surface tension, wall adhesion, drag force and gravity are all taken into consideration. The gas concentration and liquid water amount in each channel of flow field are studied to test the influence of flow field. Moreover, it is proved that increasing operating pressure is helpful to improve PEMFC performance by increasing the reactant gas concentration and membrane water content significantly. Besides, counter-flow arrangement of hydrogen and air facilitates uniform distribution of membrane content and electrochemical reaction. And in this case, the coolant flow direction designed to be the same with that of air is beneficial to PEMFC performance.

Kinetic study of single-walled carbon nanotube synthesis by thermocatalytic decomposition of methane using floating catalyst chemical vapour deposition

Abstract Single walled carbon nanotubes (SWCNTs) have been synthesized from thermocatalytic decomposition of methane by using ferrocene as the catalyst at atmospheric pressure. Floating catalyst chemical vapour deposition using a horizontal two zone reactor was employed to investigate the kinetics. The effects of temperature of synthesis (800–1000 °C), partial pressure (2–40 kPa) of reactant gas and catalyst concentration (0.2–4.6 mol m−3) on the rate of formation of SWCNTs have been studied. The rate is proportional to the partial pressure of methane indicating that the surface reaction of methane on the active site is the rate controlling step. The activation energy was found to be 80.16 kJ mol−1. Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, BET, UV–Vis-NIR and thermogravimetry have been used for the characterization of products. UV–Vis-NIR spectroscopy data reveals presence of 78–82% metallic SWCNTs (m-SWCNTs) in the synthesized samples. The BET surface area of purified SWCNTs were found to be 871 m2 g−1 with pore volume of 0.35 cc g−1 and electrical conductivity of 856–746 S cm−1.

Ozone and volatile organic compounds in the metropolitan area of Lima-Callao, Peru

This study analyzes ozone formation in the metropolitan area of Lima-Callao as a function of meteorological patterns and the concentrations of nitrogen oxides and reactive organic gases. The study area is located on the west coast of South America (12°S) in an upwelling region that is markedly affected by the Southeast Pacific anticyclone. The vertical stability and diurnal evolution of the mixing layer were analyzed from radiosondes launched daily during 1992–2014 and from two intensive campaigns in 2009. Vertical profiles show that during June–November, the subsidence inversion base ranges from 0.6 to 0.9 km above sea level (asl). In contrast, during December–May, subsidence inversion dissipates, leading to weak surface inversions from 0.1 to 0.6 km asl. At the surface level, compliance with the ozone standard of 51 parts per billion by volume (ppbv) is explained by the marine boundary layer effect and by strong inhibition of ozone formation due to titration with nitric oxide. Day-of-the-week variations in ozone and nitrogen oxides suggest a VOC-limited ozone-formation regime in the atmosphere of Lima. Furthermore, the pattern of C6–C12 species indicates that gasoline-powered vehicles are the main source of volatile organic compounds (VOCs), whereas the species with the greatest ozone-forming potential corresponded to the sum of the isomers m- and p-xylene. Mean benzene concentrations exceeded the standard of 0.63 ppbv, reaching 1.2 ppbv east of Lima. Nevertheless, the cancer risk associated with the inhalation of benzene was deemed acceptable, according to USEPA and WHO criteria.