Retrieval of trace gases vertical profile
in the lower atmosphere combining.
Differential Optical Absorption Spectroscopy
with radiative transfer models
Creators: name: Palazzi, Elisa <1978>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: FIS/06 Fisica per il sistema terra e il mezzo circumterrestre

description: The motivation for the work presented in this thesis is to retrieve profile
information for the atmospheric trace constituents nitrogen dioxide (NO2)
and ozone (O3) in the lower troposphere from remote sensing measurements.
The remote sensing technique used, referred to as Multiple AXis Differential
Optical Absorption Spectroscopy (MAX-DOAS), is a recent technique that
represents a significant advance on the well-established DOAS, especially for
what it concerns the study of tropospheric trace consituents.
NO2 is an important trace gas in the lower troposphere due to the fact that
it is involved in the production of tropospheric ozone; ozone and nitrogen
dioxide are key factors in determining the quality of air with consequences,
for example, on human health and the growth of vegetation. To understand
the NO2 and ozone chemistry in more detail not only the concentrations at
ground but also the acquisition of the vertical distribution is necessary. In
fact, the budget of nitrogen oxides and ozone in the atmosphere is determined
both by local emissions and non-local chemical and dynamical processes (i.e.
diffusion and transport at various scales) that greatly impact on their vertical
and temporal distribution: thus a tool to resolve the vertical profile
information is really important.
Useful measurement techniques for atmospheric trace species should fulfill
at least two main requirements. First, they must be sufficiently sensitive to
detect the species under consideration at their ambient concentration levels.
Second, they must be specific, which means that the results of the measurement
of a particular species must be neither positively nor negatively
influenced by any other trace species simultaneously present in the probed
volume of air. Air monitoring by spectroscopic techniques has proven to be
a very useful tool to fulfill these desirable requirements as well as a number
of other important properties. During the last decades, many such instruments
have been developed which are based on the absorption properties of
the constituents in various regions of the electromagnetic spectrum, ranging
from the far infrared to the ultraviolet. Among them, Differential Optical
Absorption Spectroscopy (DOAS) has played an important role.
DOAS is an established remote sensing technique for atmospheric trace
gases probing, which identifies and quantifies the trace gases in the atmosphere
taking advantage of their molecular absorption structures in the near
UV and visible wavelengths of the electromagnetic spectrum (from 0.25 μm
to 0.75 μm). Passive DOAS, in particular, can detect the presence of a trace
gas in terms of its integrated concentration over the atmospheric path from
the sun to the receiver (the so called slant column density). The receiver
can be located at ground, as well as on board an aircraft or a satellite platform.
Passive DOAS has, therefore, a flexible measurement configuration
that allows multiple applications.
The ability to properly interpret passive DOAS measurements of atmospheric
constituents depends crucially on how well the optical path of light
collected by the system is understood. This is because the final product of
DOAS is the concentration of a particular species integrated along the path
that radiation covers in the atmosphere. This path is not known a priori and
can only be evaluated by Radiative Transfer Models (RTMs). These models
are used to calculate the so called vertical column density of a given trace
gas, which is obtained by dividing the measured slant column density to the
so called air mass factor, which is used to quantify the enhancement of the
light path length within the absorber layers.
In the case of the standard DOAS set-up, in which radiation is collected
along the vertical direction (zenith-sky DOAS), calculations of the air mass
factor have been made using “simple” single scattering radiative transfer
models. This configuration has its highest sensitivity in the stratosphere,
in particular during twilight. This is the result of the large enhancement in
stratospheric light path at dawn and dusk combined with a relatively short
tropospheric path.
In order to increase the sensitivity of the instrument towards tropospheric
signals, measurements with the telescope pointing the horizon (offaxis
DOAS) have to be performed. In this circumstances, the light path in the
lower layers can become very long and necessitate the use of radiative transfer
models including multiple scattering, the full treatment of atmospheric
sphericity and refraction.
In this thesis, a recent development in the well-established DOAS technique
is described, referred to as Multiple AXis Differential Optical Absorption
Spectroscopy (MAX-DOAS). The MAX-DOAS consists in the simultaneous
use of several off-axis directions near the horizon: using this configuration,
not only the sensitivity to tropospheric trace gases is greatly improved,
but vertical profile information can also be retrieved by combining the simultaneous
off-axis measurements with sophisticated RTM calculations and
inversion techniques.
In particular there is a need for a RTM which is capable of dealing with
all the processes intervening along the light path, supporting all DOAS geometries
used, and treating multiple scattering events with varying phase
functions involved. To achieve these multiple goals a statistical approach
based on the Monte Carlo technique should be used. A Monte Carlo RTM
generates an ensemble of random photon paths between the light source and
the detector, and uses these paths to reconstruct a remote sensing measurement.
Within the present study, the Monte Carlo radiative transfer
model PROMSAR (PROcessing of Multi-Scattered Atmospheric Radiation)
has been developed and used to correctly interpret the slant column densities
obtained from MAX-DOAS measurements.
In order to derive the vertical concentration profile of a trace gas from
its slant column measurement, the AMF is only one part in the quantitative
retrieval process. One indispensable requirement is a robust approach to
invert the measurements and obtain the unknown concentrations, the air
mass factors being known. For this purpose, in the present thesis, we have
used the Chahine relaxation method.
Ground-based Multiple AXis DOAS, combined with appropriate radiative
transfer models and inversion techniques, is a promising tool for atmospheric
studies in the lower troposphere and boundary layer, including the retrieval of
profile information with a good degree of vertical resolution. This thesis has
presented an application of this powerful comprehensive tool for the study of
a preserved natural Mediterranean area (the Castel Porziano Estate, located
20 km South-West of Rome) where pollution is transported from remote
sources.
Application of this tool in densely populated or industrial areas is beginning
to look particularly fruitful and represents an important subject for future
studies.