Turbulence Measurements from a Near-Field Pollutants Dispersion Campaign in a Stratified Surface Layer

Pollutants dispersion in a stable atmospheric boundary layer and in complex environment is still a phenomenon that is relatively poorly described by modeling and difficult to reproduce in a wind tunnel. Nevertheless, this topic is of major interest in the field of air pollution from human activities such as industrial risks and road transportation, as stable conditions induce large fluctuations of pollutants concentrations with possible occurrence of very high values. In order to study pollutants dispersion in a stably stratified surface layer at small scales, a pollutants dispersion field experiment program is being conducted on the site SIRTA (Instrumental Site of Research by Atmospheric Remote Sensing) in the southern suburb of Paris.

The aim of this experiment is to characterize the fine structure of turbulence and associated dispersion through high temporal and spatial resolution measurements. The main features of the campaign are: experiment in near field (50 to 100 m), focus on stable thermal stratification (but may include some neutral stratification or slightly convective situations), high frequency measurements to cover the entire frequency spectrum of fluctuations, and large number of sensors measuring simultaneously turbulence and concentration of tracer gas. We have 12 ultrasonic anemometers measuring wind velocity and temperature at 10 Hz, and 6 photo-ionization detectors (PIDs) measuring gas concentration at 50 Hz. Turbulence measurements have been recorded continuously for more than 2 years, while concentration measurements have been performed during short (about 1 hour) releases of gas for specific meteorological conditions (Intensive Observation Period, IOP).

The first step in data analysis is to characterize the turbulence spatially inhomogeneous structure through statistical processing such as variances, integral length scales, velocity correlation and spectra, so that the associated pollutants dispersion can be well described in the future.

Figure (a) shows the whole measurement area in SIRTA site. The campaign is carried out in Zone 1 which is bounded in the north by a forest and in the south by a road. Figure (b) shows position of the sensors used for this experiment. Red triangles represent ultrasonic anemometers and green dots represent the PIDs. There are four anemometers around the source (NE, NW, SE and SW) and five others in a circle at a distance of 50m from the source (20N, 10N, 0, 10S and 20S). These 9 anemometers and all the PIDs (1, 2, 3, 4, 5 and “background”) are at height of 3 m. In addition, there are 3 anemometers on the two masts in the southeast and southwest of the field: 2 are at height of 10 m (10mSE and 10mSW), 1 is at height of 30 m (10mSE). So, turbulent fluctuations are measured at 3 different levels with an extensive network at 3 m's height.

Turbulence studies are made for wind measurements during IOPs in tracer tests and during several other periods chosen from continuous measurements with typical meteorological situations (Easterly /westerly wind and stable/slightly stable/near neutral condition). A strong anisotropy of turbulence in stratified surface layer has been seen in all the periods studied, by different order of magnitude in variances and integral length scales between the three wind velocity components. Propagation of turbulent structures between sensors has been demonstrated by velocity correlations at 3m height, and we obtain an advection speed higher than the average wind speed measured by sensors at this height, as previously reported in the literature. The difference between these two speeds seems to be more important for a more stable surface layer. Energy spectra show several slopes in different frequency regions. In addition to the -5/3 slope representing the inertial subrange, a -1 slope has been found in all the spectra including the vertical component's one. This intermediate spectral region has been attributed by other authors to a ‘top-down' transport mechanism of turbulent structures which are blocked and elongated by the ground. Variation of spectrum form appears to be influenced by measuring altitude and stability. Moreover, the heterogeneity of the site plays an important role for some wind sectors. In particular, the forest to the north modifies the wind velocity and direction for a large northerly sector, and some shelters influenced also the flow in the easterly sector. On the other hand, there are no significant obstacles in the westerly sector. Based on the remaining continuous turbulence measurements data set, a detailed analysis of the different turbulence characteristics mentioned above as functions of wind sector and stability will also be presented.