Abstract
Abstract. Cirrus clouds have a net warming effect on the atmosphere and cover about 30% of the Earth's area. Aerosol particles initiate ice formation in the upper troposphere through modes of action that include homogeneous freezing of solution droplets, heterogeneous nucleation on solid particles immersed in a solution, and deposition nucleation of vapor onto solid particles. Here, we examine the possible change in ice number concentration from anthropogenic soot originating from surface sources of fossil fuel and biomass burning, from anthropogenic sulfate aerosols, and from aircraft that deposit their aerosols directly in the upper troposphere. We use a version of the aerosol model that predicts sulfate number and mass concentrations in 3-modes and includes the formation of sulfate aerosol through homogeneous binary nucleation as well as a version that only predicts sulfate mass. The 3-mode version best represents the Aitken aerosol nuclei number concentrations in the upper troposphere which dominated ice crystal residues in the upper troposphere. Fossil fuel and biomass burning soot aerosols with this version exert a radiative forcing of −0.3 to −0.4 Wm−2 while anthropogenic sulfate aerosols and aircraft aerosols exert a forcing of −0.01 to 0.04 Wm−2 and −0.16 to −0.12 Wm−2, respectively, where the range represents the forcing from two parameterizations for ice nucleation. The sign of the forcing in the mass-only version of the model depends on which ice nucleation parameterization is used and can be either positive or negative. The magnitude of the forcing in cirrus clouds can be comparable to the forcing exerted by anthropogenic aerosols on warm clouds, but this forcing has not been included in past assessments of the total anthropogenic radiative forcing of climate.
Highlights
Cirrus clouds play an important role in climate
Ice shattering on inlet probes frequently leads to overestimates of number concentrations (McFarquhar et al, 2007), this problem does not appear to have greatly compromised the in Cirrus Properties From Anthropogenic Emissions (INCA) measurements since most ice crystal measurements did not include any crystals larger than 500 μm (Gayet et al, 2002) and any overestimate is probably less than a factor of two
Ice number concentrations are larger than 20 cm−3 over much of the upper troposphere near 200 hPa using the modified treatment of soot nucleation (KLm), wherein soot does not nucleate at relative humidity over ice (RHi) below that of sulfate
Summary
Cirrus clouds play an important role in climate. They trap of outgoing longwave radiation emitted by the Earth and atmosphere (a positive radiative effect) but this is partly compensated by their reflection of incoming solar radiation (a negative radiative effect). Laboratory studies of cirrus ice formation that relate ice nucleation to aerosol properties show that heterogeneous ice nucleation on mineral dust (e.g., Zuberi et al, 2002; Hung et al, 2003; Archuleta et al, 2005; Field et al, 2006; Mohler et al, 2006; Salam et al, 2006), and on soot (or black carbon, BC) particles (DeMott, 1990; DeMott et al, 1999; Gorbunov et al, 2001; Mohler et al, 2005) requires lower relative humidity over ice (RHi) than homogeneous freezing on sulfate, while coating soot with sulfate can increase the nucleation thresholds to an ice saturation ratio of about 1.3 at 230 K to 1.5 at 185 K (Mohler et al, 2005).
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
