Vera Rodrigues

CFD modelling of the aerodynamic effect of trees on urban air pollution dispersion

The current work evaluates the impact of urban trees over the dispersion of carbon monoxide (CO) emitted by road traffic, due to the induced modification of the wind flow characteristics. With this purpose, the standard flow equations with a kε closure for turbulence were extended with the capability to account for the aerodynamic effect of trees over the wind field. Two CFD models were used for testing this numerical approach. Air quality simulations were conducted for two periods of 31h in selected areas of Lisbon and Aveiro, in Portugal, for distinct relative wind directions: approximately 45° and nearly parallel to the main avenue, respectively. The statistical evaluation of modelling performance and uncertainty revealed a significant improvement of results with trees, as shown by the reduction of the NMSE from 0.14 to 0.10 in Lisbon, and from 0.14 to 0.04 in Aveiro, which is independent from the CFD model applied. The consideration of the plant canopy allowed to fulfil the data quality objectives for ambient air quality modelling established by the Directive 2008/50/EC, with an important decrease of the maximum deviation between site measurements and CFD results. In the non-aligned wind situation an average 12% increase of the CO concentrations in the domain was observed as a response to the aerodynamic action of trees over the vertical exchange rates of polluted air with the above roof-level atmosphere; while for the aligned configuration an average 16% decrease was registered due to the enhanced ventilation of the street canyon. These results show that urban air quality can be optimised based on knowledge-based planning of green spaces.

Pedestrian Exposure to Air Pollution in Cities: Modeling the Effect of Roadside Trees

The exposure of students to traffic-emitted carbon monoxide (CO) in their daily walk to school is evaluated, with a particular emphasis on the effect of trees and route choice. The study is focused on the city centre of Aveiro, in central Portugal. Time evolution of the georeferenced location of an individual is tracked with a GPS for different alternative walking routes to a school. Spatial distribution of CO concentration is simulated with a computational fluid dynamics (CFD) model. An exposure model is developed that associates the georeferenced location of the student with the computed air quality levels (at an average breathing height) for that specific grid cell. For each individual, the model calculates the instantaneous exposure at each time frame and the mean value for a given period. Results show a general benefit induced by the trees over the mean exposure of the student in each route. However, in the case of instantaneous exposure values, this is not consistent along the entire period. Also, the variability of the estimated exposure values indicates the potential error that can be committed when using a single value of air quality as a surrogate of air pollution exposure.

Impact of urban planning alternatives on air quality: URBAIR model application

In the last decades, the study of the urban structure impacts on the quality of life and on the environment became a key issue for urban sustainability. Nowadays the relevance of urban planning for the improvement of the interactions between different land uses and economic activities, and also towards a more sustainable urban metabolism, is consensually accepted. A major interest relies on understanding the role of planning on induced mobility patterns and thereafter on air quality, particularly related with the increasing use of private cars. This is one of the main objectives of BRIDGE, a research project funding by the European Commission under the 7 th Framework Programme and focused on “SustainaBle uRban plannIng Decision support accountinG for urban mEtabolism”. In this scope, and to evaluate the impact on air quality due to different city planning alternatives (PA), the urban scale air quality modelling system URBAIR was applied to selected areas in Helsinki (Finland), Athens (Greece) and Gliwice (Poland), to estimate traffic related emissions and induced pollutant concentration of different air pollutants, in a hourly basis for the entire year of 2008. For the Helsinki study case the results suggest that urban traffic and building placement considered on the different PA have an influence on local air quality despite no significant concentration levels. In the Athens case study some PA induce a decrease on traffic flows with an improvement of the air quality over the domain. On the contrary, other leads to an increase of PM10 in selected hotspots. The simulations for the Gliwice study case show minor changes between the baseline and the PA, since the proposed interventions do not imply major changes in traffic flows.

Adaptation to Climate Change at Local Scale: A CFD Study in Porto Urban Area

Green infrastructures play an essential role in urban planning, namely with their potential to reduce the impact from air pollution episodes together with extreme weather events. This chapter focuses on the assessment of green infrastructures’ benefits on current and future microclimate and air quality patterns in Porto’s urban area (Portugal). The effects of green infrastructures on flow dynamics are evaluated for the baseline scenarios by means of numerical and physical simulations, using the computational fluid dynamics (CFD) model VADIS and the wind tunnel of the University of Aveiro. The baseline morphological (BM) scenario focuses on the current morphological characteristics of Porto’s urban area, while a baseline green (BG) scenario comprises the replacement of built-up areas by green areas and parks. In addition, the benefits of green infrastructures on air quality are assessed for the baseline and under future climate scenarios. The air quality simulations focus on particulate matter, one of the most critical air pollutants with severe impacts on human health. For the BM scenario, the simulated concentrations are compared with hourly averaged PM10 concentrations measured during a weekday at the air quality station located within the study domain.

MEASURES TO REDUCE AIR POLLUTION CAUSED BY FUGITIVE DUST EMISSIONS FROM HARBOUR ACTIVITIES

Emissions from harbour-related activities have an important impact on air quality; therefore, improved knowledge about the coastal microclimate and consequent air pollution dispersion patterns is of utmost importance. In recent years, residents of the southeast urban community of the Port of Aveiro (Portugal) have identified high levels of dust in and around their residences, which has raised their concern regarding the potential effects of air pollution on public health. The citizens’ complaints were linked to fugitive dust emissions from petroleum coke (petcoke), which is usually unloaded or temporarily stored outdoors in the port prior to transportation to a nearby manufacturing plant. Following this, the air quality measurements taken in the area have shown high levels of PM10 concentrations, especially when the wind blew from north and northwest directions. Furthermore, a numerical and physical modelling study has been performed in order to assess the impacts of the transport and storage of petcoke on the local air quality. The modelling results pointed out to a set of potential mitigation measures, namely the construction upwind of different barriers from the petcoke pile. This article presents the characterization of the problem and the management strategies adopted. It also presents the results of modelling assessment to select the most potential effective barrier in order to minimize petcoke dust impact on the surrounding population.

Air quality, urban fluxes and cities resilience under climate change – a brief overview

Cities are home of about 70% of the European population, and it is expected that by 2050 this number will be achieved around the world. As a result, cities are facing important challenges related to pressures induced by changes on urban metabolism and by climate change (CC) effects. Understanding the key factors determining the vulnerability of a city provides a framework to consider resilience. These issues become more and more relevant since changes in urban structure and metabolism together with CC will start affecting atmospheric emissions, urban air quality and consequently human health. Cobenefits of an urban air quality and CC approach need to be further addressed taking into consideration that CC creates both risks and opportunities. Therefore, an integrated assessment of the interaction between resilience factors and CC effects, and its influence on the urban microclimate and air quality, is required to better adapt to future climate. This knowledge is fundamental to support policy makers and stakeholders helping them to choose the best strategies to manage resources, to mitigate extreme weather events and to predict air pollution episodes. This article offers a systematic review on CC impacts in cities, providing some adaptation strategies and suggesting future research topics.

Influence of different complexity levels of road traffic models on air quality modelling at street scale

Urban mobility accounts for 38 and 19% of nitrogen oxide (NOx) and particulate matter (PM) emissions at European urban areas, respectively. Despite of all the technological development around automobile industry, urban areas are still facing problems related to exposure to high levels of air pollutants. Increasing the accuracy of both emissions and air quality modelling from road traffic is a key-issue for the management of air pollution in road transport sector. This study assessed the influence of using different road traffic emission models on the accuracy of air quality modelling with street-level resolution, having as a case study an urban area located on the centre region of Portugal. Two emission models, with different complexity levels regarding the ability to characterise the traffic dynamics were analysed, namely, transport emission model for line sources (TREM) and vehicle-specific power (VSP), based on data obtained in an experimental campaign. To perform the air quality simulations, the pollutant dispersion in the atmosphere under variable wind conditions (VADIS) model was used and two pollutants were analysed: NOx and PM10. The results showed that the magnitude of PM10 and NOx concentrations were result of a conjoint influence of traffic dynamics and meteorological conditions. Comparison between measured and modelled data showed that the VADIS model could track the evolution of NOx levels, for both emission models considered, displaying a high correlation (> 0.8) between traffic-related NOx emissions and NOx concentrations. For PM10, VADIS model is more sensitive to the differences in the emissions calculation; however, it was observed that the traffic-related PM10 emissions accounts 1.3–8.4% to the PM10 concentration levels at the study area.

IDENTIFICATION AND ANALYSIS OF SOURCE CONTRIBUTIONS TO THE AIR QUALITY IN THE AMSTERDAM REGION

Air pollution has become a growing concern in the past few years, with an increasing number of acute air pollution episodes in many cities worldwide. Ozone (O3) is a powerful oxidising agent and one of the air pollutants of most concern in Europe. Source apportionment modelling provides valuable information on the contributions of different source sectors and source regions to ozone concentrations. This information can be useful in designing air quality management strategies and in understanding the potential benefits of reducing emissions from a particular source category. In the present work, the Comprehensive Air Quality Model with Extensions (CAMx), with its Ozone Source Apportionment Technology (OSAT), was applied over Amsterdam Region, Netherlands, for the period of 8th to 12th July 2010, when high concentration of ozone were measured in several air quality monitoring stations in the region of Amsterdam. The contributions of different source categories and the NOx/VOC limitation to the ozone formation were quantified. Results indicated a great influence of transboundary transport on the O3 simulated concentrations. The OSAT tool revealed that this is the main factor affecting O3 levels in the area of study, followed by VOC emissions from solvent use and NOx emissions from industry and other sources not individually analysed. O3 formation is slightly more NOx limited than VOC limited.

Modelling the Effects of Urban Morphology, Traffic and Pedestrian Dynamics on Students’ Exposure to Air Pollution

The United Nations Environment Programme stated that the human exposure to air pollutants primarily emitted by road traffic is associated to nearly 0.8 million premature deaths annually, especially in sensitive groups such as children. This paper evaluates the individual exposure of students in a Portuguese town accounting for their walk to school and the time spent in the classroom. The analysis was carried out for 8 hypothetical walking routes that were tracked and profiled with a GPS. Ambient air pollutant concentrations of CO and PM10 were simulated with a Computational Fluid Dynamics (CFD) model, while indoor air pollutant levels were estimated for different classrooms using the simulated outdoor concentrations and a mass transfer approach. Results show that the individual exposure of pedestrians in a city is extremely spatially dependent, as a consequence of air pollutant dispersion patterns, leading to significant disparities between the children’s exposure. A tendency between the orientation of classroom’s facade and resulting exposure was not found, but in the case of the outdoor exposure the children coming from N and NE have the highest values. The variability of the estimated exposure values shows the magnitude of the error that can be committed when using a single value of air quality as a surrogate of air pollution exposure. This problem can be overcome by using building-resolving CFD models that provide an accurate and detailed understanding of how human behaviour and habits can affect the exposure of urban citizens and ultimately their health.

Merging the Gap Between Meso and Micro Scales: Enhanced Inflow Boundary Conditions for CFD Modeling of Urban Air Quality

Despite the advent of high performance numerical tools, the spatial gap between meso and microscale models is still a challenge in atmospheric modeling. The way boundary conditions are prescribed is crucial for the correct linkage between scales, especially in inhomogeneous canopies, which induce complex airflow dynamics and pollutant concentration patterns. The purpose of this work is to evaluate the sensitivity of microscale modeling results to distinct methods of deriving inflow vertical wind profiles in urban areas. The methodology involves the comparison of three methods for prescribing inflow profiles: (1) Log wind profile based on meteorological observations, (2) the mesoscale model WRF running at 200 m resolution, and (3) the LES model ARPS. The different profiles obtained were fed into the CFD model VADIS. The comparison of results against measured SF6 concentrations shows that ARPS is capable of accounting for upwind region characteristics, providing more reliable inflow conditions. Moreover, VADIS has provided an enhanced and detailed insight over the 3D wind flow behaviour, accounting for the effects of buildings and trees, and its effect over dispersion.