Impact of urban heat island on regional atmospheric pollution

Abstract

The purpose of this work is to study the impact of an urban land cover on local meteorology and spatial distribution of atmospheric pollutants over the Paris region. One anticyclonic episode from the ESQUIF campaign was simulated using the meso-scale meteorological and chemical Meso-NHC model coupled to the town energy balance (TEB) urban canopy model. A control simulation was also performed without implementing TEB in order to quantify the effect of the urban parameterization. Both meteorological and chemical model outputs were evaluated against the data collected during the experiment and most of the results are improved when TEB is applied. The simulation indicates the formation of an urban heat island (UHI) over Paris which is stronger at night than during day. The structure of the atmospheric boundary layer is also strongly influenced by the city. The present study shows that both nocturnal and diurnal urban effects have an important impact on the primary and secondary regional pollutants, more specifically the ozone and the nitrogen oxide (NO_x). The spatial distribution and the availability of pollutants are significantly modified by the urbanized area mainly due to enhanced turbulence.

Introduction

Due to growing urbanization and pollutant emissions, it was emphasized by Crutzen (2004) that, ‘it will be important to explore the consequences of combined urban heat and pollution island effects for meso-scale dynamics and chemistry’. More precisely, it becomes a priority to understand the strong interactions between urban micrometeorology and air pollution in order to better adapt air quality policies.

Cities play a quite significant role on the local and regional scale meteorology. The morphological characteristics as well as the thermal and radiative properties of the built-up surfaces have a direct impact on the surface energy exchanges, which are quite different compared to those observed above natural soils and vegetation (Oke, 1987). At night under clear and calm conditions, a large temperature gradient develops between the city and its surroundings (Oke, 1982): such urban heat islands (UHI) were already documented for instance in Montreal (Oke and East, 1971), Paris (Escourrou, 1991), Toulouse (Estournel et al., 1983), Mexico (Oke et al., 1999) and Atlanta (Bornstein and Lin, 2000). During the day, even if the UHI can also be observed, the dynamic processes are preponderant compared to the radiative and thermal effects. The dynamic roughness of the city favours the production of turbulence and the development of the urban boundary layer (UBL) (Dupont et al., 1999).

Human activities in urban and industrial areas are large sources of atmospheric pollutants. Their spatial distribution and their temporal evolution can be largely driven by thermodynamic and dynamic processes involved above the cities. Several experimental campaigns were conducted to document the atmospheric pollution in urban areas, for instance in Athens (Kambezidis et al., 1995), in Paris (Menut et al., 2000) and in Marseilles (Cros et al., 2004). In some studies, the interactions between air pollution and dynamics have been described, such as the role of sea breeze, land breeze and orographic flows (Henne et al., 2003; Cousin et al., 2005; Corsmeier et al., 2005). However, the coupling between the urban micrometeorology and the regional atmospheric pollution has not been studied in detail, except by Martilli et al. (2003) for the city of Athens, but in combination with strong land–sea breeze flows. The role of the urban processes related to the dynamics at smaller scales (e.g. due to an urban night time shallow convective boundary layer) have not yet been investigated.

This paper deals with the interaction between urban micrometeorology and atmospheric pollution. It is focused on Paris and its surroundings. The local topography is relatively smooth and the city is located inland far from the sea. As a result, the impact of the urban processes are preponderant and can be easily distinguished (Troude et al., 2002; Lemonsu and Masson, 2002). The aim of the present paper is to study the role of the city on the evolution of the atmospheric pollutant concentrations under an anticyclonic situation (16–18 July 1999). Three-dimensional simulations were conducted with the Meso-NH atmospheric model coupled to a chemistry module. The surface is represented by using a soil and vegetation scheme (Noilhan and Planton, 1989), and also a specific urban parameterization (Masson, 2000) in order to take into account the surface perturbations induced by the urban canopy and their influences on the structure of the atmosphere. Section 2 briefly presents the ESQUIF campaign and the meteorological episode, used in this study. After a description of the model and surface schemes in Section 3, in 4 Dynamical impact of the urban area, 5 Impact of urban meteorology on pollutant distribution the numerical dynamic and chemical results are presented. In these sections, first there is an evaluation of the model against the observations, followed by a study of the inclusion of the urban processes.