Thirteen years of air pollution hourly monitoring in a large city: Potential sources, trends, cycles and effects of car-free days
Introduction
The worsening of air quality in highly anthropized environments exerts a high level of interest within the scientific community and public opinion because of the known strong relationship between exposure to many air pollutants and increased adverse short- and long-term effects on human health (e.g., Maynard, 2009, Laumbach and Kipen, 2012). In addition, air pollution seriously impairs visibility (Hyslop, 2009), may damage materials in important buildings of the cultural heritage (Watt et al., 2009) and directly and indirectly affects the climate (Seinfeld and Pandis, 2006). Recently air pollution was also included as a IARC known carcinogen to human beings (group 1). In Europe, despite the EC legislative effort has driven an overall improvement of the air quality in the last two decades (Fenger, 2009), some hot-spots still remain. It is therefore imperative to focus the research on determining the ambient levels of hazardous pollutants in the areas, where the limits proposed by international organizations (WHO, 2000) or imposed by the European Directives are not or not yet met, and a strong potential adverse effect on public health exists. This is the case of the Po Valley (Northern Italy).
This study analyzes a dataset of 13 airborne pollutants recorded at an urban background site in Mestre-Venice, a large city of the Po Valley, for 13-years (November 2000–November 2013): carbon monoxide (CO), nitrogen oxides (NOx = NO + NO2), ozone (O3), sulphur dioxide (SO2), benzene, toluene, ethylbenzene, and ortho-, meta-, and para-xylenes (collectively known as BTEX) and particulate matter with an aerodynamic diameter of ≤ 10 μm (PM10). CO is mainly generated by photochemical breakdowns of methane and nonmethane hydrocarbons as well as directly emitted by most anthropogenic combustion processes and its major sink is the oxidation by hydroxyl radical (Seinfeld and Pandis, 2006). Ozone is a reactive oxidant gas playing a key role in the photochemical air pollution and atmospheric oxidation processes. Although in the upper atmosphere it acts as a barrier for ultraviolet rays, in the troposphere it is a secondary air pollutant generated through a series of complex photochemical reactions involving solar radiation and ozone-precursors, i.e. NO2 and reactive hydrocarbons of biogenic (Curci et al., 2009) and anthropogenic (Hoor et al., 2009) origin.
Nitrogen oxides in urban environments are principally emitted from fossil fuel combustion as NO, which plays important roles in the atmospheric chemistry by rapidly reacting with ozone or radicals and forming NO2. In the presence of sunlight, NO2 is then photolyzed by a short-wave solar radiation (λ < 398 nm) to NO and free oxygen. Such equilibrium, named photostationary state, describes the complex photochemistry of the NO–NO2–O3 system in the lower troposphere, which, at a local scale, is dominated by a limited number of fast reactions including CO and many volatile organic compounds (VOCs). Because of the strong inter-conversion between ozone and NO2, the level of oxidants (OX = O3 + NO2) may give some insights into the oxidative potential of the atmosphere (Kley et al., 1999). Furthermore, increasing atmospheric NOx concentrations may favour nitric acid formation as a result of the daytime gas phase recombination reaction of ∙ OH with NO2, and have therefore a key role for the secondary inorganic aerosol generation (Finlayson-Pitts and Pitts, 2000).
Sulphur dioxide is emitted in the atmosphere from both natural (volcanoes, grassland, forest fires) and many anthropogenic sources involving the use of fossil fuels (crude oil and coal transformation processes, fossil fuel combustion, metal smelting and other industrial processes). SO2 may oxidize to S(VI) species and in this form it acts as a sulphate precursor in the aerosol system by modifying the direct and indirect radiative forcing and enhancing the acid deposition.
In urban environments BTEX are principally emitted by vehicular exhaust gases because of their presence in fuels and lubricating and heating oil. Other sources are gasoline evaporation, use of solvents and paintings, biomass burning, leakage from natural gas and liquefied petroleum gas. BTEX are highly reactive in the troposphere playing a key role in the atmospheric chemistry as important photochemical precursors for tropospheric ozone and secondary organic aerosol generation (Atkinson and Arey, 2003).
The main goals of this study are to: (1) detect the long-term trends in relation to the changes in the emission scenarios occurring in the last decade, including the technological improvements, the decline of many industrial activities and the construction of a new highway outside the city; (2) investigate the seasonal, weekly and daily cycles of pollutants to depict the most probable emission sources and photochemical processes; (3) study the inter-species relationships and their lagged correlations with some weather parameters to detect the role of micro-meteorological factors upon the air quality and (4) assess the location of the potential sources by combining atmospheric circulation and pollution data through bivariate polar plots and conditional probability functions. In addition, a new statistical procedure is presented and tested to investigate the periods when extreme mitigation measures, such as the total stop of traffic and car-free days, were adopted in the city. Results are then discussed to assess the real effects upon the air quality and to help policy-makers to adopt successful mitigation strategies. A summary of the potential harmful effects upon human health is presented in Supplementary Information Table SI1.
Section snippets
Study area: emission sources and weather
Mestre (Fig. 1) is a large mainland city where most of the population of Venice Municipality live: 271,000 inhabitants in 2001 and 263,000 in 2011 (ISTAT, 2012). It is located in the eastern part of the Po Valley, between the Adriatic Sea and a heavily anthropized mainland, 9 km WNW from the historic city centre of Venice, which lies in the middle of a ~ 550 km2 wide coastal lagoon. The main anthropogenic emissions include:
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the southern conurbation of Mestre includes Porto Marghera, one of the
Results and discussion
In urban environments the gaseous pollutants, analysed in this study, are mainly emitted by mobile and stationary combustion processes or/and are directly/indirectly involved in complex photochemical processes. While atmospheric processes are mainly driven by actinic fluxes and oxidant species (e.g., ozone, nitrogen dioxide, hydroxyl and other radicals), the anthropogenic emissions may vary as a consequence of changes in human processes and habits. Therefore, the long-term variations and the
Conclusions
By providing direct information on the levels and trends of key pollutants, this study enables some general considerations to evaluate the air pollution extent and the effects of commonly adopted mitigation strategies in a large city of Po Valley, a remaining hotspot for air pollution in Europe. Some insights are provided to drive future successful plans and actions. The main results and conclusions can be summarized as follows:
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CO, SO2 and benzene levels were relatively low in the study period
Disclaimer
This research was not funded by public or private institutions. The views expressed in this paper are exclusively of the authors and may not reflect those of ARPAV.
Acknowledgments
This study is the document no. 3 of a cooperation between the Ca' Foscari University of Venice and ARPAV. A series of weather data were provided by Ente della Zona Industriale di Porto Marghera (www.entezona.it).
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