Applied Mathematics and Applied Physics

SEAS Colloquium in Climate Science (SCiCS)

Prof. Daniel A. Knopf
Institute for Terrestrial and Planetary Atmospheres
School of Marine and Atmospheric Sciences
Stony Brook University

"The Role of the Organic Aerosol Fraction on Ice Cloud Formation"

Atmospheric aerosol particles can affect the global radiation budget and, thus, climate, by scattering and absorbing solar and terrestrial radiation, also termed the aerosol direct effect. In addition, aerosol particles can modify the radiative properties of clouds by serving as cloud condensation nuclei (CCN) and ice nuclei (IN) which commonly is referred to as the aerosol indirect effect. The importance of ice particles in the atmosphere has been well recognized, but the impact of cirrus clouds on the global radiative budget is still not well established. The radiative forcing due to the aerosol indirect effect as estimated by the IPCC 2007 (Intergovernmental Panel of Climate change) did not include ice clouds due to the lack of an accurate description of ice particles. In addition to changes in cloud radiative properties the formation of ice in the upper troposphere and lower stratosphere (UT/LS) can lead to the dehydration of the UT/LS region by sedimentation of ice particles. This process affects the water vapor distribution and, hence, also results in changes to the radiation budget. Ice particles at the tropopause will also control the water transport into the lower stratosphere with subsequent consequences for the stratospheric chemical composition. In addition, cirrus clouds can serve as heterogeneous sites for reactions of nitrogen and halogen species resulting in a destruction of ozone and as sink for the uptake of HNO₃.

We will present laboratory studies on the formation of ice particles from pre-existing aerosol particles to improve our fundamental understanding of the microphysical processes leading to the nucleation of ice in the atmosphere. Particular focus will be placed on the effect of the organic and biogenic aerosol fraction on ice formation. Particle proxies representing biomass burning aerosol (BBA) typical of forest fires, marine biogenic particles, and aqueous particles coated by organic surface active compounds are employed in our ice nucleation studies. In addition, organics containing particles sampled at an urban polluted site (Mexico City) are analyzed with respect to their ice nucleation efficiency.

Atmospheric ice particles can form by homogeneous and heterogeneous nucleation. Homogeneous ice nucleation describes the formation of ice from a supercooled purely liquid  particle. Heterogeneous ice nucleation can occur by various modi such as deposition nucleation, immersion freezing, or contact freezing. Homogeneous ice nucleation has been observed to occur in the atmosphere at high supersaturations with respect to ice. Here, we will present results on homogeneous ice nucleation, and heterogeneous ice nucleation by deposition mode and immersion mode as a function of particle temperatures as low as 205 K and relative humidities with respect to ice of up to 160% using cryo-stages coupled to optical microscopes. The freezing and melting temperatures and derived nucleation rate coefficients will be discussed in an atmospheric context.

 

Figure 1: Ice crystals formed by deposition mode nucleation onto organics containing particles sampled in Mexico City as observed using optical microscopy. Ice formation was induced at particle temperatures of -53^oC and RH_ice = 132 %.

Host: V. Faye McNeill