How to clarify pedestrian’s exposure

The TECNAIRE-CM project conducted exposure studies on air pollution, using pedestrian simulation.

The TECNAIRE-CM project conducted exposure studies on air pollution, using pedestrian simulation.

Nowadays poor urban air quality is one of the main environmental concerns worldwide generating new standards for the protection of human health with more stringent legislation. Air quality monitoring stations are used to measure pollutants concentrations in the atmosphere in urban areas. They are normally used to assess the compliance with air quality standards and they are often the only source of information about concentration levels but their temporal and spatial representativeness is unclear. Therefore, it is difficult to assess the real population exposure using the information provided from these stations.

To be able to define the representativeness of an air quality monitoring station in terms of pedestrian exposure for a defined area, it is important to know the concentration of pollutant and also the specific conditions of traffic and pedestrians in the area of interest.

For that, a methodology to assess the representativeness of an air quality monitoring station from the population exposure point of view on a preliminary case of study was developed and tested within the TECNAIRE-CM S2013/MAE-2972 project.

Figure 1. Researchers placing diffusion tubes on an air quality monitoring station.

Figure 1. Researchers placing diffusion tubes on an air quality monitoring station.

First of all, ambient concentration levels of NO2 were assessed using diffusion tubes (Figure 1). They give a good idea of spatial gradients after interpolating the results of each measuring point for the domain that they cover. Temporal variations are inferred from the measurements of an air quality monitoring station located in the modelling domain (hourly resolution). Information from both the passive samplers and the air quality monitoring station is combined to produce hourly concentration maps for a representative day.

These concentration maps were crossed with the pedestrians trajectories moving along the studied area according to the PTV Viswalk simulation to get a better understanding on the actual exposure levels within the modelling domain and their correspondence with these recorded at the air quality monitoring station used for regulatory purposes.

Figure 2. Detailed network and routes definition for traffic and pedestrian simulation.

Figure 2. Detailed network and routes definition for traffic and pedestrian simulation.

Detailed information about the road network, traffic lights locations and their phases, bus stops and schedules is needed for the traffic simulation. Whereas the pedestrian simulation also needs precise description of sidewalks, obstacles for pedestrians, position of crosswalks and number of people boarding and exiting the buses and subway stations (Figure 2). All this information was collected in an intensive field campaign in the area. PTV Vissim and the add-one module PTV Viswalk were used to generate realistic interactions between traffic and pedestrians respectively for selected representative hourly scenarios. These simulations are used to define appropriately the position of each pedestrian along the domain (with a temporal resolution of 2 seconds) taking into account their interactions with traffic, mostly in bus stops and crosswalks.

This simulation provides valuable information to assess population exposure in a particular urban hot-spot as well as the representativeness of the air quality monitoring station located in the area. This methodology is also useful to identify locations to obtain representative measures in terms of population exposure to atmospheric pollution.