Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than −36°C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼10 3 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m −2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.aerosol indirect effects | climate forcing | ice nucleation T he formation of ice in clouds is of vital importance to life on Earth, as ice formation is one of the key processes initiating precipitation. In addition, since ice nucleation is tied to the action of specific aerosol particles, natural and human impacts on ice nucleation in the atmosphere can lead to alteration of the energy and hydrological cycles (1). Ice nucleation in clouds occurs via two primary pathways: homogeneous freezing of liquid particles below about −36°C and heterogeneous ice nucleation, triggered by "ice nuclei" that possess surface properties favorable to lowering the energy barrier to crystallization. Once ice is formed, some circumstances may favor generation of ice from preexisting ice particles, or secondary ice formation (2). Heterogeneous ice nucleation remains an enigmatic topic involving multiple mechanistic processes (3) that sometimes defy ready investigation or description. Despite the lack of a complete understanding of heterogeneous ice formation processes, a variety of techniques have been developed and used to detect the presence of and quantify the number concentrations of atmospheric ice nuclei as a function of temperature (4). These measurements show that, although generally representing only 1 in 10 5 of ambient particles in the free troposphere (5), ice nuclei (IN) can nevertheless exert an influence on cold cloud microphysical processes that is disproportionate to their low number concentrations. For example, the concentrations of IN needed to explain observed precipitation rates range from as small as 10 −3 per standard liter at −10°C (6) to more typical estimates of a few IN per standard liter at −20°C (7).The sensitivity of precipitation initiation fro...

[1] Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 mm exceeded 1 cm À3 ; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.

This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were <0.03 cm ؊3 . Considering previous modeling studies, this result suggests a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus. The most common heterogeneous ice nuclei were identified as relatively pure mineral dusts and metallic particles, some of which may have origin through anthropogenic processes. Homogeneous freezing of large numbers of particles was detected above a critical relative humidity along with a simultaneous transition in nuclei composition toward that of the sulfate-dominated total aerosol population. The temperature and humidity conditions of the homogeneous nucleation transition were reasonably consistent with expectations based on previous theoretical and laboratory studies but were highly variable. The strong presence of certain organic pollutants was particularly noted to be associated with impedance of homogeneous freezing.