Initial NOx Concentration Research Articles

Theoretical implication of reversals of the ozone weekend effect systematically observed in Japan

Abstract. Systematic changes of the ozone weekend effect are found over broad areas of Japan. These changes are characterized by (1) spatial reversals from a weekend increase in the vicinity of huge precursor source areas to a weekend decrease in the surrounding rural areas, and (2) temporal reversals from a weekend increase under relatively unsuitable meteorological conditions for ozone formation to a weekend decrease under relatively suitable conditions. We developed a simple numerical advection–reaction model to explain the relationship between the duration of advection and the supplied solar energy, which causes the daily maximum ozone concentration to be lower near the precursor source. Ozone isopleth diagrams for individual advection durations (equivalent to the distance from the source) for a wide range of initial precursor conditions show that both VOC-limited and NOx-limited regimes exist for each advection duration, but the area of NOx-limited regime becomes dominant as the advection duration increases because of the increased exposure of the air mass to solar energy. For given initial VOC and NOx concentrations, the area remote from the source becomes a NOx-limited regime even if the precursor source area is in the VOC-limited regime. The rate of reduction of weekend emissions of NOx is larger than that of VOC, causing a weekend increase in ozone inside an area of VOC-limited regime near the source, but a weekend decrease in remote areas with a NOx-limited regime. The boundary between these two ozone formation regimes depends on meteorological conditions: when sunlight intensity and temperature are relatively low, the change from a VOC-limited to a NOx-limited regime occurs at a point more remote from the source than when they are relatively high, which causes a prevailing ozone weekend increase over a wide geographical area on days with lower ozone potential. Therefore, observations of ozone weekend changes can be interpreted in light of the theoretical implications of our model; they can be used for determination of ozone formation regimes, which change in different locations and under different meteorological conditions.

Secondary Organic Aerosol Formation during the Photooxidation of Toluene: NOxDependence of Chemical Composition

The photooxidation of toluene is a potential source of secondary organic aerosol (SOA) in urban air, but only a small portion of the compounds present in SOA have been identified. In this study, we analyzed the chemical compositions of SOA produced by photoirradiation of the toluene/NOx/air system in laboratory chamber experiments by a combination of liquid chromatography-mass spectrometry, hybrid high-performance liquid chromatography-mass spectrometry, and iodometry-spectrophotometry. The dependence of the chemical composition on the initial NOx concentration was examined at initial NO concentrations ([NO]0) of 0.2 and 1 ppmv. Fifteen semivolatile products, including aromatic and ring-cleavage compounds, were quantified. However, the quantified products comprised only a small portion ( approximately 1 wt %) of the total aerosol mass. The total SOA yield ( approximately 13 wt %), the ratio of organic peroxides to total SOA mass ( approximately 17 wt %), and the density of SOA ( approximately 1.4 g cm-3) were independent of the NOx level, suggesting that the reaction mechanisms of the formation of major SOA products at [NO]0 = 0.2 and 1 ppmv are essentially the same. The negative-ion mass spectra of SOA samples showed that ion signals attributed to hemiacetal oligomers and/or decomposition products of peroxy hemiacetal oligomers were detected in the range of mass-to-charge ratios (m/z) between 200 and 500. The highest signals were detected at m/z = 155 and 177, and these were tentatively assigned to C7 unsaturated oxacyclic oxocarboxylic acids and C7 unsaturated oxacyclic dicarboxylic acids, respectively. We conclude that the major chemical components of the aerosol are hemiacetal and peroxy hemiacetal oligomers and low-molecular-weight dicarboxylic acids.

A scaling analysis of ozone photochemistry

Abstract. A scaling analysis has been used to capture the integrated behaviour of several photochemical mechanisms for a wide range of precursor concentrations and a variety of environmental conditions. The Buckingham Pi method of dimensional analysis was used to express the relevant variables in terms of dimensionless groups. These grouping show maximum ozone, initial NOx and initial VOC concentrations are made non-dimensional by the average NO2 photolysis rate (jav) and the rate constant for the NO–O3 titration reaction (kNO); temperature by the NO–O3 activation energy (ENO) and Boltzmann constant (k) and total irradiation time by the cumulative javΔt photolysis rate. The analysis shows dimensionless maximum ozone concentration can be described by a product of powers of dimensionless initial NOx concentration, dimensionless temperature, and a similarity curve directly dependent on the ratio of initial VOC to NOx concentration and implicitly dependent on the cumulative NO2 photolysis rate. When Weibull transformed, the similarity relationship shows a scaling break with dimensionless model output clustering onto two straight line segments, parameterized using four variables: two describing the slopes of the line segments and two giving the location of their intersection. A fifth parameter is used to normalize the model output. The scaling analysis, similarity curve and parameterization appear to be independent of the details of the chemical mechanism, hold for a variety of VOC species and mixtures and a wide range of temperatures and actinic fluxes.

Treatment on Low NOx Concentration and DEP Collection with Barrier Discharge System Superposing TiO2

The automobiles exhaust carbon particles and NOx into the air, and then gases accumulate in expressway tunnels. DC corona discharge is used widely in electrostatic precipitator (ESP). However, the ESP with DC corona discharge can little remove NOx. Therefore, we proposed a new barrier discharge type precharger to replace corona discharge type prechargers to remove NOx. When discharge is generated in the air, NOx necessarily is generated. Then, we need to grasp the lower limit NOx concentration to remove NOx from the gas. We focus attention on photocatalyst because it is effective in NOx removal. However, photocatalyst needs ultraviolet ray. Therefore, TiO2 is painted on barrier discharge electrodes, an ultraviolet ray included in discharge luminescence was irradiated to TiO2. Furthermore, the relationship between initial NOx concentration and removal characteristic is investigated in detail.

Secondary Organic Aerosol Formation during the Photo-Oxidation of Toluene: Dependence on Initial Hydrocarbon Concentration

Abstract Aerosol formation in toluene–NOx–air–photoirradiation systems was examined in a 6-m3 photochemical reaction chamber. The initial toluene concentration was varied between 2 and 16 ppmv, whereas the initial NOx concentration was kept constant. The aerosol mass concentrations were plotted as a function of the concentration of toluene reacted. The aerosol profiles showed a curved feature at low toluene consumptions, and merged into a single line at longer reaction times. The observed characteristics were explained by assuming condensable products were formed through reactions of ozone with primary toluene photo-oxidation products.

Application of SNCR/SCR Combined process for effective operation of SCR Process

This paper have examined the optimum combination of SNCR and SCR by varying SNCR injection temperature and NSR ratio along with SCR space velocity. NOx reduction experiments using a SNCR/SCR combined process have been conducted in simple NO/NH<TEX>$_3$</TEX>/O<TEX>$_2$</TEX> gas mixtures. Total gas flow rate was kept constant 4 liter/min throughout the SNCR and SCR reactors, where initial NOx concentration was 500 ppm in the presence of 5% O<TEX>$_2$</TEX>. Commercial catalyst, sulfated V<TEX>$_2$</TEX>O<TEX>$\_$</TEX>5/-WO<TEX>$_3$</TEX>/TiO<TEX>$_2$</TEX>, was used for SCR NOx reduction. The residence time and space velocity were around 1.67 sec, 2,400 h<TEX>$\^$</TEX>-1/ and 6,000 h<TEX>$\^$</TEX>-1/ in the SNCR and SCR reactors, respectively. SNCR NOx reduction effectively occurred in a temperature window of 900-950<TEX>$^{\circ}C$</TEX>. About 88% NOx reduction was achieved with an optimum temperature of 950<TEX>$^{\circ}C$</TEX> and NSR=1.5. SCR NOx reduction using commercial V<TEX>$_2$</TEX>O<TEX>$\_$</TEX>5/-WO<TEX>$_3$</TEX>-SO<TEX>$_4$</TEX>/TiO<TEX>$_2$</TEX> catalyst occurred in a temperature window of 200-450<TEX>$^{\circ}C$</TEX> 80-98% NOxreduction was possible with SV=2400 h<TEX>$\^$</TEX>-1/ and a molar ratio of 1.0-2.0. A SNCR/SCR(SV=6000 h<TEX>$\^$</TEX>-1/) combined process has shown same NOx reduction compared with a stand-alone SCR(SV=2400 h<TEX>$\^$</TEX>-1/) unit process of 98% NOx reduction. The NH<TEX>$_3$</TEX>-based chemical could routinely achieve SNCR/SCR combined process total NOx reductions of 98% with less than 5 ppm NH<TEX>$_3$</TEX> slip at NSR ranging from about 1.5 to 2.0, SNCR temperature of 900<TEX>$^{\circ}C$</TEX>-950<TEX>$^{\circ}C$</TEX>, and SCR space velocity of 6000 h<TEX>$\^$</TEX>-1/. Particularly, more than 98% NOx reduction was possible using the combined process under the conditions of T<TEX>$\_$</TEX>SNCR/=950<TEX>$^{\circ}C$</TEX>, T<TEX>$\_$</TEX>SCR/=350<TEX>$^{\circ}C$</TEX>, 5% O<TEX>$_2$</TEX>, SV=6000 h<TEX>$\^$</TEX>-1/ and NH<TEX>$_3$</TEX>/NOx=1.5. A catalyst volume was about three times reduced by SNCR/SCR combined process compared with SCR process under the same controlled conditions.

Evolution and chemical consequences of lightning‐produced NOx observed in the North Atlantic upper troposphere

Airborne observations of NO during the Subsonics Assessment Ozone and Nitrogen Oxides Experiment (SONEX) reveal episodes of high NOx in the upper troposphere believed to be associated with lightning. Linkage to specific periods of lightning activity is possible through back trajectories and data from the National Lightning Detection Network. Lagrangian model calculations are used to explore the evolution of these high NOx plumes over the 1–2 days between their introduction and subsequent sampling by NASA's DC‐8 aircraft. Simulations include expected changes in HNO3, H2O2, CH3OOH, HO2, and OH. Depending on the time of injection and dilution rate, initial NOx concentrations are estimated to range from 1 to 7 ppbv. Similar to many previous studies, simulated HNO3 concentrations tend to be greater than observations. Several possible explanations for this difference are explored. H2O2 observations are shown to be consistent with removal in convective activity. While it is possible that upper tropospheric CH3OOH is enhanced by convection, simulations show such increases in CH3OOH can be short‐lived (e.g., &lt;12 hours) with no perceptible trace remaining at the time of sampling. High NO levels further prevent elevated levels of CH3OOH from propagating into increases in H2O2. HO2 is suppressed through reaction with NO in all cases. Simulated increases in OH exceeded a factor of 2 for some cases, but for the highest NOx levels, loss of OH via OH+NO2 offset production from HO2+NO. Additional increases in OH of 30–60% could result from convection of CH3OOH. A final point of discussion concerns how the chemistry within these plumes, their long‐range transport, and their potential importance in sustaining background NOx far from source regions present a challenge to global and regional model simulations.

Atmospheric Oxidation of Toluene in a Large-Volume Outdoor Photoreactor: In Situ Determination of Ring-Retaining Product Yields

Experiments on the photooxidation of toluene/NOx/air mixtures were performed in the European Photoreactor (EUPHORE), a large-scale outdoor reaction chamber located in Valencia/Spain. The objective of the study was the in situ determination of the yields of ring-retaining products by differential optical absorption spectroscopy (DOAS) and the elucidation of their formation pathways. The experiments were performed with toluene concentrations between 0.68 and 3.85 ppm and initial NOx concentrations ranging from 3 to 300 ppb, i.e., down to the range actually observed in the lower atmosphere. The ring-retaining product yields were found to be 5.8 ± 0.8%, 12.0 ± 1.4%, 2.7 ± 0.7%, and 3.2 ± 0.6% for benzaldehyde, o-cresol, m-cresol, and p-cresol, respectively. Under the experimental conditions, no dependency of the yields on the NOx concentration or the toluene/NOx ratio could be found. The formation kinetics of the cresols are in line with a “prompt” formation mechanism, i.e., abstraction of a hydrogen atom from the toluene−OH adduct (toluene−OH + O2 → cresols + HO2). In addition, substantial evidence was found that reaction with NO3 radicals represents an important sink for cresols in smog chamber studies conducted under conditions of NOx concentrations above the range observed in the troposphere, possibly also under tropospheric conditions.

The Reduction of NOx from the Diesel Engine Exhaust Gas by Superimposing Barrier Discharges

The reduction of NOx from diesel engine exhaust gas by superimposing barrier discharge plasma reactors is experimentally investigated. The experiments are conducted for the applied voltages from 0 to 24 kV, electrode gap distance from l.5 to 5.0 mm, and the flow rates from 0.2 to 2 L/min at initial NOx concentration 1,700 ppm. The results show that NOx reduction increases with increasing discharge power and electrode gap distance. The superimposing barrier discharges under out-of-phase operation mode shows best result, where maximum NOx reduction rate is approximately 60% at operating electric power 10 W.

Translated from Nenshonō Kagaku to Gijutsu, 3, 229–236 (1995) Study on NOx Reduction in Reburning†

NOx reduction in reburning was experimentally and numerically investigated. In the experiments, NOx reduction in simulated exhaust gas was confirmed in a flow reactor system and the concentrations of intermediate products such as HCN and N2O were measured. By controlling the temperature and air ratio, NOx reducing reactions proceeded efficiently and a 40% reduction of NOx was observed when the initial NOx concentration was 100 ppm. The calculated results agreed with the experimental results very well. Reaction analysis showed that for the HCN reducing reactions in the burn-out zone, two temperature regions were important, and in these regions HCN was reduced to N2O and finally to N2 even in an oxidizing atmosphere.

Effects of NH3 on N2O formation and destruction in fluidized bed coal combustion

The NH3 oxidation and reduction process are experimentally and kinetically studied in this paper. It is found that NH3 has contributions not only to N2O formation, but also to N2O destruction in certain conditions. The main product of homogeneous NH3 oxidation is found to be NO rather than N2O, but some bed materials and sulphur sorbents have catalytic contributions to N2O formation from NH3 oxidation. In reduction atmosphere, NH3 can promote the KC destruction. It is deduced that the ammonia injection into fluidized bed coal combustion flue gas can decrease both NOx and N2O emissions. The ammonia injection process is kinetically simulated in this study, and the reduction rates of NOx and N2O are found to depend on temperature, O2 concentration, initial NOx and N2O concentrations, and amount of injected ammonia.

Photochemical aerosol formation from α‐pinene‐ and β‐pinene

Aerosol formation from α‐pinene and β‐pinene was studied in a series of outdoor smog chamber experiments. Since a previous study focused on β‐pinene (Pandis et al., 1991), more attention was given here to α‐pinene. The initial hydrocarbon and NOx concentrations ranged from 37 to 582 ppb and 31 to 380 ppb, respectively. The aerosol carbon yield, the fraction of the carbon initially present that is converted to aerosol, varied from 0 to 5.3% for α‐pinene, depending on the initial hydrocarbon‐to‐NOx ratio. Dual‐bag experiments demonstrate that α‐pinene is more rapidly photooxidized, and produces higher yields of both aerosol and ozone in a given period of time than β‐pinene, although given sufficient time β‐pinene can produce equivalent aerosol yields. Although aerosol formation solely from isoprene photooxidation was found in a previous study to be negligible under ambient conditions, the addition of isoprene to the α‐pinene/NOx system leads to an increased aerosol yield through the enhanced photochemical activity generated.

Effect of Hydrocarbon and NOxon Photochemical Smog Formation under Simulated Transport Conditions

The body of information presented in this paper is directed to those individuals concerned with the effect of urban pollution on downwind areas. Concern has been expressed over the appropriate hydrocarbon and NOX control strategy to be used in minimizing the effects of ozone and NO2 on urban population centers and their downwind environs. O3 and NO2 formation were studied in smog chamber irradiations as a function of the initial NOX concentration at three hydrocarbon concentrations. By carrying out the irradiations for a period of time equivalent to one solar day in a continuously diluting system, smog formation in a chemically reacting pollutant system under transport was simulated. The results of this experimental simulation suggest that hydrocarbon reduction reduces O3 in urban as well as downwind areas while NOX reduction increases O3 in the urban area and has little effect on O 3 in downwind areas. Both hydrocarbon and NO* reduction will reduce atmospheric NO2 levels, with the effect of NO X reduction generally being more pronounced. In a previous study,1 the effect of NOX control on local and downwind oxidant formation was determined under conditions simulating transport of a reacting air mass. The study was performed at a constant surrogate hydrocarbon mixture concentration of 5.1 ppm C which is representative of a heavily polluted Los Angeles atmosphere today. Since hydrocarbon controls will reduce this worst case concentration, it is desirable to determine the effect of NOX control on O3 formation at lower hydrocarbon concentrations representative of future atmospheres. In the present study, O3 formation was studied at surrogate hydrocarbon mixture concentrations of 5.1, 2.7, and 1.4 ppm C, while NOX was varied from about 0.01 to 0.49 ppm. The effect of these variables on NO2 formation was also studied. Hydrocarbon mixture concentrations and compositions were also determined during the course of most irradiations.