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 (known before as NMMB/BSC-CTM) 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, friction velocity, and temperature and specific humidity at 2 m 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 and boundary meteorological conditions are derived from the fifth major global ECMWF ReAnalysis (ERA-5). To evaluate the model performances, the simulations are compared (i) to groundbased 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 surface concentration, the simulation performs well during the center of the event with major underestimation at the beginning. 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 underestimated. 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 in coastal areas, 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 play a significant role on the skills of the meteorological mesoscale model.
Hourly measurements of pollen near-surface concentration and lidar-derived profiles of volume and particle depolarization ratios during a 5-day pollination event observed in Barcelona, Spain, between 27 – 31 March, 2015, are presented. Maximum hourly pollen concentrations of 4700 and 1200 m-3 h-1 were found for Platanus and Pinus, respectively, which represented together more than 80 % of the total pollen. . The pollen concentration was found positively correlated with temperature (correlation coefficient, r, of 0.95) and wind speed (r = 0.82) and negatively correlated with relative humidity (r = -0.18). The ground concentration shows a clear diurnal cycle although pollen activity is also detected during nighttime in three occasions and is clearly associated with periods of strong wind speeds. Everyday a clear diurnal cycle caused by the vertical transport of the airborne pollen was visible on the lidar-derived profiles of the volume depolarization ratio with maxima usually reached between 12 and 15 UT. On average the volume depolarization ratios in the pollen plume ranged between 0.08 and 0.22. Except in the cases of nocturnal pollen activity, the correlation coefficients between volume depolarization ratio and near-surface concentration are high (>0.68). The dispersion of the Platanus and Pinus in the atmosphere was simulated with the Nonhydrostatic Multiscale Meteorological Model on the B grid at the Barcelona Supercomputing Center with a newly developed Chemical Transport Model (NMMB/BSC-CTM). Model near-surface daily pollen concentrations were compared to our observations at two sites: in Barcelona and Bellaterra (12 km NE of Barcelona). Model hourly pollen concentrations were compared to our observations in Barcelona. Better results are obtained for Pinus than for Platanus. Guidelines are proposed to improve the dispersion of airborne pollen by atmospheric models.
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