Abstract
Abstract. Data from both laboratory studies and atmospheric measurements are used to develop an empirical parameterization for the immersion freezing activity of natural mineral dust particles. Measurements made with the Colorado State University (CSU) continuous flow diffusion chamber (CFDC) when processing mineral dust aerosols at a nominal 105% relative humidity with respect to water (RHw) are taken as a measure of the immersion freezing nucleation activity of particles. Ice active frozen fractions vs. temperature for dusts representative of Saharan and Asian desert sources were consistent with similar measurements in atmospheric dust plumes for a limited set of comparisons available. The parameterization developed follows the form of one suggested previously for atmospheric particles of non-specific composition in quantifying ice nucleating particle concentrations as functions of temperature and the total number concentration of particles larger than 0.5 μm diameter. Such an approach does not explicitly account for surface area and time dependencies for ice nucleation, but sufficiently encapsulates the activation properties for potential use in regional and global modeling simulations, and possible application in developing remote sensing retrievals for ice nucleating particles. A calibration factor is introduced to account for the apparent underestimate (by approximately 3, on average) of the immersion freezing fraction of mineral dust particles for CSU CFDC data processed at an RHw of 105% vs. maximum fractions active at higher RHw. Instrumental factors that affect activation behavior vs. RHw in CFDC instruments remain to be fully explored in future studies. Nevertheless, the use of this calibration factor is supported by comparison to ice activation data obtained for the same aerosols from Aerosol Interactions and Dynamics of the Atmosphere (AIDA) expansion chamber cloud parcel experiments. Further comparison of the new parameterization, including calibration correction, to predictions of the immersion freezing surface active site density parameterization for mineral dust particles, developed separately from AIDA experimental data alone, shows excellent agreement for data collected in a descent through a Saharan aerosol layer. These studies support the utility of laboratory measurements to obtain atmospherically relevant data on the ice nucleation properties of dust and other particle types, and suggest the suitability of considering all mineral dust as a single type of ice nucleating particle as a useful first-order approximation in numerical modeling investigations.
Highlights
Ice nucleation by atmospheric aerosols impacts the microphysical composition, radiative properties, and precipitation processes in clouds colder than 0 ◦C
Data collected while sampling particles aerosolized from Asian and Saharan soil sources into the AIDA aerosol chamber are included together in Fig. 5 with field measurements made within Saharan and Asian dust plumes
A parameterization based on a combination of laboratory and field data was developed to quantify the immersion freezing numbers of natural mineral dust particles
Summary
Ice nucleation by atmospheric aerosols impacts the microphysical composition, radiative properties, and precipitation processes in clouds colder than 0 ◦C. Since the processes responsible for ice nucleation are not fully understood at the molecular level, and it has yet to be demonstrated that the full variety of surface property influences on ice nucleation by the many types of aerosols present in the atmosphere can be described by phenomenological models, there is need to simplify description of ice nucleation as understood through measurements. The relative importance of different ice nucleation mechanisms will be affected by INP mixing state, and the cloud formation conditions and the thermodynamic path followed by particles entering clouds. We consider that immersion freezing may be the most critical mechanism for quantifying ice nucleation in moderately supercooled clouds because most INPs reaching supercooled cloud conditions arrive having spent substantial time in clouds, and potentially serving as CCN by virtue of their sizes and/or compositions. We consider that immersion freezing may be the most critical mechanism for quantifying ice nucleation in moderately supercooled clouds because most INPs reaching supercooled cloud conditions arrive having spent substantial time in clouds, and potentially serving as CCN by virtue of their sizes and/or compositions. Murray et al (2012) summarize additional observational support for the relative major importance of immersion freezing nucleation
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