Chapter 5.6 Long-term aerosol simulation for Portugal using the CHIMERE model

Abstract Air pollution is a major environmental health problem causing approximately 3 million deaths per year in the world, as a result of exposure to particulate matter (PM). Portugal, as a European Union Member, should follow the main objectives for management and quality of ambient air, namely those related to particulate matter. An increase of scientific studies during the last years have confirmed that long-term exposure to particulate matter pollution leads to adverse health effects. These studies generally use air pollution measurements from stationary air monitoring sites to determine population exposure levels. However, because of the large local variations in pollution concentrations, the estimates are often associated with high uncertainties. Besides that, aerosols are comprised not only of primary particles emitted directly to the atmosphere, but also of products from gas-to-particle conversion (sulphur oxides, nitrogen oxides, volatile organic compounds and semi-volatile organic compounds) in clear air and clouds. Thus, considerable research is needed to better understand the complex processes of aerosols formation, transport and deposition. For an improvement on these processes knowledge and more accurate exposure estimation, air quality models can be used as important tools with the ability to provide detailed information on pollutants concentration fields. The main purpose of this study is to perform a first long-term air quality assessment for Portugal, regarding aerosols and particulate matter pollution. The CHIMERE chemistry-transport model, forced by the MM5 meteorological fields, was applied over the Continental region of Portugal for the 2001 year period, with 10 km horizontal resolution, using an emission inventory obtained from a spatial top-down disaggregation of the 2001 EMEP national database. In order to verify the ability of the model system to simulate particulate matter, an evaluation model exercise is performed based on statistical comparison between PM 10 and PM 2.5 concentration data observed at 20 air monitoring ground stations and the correspondent simulated values. Besides that, PM components (nitrate, sulphate, ammonium, etc.) are evaluated at two specific sampling (experimental) sites near two urban waste incinerators. There is a model trend to overestimate particulate pollution episodes (peaks) at urban sites, especially in winter season. This could be due to an underprediction of the winter model vertical mixing or an incorrect spatial and temporal disaggregation and consequent overestimation of local PM emissions. Time series spectral decomposition was also used to evaluate the model performance (accuracy) at different time scales. Nevertheless, as a first approach, and despite the complex topography and coastal location of Portugal affected by sea salt natural aerosols emissions, the results obtained show a modelling system able to reproduce the particulate matter levels temporal evolution and spatial patterns. The concentration maps reveal specific areas with critical particulate matter values that are not yet covered by the air quality monitoring network. In this way, this long-term simulation study could be used as a useful tool for air quality management, as well as for protecting human health.