Meteorological effects on the evolution of high ozone episodes in the greater Seoul area

Three high O3 episodes--7 days in 1992 (July 3-July 9), 9 days in 1994 (July 21-July 29), and another 3 days in 1994 (August 22-August 24)--were selected on the basis of morning (7:00 a.m.-10:00 a.m.) average wind direction and speed and daily maximum O3 concentrations in the greater Seoul, Korea, of 1990-1997. To better understand their characteristics and life cycles, surface data from the Seoul Weather Station (SWS) and surface and 850-hPa wind field data covering northeast Asia around the Korean Peninsula were used for the analysis. In the July 1992 episode, westerly winds were most frequent as a result of the influence of a high-pressure system west of the Korean Peninsula behind a trough. In contrast, in the July 1994 episode, easterly winds were most frequent as a result of the effect of a typhoon moving north from the south of Japan. Despite different prevailing wind directions, the peak O3 concentrations for each episode occurred when a sea/land breeze developed in association with weak synoptic forcing. The August 1994 episode, which was selected as being representative of calm conditions, was another typical example in which a well-developed     

Mediterranean rural ozone characteristics around the urban area of Athens

A systematic analysis of surface ozone observations in rural areas surrounding Athens is presented. The analysis is based on ozone data for the rural station Aliartos about 80 km NW of Athens center and for two stations on the northern periphery of the Athens basin: Demokritos, located 10 km NE of Athens center and Liossia, 12 km to the north. The data for these two stations are screened for cases of strong air flow from rural areas. Average hourly summer afternoon ozone mixing ratios are similar for all three stations about 60 ppb and thus exceed for the hours 12:00–20:00 LST the 55 ppb WHO guideline for human health for 8 h ozone exposure. The corresponding winter afternoon mixing ratios are at 35 ppb. However, due to the large diurnal variation, mean monthly ozone mixing ratios at Aliartos, for the months April–September vary from 32 to 40 ppb, which is comparable to the higher average ozone levels at rural stations in south-central Europe. In cases of southerly air flow in the summer in the Athens basin, afternoon ozone levels at Demokritos and Liossia are generally the highest of any in the monitoring network. Hourly average concentrations, however, are only 40% greater than rural values. A background ozone level of such magnitude will have a significant impact on estimates for the effectiveness of pollution control measures for Athens.     

Long-term meteorologically independent trend analysis of ozone air quality at an urban site in the greater Houston area

The Houston-Galveston-Brazoria (HGB) area of Texas has a history of ozone exceedances and is currently classified under moderate nonattainment status for the 2008 8-hr ozone standard of 75 ppb. The HGB area is characterized by intense solar radiation, high temperature, and high humidity, which influence day-to-day variations in ozone concentrations. Long-term air quality trends independent of meteorological influence need to be constructed for ascertaining the effectiveness of air quality management in this area. The Kolmogorov-Zurbenko (KZ) filter technique, used to separate different scales of motion in a time series, is applied in the current study for maximum daily 8-hr (MDA8) ozone concentrations at an urban site (U.S. Environmental Protection Agency [EPA] Air Quality System [AQS] Site ID: 48-201-0024, Aldine) in the HGB area. This site, located within 10 miles of downtown Houston and the George Bush Intercontinental Airport, was selected for developing long-term meteorologically independent MDA8 ozone trends for the years 1990–2016. Results from this study indicate a consistent decrease in meteorologically independent MDA8 ozone between 2000 and 2016. This pattern could be partially attributed to a reduction in underlying nitrogen oxide (NO_x) emissions, particularly lowering nitrogen dioxide (NO₂) levels, and a decrease in the release of highly reactive volatile organic compounds (HRVOCs). Results also suggest solar radiation to be most strongly correlated to ozone, with temperature being the secondary meteorological control variable. Relative humidity and wind speed have tertiary influence at this site. This study observed that meteorological variability accounts for a high of 61% variability in baseline ozone (low-frequency component, sum of long-term and seasonal components), whereas 64% of the change in long-term MDA8 ozone post 2000 could be attributed to NO_x emission reduction. Long-term MDA8 ozone trend component was estimated to be decreasing at a linear rate of 0.412 ± 0.007 ppb/yr for the years 2000–2016 and 0.155 ± 0.005 ppb/yr for the overall period of 1990–2016.

Implications: The effectiveness of air emission controls can be evaluated by developing long-term air quality trends independent of meteorological influences. The KZ filter technique is a well-established method to separate an air quality time series into short-term, seasonal, and long-term components. This paper applies the KZ filter technique to MDA8 ozone data between 1990 and 2016 at an urban site in the greater Houston area and estimates the variance accounted for by the primary meteorological control variables. Estimates for linear trends of MDA8 ozone are calculated and underlying causes are investigated to provide a guidance for further investigation into air quality management of the greater Houston area.     

Ambient formaldehyde and its contributing factor to ozone and OH radical in a rural area

Formaldehyde (HCHO), as well as correlative pollutants was measured from 1 to 31 July in 2007 at Mazhuang, a rural site located in the east of China. Gaseous HCHO was scrubbed from the air with an acidic 2,4-dinitrophenylhydrazine (DNPH) solution, which leaded to the reaction of HCHO with DNPH and produced a stable product, 2,4-dinitrophenylhydrazone, followed by online analysis by high-performance liquid chromatography (HPLC) coupled with Ultraviolet detector. During the observation period, mixing ratios of HCHO ranged from 0.2 ppbv to 6.2 ppbv, with an average of 1.5 ± 0.67 ppbv. HCHO shows an evident diurnal variation, the maximum appeared during 12:00–14:00. The average concentration diurnal variations of measured HCHO, ozone (O₃), Methylhydroperoxides (MHP, CH₃OOH), hydrogen peroxide (H₂O₂), nitrogen oxides (NO_x) and meteorological parameters were compared. The similar variations of HCHO, O₃ and radiation imply that photo-oxidation of hydrocarbons might be the major source for HCHO. Based on the maximum incremental reactivity (MIR) coefficient of HCHO, the calculation shows that HCHO contributes about 20% to total observed O₃ during the study period. In order to compare the contributions of O₃, HCHO and HONO to OH radical, photolysis rate parameters (J-values) of the three compounds were calculated by the Tropospheric Ultraviolet and Visible (TUV) Radiation Model (4.4 version). Based on the comparison, this study reaches the conclusion that O₃ is the dominant source of OH radical at Mazhuang. This study also uses P(HCHO)/P(O₃) which represents the ratio of contrbutions of HCHO and O₃ to OH radical, to discuss the action of HCHO in OH radical soucers. The result shows that P(HCHO)/P(O₃) is 12.5% on average, with the maximum of 21.0% at 13:00_P.M. and minimum of 7.5% before 9:00_A.M. and after 17:00_P.M..Therefore HCHO is also an important source of OH radical and cannot be ignored.     

Sources and sinks of ozone in rural areas

Based on data collected at rural sites in South Dakota, Louisiana and Virginia during the summers of 1978–1980, the factors controlling the diurnal behavior of O₃ near the surface were determined. At all three sites, the diurnal O₃ profile consisted of a period of O₃ increase from sunrise until mid-afternoon, and a period of O₃ decrease from late afternoon until sunrise. The daily O₃ increase was comprised of two approximately equal portions: one due to downmixing of O₃ from aloft (0600–1000 h), and the other due to photochemistry and possibly some further downmixing (1000–1400 h). Even assuming that all of the O₃ increase during the 1000–1400 h period was due to local photochemistry, only very small amounts of O₃ were formed i.e. about 6 ppb of O₃ per ppb of nitrogen oxides (NO_x). Both bag irradiation and photochemical modeling studies suggested similar, but slightly higher O₃ formation. Comparing the ambient data with the bag irradiation and modeling results, it was concluded that rural photochemistry was not controlled exclusively by NO_x, but depended on hydrocarbons as well.The O₃ decrease (1800 to 0600 h) generally occurred while nocturnal inversions isolated the lower 50 m of the atmosphere from the air aloft. Since the O₃ loss-rate could be parameterized equally well by either zero- or first-order rate laws, we could not distinguish between the two mechanisms. However, concentrations of gaseous species such as terpenes and NO_x that might lead to pseudo zero-order O₃ loss were generally of minor significance. Consequently, it is concluded that O₃ loss was controlled by surface deposition with deposition velocities ranging from 0.06 to 0.34cms⁻¹ at the three sites.     

Urban and rural ozone concentrations in Alberta,Canada

Ozone concentrations in Alberta cities typically exhibit a maximum in May (up to 35 ppb) and a minimum in November (as low as 4 ppb). This behaviour is similar to that of rural Alberta O₃ concentrations. Annual O₃ concentrations at six urban monitoring stations vary from 11 ppb to 22 ppb and are about one-half the values at rural stations. In winter, urban O₃ concentrations are always smaller than rural concentrations and the cities act as sinks for O₃. Although urban stations do not exceed Canada's maximum acceptable levels of daily (25 ppb) and annual (15 ppb) O₃ concentrations as often as rural stations, the frequency is still quite large. Canada's hourly maximum desirable level (50 ppb) is exceeded 11 times more often at the remote (rural) station than at the downtown (urban) stations.     

Increase in surface ozone at rural sites in the western US

We evaluated O₃ data for the period 1987–2004 from 11 rural and remote sites in the north and western US, including two sites in Alaska. All sites show a seasonal cycle with a spring or spring-summer maximum. By deseasonalizing the data, we are better able to identify seasonal and spatial patterns and long-term trends. For most of the locations in the western US that we considered, there are significant inter-site correlations in the deseasonalized monthly means. This indicates that there are large scale factors that influence the monthly mean O₃ concentrations across the western US. At seven out of nine sites in the western US, there is a statistically significant increase in O₃ with a mean trend of 0.26 ppbv year⁻¹ (range at the seven sites is 0.19–0.51 ppbv year⁻¹). At three of the sites, we examined the data in more detail to find that the trends are present in all seasons. At the two sites in Alaska, no clear pattern was found. At the one ozonesonde site in the western US with long-term observations (Boulder, Colorado), no significant trend was identified. However, the statistical power in the ozonesonde analysis is limited due to the low frequency of ozonesonde launches. Temperature changes can explain only a fraction of the surface O₃ trend. We consider several possible explanations for these trends, including: increasing regional emissions, changes in the distribution of emissions, increasing biomass burning or increasing global background O₃. With the available data, we are not able to unambiguously identify the cause for increasing O₃ in the western US     

Nitrogen oxides and ozone in Portugal: trends and ozone estimation in an urban and a rural site

Environmental Science and Pollution Research - This study provides an analysis of the spatial distribution and trends of NO, NO2 and O3 concentrations in Portugal between 1995 and 2010....     

Optimal reduction of the ozone monitoring network over France

Ozone is a harmful air pollutant at ground level, and its concentrations are measured with routine monitoring networks. Due to the heterogeneous nature of ozone fields, the spatial distribution of the ozone concentration measurements is very important. Therefore, the evaluation of distributed monitoring networks is of both theoretical and practical interests. In this study, we assess the efficiency of the ozone monitoring network over France (BDQA) by investigating a network reduction problem. We examine how well a subset of the BDQA network can represent the full network. The performance of a subnetwork is taken to be the root mean square error (rmse) of the hourly ozone mean concentration estimations over the whole network given the observations from that subnetwork. Spatial interpolations are conducted for the ozone estimation taking into account the spatial correlations. Several interpolation methods, namely ordinary kriging, simple kriging, kriging about the means, and consistent kriging about the means, are compared for a reliable estimation. Exponential models are employed for the spatial correlations. It is found that the statistical information about the means improves significantly the kriging results, and that it is necessary to consider the correlation model to be hourly-varying and daily stationary. The network reduction problem is solved using a simulated annealing algorithm. Significant improvements can be obtained through these optimizations. For instance, removing optimally half the stations leads to an estimation error of the order of the standard observational error (10 μg m^?3). The resulting optimal subnetworks are dense in urban agglomerations around Paris (Île-de-France) and Nice (Côte d’Azur), where high ozone concentrations and strong heterogeneity are observed. The optimal subnetworks are probably dense near frontiers because beyond these frontiers there is no observation to reduce the uncertainty of the ozone field. For large rural regions, the stations are uniformly distributed. The fractions between urban, suburban and rural stations are rather constant for optimal subnetworks of larger size (beyond 100 stations). By contrast, for smaller subnetworks, the urban stations dominate.     

Potential interference bias in ozone standard compliance monitoring

The U.S. Environmental Protection Agency has established a federal reference method (FRM) for ozone (O3) and allowed for designation of federal equivalent methods (FEMs). However, the ethylene-chemiluminescence FRM for O3 has been replaced by the UV photometric FEM by most state and local monitoring agencies because of its relative ease of operation. Accumulating evidence indicates that the FEM is prone to bias under the hot, humid, and stagnant conditions conducive to high O3 formation. This bias may lead to overreporting hourly O3 concentrations by as much as 20-40 ppb. Measurement bias is caused by contamination of the O3 scrubber, a problem that is not detected by dry air calibration. An adequate wet test has not been codified, although a procedure has been proposed for agency consideration. This paper includes documentation of laboratory tests quantifying specific interferant responses, collocated ambient FRM/FEM monitoring results, and smog chamber comparisons of the FRM and FEMs with alternative scrubber designs. As the numbers of reports on monitor interferences have grown, interested parties have called for agency recognition and correction of these biases.     

Photochemical modelling of summer ozone episodes in the Greater Madrid area

The mesoscale meteorological model TVM, coupled to a photochemical/transport module in which different chemical mechanisms (RACM, EMEP) are implemented, has been evaluated. Field measurements and numerical results are used to determine the impact of the mesoscale flows on the photochemical smog episodes observed in the Greater Madrid Area for two selected days, characterised by the presence of a thermal low-pressure system over the Iberian Peninsula. During the 14 July 1992, the synoptic flow from the southeast favoured the transport of the precursors to the Guadarrama mountain range, where high concentrations of ozone were registered, exceeding the population information threshold. On the 15 July 1995, the synoptic wind from the northwest interacted with the local thermally driven flows, pushing the pollutants far away from the metropolitan area with the result that high ozone concentrations were measured to the east-southeast of the city.     

Summer ozone episodes in the Greater Madrid area. Analyzing the ozone response to abatement strategies by modelling

The development of ozone control strategies requires analysing the sensitivity of the dispersion model used to changes in emissions of nitrogen oxides (NO_X) and volatile organic compounds. The ozone response to variations in road traffic and total anthropogenic emissions is evaluated for two different summer ozone episodes in the Greater Madrid Area (GMA). This study uses the TVM model and a transport/chemistry module in which different chemical mechanisms (EMEP, RACM) are implemented. The results show that the areas of maximum impact and ozone responses are notably influenced by the different transport and dispersion patterns established in the area. However, the contribution of anthropogenic sources other than road traffic is patent in both episodes. Strategies based only on decreasing road traffic emissions were not sufficient for an effective control of the air quality in the GMA. Moreover, certain discrepancies observed in the predicted trends, as a response to these control strategies posed, reflect the importance of variations in the precursors balance. The ozone production regime associated to these ozone episodes and the sensitivity of the ozone response to changes in this balance has been investigated. A chemical indicator has been used to deepen in that evaluation.     

Diurnal variations of ozone and other pollutants in an urban area

A theoretical model is used to describe the diurnal variations of primary and secondary pollutants, with emphasis on ozone. This is done for an urban basin with anthropogenic sources of nitrogen oxides and hydrocarbons. We propose a scheme for the decomposition of aromatic compounds. According to this scheme, each aromatic molecule gives rise to six transfers of NO to NO₂ without consumption of odd oxygen. It is concluded that it is not a good approximation to represent urban hydrocarbon emissions by one single species, neither in short term (a few hours) nor multiday simulations. Species with both high and low reactivity ought to be included. We show that the nocturnal minimum in ozone often observed in urban areas, is mainly induced by gas chemistry. It is not a good approximation to omit the chemical development during the night-time in a theoretical analysis of urban photochemical pollution. Such an omission introduces errors also in the day-time chemistry. Application of constant dissociation rate coefficients over the day gives rise to false morning and evening ozone maxima.     

Nocturnal depletion of photochemical ozone at a rural site

Nocturnal depletion of ozone during a period of photochemical pollution has been observed at a rural site by continuous ground-level measurements and the determination of vertical profiles of ozone concentration. An acoustic sounder was used to provide continuous information upon the vertical temperature structure of the atmosphere. The results are interpreted in terms of recent theories of long-range transport of photochemical ozone and the chemical and physical processes causing ozone depletion.     

Surface ozone at rural sites in the latrobe valley and Cape Grim,Australia

Ozone and other air quality data from five rural sites in the industrialized Latrobe Valley, Victoria, have been subject to statistical analyses including linear regression modelling. The behaviour of O₃ in the Latrobe Valley is explained largely in terms of natural background atmospheric processes as observed at Cape Grim, Tasmania.The maximum 1-h average concentration of naturally occurring O₃ (obtained from a 6-year record at Cape Grim) is less than 40 ppb (v/v). In contrast the industrialized Latrobe Valley sites show O₃ values exceeding 40 ppb between 1% and 3% of the time. These higher concentrations occur in conditions consistent with local photochemical production of O₃ via ‘smog’ type processes and appear preferentially at low NO_x concentrations (3–4 ppb) during the afternoon (13–18 h) and at high temperatures (above 25°C).A comparison of observations from an elevated station (750 m) with those from the valley floor shows systematic differences in seasonal and diurnal O₃ variations and the time of day of occurrence of elevated O₃ concentrations which can be explained in terms of the diurnal cycle of convective mixing and mountain/valley winds.A linear regression model incorporating this understanding has accounted for between 43% and 64% of the variance of O₃ concentration at the elevated and rural stations. The statistical model incorporates temperature, time of day, month of year, wind speed, O₃ concentration 24-h earlier and NO_x concentration as variables in the regression equation, with temperature being the dominant variable. The standard deviation of the residual O₃ values (observed minus fitted) is around 5 ppb. Auto and cross correlations are used to show that perhaps half of the unexplained variance is coherent from site to site and hence potentially could be modelled.     

Surface ozone comparison conducted in two rural areas in central-southern Spain

Purpose  

The purpose of this work is to contribute to the understanding of the photochemical air pollution analysing the levels and temporal variations of surface ozone in two rural areas situated in central-southern Spain.     

Occurrence of nitro-pah in the atmosphere in a rural area

By means of gas chromatography with nitrogen sensitive detection and negative ion chemical ionization mass spectrometric detection 7 mononitro-PAH, 9-nitroanthracene, x-nitro-4,5-methylene-phenanthrene, 3-nitrofluoranthene, 1- and 2-nitropyrene, 10-nitrobenz(a)anthracene and 6-nitrobenzo-(a)pyrene have been identied in samples of airborne particulate matter. Furthermore, 8-nitrofluoranthene has been tentatively identified. The amounts of mononitro-PAH are one to two orders of magnitude lower than the most common carcinogenic PAH, as e.g. benzo(a)pyrene. Filter effects have been studied, 9-nitroanthracene can be transformed during sampling at low atmospheric concentrations of NO₂ and photochemical oxidants to 9,10-dinitroanthracene and 10-nitroanthrone.     

The relation between ozone,NOx and hydrocarbons in urban and polluted rural environments

Research over the past ten years has created a more detailed and coherent view of the relation between O₃ and its major anthropogenic precursors, volatile organic compounds (VOC) and oxides of nitrogen (NO_x). This article presents a review of insights derived from photochemical models and field measurements. The ozone–precursor relationship can be understood in terms of a fundamental split into a NO_x-senstive and VOC-sensitive (or NO_x-saturated) chemical regimes. These regimes are associated with the chemistry of odd hydrogen radicals and appear in different forms in studies of urbanized regions, power plant plumes and the remote troposphere. Factors that affect the split into NO_x-sensitive and VOC-sensitive chemistry include: VOC/NO_x ratios, VOC reactivity, biogenic hydrocarbons, photochemical aging, and rates of meteorological dispersion. Analyses of ozone–NO_x–VOC sensitivity from 3D photochemical models show a consistent pattern, but predictions for the impact of reduced NO_x and VOC in indivdual locations are often very uncertain. This uncertainty can be identified by comparing predictions from different model scenarios that reflect uncertainties in meteorology, anthropogenic and biogenic emissions. Several observation-based approaches have been proposed that seek to evaluate ozone–NO_x–VOC sensitivity directly from ambient measurements (including ambient VOC, reactive nitrogen, and peroxides). Observation-based approaches have also been used to evaluate emission rates, ozone production efficiency, and removal rates of chemically active species. Use of these methods in combination with models can significantly reduce the uncertainty associated with model predictions.