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Title: Abundance of fluorescent biological aerosol particles at temperatures conducive to the formation of mixed-phase and cirrus clouds

Some types of biological particles are known to efficiently nucleate ice at relatively warm temperatures in the atmosphere, with the potential to influence cloud microphysical properties and climate. However, the prevalence of these particle types above the atmospheric boundary layer is not well known. Many types of biological particles fluoresce when exposed to ultraviolet light, and the Wideband Integrated Bioaerosol Sensor takes advantage of this characteristic to perform real-time measurements of fluorescent biological aerosol particles (FBAP). This relatively new instrument was flown on the National Center for Atmospheric Research Gulfstream-V aircraft to measure concentrations of fluorescent biological particles from different potential sources and at various altitudes over the U. S. western plains states in early autumn. Clear-air concentrations of FBAP larger than 0.8 µm in diameter usually decreased with height, and generally were about 10-100 L-1 in the continental boundary layer, but were always much lower in the free troposphere at temperatures colder than about 255K. Ice nucleating particle number concentrations measured during one flight also showed a decrease with height, and were consistent with a relationship to FBAP established previously at the surface at the same location. At intermediate ambient temperatures where biological ice nucleating particles may influence mixed-phasemore » cloud formation (255K≤T≤270K), concentrations of fluorescent particles were the most variable, and were occasionally near boundary layer values. Predicted vertical profiles of ice nucleating particle concentrations based on FBAP measurements in this temperature regime sometimes reached typical ice crystal concentrations in clouds, but were often much lower. If convection was assumed to lift boundary layer FBAP particles without losses to the free troposphere, better agreement between predicted ice-nucleating particle concentrations and typical ice crystal number concentrations was achieved. Measured concentrations of FBAP were also compared with concentrations of bacteria, fungal spores and pollen predicted from the EMAC global chemistry-climate model for the same region and time period. The model qualitatively reproducee the observed drop in particle concentrations with altitude above the boundary layer, and is in agreement with the observed FBAP in some cases, while underestimating observed FBAP by an order of magnitude or more in other cases.« less
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Journal Article
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Journal Name: Atmospheric Chemistry and Physics, 16(13):8205-8225
Research Org:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
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Country of Publication:
United States