Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.

Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation

Abstract. The uptake of water by contrails in ice-supersaturated air and the release of water after ice particle advection and sedimentation dehydrates the atmosphere at flight levels and redistributes humidity mainly to lower levels. The dehydration is investigated by coupling a plume-scale contrail model with a global aerosol–climate model. The contrail model simulates all the individual contrails forming from global air traffic for meteorological conditions as defined by the climate model. The computed contrail cirrus properties compare reasonably with theoretical concepts and observations. The mass of water in aged contrails may exceed 106 times the mass of water emitted from aircraft. Many of the ice particles sediment and release water in the troposphere, on average 700 m below the mean flight levels. Simulations with and without coupling are compared. The drying at contrail levels causes thinner and longer-lived contrails with about 15 % reduced contrail radiative forcing (RF). The reduced RF from contrails is on the order of 0.06 W m−2, slightly larger than estimated earlier because of higher soot emissions. For normal traffic, the RF from dehydration is small compared to interannual variability. A case with emissions increased by 100 times is used to overcome statistical uncertainty. The contrails impact the entire hydrological cycle in the atmosphere by reducing the total water column and the cover by high- and low-level clouds. For normal traffic, the dehydration changes contrail RF by positive shortwave and negative longwave contributions on the order of 0.04 W m−2, with a small negative net RF. The total net RF from contrails and dehydration remains within the range of previous estimates.

On dehydration effects from contrails in a coupled contrail-climate model

. Anthropogenic aerosols impact cirrus clouds through ice nucleation, thereby
changing the Earth's radiation budget. However, the magnitude and sign of
anthropogenic forcing in cirrus clouds is still very uncertain depending on
the treatments for ice-nucleating particles (INPs), the treatments for haze particle
freezing, and the ice nucleation scheme. In this study, a new ice nucleation
scheme (hereafter the HYBRID scheme) is developed to combine the best
features of two previous ice nucleation schemes, so that global models are
able to calculate the ice number concentration in both updrafts and
downdrafts associated with gravity waves, and it has a robust sensitivity to the
change of aerosol number. The scheme is applied in a box model, and the ice
number concentrations ( 9.52±2.08  L −1 ) are somewhat overestimated
but are in reasonable agreement with those from an adiabatic parcel model
( 9.40±2.31  L −1 ). Then, the forcing and cloud changes associated
with changes in aircraft soot, sulfur emission, and all anthropogenic
emissions between the preindustrial (PI) period and the present day (PD) are
examined using the CESM/IMPACT global model with the HYBRID scheme. Aircraft
soot emissions decrease the global average ice number concentration ( Ni ) by
 - 1.0 ± 2.4 × 10 7  m −2 ( −1  %) (over the entire
column) due to the inhibition of homogeneous nucleation and lead to a
radiative forcing of - 0.14 ± 0.07  W m −2 , while the increase in
sulfur emissions increases the global average Ni by 7.3 ± 2.9 × 10 7  m −2 (5 %) due to the increase in homogeneous nucleation and
leads to a radiative forcing of - 0.02 ± 0.06  W m −2 . The possible
effects of aerosol and cloud feedbacks to the meteorological state in remote
regions partly contribute to reduce the forcing and the change in Ni due to
anthropogenic emissions. The radiative forcing due to all increased
anthropogenic emissions from PI to PD is estimated to be - 0.20 ± 0.05  W m −2 . If newly formed secondary organic aerosols (SOAs) act as INPs and
inhibit homogeneous nucleation, the Ni formed from heterogeneous nucleation
is increased. As a result, the inclusion of INPs from SOA increases the
change in Ni to 12.0 ± 2.3 × 10 7  m −2 (9 %) and
increases (makes less negative) the anthropogenic forcing on cirrus clouds
to - 0.04 ± 0.08  W m −2 from PI to PD.

Radiative forcing of anthropogenic aerosols on cirrus clouds using a hybrid ice nucleation scheme

Climate forcing of aircraft contrails: uncertainty quantification and mitigation potential

Soot particles emitted by aircraft engines may act as ice nuclei within the atmosphere, subsequently triggering the formation of condensation trails. Such contrails might further evolve as cirrus clouds and thus greatly influence the Earth’s radiative budget and impact the amount of precipitation. In order to monitor in situ deliquescence, efflorescence, and nucleation processes followed by ice growth in the laboratory, we developed the ice and droplet nucleation experimental setup, which combines optical imaging and micro-Raman measurements to follow nucleation events in a pressure-, temperature-, and humidity-controlled optical chamber. We first compare against the literature data and later confirm the deliquescence relative humidities of micrometer-sized sodium chloride salt crystals in the −5 to −35 °C temperature range. Then, we investigate the ice nucleation activity of graphite and aircraft soot analogues, in the −15 to −45 °C temperature range, when exposed to humid nitrogen (N₂/H₂O gas flow). Soot samples exhibiting various surface chemistries, morphologies, and sizes are thoroughly examined via mass spectrometry and spectroscopic and optical techniques. All carbon-bearing samples are found to be active at nucleating ice at low ice saturation ratios (Sᵢcₑ determined when the first crystal is detected). When normalizing Sᵢcₑ to the total surface area of a sample, one can derive the ice-active surface site density (nₛ). This parameter provides a means to compare the ice nucleating behavior of various particle types with distinct surface areas. As all samples studied in our work feature large surface areas, we provide ice nucleation data (Sᵢcₑ and nₛ) for a range of surface areas that remained largely unexplored to date. We find that the interplay between surface composition and morphology (micro, meso, and macro pores, surface roughness) influences the ice onset relative humidity.

Ice Nucleation Activities of Carbon-Bearing Materials in Deposition Mode: From Graphite to Airplane Soot Surrogates

The indirect effect of aircraft soot on cirrus clouds is subject to large uncertainties due to uncertainty in the effectiveness of aircraft soot acting as heterogeneous ice nuclei (IN) and the complexity caused by background ice nucleation, which introduces two major competing ice nucleation mechanisms: homogeneous freezing that generally produces more abundant ice particles and heterogeneous nucleation that generally produces fewer ice particles. In this paper, we used the coupled Community Atmosphere Model version 5.2 (CAM5)/IMPACT model to estimate the climate impacts of aircraft soot acting as IN in large-scale cirrus clouds. We assume that only the aircraft soot particles that are preactivated in persistent contrail cirrus clouds are efficient IN. Further, we assume that these particles lose their ability to act as efficient IN when they become coated with three monolayers of sulfate. We varied the background number concentration of sulfate aerosols allowed to act as homogeneous ice nucleation sites as well as the dust concentrations that act as heterogeneous ice nuclei to examine the sensitivity of the forcing by aircraft soot to the background atmosphere. The global average effect can range from a high negative (cooling) rate, −0.35 W m−2, for the high sulfate/low dust case to a positive (warming) rate, +0.09 W m−2, for the low sulfate/low dust case (default CAM5 setup) when approximately 0.6% of total aviation soot acts as IN. The net negative forcing is caused by the addition of IN to a background atmosphere that is dominated by homogeneous nucleation (mainly in the tropic Indian Ocean, Central America, and North Atlantic Ocean). The forcings can be all positive, about +0.11 to +0.21 W m−2, when the background atmosphere is dominated by pure heterogeneous ice nucleation.

C. Zhou

Papers

Is The Indirect Forcing By Aircraft Soot Positive Or Negative