Assessing the mineral dust from North Africa over Portugal region using BSC–DREAM8b model

Over the last decade, air pollution has become a major problem in Portugal mainly due to the high concentrations of particulate matter in the atmosphere, which surpassed the daily limit values. An abundant type of natural atmospheric aerosol is related with the suspension and long–range transport of mineral dust from North Africa deserts. The main objective of this work was to assess the mineral dust over Portugal, namely in what concerns both long–term period (one year) and episode peaks. The BSC–DREAM8b v1.0 model was applied for the entire year of 2011 and the modeled surface concentrations were explored. The annual mean of the simulated dust has a magnitude of 2–6μg m–3. The monthly average analysis highlights the largest mineral dust average values in April and May (about 4μg m–3 higher than the other months). The influence of the transport of mineral dust from North Africa to Portugal is limited on time scale, since in 50% of the time this contribution is below 0.2μg m–3. Only when high percentiles are analyzed the dust surface concentrations over Portugal become relevant (>3μg m–3; with peak contribution around 10–25μg m–3). To characterize the strongest episodes of dust, a group of days with modeled surface daily concentrations above 5μg m–3 was selected, and data were extracted for 7 sites, spatially distributed along Portugal. A cluster analysis of the air parcels back trajectories that arrive at each site was performed in order to identify the mean flow patterns associated to each mineral dust episode. The prevalence of the flow regimes coming from North Africa during the episode days was different for the studied sites, with high frequency (above 70%) at south sites. This work contributes to the characterization and assessment of the dust episodes that affect Portugal, on a yearly basis and based on a modeling approach.