scholarly journals CFD modeling of reactive pollutant dispersion in simplified urban configurations with different chemical mechanisms

2016 ◽  
Vol 16 (18) ◽  
pp. 12143-12157 ◽  
Author(s):  
Beatriz Sanchez ◽  
Jose-Luis Santiago ◽  
Alberto Martilli ◽  
Magdalena Palacios ◽  
Frank Kirchner
Keyword(s):  
Chemical Reactions ◽  
Chemical Mechanism ◽  
Cfd Modeling ◽  
Background Concentration ◽  
Urban Air ◽  
Large Set ◽  
Atmospheric Conditions ◽  
Traffic Emission ◽  
Wind Speeds ◽  
Cpu Time

Abstract. An accurate understanding of urban air quality requires considering a coupled behavior between the dispersion of reactive pollutants and atmospheric dynamics. Currently, urban air pollution is mostly dominated by traffic emission, where nitrogen oxides (NOx) and volatile organic compounds (VOCs) are the primary emitted pollutants. However, modeling reactive pollutants with a large set of chemical reactions, using a computational fluid dynamic (CFD) model, requires a large amount of computational (CPU) time. In this sense, the selection of the chemical reactions needed in different atmospheric conditions becomes essential in finding the best compromise between CPU time and accuracy. The purpose of this work is to assess the differences in NO and NO2 concentrations by considering three chemical approaches: (a) passive tracers (non-reactive), (b) the NOx–O3 photostationary state and (c) a reduced complex chemical mechanism based on 23 species and 25 reactions. The appraisal of the effects of chemical reactions focuses on studying the NO and NO2 dispersion in comparison with the tracer behavior within the street. In turn, the effect of including VOC reactions is also analyzed taking into account several VOC ∕ NOx ratios of traffic emission. Given that the NO and NO2 dispersion can also be affected by atmospheric conditions, such as wind flow or the background concentration from season-dependent pollutants, in this work the influence of wind speeds and background O3 concentrations are studied. The results show that the presence of ozone in the street plays an important role in NO and NO2 concentrations. Therefore, greater differences linked to the chemical approach used are found with higher O3 concentrations and faster wind speeds. This bears relation to the vertical flux as a function of ambient wind speed since it increases the pollutant exchange between the street and the overlying air. This detailed study allows one to ascertain under which atmospheric conditions the inclusion of chemical reactions are necessary for the study of NO and NO2 dispersion. The conclusions can be applied to future studies in order to establish the chemical reactions needed in terms of an accurate modeling of NO and NO2 dispersion and the CPU time required in a real urban area.

scholarly journals CFD Modeling of Reactive Pollutants Dispersion in Simplified Urban Configurations with Different Chemical Mechanisms

2016 ◽  
Author(s):  
Beatriz Sanchez ◽  
Jose-Luis Santiago ◽  
Alberto Martilli ◽  
Magdalena Palacios ◽  
Frank Kirchner
Keyword(s):  
Wind Speed ◽  
Chemical Reactions ◽  
Atmospheric Chemistry ◽  
Urban Air Pollution ◽  
Chemical Mechanism ◽  
Cfd Modeling ◽  
Urban Air ◽  
Large Set ◽  
Atmospheric Conditions ◽  
Cpu Time

Abstract. An accurate understanding of urban air quality requires considering a coupled behavior between dispersion of reactive pollutants and atmospheric dynamics. Currently, urban air pollution is mostly dominated by traffic emissions and the primary emitted pollutants are nitrogen oxides (NOx) and Volatile Organic Compounds (VOC). Modeling reactive pollutants with a large set of chemical reactions using a computational fluid dynamics (CFD) model requires a significant amount of CPU time. In this sense, the selection of the chemical reactions needed in different conditions that gives the best compromise between CPU time and accuracy becomes essential. Three chemical approaches are considered: a) passive tracers (non-reactive), b) the NOx – O3 photostationary state, and c) a more complex chemical mechanism based on the RACM ('Regional Atmospheric Chemistry Mechanism') and it is reduced to 23 species and 25 reactions using CHEMATA software (Kirchner, 2005). The appraisal of the effects of the chemical reactions is focused on the study of NO and NO2 dispersion and the comparison with the tracer behavior within the street. Taking into consideration the VOC reactions, various VOC/NOx emission ratios of traffic are studied. In addition, the concentration of reactive pollutants is affected by many atmospheric parameters. In this work, the effect of the amount of background O3 concentration depending on the season and different wind speeds are studied. Results show that the presence of ozone in the street acquires an important role in NO and NO2 dispersion. Thus, greater differences linked to the chemical approach used are founded with higher O3 concentration and faster wind speed. This is also related with the vertical flux as a function of ambient wind speed since it improves the pollutants exchange among the street and overlying air. The joint evaluation of both parameters allows to ascertain the atmospheric conditions as from which the importance of the chemical reactions on NO and NO2 concentration is significant. This is a detailed study that aims to understand the behavior of NO and NO2 as reactive pollutants in several atmospheric conditions. The conclusions can be applied to future researches in order to determine the chemical reactions needed in terms of accuracy in modeling NO and NO2 dispersion and the CPU time required in a real urban area.

3D CFD Modeling of a Biodiesel-Fueled Diesel Engine Based on a Detailed Chemical Mechanism

2012 ◽  
Author(s):  
Junfeng Yang ◽  
Monica Johansson ◽  
Chitralkumar Naik ◽  
Karthik Puduppakkam ◽  
Valeri Golovitchev ◽  
...  
Keyword(s):  
Diesel Engine ◽  
Chemical Mechanism ◽  
Cfd Modeling ◽  
Detailed Chemical

Strategies to Enhance the Performance of Gaussian Mixture Model Fitting for Uncertainty Quantification by Conditioning to Production Data

2021 ◽  
Author(s):  
Guohua Gao ◽  
Jeroen Vink ◽  
Fredrik Saaf ◽  
Terence Wells
Keyword(s):  
Gaussian Mixture Model ◽  
Mixture Model ◽  
History Matching ◽  
Gaussian Mixture ◽  
Model Parameters ◽  
Gaussian Component ◽  
Large Set ◽  
Cpu Time ◽  
Fitting Method ◽  
New Strategies

Abstract When formulating history matching within the Bayesian framework, we may quantify the uncertainty of model parameters and production forecasts using conditional realizations sampled from the posterior probability density function (PDF). It is quite challenging to sample such a posterior PDF. Some methods e.g., Markov chain Monte Carlo (MCMC), are very expensive (e.g., MCMC) while others are cheaper but may generate biased samples. In this paper, we propose an unconstrained Gaussian Mixture Model (GMM) fitting method to approximate the posterior PDF and investigate new strategies to further enhance its performance. To reduce the CPU time of handling bound constraints, we reformulate the GMM fitting formulation such that an unconstrained optimization algorithm can be applied to find the optimal solution of unknown GMM parameters. To obtain a sufficiently accurate GMM approximation with the lowest number of Gaussian components, we generate random initial guesses, remove components with very small or very large mixture weights after each GMM fitting iteration and prevent their reappearance using a dedicated filter. To prevent overfitting, we only add a new Gaussian component if the quality of the GMM approximation on a (large) set of blind-test data sufficiently improves. The unconstrained GMM fitting method with the new strategies proposed in this paper is validated using nonlinear toy problems and then applied to a synthetic history matching example. It can construct a GMM approximation of the posterior PDF that is comparable to the MCMC method, and it is significantly more efficient than the constrained GMM fitting formulation, e.g., reducing the CPU time by a factor of 800 to 7300 for problems we tested, which makes it quite attractive for large scale history matching problems.

scholarly journals The MCM v3.3 degradation scheme for isoprene

2015 ◽  
Vol 15 (6) ◽  
pp. 9709-9766 ◽  
Author(s):  
M. E. Jenkin ◽  
J. C. Young ◽  
A. R. Rickard
Keyword(s):  
Boundary Layer ◽  
Free Radical ◽  
Degradation Mechanism ◽  
Closed Shell ◽  
Chemical Mechanism ◽  
Formation Mechanisms ◽  
Oh Radicals ◽  
Atmospheric Conditions ◽  
Radical Species ◽  
Master Chemical Mechanism

Abstract. The chemistry of isoprene degradation in the Master Chemical Mechanism (MCM) has been systematically refined and updated to reflect recent advances in understanding, with these updates appearing in the latest version, MCM v3.3. The complete isoprene degradation mechanism in MCM v3.3 consists of 1935 reactions of 605 closed shell and free radical species, which treat the chemistry initiated by reaction with OH radicals, NO3 radicals and ozone (O3). A detailed overview of the updates is provided, within the context of reported kinetic and mechanistic information. The revisions mainly relate to the OH-initiated chemistry, which tends to dominate under atmospheric conditions, although these include updates to the chemistry of some products that are also generated from the O3 - and NO3-initiated oxidation. The revisions have impacts in a number of key areas, including HOx recycling, NOx recycling and the formation of species reported to play a role in SOA-formation mechanisms. The performance of the MCM v3.3 isoprene mechanism has been compared with those of earlier versions (MCM v3.1 and MCM v3.2) over a range of relevant conditions, using a box model of the tropical forested boundary layer. The results of these calculations are presented and discussed, and are used to illustrate the impacts of the mechanistic updates in MCM v3.3.

scholarly journals What climate signal is contained in decadal- to centennial-scale isotope variations from Antarctic ice cores?

2018 ◽  
Vol 14 (12) ◽  
pp. 2053-2070 ◽  
Author(s):  
Thomas Münch ◽  
Thomas Laepple
Keyword(s):  
Climate Variability ◽  
Ice Cores ◽  
Relative Magnitude ◽  
True Temperature ◽  
Large Set ◽  
Atmospheric Conditions ◽  
Temporal Scales ◽  
Simple Method ◽  
Climate Signal ◽  
Spatial And Temporal Scales

Abstract. Ice-core-based records of isotopic composition are a proxy for past temperatures and can thus provide information on polar climate variability over a large range of timescales. However, individual isotope records are affected by a multitude of processes that may mask the true temperature variability. The relative magnitude of climate and non-climate contributions is expected to vary as a function of timescale, and thus it is crucial to determine those temporal scales on which the actual signal dominates the noise. At present, there are no reliable estimates of this timescale dependence of the signal-to-noise ratio (SNR). Here, we present a simple method that applies spectral analyses to stable-isotope data from multiple cores to estimate the SNR, and the signal and noise variability, as a function of timescale. The method builds on separating the contributions from a common signal and from local variations and includes a correction for the effects of diffusion and time uncertainty. We apply our approach to firn-core arrays from Dronning Maud Land (DML) in East Antarctica and from the West Antarctic Ice Sheet (WAIS). For DML and decadal to multi-centennial timescales, we find an increase in the SNR by nearly 1 order of magnitude (∼0.2 at decadal and ∼1.0 at multi-centennial scales). The estimated spectrum of climate variability also shows increasing variability towards longer timescales, contrary to what is traditionally inferred from single records in this region. In contrast, the inferred variability spectrum for WAIS stays close to constant over decadal to centennial timescales, and the results even suggest a decrease in SNR over this range of timescales. We speculate that these differences between DML and WAIS are related to differences in the spatial and temporal scales of the isotope signal, highlighting the potentially more homogeneous atmospheric conditions on the Antarctic Plateau in contrast to the marine-influenced conditions on WAIS. In general, our approach provides a methodological basis for separating local proxy variability from coherent climate variations, which is applicable to a large set of palaeoclimate records.

scholarly journals Investigation of processes controlling GEM oxidation at mid-latitudinal marine, coastal, and inland sites

2016 ◽  
Author(s):  
Z. Ye ◽  
H. Mao ◽  
C.-J. Lin ◽  
S. Y. Kim
Keyword(s):  
Marine Boundary Layer ◽  
State Of The Art ◽  
Elemental Mercury ◽  
Atmospheric Mercury ◽  
Chemical Mechanism ◽  
Atmospheric Conditions ◽  
Gaseous Elemental Mercury ◽  
Mixing Ratios ◽  
Marine Site ◽  
Study Sites

Abstract. A box model incorporating a state-of-the-art chemical mechanism for atmospheric mercury (Hg) cycling was developed to investigate oxidation of gaseous elemental mercury (GEM) at three locations in the northeastern United States: Appledore Island (marine), Thompson Farm (coastal, rural), and Pack Monadnock (inland, rural, elevated). The chemical mechanism improved model's ability to simulate the formation of gaseous oxidized mercury (GOM) at the study sites. At the coastal and inland sites, GEM oxidation was predominated by O3 and OH, contributing 80–99 % of total GOM production during daytime. H2O2 initiated GEM oxidation was significant (~ 33 % of the total GOM) at the inland site during nighttime. In the marine boundary layer (MBL), Br and BrO were dominant GEM oxidants contributing ~ 70 % of the total GOM production during mid-day, while O3 dominated GEM oxidation (50–90 % of GOM production) over the remaining day. Following the production of HgBr from GEM + Br, HgBr was oxidized by BrO, HO2, OH, ClO, and IO to form Hg(II) brominated GOM species. However, under atmospheric conditions, the prevalent GEM oxidants in the MBL could be Br / BrO or O3 / OH depending on Br and BrO mixing ratios. Relative humidity and products of the CH3O2 + BrO reaction possibly affected significantly the mixing ratios of Br or BrO radicals and subsequently GOM formation. Gas-particle partitioning could be potentially important in the production of GOM as well as Br and BrO at the marine site.

scholarly journals Understanding nighttime methane signals at the Amazon Tall Tower Observatory (ATTO)

2020 ◽  
Vol 20 (11) ◽  
pp. 6583-6606
Author(s):  
Santiago Botía ◽  
Christoph Gerbig ◽  
Julia Marshall ◽  
Jost V. Lavric ◽  
David Walter ◽  
...  
Keyword(s):  
Thermal Stratification ◽  
Sensible Heat Flux ◽  
Source Location ◽  
Sensible Heat ◽  
Net Radiation ◽  
Atmospheric Conditions ◽  
Wind Speeds ◽  
Mixing Ratios ◽  
Diurnal Patterns ◽  
Tall Tower

Abstract. Methane (CH4) atmospheric mixing ratio measurements are analyzed for the period between June 2013 and November 2018 at the Amazon Tall Tower Observatory (ATTO). We describe the seasonal and diurnal patterns of nighttime events in which CH4 mixing ratios at the uppermost (79 m a.g.l.) inlet are significantly higher than the lowermost inlet (4 m a.g.l.) by 8 ppb or more. These nighttime events were found to be associated with a wind direction originating from the southeast and wind speeds between 2 and 5 m s−1. We found that these events happen under specific nighttime atmospheric conditions when compared to other nights, exhibiting less variable sensible heat flux, low net radiation and a strong thermal stratification above the canopy. Our analysis indicates that even at wind speeds of 5.8 m s−1 the turbulence intensity, given by the standard deviation of the vertical velocity, is suppressed to values lower than 0.3 m s−1. Given these findings, we suggest that these nighttime CH4 enhancements are advected from their source location by horizontal nonturbulent motions. The most likely source location is the Uatumã River, possibly influenced by dead stands of flooded forest trees that may be enhancing CH4 emissions from those areas. Finally, biomass burning and the Amazon River were discarded as potential CH4 sources.

scholarly journals Evaluating Temperature Measurements of the iMET-XQ, in the Field, under Varying Atmospheric Conditions

Atmosphere ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 335 ◽  
Author(s):  
Sytske K. Kimball ◽  
Carlos J. Montalvo ◽  
Madhuri S. Mulekar
Keyword(s):  
Wind Speed ◽  
Solar Radiation ◽  
Aluminum Coating ◽  
Temperature Measurements ◽  
Atmospheric Conditions ◽  
Radiation Shield ◽  
Wind Speeds ◽  
Outdoor Setting ◽  
Accurate Temperature ◽  
Reference Sensor

Temperature measurements of InterMET Inc. aluminum-coated iMET-XQ sensors were tested in an outdoor setting under a variety of solar radiation and wind speed conditions. Twelve unshielded sensors were mounted side-by-side on the tower of a South Alabama Mesonet weather station next to a reference sensor on the tower. The iMET-XQ temperatures were most precise and accurate in solar radiation values that were close to zero, regardless of wind speed. Under overcast conditions, wind speeds of 2 m s−1 were sufficient to obtain precise and accurate temperature measurements. During the day-time, aspiration of wind speeds higher than or equal to 3 m s−1 is sufficient. An iMET-XQ was placed in a radiation shield next to the tower reference sensor to test the need for a radiation shield. A second iMET-XQ was placed unshielded on the tower. The iMET-XQ sensors with aluminum coating do not need to be shielded, but they do need to be aspirated. It is recommended that, when taking temperature measurements using unmanned aerial vehicles (UAV) with iMET-XQ sensors, the UAV either fly at 3 m s−1, be embedded in winds of those speeds, or to use the propeller wash of the UAV to aspirate the sensors.

scholarly journals Simulating regional scale secondary organic aerosol formation during the TORCH 2003 campaign in the southern UK

2006 ◽  
Vol 6 (2) ◽  
pp. 403-418 ◽  
Author(s):  
D. Johnson ◽  
S. R. Utembe ◽  
M. E. Jenkin ◽  
R. G. Derwent ◽  
G. D. Hayman ◽  
...  
Keyword(s):  
Secondary Organic Aerosol ◽  
Regional Scale ◽  
Organic Aerosol ◽  
Chemical Mechanism ◽  
Background Concentration ◽  
Aerosol Formation ◽  
Organic Species ◽  
Voc Oxidation ◽  
Partitioning Coefficients

Abstract. A photochemical trajectory model has been used to simulate the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread and prolonged photochemical pollution episode. The model incorporates speciated emissions of 124 non-methane anthropogenic VOC and three representative biogenic VOC, coupled with a comprehensive description of the chemistry of their degradation. A representation of the gas/aerosol absorptive partitioning of ca. 2000 oxygenated organic species generated in the Master Chemical Mechanism (MCM v3.1) has been implemented, allowing simulation of the contribution to organic aerosol (OA) made by semi- and non-volatile products of VOC oxidation; emissions of primary organic aerosol (POA) and elemental carbon (EC) are also represented. Simulations of total OA mass concentrations in nine case study events (optimised by comparison with observed hourly-mean mass loadings derived from aerosol mass spectrometry measurements) imply that the OA can be ascribed to three general sources: (i) POA emissions; (ii) a "ubiquitous" background concentration of 0.7 µg m-3; and (iii) gas-to-aerosol transfer of lower volatility products of VOC oxidation generated by the regional scale processing of emitted VOC, but with all partitioning coefficients increased by a species-independent factor of 500. The requirement to scale the partitioning coefficients, and the implied background concentration, are both indicative of the occurrence of chemical processes within the aerosol which allow the oxidised organic species to react by association and/or accretion reactions which generate even lower volatility products, leading to a persistent, non-volatile secondary organic aerosol (SOA). The contribution of secondary organic material to the simulated OA results in significant elevations in the simulated ratio of organic carbon (OC) to EC, compared with the ratio of 1.1 assigned to the emitted components. For the selected case study events, [OC]/[EC] is calculated to lie in the range 2.7-9.8, values which are comparable with the high end of the range reported in the literature.

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