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Noxon (1975) was the first to
analyse light measured in the field with the differential optical
absorption spectroscopy (DOAS) method, here of scattered sunlight, although
the name was given to this technique only later by Platt et al. (1979). Due
to the numerous processes a photon can undergo in the atmosphere (e.g
scattering), the absolute light intensity of a recorded spectrum cannot
easily be simulated. The lack of this information is overcome by
investigating only the narrowband absorption features to quantify
concentrations of trace gases. This is expressed by the term 'differential'
in DOAS. In general, this technique can be applied to a variety of target
species, light sources and platforms as well as observation geometries:
Occultation measurements of the sun, moon and stars. But also artificial
light sources can be used in the field in the so-called long-path DOAS
technique (see below) or for cavity-enhanced measurements where the light
path is folded several times with the help of a set of highly reflective
mirrors. This technique can as well be applied in laboratory studies.
Scattered sun-light can be observed from the ground, from aircrafts, and
from satellite platforms. With balloon-borne instruments, usually
occultation measurements are performed. Target species include OH, HO2,
HONO, HCHO, CHOCHO, O3, NO2, NO3, ClO,
OClO, IO, OIO, I2, and BrO. In practice, concentrations can be
retrieved from the measured spectra using a modified Lambert-Beer's law.
For that, the absorption cross sections off all relevant absorbers in a
certain wavelength interval are simultaneously fitted to the measured
optical density after a polynomial has been subtracted to account for
broad-band features.
we run two
different DOAS systems for field measurements: a long-path DOAS and a
multi-axis DOAS. Both systems have been used in several campaigns (e.g.
Plane and Smith, 1995; Saiz-Lopez and Plane, 2004; Allan et al., 2001)
which will be further described below. In general, our group focusses on
the chemistry of halogen species in the marine boundary layer and it's
effects on ozone. However, recently we participated in the OP3 project in
the Bornean rainforest where we successfully detected glyoxal.
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MAX-DOAS
The multi-axis
DOAS is an advancement to the classical zenith-sky scattered-sunlight
spectroscopy method (e.g Solomon et al., 1987; Perliski and Solomon, 1993).
Whereas zenith measurements are most sensitive towards stratospheric
absorbers, pointing the telescope to the horizon significantly enhances the
light path in the lower troposphere while not effecting the stratospheric
path. This is illustrated in the figure below. The absorption by a certain
molecule along the light path can be converted into a slant column density
by spectral de-convolution as described above.
The Leeds MAX-DOAS system utilises an optical fibre bundle mounted to a
stepper motor (see photo below). This enables us to flexibly scan the
horizon under different elevation angles. A lens is placed in front of the
glass fibre to limit the field of view to 1°. The same
spectrometer/detector system is used here as for the LP-DOAS.
By combining the slant column densities from several lines of sight
including the zenith, the concentration of an absorber in the boundary
layer can be obtained by simulating the light path through the atmosphere
with a radiative transfer model taking into account multiple scattering and
the correct treatment of the aerosol load. For the interpretation of the
measurements, a programme has been developed to retrieve profiles. The
radiative transfer model SCIATRAN (e.g. Rozonov et al. (2005), http://www.iup.uni-bremen.de/sciatran)
in combination with a retrieval based on the method of optimal estimation
is used for that (Rodgers, 2000).
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Former Projects
The Leeds DOAS
group has been involved in various projects understanding the role of
halogens and the nitrate radical in the atmosphere. In the following, some
of the campaigns are described in more detail. Please also refer to the
publication section.
Please bear with us while we update the information about the
individual projects.
- Laminariae
Emissions in Galicia: Observation by Flourescence and Absorption
Spectroscopy (LEGOLAS) – a two week campaign in Galicia on the
North-West coast of Spain in collaboration with CIAC.
- NCAS-funded
10 months long campaign at Sao
Vicente (Cape Verde Islands) from February 2009.
- Reactive
Halogens in the Marine Boundary Layer (RHaMBLe) at the Station Biologique in
Roscoff (France): RHaMBLe is a part of the NERC sponsored UK SOLAS
project aiming at understanding the impact of halogen chemistry. The
campaign at Roscoff in summer 2006 was a collaborative effort of six
universities which did conduct a five week intensive studying the
different halogen species in the mid-latitudinal marine boundary
layer.
- RHaMBLe
at Sao Vicente (Cape Verde
Islands) : A new atmospheric monitoring site has been developed at
Cape Verde as a part of the SOLAS project which is used as a base to
increase our understanding of the oceanic atmosphere. We ran a
year-long (2006-2007) monitoring of different halogen species at this
location to study in detail the global imapct of halogens.
·
Impact of combined iodine and bromine release on the Arctic
atmosphere (COBRA) at Kuujjuarapik, Hudson
Bay (Canada) in spring 2008. Also, have a look at Roisin's blog.
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Oxidant and
Particle Photochemical Processes (OP3-DANUM-08) above a South-East Asian
tropical rain forests at Borneo (Malaysia) in summer 2008. Also, have a
look at Kate's blog.
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The Chemistry of the Antarctic Boundary Layer and the Interface with
Snow (CHABLIS) project at Halley Bay, Antarctica was an initiative to
explore the atmospheric chemistry of the coastal Antarctic boundary layer
in far greater detail and for a longer period of time than has been
achieved hitherto. Our role in this campaign has been to help understand
the role of halogens in the Antarctic MBL.
- North
Atlantic Marine Boundary Layer EXperiment (NAMBLEX) at Mace Head,
Ireland
- Land
Ocean Interaction Study (LOIS) at Weybourne, UK
- Atmospheric
Chemistry Studies in the Oceanic Environment (ACSOE) at Mace Head,
Ireland
- Aerosol
Characterization Experiment (ACE) at Tenerife, Canarias
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