Study of the influence of surface roughness and topography on wind profiles and the dispersion of atmospheric pollutant in two populated hills in Cusco, Peru

The urban areas of many developing cities are suffering from environmental problems due to overpopulation and inadequate public services, in that sense, air pollution is one of the biggest problems. In general, Latin American cities have a higher density of vehicles and are therefore prone to experience high contributions of vehicular pollution. Considering also that the vehicle fleet is old and, in many cases, poorly maintained compared to more developed cities.

The dispersion of pollutants is mainly influenced by wind characteristics, which in turn are influenced by surface roughness (urban coverage) and mountain topography. The objective of the study is to evaluate the influence of surface roughness and topography on wind profiles and the dispersion of atmospheric pollutants in two populated hills located in the city of Cusco, the first called UNSAAC and the second Independencia Hill.

The analysis will be carried out using the numerical model RANS ENVI-met, which determines the dispersion of air pollutant taking into account the interaction between the cover and the atmosphere. The input of the model will be the topographic information, hourly meteorological data and the concentration of pollutants (NO₂, SO₂, O₃, PM10) measured in the field for two months.

In the UNSAAC area, the urban coverage extends along one of the faces of a mountain with a 21 % slope and in the Indepencia area, the urban coverage is located between two mountains with a slope of 15 % (see Figure 1). Regarding roughness, 3 cases were evaluated: zero roughness (topography without buildings), normal roughness (topography with buildings) and increased roughness (topography with doubled-height buildings). Two wind directions were evaluated: 180° and 360°.

Figure 1: Northern axis of evaluation in the Independencia and UNSAAC area

According to the results, the velocity in the boundary layer is lower when the roughness is increased for both study areas; this difference is greater when the wind direction is 360° (see Figure 2). It can also be observed that the height of the boundary layer is higher in the urban area of Independencia. Here, the velocity exceeds 2 m/s at a height of 20 m, while, in the other profiles it exceeds this value at a height less than 5 m. On the other hand, a peak in the NO₂ concentration values ​​with 180 µg/m³ can be observed in the urban area of ​​Independencia (see Figure 3).

The results of the study may be useful to buid a risk map of both areas, in order to identify areas with high concentrations of pollutants, and propose measures to reduce pollution, such as limiting the number of vehicles on certain roads.

Figure 2: Wind profiles for a) UNSAAC zone WD= 180° b) UNSAAC zone WD= 360° c) Independencia zone WD= 180° d) Independencia zone WD= 360°

Figure 3: NO₂ concentration for a) UNSAAC zone  and b) Independencia zone