Lidar-Based Observations of Pollen Above a Mixed Hardwood Forest in the United States

We present lidar (LIght Detection And Ranging) and particle measurements from a mixed hardwood forest in northern Michigan over a 74-day period in April–July 2016, capturing the peak spring tree pollen emissions season. Mini Micropulse Lidar (MPL) observations of the normalized ratio backscatter and depolarization ratios show pollen plumes emanating from the forest on 9 days during the pine and oak pollination time periods, with highly depolarizing particles within the boundary layer up to about 500 m–1.6 km depending on the meteorological conditions. We calculate the total particle backscatter and aerosol optical depth (AOD) on the 9 pollen plume days and find that during pollen plume events, the pollen contributes approximately 50% to the total AOD, with daily contributions ranging from 25% to 97%. Single particle measurements from passive samplers at three heights in and above the forest canopy (1.5, 15, and 30 m) observe pine and oak pollen grains, with biological aerosol particles comprising a large component of supermicron aerosols. Together, these results provide time-resolved observational evidence of the diurnal cycle of pollen and their correlation with meteorological conditions, as well as the contribution of pollen to the total AOD, indicating its potential impact on climate.