Local NO Concentration Research Articles

Nitrate formation mechanisms causing high concentration of PM2.5 in a residential city with low anthropogenic emissions during cold season

During the cold season in South Korea, NO3− concentrations are known to significantly increase, often causing PM2.5 to exceed air quality standards. This study investigated the formation mechanisms of NO3− in a suburban area with low anthropogenic emissions. The average PM2.5 was 25.3 μg m−3, with NO3− identified as the largest contributor. Ammonium-rich conditions prevailed throughout the study period, coupled with low atmospheric temperature facilitating the transfer of gaseous HNO3 into the particulate phase. This result indicates that the formation of HNO3 played a crucial role in determining particulate NO3− concentration. Nocturnal increases in NO3− were observed alongside increasing ozone (O3) and relative humidity (RH), emphasizing the significance of heterogeneous reactions involving N2O5. NO3− concentrations at the study site were notably higher than in Seoul, the upwind metropolitan area, during a high concentration episode. This difference could potentially attributed to lower local NO concentrations, which enhanced the reaction between O3 and NO2, to produce NO3 radicals. High concentrations of Cl− and dust were also identified as contributors to the elevated NO3− concentrations.

Characterizing turbulent non-premixed flame structure and pollutant formation of cracked ammonia jet flames using simultaneous NH and NO PLIF

The combustion of cracked ammonia is crucial for enhancing flame stability and reducing pollutant emissions in ammonia-based combustion systems. This study investigated the flame structure and pollutant formation in cracked ammonia jet flames (CAJFs) to improve our understanding of the turbulence-chemistry interactions in ammonia combustion. A simultaneous NH and NO planar laser-induced fluorescence (PLIF) technique was employed to analyze the flame structure of turbulent non-premixed CAJFs, emulated using ammonia-hydrogen–nitrogen mixtures. Experimental measurements were conducted across a range of pressures (1–5 bar) and cracking ratios (7 %-28 %). The results revealed the significant influence of the cracking ratio on the chemical reactivity and NH-layer characteristics of ammonia-hydrogen flames. NH effectively marked the heat release layer of CAJFs. Reducing the cracking ratio from 28 % to 7 % resulted in increased local extinction-induced NH-layer fragmentation and reduced reaction area. The reaction layer thickness exhibited low sensitivity to the cracking ratio and pressure, while the broadening of the NH layer displayed a slowdown pattern with increasing height, differing from premixed flames. The pollutant NO rapidly formed within the reaction zone, persisting in the outer hot products until reduced by turbulent mixing. The fragmented flame structure promoted decreased NO formation and potentially lower NO concentration. A novel approach of multiplying NH and NO was proposed to reflect the formation characteristics of another important pollutant N2O. The effect of turbulent disturbances on N2O formation needs to be considered in turbulent CAJFs because N2O formation is strongly suppressed when turbulent transport reduces local NO concentration. This research enhances understanding of turbulence-chemistry interactions in cracked ammonia combustion, benefiting ammonia flame stability and pollutant emission control.

Inhaled nitric oxide: well-known and new therapeutic indications

The use of nitric oxide is widespread in medical practice. Inhaled form of NO in patients with respiratory disease is especially interesting. As an inhalation therapy, nitric oxide reaches well-ventilated areas of the lungs, having a negligible systemic effect due to its rapid inactivation in the bloodstream. It has long been known that inhaled NO improves oxygenation in various pathological conditions. Low doses of inhaled nitric oxide are known as a vasodilator in patients with pulmonary hypertension, its use is possible even in newborns with hypoxemic respiratory failure and persistent pulmonary hypertension. In turn, high doses are used in the treatment of infectious lung diseases, besides, another surge of scientific interest in nitric oxide arose during the coronavirus pandemic. Although the antimicrobial potential of nitric oxide is widely known, it is little used in clinical practice. Endogenous antimicrobial activity is largely mediated by high local concentrations of NO. Numerous preclinical studies have demonstrated that NO has general static and bactericidal activity against viruses, bacteria, protozoa and fungi/yeast in vitro. Due to the dose-dependent positive and negative effects of NO, safety and efficacy tests of NO and its donors are needed to assess their role in the prevention and treatment of infections. This review provides information on the chemical structure, features of metabolism and delivery of inhaled NO, and also pays special attention to the antimicrobial effect in various diseases of the pulmonological profile.

Ozone in the boreal forest in the Alberta Oil Sands Region

Abstract. Measurements of ozone were made using an instrumented tower and a tethersonde located in a forested region surrounded by oil sands production facilities in the Athabasca Oil Sands Region (AOSR). Our observations and modeling show that the concentration of ozone was modified by vertical mixing, photochemical reactions, and surface dry deposition. Measurements on the tower demonstrated that when winds are from the direction of anthropogenic emissions from oil sand extraction and processing facilities, there is no significant increase in ozone mixing ratio compared to when winds are from the direction of undisturbed forest. This suggests that ozone is destroyed by reaction with NOx from oil sands extraction operations (as well as NO resulting from photolysis of NO2). Vertical gradients of ozone mixing ratio with height were observed using instruments on a tethered balloon (up to a height of 300 m) as well as a pulley system and two-point gradients within the canopy. Strong gradients (ozone increasing with height near 0.35 ppb m−1) were measured in the canopy in the evening and overnight, while morning and daytime gradients were weaker and highly variable. A 1D canopy model was used to simulate the diurnal variation of the in-canopy gradient. Model results suggest an ozone dry deposition velocity between 0.2 and 0.4 cm s−1 for this location. Sensitivity simulations using the model suggest that the local NO concentration profile and coefficients of vertical diffusivity have a significant influence on the O3 concentrations and profiles in the region.

LES Simulation of a Model Gas-Turbine Lean Combustor: Impact of Coherent Flow Structures on the Temperature Field and Concentration of CO and NO

The present paper reports on the numerical simulation of flow dynamics in a model gas-turbine combustor by large eddy simulation in order to evaluate the effect of coherent flow structures on the local fluctuations of gas temperature and local concentrations of NO and CO. The simulations were performed for a generic swirler, based on the design by Turbomeca, for a Reynolds number of 15,000 at normal and elevated inlet temperature and pressure (up to 500 K and 3.4 atm). The simulation data were validated based on the velocity measurements by stereoscopic particle image velocimetry. In order to reveal coherent flow structures, the velocity snapshots were processed by the proper orthogonal method. The temporal coefficients of the decomposition were used to evaluate the conditional sampled spatial distributions of the temperature and species concentration. It is shown that the coherent fluctuations of temperature can locally reach up to 200 K with the fluctuations of NO up to 20%. Thus, the results demonstrate that coherent flow structures in a lean swirl combustor can sufficiently contribute to NOx emission.

Emission prediction and analysis on CH4/NH3/air swirl flames with LES-FGM method

Ammonia, made up of 17.8% hydrogen, is one of carbon free fuels which can be used in large-scale power plants. However, ammonia combustion faces some challenges due to the high NOx emission. This study investigates the emission characteristics of CH4/NH3/air co-firing premixed swirling flames with NH3 mole fraction up to 60% in a model combustor. Large eddy simulation (LES) coupled with flamelet generated manifold (FGM) model which considers the transport equations of slow chemical emission component was performed, with experimental validation in both flame structure and global emission. The simulation shows an overall good agreement with experimental results. It is found that the NO emission is improved with the NO transport equation calculated during the simulation. The results imply that the nitrogenous components, residence time and temperature are important factors that influence NO concentration profile. The residence time of inner recirculation indicates the NO level for CH4/NH3/air flames, and local high NO concentration is mainly caused by local long residence time. The OH and NO are correlative in premixed CH4/NH3/air flames because these components are both strongly related to the temperature. N2O has a correlation with NH and HNO components. The radical components such as HNO, O, OH etc. also influence the emission formation process. When the NH3 ratio is closed to 40%, the concentration of H, OH, O2 and HNO are all adequate to produce maximum NO.

Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH4)/ammonia (NH3)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH3 (ENH3) in CH4/NH3 mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH4/air and NH3/air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (ϕ) of 1.3 to 1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH4/NH3/air flames indicated that the local temperature and NO and OH concentration distributions with local ϕ were qualitatively similar to those in NH3/air flames. That is, the maximum local NO and OH concentrations appeared at local ϕ of 0.9, although the maximum temperature was observed at local ϕ of 1.0. Both the temperature and OH concentration were found to gradually decrease with the partial replacement of CH4 with NH3. Consequently, NO emissions from CH4/NH3 flames were maximized at ENH3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH4/NH3 flames increased exponentially with temperature, which was not observed in NH3/air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH4/NH3 flames was occurred at a temperature slightly below the maximum temperature, just as in NH3/air flames. The apparent exponential increase in NO emissions from CH4/NH3 flames is attributed to a similar trend in the OH concentration at high temperatures.

The temperature dependence of the laminar burning velocity and superadiabatic flame temperature phenomenon for NH3/air flames

Combustion of ammonia (NH3) as a carbon-free alternative fuel has been recently widely studied, with vast majority of the burning velocity data obtained at room temperature. In the present study, the laminar burning velocity SL of NH3/air mixtures has been measured at unburnt gas temperature Tu from 298 K to 448 K, covering equivalence ratios from 0.85 to 1.25 and at 1 atm using the heat flux method. Kinetic simulations were made with five literature mechanisms developed for NH3 combustion, i.e., Nakamura et al., Otomo et al., San Diego, Okafor et al., and Mei et al. mechanisms, and the influence of radiation heat losses was considered. Using the obtained burning velocity data at different temperatures, the temperature dependence coefficients α in SLSL0=(TuTu0)α were derived, and compared with different models’ predictions. Further analyses of the temperature dependence of SL were carried out through examination of the overall activation energy, temperature and species profiles as well as the reaction paths, and a unique flame structure at the rich side of adiabatic NH3/air flames was found, which resembles ‘over-rich’ phenomena in hydrocarbon flames. At equivalence ratio larger than 1.1 ± 0.05, the NH3/air flames become so rich that (1) the NH2 radical overwhelms the H and OH radicals in maximum mole fraction; (2) after the flame front, H2O converts back to H2 with NO formed at the same time, causing the superadiabatic flame temperature phenomena, i.e. adiabatic flame temperature being lower than the maximum achieved in the flame. Moreover, local minimum NO concentration is found right after the over-rich NH3/air flame front, which may be helpful in reducing NO emissions from NH3 flames in practical applications.

Emission characteristics of turbulent non-premixed ammonia/air and methane/air swirl flames through a rich-lean combustor under various wall thermal boundary conditions at high pressure

This study is dedicated to understanding the combustion and emission characteristics of turbulent non-premixed ammonia (NH3)/air and methane (CH4)/air swirl flames in a rich-lean gas turbine−like combustor at high pressure under various wall thermal boundary conditions. In this study, the emission characteristics of both flames were obtained through numerical simulations using large−eddy simulations with the finite-rate chemistry technique. In addition, for NH3/air flames, simultaneous NO and OH planar laser−induced fluorescence (PLIF) images were acquired in order to qualitatively verify the numerical results. The results show that the minimum NO emission could be obtained when the primary zone global equivalence ratio (ϕglobal/pri) was 1.1, irrespective of the wall thermal condition, using a rich-lean combustor in NH3/air flames, whereas in CH4/air flames, the maximum NO emission was obtained with a ϕglobal/pri value of 1.0. Moreover, in NH3/air flames, the local NO concentration is largely dependent on the local OH concentration, whereas in CH4/air flames, the local NO concentration is largely dependent on the local temperature. The NO−OH correlation in NH3/air flames was experimentally verified using simultaneous NO and OH PLIF images. Additionally, it was found that, in NH3/air flames, the wall heat losses due to the combustor wall cooling greatly affected the NH3 oxidation and led to significant emissions of unburnt NH3, although lower NO emission resulted from the combustor wall cooling. This was primarily because of the lower OH concentration level in the flame region owing to the wall heat losses.

Modelling of ammonia/air non-premixed turbulent swirling flames in a gas turbine-like combustor at various pressures

This study simulated bluff body stabilised non-premixed turbulent ammonia (NH3)/air flames using swirling flows. Although the space and time-averaged emission (STAE) of NO was found to decrease with increase in the global equivalence ratio (global ) under stoichiometric and rich conditions, similar to the behaviour of premixed flames, the local NO concentration distribution within the combustor was heterogeneous, unlike the almost uniform NO distribution in premixed flames. Zones of high NO concentration were identified near the combustor wall boundaries, whereas there was almost nil NO in the combustor centre, irrespective of the global . The localised NO concentrations in non-premixed flames were shown to depend on the local within the combustor and the model reproduced the NO emission characteristics of premixed flames in terms of . This study also found that the effect of pressure on NO emissions is significant irrespective of the combustion type, because reducing the OH radical concentration in NH3/air flame at high pressure limits the NO generation. However, the STAE of NO of non-premixed flames were slightly higher than those of premixed flames, especially under rich conditions, possibly owing to zones of elevated NO concentrations near the wall boundaries. The introduction of an additional NH3 stream external to the air stream (by splitting the original NH3 flow) may mitigate this effect by creating an almost uniform distribution within the combustor and thus a homogeneous NO distribution. The STAE of NO was minimised (and equalled that of a premixed flame) by applying a volumetric flow ratio between the innermost and outermost NH3 flows of 0.6:0.4.

NOX formation of biodiesel in utility power plant boilers; Part B. Comparison of NO between biodiesel and petrodiesel

This paper evaluates the formation of NO in a non-pressurized, water-cooled combustion chamber. Thermal and prompt NO were roughly estimated from local concentrations of NO in the fumes, and were studied at different burner operating points (combustion pressure, equivalence ratio, fuel spray pattern, and swirl angle). Afterwards, thermal and prompt NO for six different methyl esters and petrodiesel were measured, compared and what mostly influences and differentiates thermal and prompt NO between fuels was identified. Results reveal that the level of thermal NO for fuels is almost the same order of magnitude, while prompt NO varies markedly among the fuels. The relative level of thermal and prompt NO is much different depending on the operating points of the burner varying from 16/84 to up to 73/27.

An experimental study of combustion and emissions of two types of woody biomass in a 12-MW reciprocating-grate boiler

The gaseous emissions of primary concern from biomass combustion are nitrogen oxides (NOX), carbon monoxide, and various unburned gaseous components. Detailed characterization of the gas in the hot reaction zones is necessary to study the release, formation, and evolution of the gas components. In the present study, gas temperature and concentration were measured in a 12-MWth biomass-fired reciprocating-grate boiler operated with over-fire air and flue-gas recirculation. Temperature measurement was combined with flue gas quenching and sample gas extraction using two water-cooled stainless-steel suction pyrometers. The concentration profiles of O2, NO, and CO were experimentally determined throughout the furnace, and the profile gas temperature was measured in several positions inside the furnace for the two types of woody biomass studied. For both fuels, the gas temperature varied between approximately 450°C (average primary chamber temperature) and 1200°C (average secondary chamber temperature). The concentration profiles of CO and O2 suggested no conclusive difference between the two types of biomass. However, the local mean concentrations of NO and NOX emission factors (measured in the stack) were higher for Greenery fuel due to its higher nitrogen content than that of Standard fuel.

Biomass Char Nitrogen Oxidation—Single Particle Model

In this study, the release of NO during the combustion of single char particles of spruce bark was investigated. Modeled NO release rates were compared to experimentally determined NO rates from a single particle reactor of quartz glass. The operating conditions were between 1073 and 1323 K with 3–19 vol % O2 and with 0% NO in the surrounding gas. The NO release rates from the single particles were determined from measured molar fractions of NO at the outlet from the reactor system. In the model, the char-N is assumed to be oxidized proportionally to the char-C oxidation rate, which is measured. The formed NO is assumed to be reduced as a function of the local NO concentration inside the particle. The NO concentration profile inside the char particle is numerically calculated by taking into account the transport of NO inside and outside the particle and the reduction rates of NO inside the char particle. The modeled NO release rates were in good agreement with the measurements. The results show that the fractional conversion of char-N to NO increases as functions of decreasing particle size and conversion. The most likely explanation for this is that the formed NO diffuses away at an increasing rate as the particle decreases in size. Further, a suggested analytical model for the fractional conversion of char-N to NO is tested.

Economic crisis detected from space: Air quality observations over Athens/Greece

Using both satellite observations of tropospheric NO2 columns and a number of economic metrics, we investigate the impact of the economic crisis (from 2008 onward) on air quality over Greece, and Athens in particular. The multiannual analysis shows that NO2 columns over Athens have been significantly reduced in the range 30–40%. This decline is further supported by surface measurements of atmospheric NO2 mixing ratios. Additionally, the declining local concentrations of NO, CO, and SO2 are associated with an increase in ozone due to reduced titration by NO. In particular, regression analysis revealed that the reduction of NO2 (0.3 ± 0.2 ppbv y−1) and SO2 (0.2 ± 0.1 ppbv y−1) during the period 2000–2007, significantly accelerated during the economic crisis period (from 2008 onward), reaching 2.3 ± 0.2 ppbv y−1 and 0.7 ± 0.1 ppbv y−1, respectively. The strong correlations between pollutant concentrations and economic indicators show that the economic recession has resulted in proportionally lower levels of pollutants in large parts of Greece.

Kinetic behaviour of HC-SCR over Ag/alumina catalyst using a model paraffinic second generation biodiesel compound

The effect of longer paraffins on the mechanism of the HC-SCR reaction over a 1.91wt.% Ag/alumina catalyst was investigated by kinetic studies. Hexadecane (C16H34) was chosen as a model compound as it is also a representative molecule for a second generation biodiesel consisting of only long-chain paraffins. The kinetic behaviour of the catalytic reduction of NOx was examined at steady-state conditions in the temperature range 250–550°C (50°C ramping) and by using the following gas concentrations: PNO=100, 250, 500, 750 or 1000ppm, Phexadecane=31, 94, 188, 281 or 375ppm and PO2=1.5, 3, 4.5, 6 or 9vol.%. Results showed that in the temperature range 250–425°C high hexadecane concentration had an inhibiting effect on the NO reduction. At temperatures above 350°C the apparent reaction orders for hexadecane with respect to hexadecane increased to close or above 1. Reaction orders towards NO were close to 0.55 indicating that NO adsorption on the catalyst surface is stronger than hexadecane adsorption. Based on the experimental data it is proposed that small clusters alone cannot be the active sites for HC-SCR over Ag/Al2O3 but the important requirement for high activity over the catalyst is the local concentrations of hydrocarbon and NO on the interface of silver and the support.

Brown Norway rats show impaired nNOS-mediated information transfer in renal autoregulationThis article is part of a Special Issue on Information Transfer in the Microcirculation.

Nonselective inhibition of NO synthase (NOS) augments myogenic autoregulation of renal blood flow (RBF) and profoundly reduces RBF. Previously in Wistar rats, we showed that augmented autoregulation, but not vasoconstriction, is duplicated by intrarenal inhibition of neuronal NOS (nNOS), whereas intrarenal inhibition of inducible NOS (iNOS) has no effect on RBF or on RBF dynamics. Thus macula densa nNOS transfers information from tubuloglomerular feedback to the afferent arteriole. This information flow requires that macula densa nNOS can sufficiently alter ambient NO concentration, that is, that endothelial NOS (eNOS) and iNOS do not alter local NO concentration. Because the Brown Norway rat often shows exaggerated responses to NOS inhibition and has peculiarities of renal autoregulation that are related to NO, we used this strain to study systemic and renal vascular responses to NOS inhibition. The first experiment showed transient blood pressure reduction by bolus i.v. acetylcholine that was dose-dependent in both strains and substantially prolonged in Brown Norway rats. The depressor response decayed more rapidly after nonselective NOS inhibition and the difference between strains was lost, indicating a greater activity of eNOS in Brown Norway rats. In Brown Norway rats, selective inhibition of iNOS reduced RBF (-16% +/- 7%) and augmented myogenic autoregulation, whereas nNOS inhibition reduced RBF (-25% +/- 4%) and did not augment myogenic autoregulation. The significant responses to intrarenal iNOS inhibition, the reduced modulation of autoregulation by nNOS inhibition, and the enhanced endothelial depressor response suggest that physiological signalling by NO within the kidney is impaired in Brown Norway rats because of irrelevant or inappropriate input of NO by eNOS and iNOS.

Combustion Characteristics and NO x Emissions of Two Kinds of Swirl Burners in a 300-MWe Wall-Fired Pulverized-Coal Utility Boiler

Measurements were performed in a 300-MWe wall-fired pulverized-coal utility boiler. Enhanced ignition-dual register (EI-DR) burners and centrally fuel rich (CFR) swirl coal combustion burners were installed in the bottom row of the furnace during experiments. Local mean concentrations of O2, CO, CO2 and NO x gas species, gas temperatures, and char burnout were determined in the region of the two types of burners. For centrally fuel rich swirl coal combustion burners, local mean CO concentrations, gas temperatures and the temperature gradient are higher and mean concentrations of O2 and NO x along the jet flow direction in the burner region are lower than for the enhanced ignition-dual register burners. Moreover, the mean O2 concentration is higher and the gas temperature and mean CO concentration are lower in the side wall region. For centrally fuel rich swirl coal combustion burners in the bottom row, the combustion efficiency of the boiler increases from 96.73% to 97.09%, and NO x emission decreases from 411.5 to 355 ppm @ 6% O2 compared to enhanced ignition-dual register burners and the boiler operates stably at 110 MWe without auxiliary fuel oil.

Urban and Background Ozone Trend in 1984-1999 at Subtropical Hong Kong, South China

There has been a growing concern for the impact of increasing Asian pollutant emissions on climate, atmospheric chemistry and air quality of the downwind region. In this study, we examined the long-term records of surface ozone (O3) and its precursors (NO and NO2) measured in Hong Kong from the early 1980s to 2000. The urban O3 concentration has shown more than two-fold increase since the early 1980s and the increase was especially apparent after the late 1980s. The O3 increases led to frequent O3 pollution episodes and caused deterioration of local air quality. Such increases were not accompanied by local NO and NO2 concentration changes in Hong Kong but coincided with the increase of NOx emissions from Asia. We derived the background O3 concentration for the South China region using the early morning (1:00–5:00 a.m. local standard time) data and linked the transport of Asian emissions and background O3 change using back air trajectory and local meteorology. The derived background O3 concentration shows an increase rate of 1.5% per year over the 15-year period from 1984 to 1999, which is close to that in the midlatitudes of East Asia at Japan. The sharp O3 increase is related to the regional O3 built-up in South China and the East Asia region due to the increases in the pollutant emissions as a result of rapid urban and industrial development in the region.

동축 확산 부상화염의 Blow-off와 연소 특성

An experiment was performed to investigate lift-off, blow-off and combustion characteristics of a lifted coaxial diffusion flame according to fuel jet and air velocity. A jet diffusion flame which is attached on the nozzle rim begins to be lifted with increase of air velocity, and finally becomes blow-off at higher air velocity. In experiment, blow-off limit increased with increase of fuel jet velocity, however lift-off occurred at lower air velocity. Flame structure and combustion characteristics were examined by schlieren photos, temperature distributions and emisions concentration distributions. Flame temperature became higher at midstream and its RMS became larger at up and downstream with increase of air velocity. Local NO concentration decreased but CO₂ concentration increased with increase of air velocity, which shows combustion reaction becomes close to be stoichiometric at higher air velocity in spite of lift-off.

Power-station contributions to local concentrations of NO 2 at ground level

New air quality guidelines and standards for the protection of human health have recently been produced by various UK and European advisory and regulatory bodies. The new standards for NO 2 are more stringent than previously and have reawakened interest in the impact of NO x emissions from large point sources, such as power stations, on ambient concentrations of NO 2 at ground level. Theoretical considerations of the main chemical and dispersion processes which determine the concentration of NO 2 in ambient air and analysis of 75 site-years of monitoring data by meteorological category, time of day and time of year indicate that concentrations above the WHO hourly mean guideline of 104.6 ppb due to power-station plumes are extremely unlikely in the UK. This conclusion is supported by data from sites in the vicinity of power stations which show no exceedances of the WHO NO 2 guideline with a dominant local power-station contribution.