NO NO2 Research Articles

Modeling the diurnal cycle of conserved and reactive species in the convective boundary layer using SOMCRUS

Abstract. We have developed a one-dimensional second-order closure numerical model to study the vertical turbulent transport of trace reactive species in the convective (daytime) planetary boundary layer (CBL), which we call the Second-Order Model for Conserved and Reactive Unsteady Scalars (SOMCRUS). The temporal variation of the CBL depth is calculated using a simple mixed-layer model with a constant entrainment coefficient and zero-order discontinuity at the CBL top. We then calculate time-varying continuous profiles of mean concentrations and vertical turbulent fluxes, variances, and covariances of both conserved and chemically reactive scalars in a diurnally varying CBL. The set of reactive species is the O3–NO–NO2 triad. The results for both conserved and reactive species are compared with large-eddy simulations (LES) for the same free-convection case using the same boundary and initial conditions. For the conserved species, we compare three cases with different combinations of surface fluxes, and CBL and free-troposphere concentrations. We find good agreement of SOMCRUS with LES for the mean concentrations and fluxes of both conserved and reactive species except near the CBL top, where SOMCRUS predicts a somewhat shallower depth, and has sharp transitions in both the mean and turbulence variables, in contrast to more smeared-out variations in the LES due to horizontal averaging. Furthermore, SOMCRUS generally underestimates the variances and species–species covariances. SOMCRUS predicts temperature–species covariances similar to LES near the surface, but much smaller magnitude peak values near the CBL top, and a change in sign of the covariances very near the CBL top, while the LES predicts a change in sign of the covariances in the lower half of the CBL. SOMCRUS is also able to estimate the intensity of segregation (the ratio of the species–species covariance to the product of their means), which can alter the rates of second-order chemical reactions; however, for the case considered here, this effect is small. The simplicity and extensibility of SOMCRUS means that it can be utilized for a broad range of turbulence-mixing scenarios and sets of chemical reactions in the planetary boundary layer; it therefore holds great promise as a tool to incorporate these processes within air quality and climate models.

Experimental study on the role of ethanol on performance emission trade-off and tribological characteristics of a CI engine

An experimental study of engine combustion, performance and emission characteristics using diesel–ethanol blends along with investigation of tribological effects of ethanol on engine oil was done in present work using 1-butanol as emulsifier. Thorough observations of diesel–ethanol miscibility resulted that 25% v/v ethanol is miscible with diesel using only 3% v/v emulsifier. Tribological effects of ethanol on engine oil were investigated by analyzing the engine oil samples through FT-IR (Fourier Transform Infrared Spectroscopy). Overall experimentation re-evaluated the potential of ethanol in reduction of NOx, Soot and in-cylinder temperature with slight penalty for HC, CO and BSEC prominently at low load. All fuels produced more NO but lesser NO2 at higher load satisfying Zeldovich mechanism. A comparative trade-off analysis was done in between NHC, Soot and BSEC to reflect the performance and emission characteristics at a time. Trade-off study revealed D78E20B02 (78% diesel 20% ethanol 2% butanol) as optimal blend among all fuels used in present work. FT-IR analysis depicted negligible variation in the compounds in engine oil samples for the specified operational period. Statistical analysis showed larger Coefficient of variation for D78E20B02 blend due to higher absorbance of a particular compound.

Impact of biodiesel fuel on engine emissions and Aftertreatment System operation

The effect of biofuel use on the operation of Diesel Exhaust Aftertreatment System is investigated both numerically and experimentally, by focusing on the contribution of three main factors: raw PM–NOx emissions trade-off, NO–NO2 conversion efficiency of the Diesel Oxidation Catalyst (DOC) and PM reactivity toward oxidation.The possibility of limited interventions on assessed engine technologies is key toward the deployment of the potential related to fuel based greenhouse reduction policies, for both road and non-road markets. To verify its impact on the Aftertreatment System, a Diesel engine for non-road applications has been tested with Waste Cooking Oil (WCO) biodiesel blended with commercial fossil fuel at 6% and 30% v/v. Six engine operating modes have been selected as the most representative of the reference standard cycle (NRTC) for non-road Diesel engines and have been run to evaluate the biodiesel impact on engine emissions. Experimental results indicate a significant reduction of soot emissions, in line with literature trends, especially at high loads, as fuel oxygen enhances oxidation in the fuel rich regions of the combustion chamber. On the other side, only a slight increase in NOx emissions has been observed, along with a similar trend of the equivalence ratio due to both the lower heating value and stoichiometric air/fuel ratio of biodiesel in comparison with fossil fueling.The study has been also focused on the analysis of PM/NO2 ratio, in presence of the Diesel Oxidation Catalyst, when biodiesel is used. Engine tests have in fact demonstrated that, although the NO2/NOx ratio on raw exhaust is almost unaffected, a slight reduction of the NO2/NOx light-off temperature of the DOC is observed.This gives, along with a greater PM reactivity oxidation, more favorable conditions for the Diesel Particulate Filter passive regeneration process, with the final aim of a higher engine conversion efficiency.

Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions

In this study, the ammonia-SCR process was investigated using 2.5wt.% Cu/BEA, 2.6wt.% Cu/SAPO-34 and 3.1wt.% Cu/SSZ-13. Several reactions such as NO oxidation, ammonia oxidation, standard SCR, fast SCR and NO2 SCR were studied to understand the effect of zeolite type. It was found that the small-pore zeolites/silicoaluminophosphates with CHA structure (Cu/SAPO-34 and Cu/SSZ-13) exhibited higher SCR activity at 150°C and lower selectivity toward N2O formation during standard SCR conditions than Cu/BEA. However, formation of ammonium nitrate species during fast SCR conditions at 150°C occurred over Cu/CHA catalysts, which resulted in a gradual decrease of the NOx conversion. Such blocking was also observed over Cu/BEA, albeit to a minor extent. The ammonium nitrate formation and its decomposition temperature regimes resulted in that the Cu/BEA was catalytically more active at lower temperature than either Cu/SAPO-34 or Cu/SSZ-13 during fast SCR conditions. Additionally, our results show that the ammonium nitrate species were more stable on the small-pore zeolites than on Cu/BEA. Comparing the two Cu/CHA catalysts, Cu/SAPO-34 and Cu/SSZ-13, it was found that ammonia oxidation at high temperatures and ammonia SCR at 150°C was higher on Cu/SAPO-34. Further, TPR experiments showed that Cu in Cu/SAPO-34 is more easily reduced compared to Cu/SSZ-13. This can facilitate the redox processes and can thereby be a reason for the higher activity at 150°C for Cu/SAPO-34. In addition, Cu/SAPO-34 forms less N2O and this might be a result of the formation of more stable ammonium nitrates. To conclude, the choice of the type of zeolite/silicoaluminophosphates affects the activity and selectivity of the different steps in the SCR process.

High-Pressure Study of Methyl Formate Oxidation and Its Interaction with NO

An experimental and modeling study of the influence of pressure on the oxidation of methyl formate (MF) has been performed in the 1–60 bar pressure range, in an isothermal tubular quartz flow reactor in the 573–1073 K temperature range. The influence of stoichiometry, temperature, pressure, and presence of NO on the conversion of MF and the formation of the main products (CH2O, CO2, CO, CH4, and H2) has been analyzed. A detailed kinetic mechanism has been used to interpret the experimental results. The results show that the oxidation regime of MF differs significantly from atmospheric to high-pressure conditions. The impact of the NO presence has been considered, and results indicate that no net reduction of NOx is achieved, even though, at high pressure, the NO–NO2 interconversion results in a slightly increased reactivity of MF.

Deviations from the O3–NO–NO2 photo-stationary state in Delhi, India

A network of air quality and weather monitoring stations was set-up across Delhi, India, under the System of Air quality Forecasting And Research (SAFAR) project. The objective of this network was to enable better understanding of air quality in terms of atmospheric chemistry, emissions and forecasting in Delhi, one of the largest metropolises in the world. In this study, we focus on the O3–NO–NO2-triad Photo Stationary State (PSS), and investigate site-specific deviations in the Leighton Ratio (Φ) during a short period in 2012 (1–31 December). Large variations were observed in the NO (<1 ppbv to a peak of 295 ppbv), NO2 (<2 ppbv–47 ppbv) and O3 (4 ppbv–95 ppbv) mixing ratios, all of which showed strong diurnal variation. The Φ values showed large deviations from unity over the measurement period, with mostly negative deviations (Φ < 1), showing that the air masses were dominated by local sources of NOx and that the PSS was not achieved. Positive deviations (Φ > 1) were also observed occasionally, and these data were used to estimate the total peroxy radical (PO2) mixing ratios. This is the first estimate of PO2 reported for the city of Delhi and compares well with the results in the literature.

Comprehensive modeling study analyzing the insights of the NO–NO2 conversion process in current diesel engines

Multiple researches have focused on reducing the NOx emissions and the greatest results have been achieved when lowering the combustion temperature by employing massive exhaust gas recirculation rates (LTC). Despite this benefit, a substantial increase in the NO2 contribution to the NOx emissions has also been observed, which is the most harmful specie and is important for the design and positioning of the after-treatment devices. To understand how NO2 behaves and how it contributes to the total NOx (NO2/NOx), not only under LTC but also for CDC conditions, a stepwise computational research study was performed with Chemkin Pro software, due to the complexity of isolating the different phenomena studied, to analyze: (1) general equilibrium conditions and (2) the influence of typical diesel engine phenomena (combustion and cooling effects) under non-equilibrium conditions.The results obtained under equilibrium state confirm the theoretical guidelines established for the NO2 formation process. When considering a combustion process (HCCI-like mode), the previous results were corroborated as well as the fact that only poor or slow combustion processes are responsible for the NO2 formation. Additionally, it reflected a cyclic process between NO and NO2, or in other words, it is suffice to just concentrate on NO to be able to predict NO2. Finally, the results yield after analyzing some cooling effects, inherent to how diesel engines work (the expansion stroke, dilution of combustion products with the rest of in-cylinder charge and the one caused by wall impingement), reflect that: (1) the dilution effect explains the 10% of the NO2/NOx ratio under CDC conditions and (2) the coupling of the dilution with the expansion stroke cooling effects can explain the NO2 increase typical of LTC conditions. These results were also supported by some experiments performed in a single-cylinder diesel engine. Consequently, the cooling effect caused by dilution should be considered when modeling the NO2 formation just like the expansion stroke.

Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NOx formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NOx emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood+13% DDGS (dried distiller grain soluble) gas and wood+40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NOx emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NOx is very sensitive to the ammonia content in the producer gas. Results show that although NO–NO2 interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NOx reduction. The results indicate that NOx can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NOx burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NOx is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.

Evaluation of aromatic oxidation reactions in seven chemical mechanisms with an outdoor chamber

Environmental context Regulatory air quality models used to develop strategies to reduce ozone and other pollutants must be able to accurately predict ozone produced from aromatic hydrocarbons. In urban areas, major sources of aromatic hydrocarbons are gasoline and diesel-powered vehicles. Our findings show that the representation of aromatic hydrocarbon chemistry in air quality models is an area of high uncertainty Abstract Simulations using seven chemical mechanisms are intercompared against O3, NOx and hydrocarbon data from photooxidation experiments conducted at the University of North Carolina outdoor smog chamber. The mechanisms include CB4–2002, CB05, CB05-TU, a CB05 variant with semi-explicit aromatic chemistry (CB05RMK), SAPRC07, CS07 and MCMv3.1. The experiments include aromatics, unsaturated dicarbonyls and volatile organic compound (VOC) mixtures representing a wide range of urban environments with relevant hydrocarbon species. In chamber simulations the sunlight is characterised using new solar radiation modelling software. A new heterogeneous chamber wall mechanism is also presented with revised chamber wall chemical processes. Simulations from all mechanisms, except MCMv3.1, show median peak O3 concentration relative errors of less than 25% for both aromatic and VOC mixture experiments. Although MCMv3.1 largely overpredicts peak O3 levels, it performs relatively better in predicting the peak NO2 concentration. For aromatic experiments, all mechanisms except CB4–2002, largely underpredict the NO–NO2 crossover time and over-predict both the absolute NO degradation slope and the slope of NO2 concentration rise. This suggests a major problem of a faster and earlier NO to NO2 oxidation rate across all the newer mechanisms. Results from individual aromatic and unsaturated dicarbonyl experiments illustrate the unique photooxidation chemistry and O3 production of several aromatic ring-opening products. The representation of these products as a single mechanism species in CB4–2002, CB05 and CB05-TU is not adequate to capture the O3 temporal profile. In summary, future updates to chemical mechanisms should focus on the chemistry of aromatic ring-opening products.

Hydrocarbon Assisted NO Oxidation with Non-thermal Plasma in Simulated Marine Diesel Exhaust Gases

The NO oxidation performance in a non-thermal plasma (NTP) reactor under realistic synthetic exhaust gas compositions is investigated. The gas compositions differ mainly in the NO–NO2 ratio and represent different modes of operation of a marine diesel engine. It is found that the maximum NO oxidation efficiency is independent on the NO–NO2 ratio. Up to 55 % of the NO is mainly oxidised to NO2 in all gas mixtures being analysed. However, the specific energy density needed to reach the highest NO oxidation varies with the gas composition between 15 and 60 J/L. The performance of the NTP-reactor was significantly improved by the addition of propene (C3H6) acting as an additional oxidising agent. The energy consumption for NO–NO2 conversion was found to be between 20 and 45 eV/NO, depending on the ratio of the added propene as well as the initial concentrations of nitrogen oxides.

Ozone Weekend Effect Analysis in Three European Urban Areas

AbstractThis paper aims to apply statistical procedures to analyse the ozone weekend effect (OWE) in three European urban areas: Lisbon, Oporto and London. Seasonal variations of surface ozone (O3) and total oxidant species (Ox) concentrations are presented. The studied period was from January 2007 to December 2009. The O3 annual average profile in London was significantly different from the ones achieved for Lisbon and Oporto, due to the high emissions of nitrogen oxides and low temperatures (associated with low solar radiations). The influence of nitrogen oxides in atmospheric chemistry of London was analysed by linear correlations between O3 concentrations and NO2/NO ratio. High values during almost all period showed that O3 concentrations are mainly influenced by NONO2O3 chemical reaction. On the other hand, the linear correlations between O3 and NO2 concentrations only presented positive values in Lisbon and Oporto. This behaviour is associated with photo‐dissociation of NO2, producing O3 during the spring and summer periods. Additionally, cluster analysis was applied to group the days of week according the correspondent O3 behaviour. The weekend and some weekdays (immediately after or before) were grouped in the same cluster, showing that OWE occurs in all urban areas selected in this study. These findings suggest that the applied statistical procedures should be used to analyse the OWE and that the causes for the different O3 behaviours during the week may be influenced by meteorological variables.

One way coupling of CMAQ and a road source dispersion model for fine scale air pollution predictions

In this paper we have coupled the CMAQ and ADMS air quality models to predict hourly concentrations of NOX, NO2 and O3 for London at a spatial scale of 20 m × 20 m. Model evaluation has demonstrated reasonable agreement with measurements from 80 monitoring sites in London. For NO2 the model evaluation statistics gave 73% of the hourly concentrations within a factor of two of observations, a mean bias of −4.7 ppb and normalised mean bias of −0.17, a RMSE value of 17.7 and an r value of 0.58. The equivalent results for O3 were 61% (FAC2), 2.8 ppb (MB), 0.15 (NMB), 12.1 (RMSE) and 0.64 (r). Analysis of the errors in the model predictions by hour of the week showed the need for improvements in predicting the magnitude of road transport related NOX emissions as well as the hourly emissions scaling in the model. These findings are consistent with recent evidence of UK road transport NOX emissions, reported elsewhere. The predictions of wind speed using the WRF model also influenced the model results and contributed to the daytime over prediction of NOX concentrations at the central London background site at Kensington and Chelsea. An investigation of the use of a simple NO–NO2–O3 chemistry scheme showed good performance close to road sources, and this is also consistent with previous studies. The coupling of the two models raises an issue of emissions double counting. Here, we have put forward a pragmatic solution to this problem with the result that a median double counting error of 0.42% exists across 39 roadside sites in London. Finally, whilst the model can be improved, the current results show promise and demonstrate that the use of a combination of regional scale and local scale models can provide a practical modelling tool for policy development at intergovernmental, national and local authority level, as well as for use in epidemiological studies.

Catalytic NO Oxidation Pathways and Redox Cycles on Dispersed Oxides of Rhodium and Cobalt

AbstractThe elementary steps and site requirements for the oxidation of NO on Rh and Co and the oxidation state of the catalysts were probed by isotopic tracers, chemisorption methods, and kinetic measurements of the effects of the pressures of NO, O2, and NO2 on turnover rates. On both catalysts, NO oxidation rates were first order in NO and O2 and were inversely proportional to NO2 pressure, as observed on Pt and PdO. These data implied that O2 activation on an isolated vacancy (*) on the catalyst surfaces that were saturated with oxygen (O*) was the kinetically relevant step. Quasi‐equilibrated NO–NO2 interconversion steps established the coverage of * and O* and the chemical potential of oxygen during the catalysis. These chemical potentials set the oxidation state of Rh and Co clusters and were described by an O2 virtual pressure, which was determined from the formalism of non‐equilibrium thermodynamics. RhO2 and Co3O4 were the phases that were present during NO oxidation, which had several consequences for catalysis. Turnover rates increased with increasing cluster size because the vacancies that were needed for O2 activation were more abundant on large oxide clusters, which delocalized electrons better than small clusters. NO oxidation turnover rates on RhO2 and Co3O4 were higher than expected from the oxygen‐binding energy on Rh and Co metal surfaces and from the reduction potentials of Rh3+ and Co2+. These NO oxidation rates were consistent with the rates on Pt and PdO when one‐electron‐reduction processes, which were accessible for Rh4+ and Co3+ but not for Pt2+ and Pd2+, were used to describe the reactivity of RhO2 and Co3O4. One‐electron redox cycles caused the 16O2–18O2 exchange rates to be higher than the NO oxidation rates, in contrast with their analogous values on Pt and PdO, although O2 activation on the vacancies limited NO oxidation and O2 exchange on all of the catalysts. One‐electron redox cycles allowed electron sharing between metal cations and a facile route to form vacancies on RhO2 and Co3O4. This interpretation of the data highlighted the role of vacancies in kinetically relevant O2‐activation steps to explain the higher reactivity of larger metal and oxide clusters and to provide a common framework to describe NO oxidation and the active species on catalysts of practical interest.

Study of NO oxidation reaction over the Pt cluster supported on γ-Al2O3(111) surface

NO oxidation reaction on the Pt cluster supported on γ-Al2O3(111) surface is investigated using the climbing image nudged elastic band (CI-NEB) calculation based on the density functional theory. This is to verify the possibility of using Pt-based catalyst for NO oxidation reaction which is the rate-limiting step for the NOx oxidation reaction. From investigations of NO adsorption and NO2 desorption on the Pt cluster supported on γ-Al2O3 (111) surface where the dissociated O atoms already formed and lie on the Pt cluster sites, we find the process requires no activation barrier. Furthermore, we also performed the first principles of molecular dynamics simulation to investigate NO oxidation reaction at certain range temperatures (400 K, 600 K and 800 K) for complement of CI-NEB calculation. From the results, we discussed the temperature dependence of NO oxidation reaction.

Numerical study of a butanol/heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine utilizing negative valve overlap

The calibration and comparison to experimental data of a parallel computing multi-zone combustion model for simulating operational characteristics of an n-butanol/n-heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine utilizing the negative valve overlap (NVO) technology is described. The model is calibrated using one experimentally characterized operating point and by taking into account the major features of NVO. The model simulations at other four operating points closely match the cylinder pressure trace and heat release rates of the experiments. The unburned hydrocarbon emission is predicted to a reasonable level while NOx (nitric oxide NO and nitrogen dioxide NO2) formation is under-predicted. The difficulty in mapping the NOx emission is attributed to the fact that the combustion model operates within the closed cycle period of engines and therefore does not capture the complexity of the charge stratification within the NVO equipped engine. Nevertheless, the trend of increasing NOx levels with the increasing fraction of butanol in the fuel mixture is captured and the model is able to predict the pressure, heat release rates, and combustion phasing for the three fuel blends tested.

Dealumination and characterization of chabazite for catalytic application

Dealumination of the small-pore zeolite chabazite (CHA) was performed with ammonium hexafluorosilicate under solid and liquid-state conditions to increase the Si-to-Al ratio from 2.0 to 6.0. In the solid state, the mesopore fraction increased with repeating hydrothermal synthesis at 423 K, which was confirmed by nitrogen adsorption–desorption. In the liquid state, the formation of mesopores decreased substantially and the resulting CHA had an Si-to-Al ratio of ca. 5. The result of desorption of NO–NO2 from the Cu ion-exchanged dealuminated CHA in the solid state indicated that the presence of mesopores reduced NO–NO2 adsorption and desorption of NO2 occurred at 383 and 683 K whereas for the high-silica analog SSZ-13 desorption of NO2 occurred mostly at 473 and at 673 K.

Modeling Species Inhibition of NO Oxidation in Urea-SCR Catalysts for Diesel Engine NOx Control

Urea-selective catalytic reduction (SCR) catalysts are regarded as the leading NOx aftertreatment technology to meet the 2010 NOx emission standards for on-highway vehicles running on heavy duty diesel engines. However, issues such as low NOx conversion at low temperature conditions still exist due to various factors, including incomplete urea thermolysis, inhibition of SCR reactions by hydrocarbons, and H2O. We have observed a noticeable reduction in the standard SCR reaction efficiency at low temperature with increasing water content. We observed a similar effect when hydrocarbons are present in the stream. This effect is absent under fast SCR conditions where NO∼NO2 in the feed gas. As a first step in understanding the effects of such inhibition on SCR reaction steps, kinetic models that predict the inhibition behavior of H2O and hydrocarbons on NO oxidation are presented in the paper. A one-dimensional SCR model was developed based on the conservation of species equations and was coded as a C-language S-function and implemented in MATLAB/SIMULINK environment. NO oxidation and NO2 dissociation kinetics were defined as a function of the respective adsorbate’s storage in the Fe–zeolite SCR catalyst. The corresponding kinetic models were then validated on temperature ramp tests that showed good match with the test data. Such inhibition models will improve the accuracy of the model based control design for integrated diesel particulate filter-SCR aftertreatment systems.

Temporal and Spatial Variations of NOx and Ozone Concentrations in Seoul during the Solar Eclipse of 22 July 2009

Abstract The temporal and spatial variations of NO, NO2, and O3 concentrations in Seoul, South Korea, during the solar eclipse of 22 July 2009 are investigated by analyzing data measured at 25 environmental monitoring stations. The NO2 concentration increases and the NO and O3 concentrations decrease because the efficiency of NO2 photolysis decreases during the solar eclipse. About an hour after the maximum obscuration, the reduction in the average O3 concentration over Seoul is estimated to be 45%. The maximum reduction in the O3 concentration downwind of the NOx source area is higher and occurs later than that in the downtown region. Deviations from the NO–NO2–O3 photostationary state in the downwind region are larger than those in the downtown region. This result implies that, in addition to the photochemical effect, the effect of transport by winds increases the reduction potential of the O3 concentration in the downwind region during the solar eclipse.

Development and evaluation of GRAL-C dispersion model, a hybrid Eulerian–Lagrangian approach capturing NO–NO2–O3 chemistry

Abstract Based on two recent publications using Lagrangian dispersion models to simulate NO–NO2–O3 chemistry for industrial plumes, a similar modified approach was implemented using GRAL-C (Graz Lagrangian Model with Chemistry) and tested on two urban applications. In the hybrid dispersion model GRAL-C, the transport and turbulent diffusion of primary species such as NO and NO2 are treated in a Lagrangian framework while those of O3 are treated in an Eulerian framework. GRAL-C was employed on a one year street canyon simulation in Berlin and on a four-day simulation during a winter season in Graz, the second biggest city in Austria. In contrast to Middleton D.R., Jones A.R., Redington A.L., Thomson D.J., Sokhi R.S., Luhana L., Fisher B.E.A. (2008. Lagrangian modelling of plume chemistry for secondary pollutants in large industrial plumes. Atmospheric Environment 42, 415–427) and Alessandrini S., Ferrero E. (2008. A Lagrangian model with chemical reactions: application in real atmosphere. Proceedings of the 12th Int. Conf. on Harmonization within atmospheric dispersion modelling for regulatory purposes. Croatian Meteorological Journal, 43, ISSN: 1330-0083, 235–239) the treatment of ozone was modified in order to facilitate urban scale simulations encompassing dense road networks. For the street canyon application, modelled daily mean NOx/NO2 concentrations deviated by +0.4%/−15% from observations, while the correlations for NOx and NO2 were 0.67 and 0.76 respectively. NO2 concentrations were underestimated in summer, but were captured well for other seasons. In Graz a fair agreement for NOx and NO2 was obtained between observed and modelled values for NOx and NO2. Simulated diurnal cycles of NO2 and O3 matched observations reasonably well, although O3 was underestimated during the day. A possible explanation here might lie in the non-consideration of volatile organic compounds (VOCs) chemistry.

Primary NO2 emissions and their role in the development of NO2 concentrations in a traffic environment

Abstract An assessment of the formation of NO2 concentrations in heavily traffic-influenced environments in Helsinki, Finland was carried out. The proportion of primary NO2 emissions from road traffic was estimated using a statistical model for the relationship between the mixing ratios of nitrogen oxides (NO + NO2) and total oxidant (O3 + NO2) measured in 1994–2009. Based on this analysis, a quantitative estimate was derived for the relative importance of the primary NO2 emissions, ambient NO–NO2–O3 equilibrium and background concentrations in the observed NO2 concentrations. The proportion of primary NO2 in the vehicular NOx emissions increased from below 10% in the 1990s to about 20% in 2009, with a more distinctive increase during the most recent years. This development was related to the changes in the proportion of diesel-powered passenger cars in Finland. Between 1994 and 2004, the photochemical NO-to-NO2 conversion comprised on average 51% of the mean NO2 concentration, while the primary NO2 emissions contributed 31%. The role of the primary NO2 emissions was limited by the steeply-decreasing total NOx emissions. More recent data (2005–2009) yielded higher primary NO2 emission fractions (15 –21%), with a clearly increasing trend. As a result, the contribution of chemical conversion steadily decreased from 54% in 2005 to 43% in 2009, while that of the primary NO2 emissions increased from 32 to 44%. In order not to exceed in future the annual limit of NO2 concentration, set by the European Union, in the busiest street canyons in downtown Helsinki, the primary NO2 emissions need to be addressed alongside the total NOx emissions.