Green infrastructures have been pointed out as innovative solutions to deal with current and future challenges related to air pollution and climate change. Although the potential of green infrastructures, such as green walls and green roofs, to mitigate air pollution has been documented, evidence at a local scale is still limited. This work aims to increase knowledge about the potentialities of green infrastructures in improving local air quality, focusing on particulate matter, nitrogen dioxide and ozone pollutants, and by using a local-scale computational fluid dynamics model. The ENVI-met model was applied to a particular hour of a summer day over a built-up environment centred on a main avenue in the city of Lisbon (Portugal). The dimensions of the computational domain are 618 m × 594 m × 143 m, and it contains 184 buildings, with the tallest building being 56 m. In addition to the baseline simulation, modelling was also done considering the application of green walls and green roofs to specific buildings located near the main avenue, together with a green corridor. The overall results show no disturbances exerted by green walls on the turbulent flow dynamics and on the air quality levels when compared to the baseline scenario (without green walls). The integrated scenario, which includes green walls, green roofs and a green corridor, will lead to potential local benefits of green infrastructures on O3 concentrations, followed by variable impacts on NO2 and particulate matter concentrations.

European cities have made significant progress over the last decades towards clean air. Despite this progress, several cities are still facing acute air pollution episodes, with various urban areas frequently exceeding air quality levels allowed by the European legal standards and WHO guidelines. In this paper, six European cities/ regions (Bristol, UK; Amsterdam, NL; Sosnowiec, PL; Ljubljana, SI; Aveiro, PT; Liguria, IT) are studied in terms of air quality, namely particulate matter, nitrogen dioxide and ozone. The concentrations trends from 2008 to 2017 in the different typology of monitoring stations are addressed, together with the knowledge of daily, weekly and seasonal pollution patterns to better understand the city specific profiles and to characterize pollutant dynamics and variations in multiple locations. Additionally, an analysis of the duration and severity of air pollution episodes is also discussed, followed by an analysis of the fulfillment of the legislated limit values. Each of our 6 case study locations face different air pollution problems, but all these case studies have made some progress in reducing ambient concentrations. In Bristol, there have been strong downward trends in many air pollutants, but the levels of NO2 remain persistently high and of concern. In recent years, decreasing concentration levels point to some success of Amsterdam air quality policies. PM10 exceedances are a seasonal pollution problem in Ljubljana, Sosnowiec and Aveiro region (even if with different levels of severity). While, exceedances of NO2 and O3 concentrations are still problematic in Liguria region. The main findings of this paper are particular relevant to define and compare future citizen-led strategies and policy initiatives that may be implemented to improve and fulfill the EU legislation and the WHO guidelines.

Green roofs, as part of urban green structures, have been pointed out as the solution to pursuit the goal of healthy cities. This study aims to investigate the direct, focused on meteorological changes, and indirect, related to both meteorological and emissions changes, impacts of green roofs on air quality (PM10, NO2 and O3). For that, the numerical modelling system composed by the WRF-SLUCM-CHIMERE models was applied to a 1-year period (2017), having as case study the Porto urban area. The EnergyPlus model was also applied to estimate the green roofs impacts on the building's energy needs and related impacts on air quality and atmospheric emissions. The analysis of the direct impacts showed that green roofs promote a temperature increase during the autumn and winter seasons and a temperature decrease during the spring and summer seasons. Both negative - concentrations increase - and positive - concentrations decrease - impacts were obtained for the primary, PM10 and NO2, and secondary, O3, air pollutants, respectively, due to changes in the dynamical structure of the urban boundary layer. The indirect effects of green roofs showed their potential to enhance the buildings energy efficiency, reducing the cooling and heating needs. These changes in energy consumption promoted an overall decrease of the environmental and economic indicators. Regarding air quality, the impact was negligible. The obtained results highlight the need for a multipurpose evaluation of the impacts of green roofs, with the different effects having to be traded off against each other to better support the decision-making process.

The effect of different "green" measures, such as the increase of urban green areas, the application of green roofs and the increase of surfaces albedo on urban air quality were evaluated with the WRF-CHIMERE modelling system. In order to account for the heterogeneity of urban areas, a single layer urban canopy model was coupled to the WRF model. The case study consists of a heat wave occurring in the Porto (Portugal) urban area in a future climate scenario, considering the Representative Concentration Pathway RCP8.5. The influence of the selected measures on PM10, NO2 and O-3 concentrations was quantified and compared with a control run (without measures) simulation scenario. The results revealed that all the measures are able to mitigate the effects of heat waves by reducing the air temperature between -0.5 degrees C and -1 degrees C (maximum differences for the mean of the episode). Positive and negative effects were found in terms of air quality. The implementation of green roofs and the increase of surfaces albedo promoted an overall increase of PM10 (between +0.6% and +1.5%) and NO2 (between +0.8% and 3.5%) concentrations, which are closely related to a decrease of vertical mixing in the urban boundary layer. The increase of green urban areas promoted an overall decrease (on average) of both PM10 and NO2, by around -1% and -3%, respectively. The O-3 levels increased with the increase of urban green areas, mostly located over the Porto urban area. Slight differences were promoted by the implementation of green roofs. For the increase of surfaces albedo, both increases and decreases of O-3 concentrations were observed. The obtained results contribute to the knowledge of the chemical composition Of the urban atmosphere and can be of great importance for stakeholders and decision-makers to deal with climate change impacts.

The purpose of this study is to investigate how two different atmospheric 3D modelling systems, with different dust modules, simulate a Saharan dust episode, using satellite data and in-situ observations to validate their performances. The episode occurred during 19-23 February 2016 and impacted the Iberian Peninsula. The two numerical modelling systems applied are the CHIMERE and the EURAD-IM chemistry transport models with different dust modules, both forced by the same WRF meteorological input. A common domain and resolution (27 x 27 km(2)) was adopted for the modelling setup. The comparison and evaluation of the two modelling results have shown that both models are able to capture the occurrence of the natural event, which was initiated by a cut-off low above the coast of Morocco, inducing a strong meridional transport of dust loaded air from Algeria straight towards eastern parts of the Iberian Peninsula. The most notable differences between the two model outputs concern the emission strengths and the emission source regions. In fact, different emission patterns and strengths are simulated by each model despite they use the same soil database, identical clay/silt/sand contribution for each soil type, and the same meteorological simulation. In general, CHIMERE simulates higher PM10, PM2.5, and dust concentrations than EURAD-IM for this event. In the South of Portugal, CHIMERE shows better agreement with observations, while in Central Portugal, EURAD-IM is closer to particle related measurements.