engleski

Atmospheric conditions during high ragweed pollen concentrations in Zagreb, Croatia

We examine the atmospheric conditions favourable to the occurrence of maximum concentrations of extremely allergenic ragweed pollen. Over the 2002-2009 period, daily pollen data collected at Zagreb were used to identify two periods of high pollen concentration (> 600 grains/m3 of air) for our analysis: period A (3-4 September 2002) and period B (6-7 September 2003). Synoptic conditions in both periods were very similar: Croatia was under the influence of a lower sector high pressure system moving slowly eastward over Eastern Europe. During the 2002-2009 period, this type of weather pattern (in ~ 70% of days), in conjunction with almost non-gradient surface pressure conditions in the area (in ~ 30% of days) characterized days when the daily pollen concentrations were higher than 400 grains/m3 of air. Numerical experiments using a mesoscale model at fine resolution showed successful multi-day simulations reproducing the local topographic influence on wind flow and a reasonable agreement with available observations. According to the model, the relatively weak synoptic flow (predominantly from the eastern directions) allowed local thermal circulations to develop over Zagreb during both high pollen periods. Two-hour pollen concentrations and 48-h back-trajectories indicated that regional-range transport of pollen grains from the central Pannonian Plain was the cause of the high pollen concentrations during period A. During period B, the north-westward regional-range transport in Zagreb was significantly supplemented by pronounced horizontal recirculation of the pollen grains. This recirculation happened within the diurnal local circulation over the city, causing a late-evening increase in pollen concentrations.

slope winds; urban heat island circulation; recirculation of pollen grains; WRF model; regional transport

nije evidentirano