Insights into the role of soot aerosols in cirrus cloud formation

Abstract

Abstract.Cirrus cloud formation is believed to be dominated by homogeneous freezing of supercooled liquid aerosols in many instances. Heterogeneous ice nuclei such as mineral dust, metallic, and soot particles, and some crystalline solids within partially soluble aerosols are suspected to modulate cirrus properties. Among those, the role of ubiquitous soot particles is perhaps the least understood. Because aviation is a major source of upper tropospheric soot particles, we put emphasis on ice formation in dispersing aircraft plumes. The effect of aircraft soot on cirrus formation in the absence of contrails is highly complex and depends on a wide array of emission and environmental parameters. We use a microphysical-chemical model predicting the formation of internally mixed, soot-containing particles up to two days after emission, and suggest two principal scenarios: high concentrations of original soot emissions could slightly increase the number of ice crystals; low concentrations of particles originating from coagulation of emitted soot with background aerosols could lead to a significant reduction in ice crystal number. Both scenarios assume soot particles to be moderate ice nuclei relative to cirrus formation by homogeneous freezing in the presence of few efficient dust ice nuclei. A critical discussion of laboratory experiments reveals that the ice nucleation efficiency of soot particles depends strongly on their source, and, by inference, on atmospheric aging processes. Mass and chemistry of soluble surface coatings appear to be crucial factors. Immersed soot particles tend to be poor ice nuclei, some bare ones nucleate ice at low supersaturations. However, a fundamental understanding of these studies is lacking, rendering extrapolations to atmospheric conditions speculative. In particular, we cannot yet decide which indirect aircraft effect scenario is more plausible, and options suggested to mitigate the problem remain uncertain.

On Source Identification and Alteration of Single Diesel and Wood Smoke Soot Particles in the Atmosphere; An X-Ray Microspectroscopy Study

“…Aging is also the likely reason for the differences between the spectra of emission and atmospheric particles. Reaction with atmospheric gases and adsorption of new species will change the nature of the carbon bonds in particles (e.g., ref 39). This is very well illustrated by the ozone aging experiments of jeep soot emission samples.…”

Effects of pre-existing ice crystals on cirrus clouds and comparison between different ice nucleation parameterizations with the Community Atmosphere Model (CAM5)

“…Laboratory experiments and field observations show that various insoluble or partly insoluble aerosol particles can act as IN under cirrus formation conditions, such as mineral dust, fly ash, and metallic particles (DeMott et al, 2003(DeMott et al, , 2011Cziczo et al, 2004;Hoose and Möhler, 2012). Understanding the role of different aerosol types serving as heterogeneous IN in cirrus clouds remains challenging (Szyrmer and Zawadzki, 1997;Kärcher et al, 2007;Hendricks et al, 2011;Hoose and Möhler, 2012;Cziczo et al, 2013). Compared to heterogeneous nucleation, homogeneous nucleation is relatively better understood (Koop et al, 2000;Koop, 2004).…”

Two-moment bulk stratiform cloud microphysics in the GFDL AM3 GCM: description, evaluation, and sensitivity tests

“…various processes involving solid nuclei; see e.g. DeMott et al, 1997;Cziczo et al, 2004;Kärcher et al, 2007), which is the dominant ice nucleation pathway at temperatures above −35 • C. Heterogeneous nucleation involving suitable IN requires a smaller supersaturation than homogeneous nucleation. Thus, the presence of a relatively small number of IN at low temperatures could, in principle, lead to a reduction in ice crystal number compared to a homogeneous nucleation scenario (e.g.…”