The formation of ice particles in the Earth's atmosphere strongly affects the properties of clouds and their impact on climate. Despite the importance of ice formation in determining the properties of clouds, the Intergovernmental Panel on Climate Change (IPCC, 2007) was unable to assess the impact of atmospheric ice formation in their most recent report because our basic knowledge is insufficient. Part of the problem is the paucity of quantitative information on the ability of various atmospheric aerosol species to initiate ice formation. Here we review and assess the existing quantitative knowledge of ice nucleation by particles immersed within supercooled water droplets. We introduce aerosol species which have been identified in the past as potentially important ice nuclei and address their ice-nucleating ability when immersed in a supercooled droplet. We focus on mineral dusts, biological species (pollen, bacteria, fungal spores and plankton), carbonaceous combustion products and volcanic ash. In order to make a quantitative comparison we first introduce several ways of describing ice nucleation and then summarise the existing information according to the time-independent (singular) approximation. Using this approximation in combination with typical atmospheric loadings, we estimate the importance of ice nucleation by different aerosol types. According to these estimates we find that ice nucleation below about −15 °C is dominated by soot and mineral dusts. Above this temperature the only materials known to nucleate ice are biological, with quantitative data for other materials absent from the literature. We conclude with a summary of the challenges our community faces.

/pdf/ice-nucleation-by-particles-immersed-in-supercooled-cloud-4c88zmbtg2.pdf

Ice nucleation by particles immersed in supercooled cloud droplets

The 39-day long eruption at the summit of Eyjafjallajokull volcano in April–May 2010 was of modest size but ash was widely dispersed. By combining data from ground surveys and remote sensing we show that the erupted material was 4.8±1.2·1011 kg (benmoreite and trachyte, dense rock equivalent volume 0.18±0.05 km3). About 20% was lava and water-transported tephra, 80% was airborne tephra (bulk volume 0.27 km3) transported by 3–10 km high plumes. The airborne tephra was mostly fine ash (diameter <1000 µm). At least 7·1010 kg (70 Tg) was very fine ash (<28 µm), several times more than previously estimated via satellite retrievals. About 50% of the tephra fell in Iceland with the remainder carried towards south and east, detected over ~7 million km2 in Europe and the North Atlantic. Of order 1010 kg (2%) are considered to have been transported longer than 600–700 km with <108 kg (<0.02%) reaching mainland Europe.

/pdf/ash-generation-and-distribution-from-the-april-may-2010-2nud1l5m62.pdf

Ash generation and distribution from the April-May 2010 eruption of Eyjafjallajökull, Iceland

. The April–May, 2010 volcanic eruptions of Eyjafjallajokull, Iceland caused significant economic and social disruption in Europe whilst state of the art measurements and ash dispersion forecasts were heavily criticized by the aviation industry. Here we demonstrate for the first time that large improvements can be made in quantitative predictions of the fate of volcanic ash emissions, by using an inversion scheme that couples a priori source information and the output of a Lagrangian dispersion model with satellite data to estimate the volcanic ash source strength as a function of altitude and time. From the inversion, we obtain a total fine ash emission of the eruption of 8.3 ± 4.2 Tg for particles in the size range of 2.8–28 μm diameter. We evaluate the results of our model results with a posteriori ash emissions using independent ground-based, airborne and space-borne measurements both in case studies and statistically. Subsequently, we estimate the area over Europe affected by volcanic ash above certain concentration thresholds relevant for the aviation industry. We find that during three episodes in April and May, volcanic ash concentrations at some altitude in the atmosphere exceeded the limits for the "Normal" flying zone in up to 14 % (6–16 %), 2 % (1–3 %) and 7 % (4–11 %), respectively, of the European area. For a limit of 2 mg m−3 only two episodes with fractions of 1.5 % (0.2–2.8 %) and 0.9 % (0.1–1.6 %) occurred, while the current "No-Fly" zone criterion of 4 mg m−3 was rarely exceeded. Our results have important ramifications for determining air space closures and for real-time quantitative estimations of ash concentrations. Furthermore, the general nature of our method yields better constraints on the distribution and fate of volcanic ash in the Earth system.

/pdf/determination-of-time-and-height-resolved-volcanic-ash-n9xc7uu1ci.pdf

Determination of time- and height-resolved volcanic ash emissions and their use for quantitative ash dispersion modeling: the 2010 Eyjafjallajökull eruption

https://www.kau.edu.sa/Files/188/Researches/65975_37398.pdf

Aircraft engine exhaust emissions and other airport-related contributions to ambient air pollution: A review

[1] To better represent the indirect effect of aerosols on climate, a size-resolved simulation of aerosol microphysics, size distributions, number and mass concentrations has been incorporated into the GISS general circulation model (GCM) The TwO-Moment Aerosol Sectional (TOMAS) microphysics model used here conserves aerosol number as well as mass It has high size resolution, 30 bins between 001 and 10 μm diameter As a first application, a size-resolved simulation of sulfate has been performed The model reproduces important features of the atmospheric aerosol such as number concentrations that increase with altitude and land-sea contrasts in aerosol number concentrations and size distributions Comparisons with observations show that simulated size distributions are realistic and condensation nuclei (CN) concentrations agree with observations within about 25% Predicted cloud condensation nuclei (CCN) concentrations are also in reasonable agreement with observations, although there are locations for which agreement would be improved by including other aerosol components such as sea salt and carbonaceous aerosols Sensitivity scenarios show that uncertainties in nucleation and primary emissions from fossil fuels can have significant effects on predictions of CN and CCN concentrations

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JD001010

Predicting global aerosol size distributions in general circulation models

. Airborne lidar and in-situ measurements of aerosols and trace gases were performed in volcanic ash plumes over Europe between Southern Germany and Iceland with the Falcon aircraft during the eruption period of the Eyjafjalla volcano between 19 April and 18 May 2010. Flight planning and measurement analyses were supported by a refined Meteosat ash product and trajectory model analysis. The volcanic ash plume was observed with lidar directly over the volcano and up to a distance of 2700 km downwind, and up to 120 h plume ages. Aged ash layers were between a few 100 m to 3 km deep, occurred between 1 and 7 km altitude, and were typically 100 to 300 km wide. Particles collected by impactors had diameters up to 20 μm diameter, with size and age dependent composition. Ash mass concentrations were derived from optical particle spectrometers for a particle density of 2.6 g cm−3 and various values of the refractive index (RI, real part: 1.59; 3 values for the imaginary part: 0, 0.004 and 0.008). The mass concentrations, effective diameters and related optical properties were compared with ground-based lidar observations. Theoretical considerations of particle sedimentation constrain the particle diameters to those obtained for the lower RI values. The ash mass concentration results have an uncertainty of a factor of two. The maximum ash mass concentration encountered during the 17 flights with 34 ash plume penetrations was below 1 mg m−3. The Falcon flew in ash clouds up to about 0.8 mg m−3 for a few minutes and in an ash cloud with approximately 0.2 mg m−3 mean-concentration for about one hour without engine damage. The ash plumes were rather dry and correlated with considerable CO and SO2 increases and O3 decreases. To first order, ash concentration and SO2 mixing ratio in the plumes decreased by a factor of two within less than a day. In fresh plumes, the SO2 and CO concentration increases were correlated with the ash mass concentration. The ash plumes were often visible slantwise as faint dark layers, even for concentrations below 0.1 mg m−3. The large abundance of volatile Aitken mode particles suggests previous nucleation of sulfuric acid droplets. The effective diameters range between 0.2 and 3 μm with considerable surface and volume contributions from the Aitken and coarse mode aerosol, respectively. The distal ash mass flux on 2 May was of the order of 500 (240–1600) kg s−1. The volcano induced about 10 (2.5–50) Tg of distal ash mass and about 3 (0.6–23) Tg of SO2 during the whole eruption period. The results of the Falcon flights were used to support the responsible agencies in their decisions concerning air traffic in the presence of volcanic ash.

/pdf/airborne-observations-of-the-eyjafjalla-volcano-ash-cloud-3dz485lnbg.pdf

Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010

The impact of sulfur oxides on particle formation and contrails is investigated in the exhaust plumes of a twin-engine jet aircraft. Different fuels were used with sulfur mass fractions of 170 and 5500 ppm in the fuel, one lower than average, the other above the specification limit of standard Jet-Al fuel. During various phases of the same flight, the two engines burnt either high- or low-sulfur fuel or different fuels in the two engines. Besides visual, photographic, and video observations from close distance, in situ measurements were made within the plumes at plume ages of 20 to 30 s, at altitudes between 9 and 9.5 km, and temperatures between −49 and −55°C, when the visible contrail was about 2 km long. The data include particle number densities for particles larger than 7 nm, 18 nm, 120 nm, and 1 μm in diameter, together with wind, temperature and humidity measurements. The observations show visible and measurable differences between contrails caused by the different sulfur levels. At ambient temperatures 5 K below the threshold temperature for contrail onset, the plume became visible about 10 m after the engine exit for high sulfur content, but 15 m after the engine exit for low sulfur content. The higher sulfur emission caused a larger optical thickness of the contrail shortly after onset, with slightly brown-colored contrail when the Sun was behind the observer, and more contrast when viewed against the Sun. The high-sulfur contrail grew more quickly but also evaporated earlier than the low-sulfur contrail. At plume ages of about 20 s, each engine plume was diluted to an effective diameter of 20 m. The plumes contained many sub visible particles. Peak number densities were 30,000 cm−3 for particles of diameter above 7 nm and 15,000 cm−3 above 18 nm. The latter is a little larger than the estimated number of soot particles emitted. The high-sulfur plume shows more particles than the low-sulfur plume. The differences are about 25% for particles above 7 nm and about 50% above 18 nm. The results indicate that part of the fuel sulfur is converted to sulfuric acid which nucleates with water vapor heterogeneously on soot or nucleates acid droplets homogeneously which then coagulate partly with soot. During descent through the level of contrail onset, the high-sulfur contrail remained visible at slightly lower altitude (25 to 50 m) or higher temperature (0.2 to 0.4 K). At least for average to high sulfur contents, aircraft generate an invisible aerosol trail which enhances the background level of condensation nuclei, in particular in regions with dense air traffic at northern latitudes and near the tropopause.

/pdf/in-situ-observations-of-particles-in-jet-aircraft-exhausts-4e2mn1mdpo.pdf

M. Krautstrunk

Papers

Airborne observations of the Eyjafjalla volcano ash cloud over Europe during air space closure in April and May 2010

In situ observations of particles in jet aircraft exhausts and contrails for different sulfur-containing fuels