The impact of rainfall on the diurnal patterns of atmospheric pollen concentrations

Atmospheric precipitation removes the majority of biological and chemical pollutants from the lower parts of the troposphere. However, some studies infer increasing atmospheric pollen concentrations during precipitation events and/or thunderstorms, which are known to trigger allergic reactions (referred to as thunderstorm-triggered asthma). The aim of this study was to determine the impacts of precipitation on the diurnal variability of airborne pollen concentrations, with particular focus on the timing and intensity of precipitation events. We also considered the impacts of other meteorological elements on atmospheric pollen concentrations. The research was conducted in Rzeszów (SE Poland) from 2016 to 2018. We used two aerobiological and two meteorological stations in this study to determine potential spatial variability in the pollen response to rainfall, the diurnal precipitation changes, and the precipitation intensity in the city area. We determined the pollen response of four pollen types: Betula, Pinus, Poaceae, and Urtica, and we only included data from days that fulfilled restrictive criteria throughout all pollen seasons. We analysed a total of 81 days, which were divided into five groups according to precipitation intensity, the time of day of a precipitation event, and the type of pollen response to precipitation. Our results suggest that precipitation intensity was the dominant control on pollen concentrations; concentrations only decreased clearly under rainfall intensities of at least 5 $mm•h^{-1}$ and this value we recommended as threshold value for long lasting decrease of pollen concentrations in our region. Lower intensity rainfall events resulted in no change in atmospheric pollen concentrations throughout the day. We noted the occurrence of increased pollen concentration immediately before and during rainfall events. Further, other parameters were also found to influence pollen concentrations in the atmosphere, including relative humidity, the time of day, and biological factors such as the plant phenophase or the phase of a pollen season.