Atmospheric Particles and Their Effects on Cloud Formation and Climate

Abstract

Under humid atmospheric conditions, pollen grains can rupture creating pollen grain fragments referred to as subpollen particles (SPPs). This study evaluated environmental conditions needed to emit SPPs and measure the concentration of SPPs produced. Conventional SPP release was considered where live oak branches were exposed to high relative humidity (>95%), followed by reduced relative humidity (73.5%-76.3%) and wind (up to 1.8 m s^-1). In contrast, wind-driven SPP release experiments were conducted by exposing branches to constant relative humidity and wind. Wind-driven SPP release was the mechanism considered for subsequent experiments. SPP emission factors were determined for wind-driven SPP release for live oak, Quercus virginiana, ryegrass, Lolium sp., and giant ragweed, Ambrosia trifida, in terms of SPPs produced per pollen grain and SPPs produced per m^2. The SPPs produced per m^2 were 1.1x10^15 ± 1.6x10^15 for live oak, 4.9x10^13 ± 4.3x10^13 for ryegrass, and 1.3x10^15 ± 1.1x10^15 for giant ragweed. SPPs and pollen grains from these species were evaluated for their ice nucleation efficiency in immersion and contact mode freezing. Measurements indicate that SPPs are weakly effective INPs in immersion mode, but that pollen grains represent a source of moderately efficient INPs in immersion and contact modes. Additionally, viscous solution droplets were tested for their ability to contribute to homogeneous freezing and ice cloud formation. In this study, viscosity was determined for aqueous organic solutions (citric acid, sucrose, maltose, glyoxal, and methylglyoxal) at varying weight percent (40%, 50%, and 65%), saturated solutions, and three eutonic solutions. In high viscosity droplets, the time required for initial onset of freezing was increased to 4.4±3.9 hours and 5.2±4.1 hours for complete freezing at a temperature of -40 °C. While low viscosity droplets had onset freezing as quickly as 1.9±2.2 hours and complete freezing in 2.6±4.0 hours at a temperature of -40 °C. Given the timescales of freezing observed here, the phase of aerosol containing high concentrations of organic compounds will depend on updraft velocity. Overall, our measurements suggest aerosol containing high concentrations of organic compounds will be present as supercooled liquids in the upper troposphere.