Ground level ozone concentrations at a rural location in northern Spain

Effect of restricted emissions during COVID-19 on atmospheric aerosol chemistry in a Greater Cairo suburb: Characterization and enhancement of secondary inorganic aerosol production

To prevent the rapid spreading of the COVID-19 pandemic, the Egyptian government had imposed partial lockdown restrictions which led emissions reduction. This served as ideal conditions for a natural experiment, for study the effect of partial lockdown on the atmospheric aerosol chemistry and the enhanced secondary inorganic aerosol production in a semi-desert climate area like Egypt. To achieve this objective, SO₂, NO₂, and PM_2.5 and their chemical compositions were measured during the pre-COVID, COVID partial lockdown, and post-COVID periods in 2020 in a suburb of Greater Cairo, Egypt. Our results show that the SO₂, NO₂, PM_2.5 and anthropogenic elements concentrations follow the pattern pre-COVID > post-COVID > COVID partial lockdown. SO₂ and NO₂ reductions were high compared with their secondary products during the COVID partial lockdown compared with pre-COVID. Although, PM_2.5, anthropogenic elements, NO₂, SO₂, SO₄ ^2-, NO₃ ^-, and NH₄ ⁺ decreased by 39%, 38-55%, 38%, 32.9%. 9%, 14%, and 4.3%, respectively, during the COVID partial lockdown compared with pre-COVID, with the secondary inorganic ions (SO₄ ^2-, NO₃ ^-, and NH₄ ⁺) being the dominant components in PM_2.5 during the COVID partial lockdown. Moreover, the enhancement of NO₃ ^- and SO₄ ^2- formation during the COVID partial lockdown was high compared with pre-COVID. SO₄ ^2- and NO₃ ^- formation enhancements were significantly positive correlated with PM_2.5 concentration. Chemical forms of SO₄ ^2- and NO₃ ^- were identified in PM_2.5 based on their NH₄ ⁺/SO₄ ^2- molar ratio and correlation between NH₄ ⁺ and both NO₃ ^- and SO₄ ^2-. The particles during the COVID partial lockdown were more acidic than those in pre-COVID.

Variation and correlation between ultraviolet index and tropospheric ozone during COVID-19 lockdown over megacities of India

Worldwide spread out of COVID-19 in a short-time has brought a significant decline of road traffic, tourist flow and industrial ventures. During this emergency period, the restricted human dealings with nature have appeared as blessing for health of the total environment. The variation of atmospheric O₃ may modulate the range of UV index (UVI) at any region of the earth. The objective of the study is to examine the variation of UV index over the megacities of India with respect to tropospheric O₃ level modification during COVID-19 lockdown. The meteorological or environmental data (temperature in °C, gust in km/h, wind speed km/h, relative humidity in %, air pressure in mb and cloud cover in okta) of four selective megacities of India (Kolkata, Chennai, Delhi, Mumbai) during and pre lockdown period have been obtained to comprehend about the variation of UV index and tropospheric O₃. The descriptive statistical applications i.e. standard deviation, standard errors and K-means clustering have been done through standard statistical software. In the present study, t-test has been used to understand level of significance of surface O₃ and UVI during pre-lockdown (2019) and lockdown (2020) phase. The result shows that the four major megacities in India namely New Delhi, Mumbai, Kolkata and Chennai have experienced the vibrant diminution in terms of the concentration of UV index with slightly increasing the tropospheric O₃ level during the lockdown phase. The higher accumulation of O₃ during the lockdown in the lower atmosphere of four megacities does not exceed the permissible limit. The excess amount of O₃ has remarkably contributed to trap the harmful UV radiation which has lowered the UVI in these worst polluted megacities of India. In the meantime, the prominent reduction of NO_x during the lockdown period decreases the titration impact to O₃ and this mechanism helps to revitalize the ozone concentration level. The uniqueness of the current study is highlighted the ground reality regarding reduction of UV index and amplification of tropospheric O₃ concentration during lockdown phase. This study definitely assists to make new environmental policy, act and law for recover the health of the total environment.

A Predictive Model for Steady State Ozone Concentration at an Urban-Coastal Site

Ground level ozone (O₃) plays an important role in controlling the oxidation budget in the boundary layer and thus affects the environment and causes severe health disorders. Ozone gas, being one of the well-known greenhouse gases, although present in small quantities, contributes to global warming. In this study, we present a predictive model for the steady-state ozone concentrations during daytime (13:00⁻17:00) and nighttime (01:00⁻05:00) at an urban coastal site. The model is based on a modified approach of the null cycle of O₃ and NO_x and was evaluated against a one-year data-base of O₃ and nitrogen oxides (NO and NO₂) measured at an urban coastal site in Jeddah, on the west coast of Saudi Arabia. The model for daytime concentrations was found to be linearly dependent on the concentration ratio of NO₂ to NO whereas that for the nighttime period was suggested to be inversely proportional to NO₂ concentrations. Knowing that reactions involved in tropospheric O₃ formation are very complex, this proposed model provides reasonable predictions for the daytime and nighttime concentrations. Since the current description of the model is solely based on the null cycle of O₃ and NO_x, other precursors could be considered in future development of this model. This study will serve as basis for future studies that might introduce informing strategies to control ground level O₃ concentrations, as well as its precursors' emissions.

The spatial variation of O3, NO, NO2 and NO x and the relation between them in two Swedish cities

Ozone and nitrogen oxides (NO _x ) are air pollutants with known associations to adverse health effects on humans. Few studies have simultaneously measured ozone and nitrogen oxides with high spatial resolution. The main aim of this paper was to assess the levels and variation of ground-level ozone, NO₂ and NO _x in two Swedish cities. An additional aim was to describe the levels of these pollutants within and between three different types of measurement sites (regional background, urban background and traffic sites) and within and between different measurement periods of the year. Three weekly sampling campaigns of NO _x and ozone were conducted simultaneously at 20 sites in two Swedish regions using Ogawa badges. Ozone was measured at 20 additional sites in each area. The median ozone concentration for all measurements was statistically significantly higher in Malmö (67 μg/m³) compared to Umeå (56 μg/m³), and in both cities, ozone levels were highest in April. Measurement period was a more important factor for describing the variation in ozone concentrations than the type of measurement site. The levels of NO₂ and NO _x were statistically significantly higher in the Malmö area (8.1 and 12 μg/m³) compared to the Umeå area (4.5 and 8.9 μg/m³). The levels were generally highest at the sites categorized as traffic, while the variability between different seasons was sparse.

Surface ozone levels in the forest and vegetation areas of the Biga Peninsula, Turkey

Spatial and temporal variability of surface ozone in the rural, mountainous and suburban sites of Biga Peninsula, at the northwest of Turkey which is about 300km southwest of Istanbul was investigated using passive samplers and continuous analyzers. A total 10 passive samplers and two continuous analyzers were used between 1.1.2013 and 31.12.2014. OX levels in the study region were examined to understand NOx dependent or independent contribution to ozone. The influences of the meteorological parameters on ozone levels were also examined by wind speed and ambient temperature. The results clearly show that mountainous areas have higher cumulative exposure to ozone than suburban locations. In order to understand the long range transport sources contributing to the high ozone levels in the region backward trajectories were computed using HYSPLIT model and then clustering of trajectories are performed. The results clearly show the characteristics of pollutant transport from north to Biga Peninsula. Additionally, AOT40 (Accumulated hourly O3 concentrations Over a Threshold of 40ppb) cumulative index was calculated using daytime hourly measurements. The results indicate that the ozone values in the study area are much higher than the critical levels for forest and vegetation based on EU Directive 2008/50/EC.

Temporal Variations of O3 and NO x in the Urban Background Atmosphere of Nanjing, East China

Rapid economic growth has given rise to a significant increase in ozone (O3)-precursor emissions in many regions of China. An improved understanding of O3 formation in response to different precursor emissions is imperative to address the highly nonlinear O3 problem and to provide a solid scientific basis for efficient O3 abatement in these regions. To this end, this study was performed in Nanjing using a set of observational data from June 1, 2013, to May 31, 2014. The results showed that O3 concentrations were positively correlated with wind speed and temperature and were significantly negatively correlated with relative humidity. The highest monthly daytime, nighttime, and daily average O3 concentrations were observed in summer with values of approximately 46, 18 and 30 ppb, respectively. The lowest O3 concentrations were observed in November through January with values as low as 17, 4, and 9 ppb for the daytime, nighttime, and daily concentrations, respectively. The highest daytime average NO and NO2 concentrations were observed in December, whereas the lowest concentrations were observed in July. A unimodal O3 peak was observed with the highest O3 levels in summer followed by spring and then autumn; the lowest levels observed in the winter. The O3 concentration reached maximum levels at 14:00 to 15:00 h (local standard time). It was found that the crossover occurred with approximately several hours difference with the earliest occurring in summer (06:00 h) followed by spring (08:00 h), autumn (09:00 h), and winter (10:30 h). Furthermore, the highest constant rate of O3 accumulation was observed in summer (5.6 ppb/h) followed by autumn (4.8 ppb/h), spring (4.5 ppb/h), and winter (2.7 ppb/h). The oxidant intercept ranged from 28.4 ppb in January to 58.6 ppb in June, although the slope also shows substantial variation from 0.18 in June to 0.67 in August. The weekend effect is stronger in spring and summer than in autumn and winter and is more intense on Sundays than on Saturdays. Thus, the decrease of O3 levels during weekends suggests that it may be NO x -sensitive.

Influence of atmospheric stability and transport on CH4 concentrations in northern Spain

Continuous methane (CH4) concentrations were measured in Northern Spain over two years (2011-2012) by multi-point sampling at 1.8, 3.7 and 8.3m using a Picarro analyser. The technique is based on cavity ring-down spectroscopy. The contrast in mean concentrations was about 1.2ppb, with 95th percentiles differing by 2.2ppb and mean minimum concentrations proving similar. Temporal variations of CH4 were also analysed, with a similar seasonal variability being found for the three heights. The highest CH4 concentrations were obtained in late autumn and winter and the lowest in summer, yielding a range of 52ppb. This variation may depend on the active photochemical reaction with OH radical during a period of intense solar radiation and changes in soil conditions together with variations in emissions. Peak concentration levels were recorded at night-time, between 5:00-7:00 GMT, with mean values ranging between 1920 and 1923ppb. The lowest value, around 1884ppb, was obtained at 16:00 GMT. This diurnal variation was mainly related to vertical mixing and photochemistry. Therefore, CH4 concentrations were also examined using the bulk Richardson number (RB) as a stability indicator. Four groups were distinguished: unstable cases, situations with pure shear flow, transitional stages and drainage flows. The highest contrast in mean CH4 concentrations between lower and upper heights was obtained for the transition and drainage cases, mainly associated to high concentrations from nearby sources. The impact of long range transport was analysed by means of 3-day isobaric backward air mass trajectories, which were calculated taking into account origins from Europe, Africa, the Atlantic Ocean and Local conditions. Assessment of the results showed the influence of S and SE wind sectors, especially with Local conditions associated with low winds. Finally, an estimation of the background CH4 concentration in the study period provided an average value of about 1892ppb.

Exploring the interaction between O₃ and NOx pollution patterns in the atmosphere of Barcelona, Spain using the MCR-ALS method

This work focuses on understanding the behaviour and patterns of three atmospheric pollutants namely, nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) along with their mutual interactions in the atmosphere of Barcelona, North Spain. Hourly samples were collected for NO, NO2 and O3 from the same city location for three consecutive years (2010-2012). The study explores the seasonal, annual and weekday-weekend variations in their diurnal profiles along with the possible identification of their source and mutual interactions in the region. Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) was applied to the individual datasets of these pollutants, as well as to all of them simultaneously (augmented mode) to resolve the profiles related to their source and variation patterns in the atmosphere. The analysis of the individual datasets confirmed the source pattern variations in the concerned pollutant's profiles; and the resolved profiles for augmented datasets suggested for the mutual interaction of the pollutants along with their patterns variations, simultaneously. The study suggests vehicular pollution as the major source of atmospheric nitrogen oxides and presence of weekend ozone effect in the region.

Decreased rates of terpene emissions in Ornithopus compressus L. and Trifolium striatum L. by ozone exposure and nitrogen fertilization

Increasing tropospheric ozone (O3) and nitrogen soil availability (N) are two of the main drivers of global change. They both may affect gas exchange, including plant emission of volatiles such as terpenes. We conducted an experiment using open-top chambers to analyze these possible effects on two leguminous species of Mediterranean pastures that are known to have different O3 sensitivity, Ornithopus compressus and Trifolium striatum. O3 exposure and N fertilization did not affect the photosynthetic rates of O. compressus and T. striatum, although O3 tended to induce an increase in the stomatal conductance of both species, especially T. striatum, the most sensitive species. O3 and N soil availability reduced the emission of terpenes in O. compressus and T. striatum. If these responses are confirmed as a general pattern, O3 could affect the competitiveness of these species.

Spatial and temporal variation of surface ozone, NO and NO₂ at urban, suburban, rural and industrial sites in the southwest of the Iberian Peninsula

Surface ozone is one of the most important photochemical pollutants in the low atmosphere, causing damage to human health, vegetation, materials and climate. The weather (high temperatures and high solar radiation), orography (presence of the Guadalquivir valley) and anthropogenic (the cities of Cádiz, Córdoba, Huelva and Seville and two important industrial complexes) characteristics of the southwestern Iberian Peninsula make this region ideal for the formation and accumulation of ozone. To increase the knowledge of ozone behaviour in this area, the monthly, daily and weekly variations of ozone and its precursors, nitrogen oxides (NO(x) = NO + NO2), were analysed over a 4-year period (2003 to 2006). Using the k-means cluster technique, 12 representative stations of five different areas with different ozone behaviour were selected from a total of 29 monitoring sites. This is the first time that the analysis of these atmospheric pollutants has been carried out for the whole area, allowing therefore a complete understanding of the dynamics and the relationships of these compounds in this region. The results showed an opposite behaviour among ozone and NO and NO2 concentrations in urban and suburban zones, marked by maximums of ozone (minimums NO(x)) in spring and summer and minimums (maximums) in autumn and winter. A seasonal behaviour, with lower amplitude, was also observed in rural and industrial areas for ozone concentrations, with the NO and NO2 concentrations remaining at low and similar values during the year in rural zones due to the absence of emission sources in their surroundings. The daily cycles of ozone in urban, suburban and industrial sites registered a maximum value in the early afternoon (14:00-17:00 UTC) while for NOx two peaks were observed, at 7:00-10:00 UTC and 20:00-22:00. In the case of rural stations, no hourly peak of ozone or NO(x) was registered. The weekend effect was studied by using a statistical contrast tests (Student's t). The results indicated that only areas influenced by important traffic emissions presented a weekend effect for NO and NO2, whereas an ozone weekend effect was not detected in any case.

The role of transboundary air pollution over Galicia and North Portugal area

In summer, high levels of ozone (O3) are frequently measured at both Galicia and Northern Portugal air quality monitoring stations, even exceeding the limit values imposed by legislation. This work aims to investigate the origin of these high O3 concentrations by the application of a chemical transport modelling system over the northwestern area of the Iberian Peninsula. The WRF-CHIMERE modelling system was applied with high resolution to simulate the selected air pollution episodes that occurred simultaneously in Galicia and North Portugal and in order to study both the contribution of local emission sources and the influence of transboundary pollution. Emission inputs have been prepared based on the development of the Portuguese and Galician emission inventories. The obtained results for O3 have been evaluated and validated against observations. Modelling results show possible contribution of the transboundary transport over the border of two neighbour regions/countries, indicating that the O3 episode starts over the urban and industrialised area of North coast of Portugal, reaching the maximum peaks over this region; at the same time, O3 levels increased over Galicia region, where lower concentrations, but still high, were observed. These results pointed out that air quality management should not be driven by political boundaries and highlight the importance of joining efforts between neighbouring countries.

Analysis of surface ozone and nitrogen oxides at urban, semi-rural and rural sites in Istanbul, Turkey

Ozone (O(3)) mixing ratios were measured at three different sites (urban/traffic, semi-rural and rural/island) in Istanbul from September 2007 to December 2009 in order to determine the diurnal, monthly and seasonal variations of O(3) and nitrogen oxides (NO(x)) and to study the local and regional impacts. This is the first study that evaluates the O(3) levels in semi-rural and rural sites in Istanbul in addition to the urban sites. The diurnal O(3) variations are generally characterized by afternoon maxima (64 ppb at the urban, 80 ppb at the semi-rural and 100 ppb at the rural site) and the nighttime minimum being more pronounced at the polluted urban site. The monthly mean O(3) mixing ratios start to increase in March, reaching their maximum values in August for the urban (~25 ppb) and semi-rural sites (30 ppb). However, at the rural site, the monthly mean O(3) levels reach their maximum value in June (35 ppb). The O(3) mixing ratios for weekends were higher than those on weekdays at each site by up to 28%, possibly due to changes in VOC sensitivity and reduction in NO(x) levels. In order to better understand and characterize the relationship between air masses and O(3) levels, cluster analysis was applied to the back-trajectories calculated by the HYSPLIT model for the semi-rural site. The analyses clearly showed that major transport is characterized by northern and western clusters, particularly from the Eastern Europe and the Mediterranean region, as well as recirculation over Istanbul due to high pressure systems leading to accumulated levels of O(3). The results clearly suggest that extended measurement networks from urban to rural sites should be considered for a more comprehensive evaluation of O(3) levels.

Synoptic patterns and air mass transport during ozone episodes in northwestern Iberia

High levels of ozone are frequently measured at the Galicia (NW Iberian Peninsula) air quality monitoring stations from March to October. However, there have been very few studies on surface ozone in the northwestern Iberian Peninsula, most likely because the climate of this region is not favourable to photochemical ozone generation. The occurrence of these episodes may be related to either local-scale photochemical pollution or regional-scale transport from other polluted regions. In addition, high ozone episodes usually are developed under specific synoptic conditions. The main purposes of this study are to characterise the atmospheric conditions that lead to the ozone episodes in this region and to identify possible advection paths of ozone and precursors. A surface hourly ozone dataset (2002-2007) measured at rural sites in Galicia was analysed to identify high ozone episodes together with their associated synoptic patterns using a subjective classification with 23 different synoptic types. The synoptic weather patterns revealed that most of the episodes occur with high surface pressures centred over the British Isles and/or Central Europe while a high-altitude anticyclonic ridge crosses the Peninsula from North Africa, causing easterly or southeasterly winds. This analysis was completed with 3-day backward air mass trajectories obtained with HYSPLIT to assess the contribution of long-range transport, resulting in the following main routes: Mediterranean-Peninsular, South Atlantic-Portuguese, local and French-Cantabric.

Daily, seasonal and monthly variations in ozone levels recorded at the Turia river basin in Valencia (Eastern Spain)

INTRODUCTION

The Turia river basin, located in the east of the Iberian Peninsula, drains into the Mediterranean Sea near the city of Valencia (population, 814,208). The predominance of sea-breeze fluxes favours the inland transport of pollutants from the city up the basin where ozone concentrations exceeding the threshold for protection of human health are systematically recorded during the summer months.

METHODS

This work analyses the variability in ozone levels by examining their spatial and temporal distribution in a Mediterranean river basin downwind from a city within the period 2005-2008. Orographic determinants and atmospheric fluxes induce strong variations in ozone measurements, even on relatively close locations.

CONCLUSIONS

Results show a different behaviour of the monthly means and the daily cycles depending on the season of the year and the measuring environment, with summer/winter ratios ranging from 2.4 in cities to 1.6 inland, and mean values always higher in the interior of the basin. Daily cycles show significant summer/winter differences related to the predominant situations of anticyclonic stability in winter, which limit ventilation, and the predominant breeze circulations in summer. Results also show a "weekend effect" at urban and medium-distance stations. At the most inland station, the weekend/weekday behaviour differs according to the season of the year; weekend ozone levels are higher in spring, autumn and winter, and lower in summer, coinciding with the predominance of local wind cycles that favour air mass penetration inland from the coast.

Implementation of a module for risk of ozone impacts assessment to vegetation in the Integrated Assessment Modelling system for the Iberian Peninsula. Evaluation for wheat and Holm oak

A module to estimate risks of ozone damage to vegetation has been implemented in the Integrated Assessment Modelling system for the Iberian Peninsula. It was applied to compute three different indexes for wheat and Holm oak; daylight AOT40 (cumulative ozone concentration over 40 ppb), cumulative ozone exposure index according to the Directive 2008/50/EC (AOT40-D) and POD(Y) (Phytotoxic Ozone Dose over a given threshold of Y nmol m(-2) s(-1)). The use of these indexes led to remarkable differences in spatial patterns of relative ozone risks on vegetation. Ozone critical levels were exceeded in most of the modelling domain and soil moisture content was found to have a significant impact on the results. According to the outputs of the model, daylight AOT40 constitutes a more conservative index than the AOT40-D. Additionally, flux-based estimations indicate high risk areas in Portugal for both wheat and Holm oak that are not identified by AOT-based methods.

Trends of rural tropospheric ozone at the northwest of the Iberian Peninsula

Tropospheric ozone levels around urban and suburban areas at Europe and North America had increased during 80's–90's, until the application of NO_x reduction strategies. However, as it was expected, this ozone depletion was not proportional to the emissions reduction. On the other hand, rural ozone levels show different trends, with peaks reduction and average increments; this different evolution could be explained by either emission changes or climate variability in a region. In this work, trends of tropospheric ozone episodes at rural sites in the northwest of the Iberian Peninsula were analyzed and compared to others observed in different regions of the Atlantic European coast. Special interest was focused on the air quality sites characterization, in order to guarantee their rural character in terms of air quality. Both episodic local meteorological and air quality measurements along five years were considered, in order to study possible meteorological influences in ozone levels, different to other European Atlantic regions.

Surface ozone measurements in the southwest of the Iberian Peninsula (Huelva, Spain)

INTRODUCTION

Photochemical ozone pollution of the lower troposphere (LT) is a very complex process involving meteorological, topographic emissions and chemical parameters. Ozone is considered the most important air pollutant in rural, suburban and industrial areas of many sites in the world since it strongly affects human health, vegetation and forest ecosystems, and its increase during the last decades has been significant. In addition, ozone is a greenhouse gas that contributes to climate change. For these reasons, it is necessary to carry out investigations that determine the behaviour of ozone at different locations. The aim of this work is to understand the levels and temporal variations of surface ozone in an industrial-urban region of the Southwest Iberian Peninsula.

MATERIALS AND METHODS

The study is based on ozone hourly data recorded during a 6-year period, 2000 to 2005 at four stations and meteorological data from a coastal station. The stations used were El Arenosillo and Cartaya--both coastal stations, Huelva--an urban site and Valverde--an inland station 50 km away from the coastline. The general characteristics of the ozone series, seasonal and daily ozone cycles as well as number of exceedances of the threshold established in the European Ozone Directive have been calculated and analysed.

RESULTS

Analysis of the meteorological data shows that winter-autumn seasons are governed by the movement of synoptic weather systems; however, in the spring-summer seasons, both synoptic and mesoescale conditions exist. Average hourly ozone concentrations range from 78.5 +/- 0.1 microg m(-3) at Valverde to 57.8 +/- 0.2 microg m(-3) at Huelva. Ozone concentrations present a seasonal variability with higher values in summer months, while in wintertime, lower values are recorded. A seasonal daily evolution has also been found with minimum levels around 08:00 UTC, which occurs approximately 1-1.5 h after sunrise, whereas the maximum is reached at about 16:00 UTC. Furthermore, during summer, the maximum value at El Arenosillo and Valverde stations remains very uniformed until 20:00 UTC. These levels could be due to the photochemical production in situ and also to the horizontal and vertical ozone transport at El Arenosillo from the reservoir layers in the sea and in the case of Valverde, the horizontal transport, thanks to the marine breeze. Finally, the data have been evaluated relative to the thresholds defined in the European Ozone Directive. The threshold to protect human health has been exceeded during the spring and summer months mainly at El Arenosillo and Valverde. The vegetation threshold has also been frequently exceeded, ranging from 131 days at Cartaya up to 266 days at Valverde.

DISCUSSION

The results in the seasonal and daily variations demonstrate that El Arenosillo and Valverde stations show higher ozone concentrations than Cartaya and Huelva during the spring and summer months. Under meteorological conditions characterized by land-sea breeze circulation, the daytime sea breeze transports the emissions from urban and industrial sources in the SW further inland. Under this condition, the area located downwind to the NE is affected very easily by high ozone concentrations, which is the case for the Valverde station. Nevertheless, according to this circulation model, the El Arenosillo station located at the coast SE from these sources is not directly affected by their emissions. The ozone concentrations observed at El Arenosillo can be explained by the ozone residual layer over the sea, similar to other coastal sites in the Mediterranean basin.

CONCLUSIONS

The temporal variations of the ozone concentrations have been studied at four measurement sites in the southwest of the Iberian Peninsula. The results obtained point out that industrial and urban emissions combined with specific meteorological conditions in spring and summer cause high ozone levels which exceed the recommended threshold limits and could affect the vegetation and human health in this area.

RECOMMENDATIONS AND PERSPECTIVES

This work is the first investigation related to surface ozone in this region; therefore, the results obtained may be a useful tool to air quality managers and policy-makers to apply possible air control strategies towards a reduction of ozone exceedances and the impact on human health and vegetation. Due to the levels, variability and underlying boundary layer dynamics, it is necessary to extend this research in this geographical area with the purpose of improving the understanding of photochemical air pollution in the Western Mediterranean Basin and in the south of the Iberian Peninsula.

Diurnal, seasonal and weekdays-weekends variations of ground level ozone concentrations in an urban area in greater Cairo

Ground level ozone (O3) concentration was monitored during the period of December 2004 to November 2005 in an urban area in Greater Cairo (Haram, Giza). During the winter and summer seasons, nitrogen dioxide (NO2) and nitric oxide(NO) concentrations and meteorological parameters were also measured. The mean values of O3 were 43.89, 65.30, 91.30 and 58.10 ppb in daytime and 29.69, 47.80, 64.00 and 42.70 ppb in whole day (daily) during the winter, spring, summer and autumn seasons, respectively. The diurnal cycles of O3 concentrations during the four seasons revealed a uni-modal peak in the mid-day time, with highest O3 levels in summer due to the local photochemical production. The diurnal variations in NO and NO2 concentrations during the winter and summer showed two daily peaks linked to traffic density. The highest levels of NOx were found in winter. Nearly, 75%, 100%, 34.78% and 52.63% of the mean daytime concentrations of O3 during spring,summer, autumn and the whole year, respectively, exceeded the Egyptian and European Union air quality standards (60 ppb) for daytime (8-h) O3 concentration. About, 41.14% and 10.39% of the daytime hours concentrations and 14.93% and 3.77% of the daily hour concentrations in summer and the whole year, respectively, exceeded the Egyptian standard (100 ppb) for maximum hourly O3 concentration, and photochemical smog is formed in the study area (Haram) during a periods represented by the same percentages. This was based on the fact that photochemical smog usually occurs when O3 concentration exceeds 100 ppb. The concentrations of O3 precursors (NO and NO2) in weekends were lower than those found in weekdays, whereas the O3 levels during the weekends were high compared with weekdays. This finding phenomenon is known as the "weekend effect". Significant positive correlation coefficients were found between O3 and temperature in both seasons and between O3 and relative humidity in summer season, indicating that high temperature and high relative humidity besides the intense solar radiation (in summer) are responsible for the formation of high O3 concentrations.