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Aerosols – cirrus - contrails
Upper troposphere aerosols-cirrus-contrails optical properties
The effects of aerosols on the Earth’s radiative balance (direct, indirect via
clouds or semi-direct) are still unknown both in terms of sign and magnitude.
Global models’ uncertainties due to the aerosols’ effects are unacceptably high.
In particular, the radiative forcing due to natural and aviation-induced (contrails)
cirrus clouds is still poorly known.
As a contribution to this much-needed knowledge, this chapter deals with the
optical properties of aerosols and cirrus clouds in the upper troposphere (UT)
region based on a multi wavelength lidar measurement. These addressed optical
properties refer to high-resolution vertical profiles of the backscatter
) and extinction (
) coefficients and their integrated column extinction
(AOD- aerosols optical depth). These retrievals may be estimated by using both
elastic (Mie) lidar signals (355, 532 and 1064 nm) and inelastic (Raman) lidar
signals (387 and 532 nm) in a combined methodology. A 2-year statistical
analysis of the regular measurements within the EARLINET project is discussed
based on the determination of UT aerosol optical properties using only the
elastic (Mie) lidar signals. The comparisons with the co-located sun photometer
measurements (PFR) show relatively good and realistic agreement in terms of
AOD and Angstrom coefficients. The use of the Angstrom law on lidar and
complementary sun-photometer retrievals made it possible to distinguish and to
define different UT aerosol load degrees: aerosol-free reference, typical UT
aerosols, and cirrus-contrails load.
Range corrected signals, Mie-Raman combined methods, and measures of the
depolarization ratio at 532 nm allow a lidar-based classification of the UT cirrus
clouds. A pure contrail case study that examines its geometrical, optical, and
microphysical properties is also presented.
Aerosols – cirrus - contrails
1.1 Aerosols and cirrus clouds: climatic significance
are liquid or solid particles, 10
m, suspended in the
atmosphere. Particles larger than 2.5
m (i.e. coarse mode) are easily removed
by wet and dry deposition from the atmosphere. Particles between 0.1 to 2.5 µm
(i.e. accumulation mode) form the largest amount of atmospheric aerosols.
Particles smaller than 0.1
m (i.e. Aitken nuclei mode) serve as condensation
nuclei for forming larger particles and then they will finally migrate to the
accumulation mode. These particles remain longer in the atmosphere, and they
have various origins and types. One may divide the aerosols sources into
anthropogenic (particles from industrial emissions and photochemical
transformation in urban pollution plumes), and natural (stratospheric aerosols of
sulphuric acid, mainly from volcanic eruptions, tropospheric marine aerosols
from the oceans, mineral aerosols from desert or semi-desert areas, forest fires,
pollens, etc). An important topic in the Global Change program  is the study
of the biogeochemical cycle of tropospheric aerosols, and specifically the
generation of aerosols from the surface, their uplift and transport as well as their
interaction with other cycles [1, 2]. The aerosols have both a direct and indirect
impact on climate. Their impact is direct through the diffusion and absorption of
solar radiation, leading to cooling (e.g sulfuric) or warming effects (e.g
carbonaceous). The indirect effect is related to their role in forming and
interacting with clouds. Aerosols act as condensation nuclei and affect the
microphysical properties of clouds, which in turn modulate the Earth's radiation
budget (i.e. brighter clouds formed in this way would reflect more solar
radiation) . The aerosols decrease precipitation efficiency by increasing the
number of droplets in warm clouds thereby increasing the clouds’ lifetimes and
enhancing the indirect radiative forcing associated with these changes in cloud
properties . There is also a semi - direct effect that is related to the aerosols
capacity to absorb the solar radiation and produce local heating, which in turn
will evaporate the surrounding clouds.
The aerosols have different physical and optical properties, depending on their
chemical composition, size, and other intrinsic factors as their hygroscopic
behavior. Condensation of water vapor on atmospheric aerosol particles
significantly affects the size, shape, and chemical composition of these particles,
and therefore modifies their optical properties and thus further affects the direct
radiative forcing. As the distribution of aerosol concentrations is highly space -
time dependent, with short atmospheric lifetimes (i.e. days to weeks) they cannot
be considered responsible for a long-term offset to the warming such as in the
case with the greenhouse gases (i.e. CO
, CFC, etc).
See diagram in annex A12
Aerosols – cirrus - contrails
Cirrus/contrails are aggregations of particles of water or ice suspended in the air,
formed when air containing water vapor is cooled below a critical temperature
(i.e. dew point) and the moisture condenses into droplets on microscopic
particles (condensation nuclei) in the atmosphere. The air is cooled either by
expansion during the upward convection resulting from intense solar heating of
the ground; by a cold wedge of air (cold front) near the ground causing a mass of
warm air to be forced aloft; by orographic movements, and occasionally by a
reduction of pressure aloft or by the mixing of warmer and cooler air currents
(i.e. aircraft exhausts) . Cloudiness (the proportion of the sky covered by any
form of cloud), measured in tenths, is a key element in the estimation of radiative
global forcing. One may observe high clouds (6-12 km, cirrus, cirrostratus and
cirrocumulus); intermediate clouds (2-6 km, cumulus, altostratus and
altocumulus); low clouds (< 2 km, stratus, nimbostratus, stratocumulus) and
clouds with vertical development, (0.5–6 km, cumulonimbus) . The effect of
the upper troposphere clouds (i.e. natural cirrus or contrails) on chemistry and
radiative forcing has recently become a focus of scientific interest. Cirrus clouds
are increasing the Earth's albedo and at the same time trapping the infrared
radiation that the Earth is emitting to space. The warming or cooling effects are
both possible depending on clouds location, cover, composition and structure.
The greenhouse effect is weak for low altitude clouds, so their albedo effect
dominates and they have a net cooling effect on the Earth's climate. In contrast,
cold high altitude cirrus clouds may either cool or warm the air. They have a
strong greenhouse effect, which may outweigh their albedo effect . Generally
the cirrus greenhouse effect (warming) is expected to prevail over the albedo
effect (cooling). In addition, the effect of the multiple scattering within a cloud is
more important for the long wavelength waves (i.e. albedo 0.4-0.7) augmenting
the warming effect in the lower atmosphere by as much as 2° . Cirrus clouds
may also play a role in heterogeneous chemistry in the upper troposphere,
particularly in mid-latitude ozone depletion. The tropopause cirrus may also
contribute to the adiabatic heating of the upper troposphere, modifying the
temperature profile at the tropopause regions .
It is thought that cirrus clouds form naturally in the upper troposphere, when
highly dilute sulfate aerosols cool and become supersaturated with respect to ice.
These cloud particles freeze homogeneously when water vapor reaches ice super-
saturations of around 150%. It has been shown (i.e. MOZAIC experiment, )
that the free upper troposphere contains regions which present a super-saturation
state with respect to the ice. Thus it is important to analyze the relationships
between the number, concentration, and type of aerosols as well as temperature
and relative humidity conditions when examining the homogeneous or
heterogeneous freezing (condensation) processes with respect to ice or water
saturation pressure . It has been suggested that cirrus clouds could also be
formed from heterogeneous nucleation on insoluble solids (e.g sulfates). A
Aerosols – cirrus - contrails
recent focus has thus been made on the formation of ice clouds on soot particles
which are by-products of fossil fuel combustion at the Earth surface and of
aircraft emissions throughout the atmosphere .
Contrails are aircraft trace plumes producing a cloudiness up to ~ 0.1-0.2%
(1992). This is estimated to increase up to ~0.5-0.8% by the year 2050 . Their
formation and influence on the radiation budget are becoming an important
scientific topic. Like natural cirrus, the contrails reflect short wave (0.2-5
and they absorb long wave (5-50
m) radiation having thus an overall positive
effect (warming). The contrails’ formation and persistence is due to the injection
of the warm water vapor, soot, nitrogen oxides, sulfates, carbon dioxide,
unburned hydrocarbons, metallic particles, etc in the supersaturated over ice (~
125-150 %) upper troposphere regions. These emissions may enhance the ozone
formation and the decrease in methane. The carbon dioxide emitted is ~ 2% of
the total amount produced by anthropogenic activities. Large numbers (about
1017 particles/kg fuel) of small (radius 1 to 10 nm) volatile particles are formed
in the exhaust plumes of cruising aircraft (8 -13 km ASL altitudes), as shown by
in situ observations and model calculations [8, 13]. The global radiative forcing
by persistent contrails was estimated to be some ~ 0.02 Wm
in 1992 increasing
to ~ 0.1 Wm
in 2050 . Their impact on increasing the daytime maxima and
decreasing the nighttime minima of temperatures was observed during the 11
September aviation traffic break . Extensive aircraft-induced cirrus clouds
have been observed after the formation of persistent contrails. However, the
mechanisms associated with increases in cirrus cover are not well understood
and need further investigation.
1.2 Optical properties of aerosols and cirrus clouds: considerations
The extremely variable nature of physical and chemical properties and their
distribution over time and space make the study of aerosols and cirrus clouds
quite complex. In addition to laboratory measurements of their chemical and
physical properties, climate models require “real atmosphere” measurements of
aerosol size distribution and optical properties for their radiative budget (forcing)
calculations cf. . Global measurements are not available for many aerosol
properties, so models must be used to interpolate and extrapolate the available
data. Such models now include the types of aerosols that are most important for
climate change, but there are large discrepancies between the different models
concerning the estimation of sources and spatial distribution of different types of
aerosols. Despite this complex but essential influence there is still a big
uncertainty on the direct and indirect effects that aerosols have on radiative
forcing, as concluded in the last IPCC report . The models’ uncertainty is
due to: (a) extrapolation of experimentally determined source strengths to other
regions and seasons, (b) secondary aerosols (precursors and atmospheric
processes), (c) optical properties and (d) aerosol-cloud interaction. More
Aerosols – cirrus - contrails
scientific investigations (e.g. field measurements) concerning chemical and
physical properties of aerosols and their involved processes  are required to
estimate and predict direct and indirect climate forcing.
The aerosols – light interaction can be quantified based on a set of measured or
: the extinction (
), the scattering (
), the lidar ratio (LR, i.e. the extinction to backscatter ratio),
the single-scattering albedo (
, i.e. the scattering to extinction ratio),
the absorption coefficient (
), the functional dependence of
light-scattering on relative humidity (i.e. f(RH)), the complex refractive index
(m), the asymmetry parameter (g), …( and chapter II, section 2.2 for more
details and definitions). One of the most often-used parameters is the aerosols’
optical depth or thickness (AOD or AOT), which is the extinction coefficient
integrated on an atmosphere path cf. Eq. (1) generally scaled at the zenith
The clouds’ albedo depends on their AOD, the droplet effective radius (r
the geometrical thickness .
The wavelength dependence of these parameters is critical and is generally
known as Ångstrom’s  turbidity formula cf. Eq. (2) described already in the
chapter II section 2.2.2.
The measurement of the light depolarization degree (
), see chapter II section
2.2.3 by aerosols/cirrus is giving a good estimation of the particles shape
(spherical or non-spherical) and indirectly of their physical phase (water or ice
The above-defined aerosol parameters are measured via various complementary
techniques at global (i.e. satellites) and local (i.e. ground based) scales. The
ground based measurements can be realized in situ (e.g. nephelometer ,
aethalometer , epiphaniometer  on a atmospheric integrated path (e.g.
sun photometer ), atmospheric profiling (i.e. lidar and radar techniques [24-
26]). Generally the ground based observations are made within a network (e.g.
AERONET , EARLINET ). At the global scale these observations are
performed via various satellite observations such as AVHRR (Advanced Very
High Resolution Radiometer) Ångström coefficient, TOMS (Total Ozone
Mapping Spectrometer) aerosol index (AI) and MODIS (Moderate Resolution
Imaging Spectroradiometer) AOT data, SAGE (Stratospheric Aerosol and Gas
Indexes: a = aerosol, m = molecular, t = total, scat = scattering, abs = absorption
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