Contribution of anthropogenic pollutants to the increase of tropospheric ozone levels in the Oporto Metropolitan Area, Portugal since the 19th century

Abstract

The main purpose of this study was to evaluate the contribution of anthropogenic pollutants to the increase of tropospheric ozone levels in the Oporto Metropolitan Area (Portugal) since the 19th century. The study was based on pre-industrial and recent data series, the results being analyzed according to the atmospheric chemistry. The treatment of ozone and meteorological data was performed by classical statistics and by time-series analysis. It was concluded that in the 19th century the ozone present in the troposphere was not of photochemical origin, being possible to consider the respective concentrations as reference values. For recent data a cycle of 8 h for ozone concentrations could be related to traffic. Compared to the 19th century, the current concentrations were 147% higher (252% higher in May) due to the increased photochemical production associated with the increased anthropogenic emissions.

Introduction

Increased tropospheric ozone levels have been affecting human health, climate, vegetation, materials and atmospheric composition. Respiratory and ocular damage are the most significant effects on human health. Concerning climate, a temperature increase is expected to be related to the tropospheric ozone increase, because it is a greenhouse gas. In vegetation it causes leaf injury, growth and yield reduction, and changes in the sensitivity to biotic and abiotic stresses.

During the pre-industrial era, the ozone found in the troposphere came essentially from the stratosphere, through intrusion of stratospheric air. Recent estimates indicate that stratospheric–tropospheric exchanges only account for 20% of the current total tropospheric ozone, because now it is mainly produced by complex photochemical reactions involving solar radiation and anthropogenic pollutants (Marenco et al., 1994). Hence increased tropospheric ozone levels of photochemical origin can be correlated with the increased emissions of anthropogenic pollutants.

Photochemical ozone is formed by reactions involving solar radiation and anthropogenic pollutants (methane, non-methane volatile organic compounds, carbon monoxide) in the presence of nitrogen oxides.

In less polluted environments, ozone is produced in the presence of sunlight (at wavelengths <424 nm), through the photolysis of NO₂:

$${\text{NO}}_2+h\upsilon \to \text{NO}+\text{O}$$

$$\text{O}+{\text{O}}_2+M\to {\text{O}}_3+M$$

where M represents N₂, O₂ or other molecules that can absorb the excess of vibrational energy, allowing the stabilization of O₃. Once formed, O₃ quickly reacts with NO regenerating NO₂:

$${\text{O}}_3+\text{NO}\to {\text{NO}}_2+{\text{O}}_2$$

In the absence of other chemical species this cycle reaches a photo-stationary equilibrium between NO, NO₂ and O₃, without effective ozone formation, and with an ozone concentration proportional to the ratio [NO₂]/[NO]. In less polluted environments ozone production may also involve the radicals hydro-peroxile (HO₂) and methyl peroxile (CH₃O₂), intermediate products of CO and CH₄ oxidation:

$${\text{HO}}_2+\text{NO}\to \text{OH}+{\text{NO}}_2$$

$${\text{CH}}_3{\text{O}}_2+\text{NO}\to {\text{CH}}_3\text{O}+{\text{NO}}_2$$

These reactions with NO occur when the ratio [NO]/[O₃] is significantly high. The global reactions of oxidation of CO to CO₂ and CH₄ to HCHO are the following:

$$\text{CO}+2{\text{O}}_2+h\upsilon \to {\text{CO}}_2+{\text{O}}_3$$

$${\text{CH}}_4+4{\text{O}}_2+h\upsilon \to \text{HCHO}+{\text{H}}_2\text{O}+2{\text{O}}_3$$

In polluted environments the photochemical production of ozone cannot be explained solely by the CO–CH₄–NO_x equilibrium, mainly due to the additional presence of volatile organic compounds (VOC). The VOC oxidation cycle disturbs the natural equilibrium NO_x–O₃, allowing alternative paths for the oxidation of NO to NO₂ without consumption of O₃. VOC oxidation mechanisms are mainly induced by OH radical during the day and by NO₃ radical during the night, leading to the production of HO₂ and RO₂ radicals. These radicals are able to oxidize NO to NO₂ avoiding reaction (3), and leading to the accumulation of ozone through reactions (1), (2). These mechanisms explain the high concentrations of ozone in polluted areas, since the radiation is high enough to initiate the photodissociation process. The reactions involved can be simplified as follows:

where CARB is either a carbonyl species (RCHO) or a ketone (RCRO).

The destruction of ozone is accomplished by chemical and photochemical degradation, by the oxidative cycle of CO and CH₄, and by dry deposition on the soil. The main reactions are the following:

$${\text{O}}_3+h\upsilon \to {\text{O}}_2+\text{O}\left({}^1\text{D}\right)\lambda <310\text{nm}$$

$$\text{O}\left({}^1\text{D}\right)+{\text{H}}_2\text{O}\to 2\text{OH}$$

$${\text{HO}}_2+{\text{O}}_3\to \text{OH}+2{\text{O}}_2$$

$$\text{OH}+{\text{O}}_3\to {\text{HO}}_2+{\text{O}}_2$$

$$\text{CO}+{\text{CH}}_4+{\text{O}}_3+h\upsilon \to {\text{CO}}_2+{\text{CH}}_3\text{OOH}$$

Previous studies showed that NO_x emissions are mainly responsible for ozone formation in rural areas, whilst VOC are responsible in urban areas (EEA, 1998). There is a competition between VOC and NO_x for the OH radical. When [VOC]/[NO_x] is high, OH will react mainly with VOC (NO_x limited), generating new radicals and accelerating O₃ production. Under these conditions, typical of rural areas, an increase in NO_x concentration accelerates O₃ formation. When [VOC]/[NO_x] is low, the reaction of OH with NO_x can predominate (VOC limited), removing OH from the VOC oxidation cycle and retarding the further production of O₃. Under these conditions, typical of polluted areas, an increase in NO_x concentration leads to O₃ decrease. Therefore, while increasing of VOC concentration always increases ozone formation, increasing of NO_x leads to more or less ozone, depending on the prevailing ratio between [VOC] and [NO_x] (Barros, 1999, Guicherit and Roemer, 2000, Pereira et al., 2005, Sadanga et al., 2003, Seinfeld and Pandis, 1998).

Although ozone is now considered a pollutant in the troposphere, when it was identified as an atmospheric trace constituent, by F. Schönbein around 1840, medical doctors considered it as an index of healthy air, attributing to it an important role on the control of epidemic diseases due to its strong oxidative properties. Therefore, many stations were installed for ozone monitoring in places that wished to be considered healthy. During the decades 50 and 60 of the 19th century, ozone was a fashionable subject. Many meteorologists, physicians, chemists and medical doctors developed studies to prove its existence, to discover its function in the atmosphere and to evaluate its erroneously presumed role in the spread of epidemics. The first semi-quantitative analytical methodology for measuring ozone concentrations in the atmosphere, expressed in ozonometric degrees, was developed by Schönbein. Improved versions were first introduced by Bérigny into France in 1856, being used until early 20th century. Albert-Lévy began ozone measurements at Parc Montsouris (Paris) in 1876 using simultaneously a new quantitative method and the improved Schönbein's method. These simultaneous measurements, which were conducted for 34 years, allowed the conversion between the two methods, being the basis of all the methods since developed to convert ozonometric degrees into modern units (Pavelin et al., 1999). The Schönbein-type tests have recognized interferences that were attempted to compensate for their effects, in order to improve the quality of the measurements (Bojkov, 1986, Volz and Kley, 1988). Humidity is the most important interference that was analyzed by Linvill et al. (1980), obtaining a chart later modified by Anfossi et al. (1991), relating ozone levels and relative humidity (Marenco et al., 1994, Pavelin et al., 1999). A precision between 25% and 33% can be expected for the ozone concentrations deduced from ozonometric degrees (Anfossi and Sandroni, 1997, Marenco et al., 1994). The increased tropospheric ozone enhanced the relevance of pre-industrial ozone levels. Therefore, in spite of the conversion precision, the pre-industrial data are of great interest if they are used cautiously. Trends in tropospheric ozone are highly variable and depend on region (Logan et al., 1999). Nevertheless, several studies have showed the increase of tropospheric ozone concentrations (Baldassano et al., 2003, Dueñas et al., 2002, Guicherit and Roemer, 2000). Some scientists have pointed out the need for more detailed studies about tropospheric ozone, not only because the actual knowledge of their effects, but also because the chemical processes in the troposphere are important to interpret stratospheric processes (Bojkov, 1986). Furthermore, as pre-industrial ozone data were measured in an atmosphere not significantly influenced by human activities, the interest in the search of those data was strongly stimulated since they can provide a reference for the levels and behavior of ozone (Anfossi et al., 1991). The main purpose of this study was to evaluate the contribution of anthropogenic pollutants in the increased surface ozone levels in the Oporto Metropolitan Area (Oporto-MA) since the 19th century. The study developed was based on pre-industrial and recent data series. The results were analyzed according to the atmospheric chemistry characteristic of the periods corresponding to the two series. The treatment of ozone and meteorological data was performed by classical statistics and by time-series analysis.