Measurements of the concentration and composition of nuclei for cirrus formation

Abstract

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.

Fig. 1.

Seven-day back-trajectory analyses from the measurement site initialized at 12:00 GMT (except 00:00 GMT on November 18). Color-coded time periods are discussed in Results and Discussion. Filled circles mark daily units of each trajectory.

Fig. 2.

Ice concentrations nucleated by or within aerosol particles as a function of water vapor supersaturation (Upper) and ice supersaturation (Lower) for the project period. Data are color-coded to differentiate measurements in different temperature regimes. Increases in nucleated concentrations at near water saturation and at higher ice supersaturation conditions are indicative of homogeneous freezing at temperatures below –38°C.

Fig. 3.

Heterogeneous IN concentrations (filled symbols) for selected conditions (–42 to –46°C and 90–92% RH_W) and ambient aerosol concentrations (open symbols) in a selected size range (200–800 nm) as a function of time.

Fig. 4.

Statistics of different particle populations based on cluster analysis of PALMS mass spectra. The total aerosol composition is shown (Left), and the composition of nucleated ice-crystal residuals are shown in the regime under which homogeneous freezing was dominating heterogeneous nucleation (Center) and under conditions favorable only to heterogeneous nucleation (Right).

Fig. 5.

Comparison of the conditions required for homogeneous freezing of ambient aerosols on November 3 (–51°C nominal processing temperature) and November 17 (–44°C nominal processing temperature). The shaded regions indicate the expected conditions for homogeneous freezing (onset to complete freezing) of pure sulfate aerosols for the range of CFDC processing temperatures on each day.

Fig. 6.

PALMS negative ion mass spectra of ice-crystal residual particles typical of homogeneous-freezing conditions in two different types of air masses. (Upper) A negative polarity spectrum obtained on November 16. This particle, as well as the background aerosol, had minimal organics, instead being relatively pristine sulfates. Conversely, Lower shows a negative polarity spectrum obtained on November 17 that also coexhibits several organic fragments (as did the background aerosol) with sulfate.