Modelling of airborne pollen dispersion in the atmosphere in the Catalonia region, Spain: model description, emission scheme and evaluation of model performance for the case of Pinus

Pollination is a biologically-relevant process that affects the structure of ecosystems since pollen contributes to determine the spatial distribution of plant species. It is thus of interest for mapping ecosystem services for policy support and decision making to increase our knowledge of pollen grain behavior in the atmosphere (source, emission, processes involved during their transport, etc.) at fine temporal and spatial scales. First simulations with the Barcelona Supercomputing Center MONARCH dispersion model of Pinus pollen in the atmosphere were performed during a 5-day pollination event observed in Barcelona, Spain, between 27 – 31 March, 2015. MONARCH is an online atmospheric composition model that solves the life cycle of water vapor, gases and aerosols within a meteorological model. A new aerosol emission scheme for pollen grains has been implemented in the system. The emission scheme considers wind speed at 10 m and friction velocity as main drivers of the mobilization of Pinus pollen grains. The meteorological information is available for the emission scheme at each meteorological integration time step. The spatial distribution of the pine species (P. halepensis, P. pinea) that pollinate from February to April in Catalonia has been derived from the Cartography of habitats of Catalonia and the tree density was obtained from the Forest Inventory of Catalonia. A domain over north-east Spain at 9 km x 9 km horizontal resolution covering Catalonia is designed with 48 vertical layers. The initial meteorological conditions are derived from ERA-5. To evaluate the model performances, the simulations are compared (i) to ground-based concentration measurements performed with a Hirst collector in Barcelona downtown, and (ii) to vertically-resolved measurements performed 4 km west of Barcelona downtown with a Micro Pulse Lidar (MPL). A method based on the lidar polarization capabilities was used to retrieve the contribution of the pollen to the total signal. The conversion from optical lidar-retrieved properties to concentration was optimized by minimizing the sum of the squared deviations between the lidar-retrieved concentration at the first height and the true (Hirst) concentration measured at the ground. In terms of ground-based concentration, the simulations look usually good in the morning and worsen in the afternoon. As far as the vertical distribution of airborne Pinus pollen is concerned, simulations reproduce well the shape of the profiles but the intensity tends to be under estimated. Three major limitations are identified with the model runs: (1) the poorly known phenology emission function, (2) the temporal development of the convective planetary boundary layer, which directly affects the vertical structure of the pollen dispersion; (3) the development of the sea breeze and a proper representation of the sea coast line, that plays a significant role on the skills of the meteorological mesoscale model.