Boreal tree pollen sensed by polarization lidar: Depolarizing biogenic chaff

Abstract

[1] Polarization (0.694 mm) lidar measurements show that tree pollen can generate strong laser depolarization in the backscatter from the lower atmosphere. Examples are given illustrating that linear depolarization ratios up to 0.3 are measured in plumes of paper birch pollen at the onset of boreal forest green-out. These pollen are $25 mm in diameter and near-spherical in shape, but with lobes protruding from a surface membrane, which appears to produce the depolarization. Similar lidar findings are frequently observed during the summer at Fairbanks, Alaska, indicating that various types of seasonal pollen releases may be identified by polarization lidar. This scattering behavior is likely a general attribute of pollen and other suspended biogenic debris, which has implications for benefiting human health. This source of laser depolarization should not be confused with the presence of airborne dust or certain pollution particles, but is a natural background aerosol component caused by plant reproduction, as should be recognized in current global polarization lidar aerosol research using the CALIPSO satellite. Citation: Sassen, K. (2008), Boreal tree pollen sensed by polarization lidar: Depolarizing biogenic chaff, Geophys. Res. Lett., 35, L18810,

Assessment of WRF/Chem Simulated Vertical Distributions of Particulate Matter from the 2009 Minto Flats South Wildfire in Interior Alaska by CALIPSO Total Backscatter and Depolarization Measurements

“…Previous studies found that depolarization values of 6% typically occur in the upper part of a smoke layer, and comparatively lower depolarization percentages occur below the active layer [50] [52]. Depolarizations of 3% and 5% indicate fresh smoke and smoke layers at higher altitudes, respectively [48].…”

“…In theory, if particles are perfectly spherical, no backscattering in the perpendicular, or orthogonal plane, should occur. This means irregularly shaped particles cause perpendicular backscattering [7] [48].…”

“…Our analysis focused on overpasses with as few clouds as possible, as clouds affect the depolarization ratio and overshadow smoke signatures [48]. Moreover, our analysis used nighttime CALIPSO data to reduce possible interference from the Sun.…”

Primary biological aerosol particles in the atmosphere: a review

“…The detection of fluorescent ambient aerosol was reported for the first time using a LIDAR system (Immler et al, 2005), attributing the signals to a combination of PAH-containing particles and/or PBAP. Furthermore, it was shown (Sassen, 2008) that pollen can generate strong laser depolarisation in LIDAR backscatter during Alaskan springtime measurements and suggested that pollen plumes may be mistaken for upper cirrus clouds and therefore introduce important errors into identifying aerosols in the atmosphere. Atmospheric optical phenomena caused by pollen also have been reported, 6 and will be discussed briefly in Section 5.…”

Raman Lidar Observations of Aerosol Optical Properties in 11 Cities from France to Siberia

“…The PDR profile in Berlin showed a maximum in the lowest visible layer and decreased regularly with altitude throughout the PBL. The particularly large amount of pollens reported this day might explain this profile: pollens are very coarse particles that are too heavy to reach the upper levels of the PBL, but have a high depolarizing power [55,56]. This combined with the effect of relative humidity, as aerosols reaching the upper levels of the PBL became less depolarizing due to water-coating.…”