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subtracting it. Obviously, this will not produce an exact equality between the measured optical depths and those calculated with the differential cross-sections but the difference is usually small. Alternatively a common method which is applied to remove broad-band structures from the optical density are binomial high-pass filters.
1162:{\displaystyle \delta _{d}+\delta _{c}=\ln \left({\frac {I_{1d}}{I_{2d}}}\right)+\ln \left({\frac {I_{1c}}{I_{2c}}}\right)=\sum \left(\beta _{i}^{*}+\alpha _{i}\right)\left(\sigma _{i2}-\sigma _{i1}\right)=\sum _{i}\beta _{i}^{*}\left(\sigma _{i2}-\sigma _{i1}\right)+\sum _{i}\alpha _{i}\left(\sigma _{i2}-\sigma _{i1}\right)}
651:
1695:
is the angle at the second. Note that with this method, the column along the common path will be subtracted from our measurements and cannot be recovered. What this means is that, only the column density in the stratosphere can be retrieved and the lowest point of scatter between the two measurements
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will be meaningless. The typical measurement geometry will be as follows: the instrument is always pointing straight up. Measurements are taken at two different times of day: once with the sun high in the sky, and once with it near the horizon. In both cases the light is scattered into the instrument
87:
DOAS instruments are often divided into two main groups: passive and active ones. The active DOAS system such as longpath(LP)-systems and cavity-enhanced(CE) DOAS systems have their own light-source, whereas passive ones use the sun as their light source, e.g. MAX(Multi-axial)-DOAS. Also the moon can
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What this means is that before performing the inversion, the continuum components from both the optical depth and from the species cross sections must be removed. This is the important “trick” of the DOAS method. In practice, this is done by simply fitting a polynomial to the spectrum and then
489:
between the two columns (Alternative a solar atlas can be employed, but this introduces another important error source to the fitting process, the instrument function itself. If the reference spectrum itself is also recorded with the same setup, these effects will eventually cancel out):
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To deal with this, we introduce a quantity called the airmass factor which gives the ratio between the vertical column density (the observation is performed looking straight up, with the sun at full zenith) and the slant column density (same observation angle, sun at some other angle):
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Also, unless the path difference between the two measurements can be strictly determined and has some physical meaning (such as the distance of telescope and retro-reflector for a longpath-DOAS system), the retrieved quantities,
28:. When combined with basic optical spectrometers such as prisms or diffraction gratings and automated, ground-based observation platforms, it presents a cheap and powerful means for the measurement of trace gas species such as
185:
1412:
88:
be used for night-time DOAS measurements, but here usually direct light measurements need to be done instead of scattered light measurements as it is the case for passive DOAS systems such as the MAX-DOAS.
36:. Typical setups allow for detection limits corresponding to optical depths of 0.0001 along lightpaths of up to typically 15 km and thus allow for the detection also of weak absorbers, such as
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denotes different species, assuming that the medium is composed of multiple substances. Several simplifications can be made. The first is to pull the absorption cross section out of the
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1715:
Richter, A.; M. Eisinger; A. Ladstätter-Weißenmayer & J. P. Burrows (1999). "DOAS zenith sky observations. 2. Seasonal variation of BrO over Bremen (53°N) 1994–1995".
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1199:
the differential optical depth (DOD). Removing the continuum components and adding in the wavelength dependence produces a matrix equation with which to do the inversion:
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474:. Active DOAS variants can use the spectrum of the lightsource itself as reference. Unfortunately for passive measurements, where we are measuring from the bottom of the
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646:{\displaystyle \delta =\ln \left({\frac {I_{1}}{I_{2}}}\right)=\sum _{i}\beta _{i}\left(\sigma _{i2}-\sigma _{i1}\right)=\sum _{i}\beta _{i}\,\Delta \sigma _{i}}
1724:
Eisinger, M., A. Richter, A. Ladstätter-Weißmayer, and J. P. Burrows (1997). "DOAS zenith sky observations: 1. BrO measurements over Bremen (53°N) 1993–1994".
67:
101:
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A significant component of a measured spectrum is often given by scattering and continuum components that have a smooth variation with respect to
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485:. Rather, what is done is to take the ratio of two measurements with different paths through the atmosphere and so determine the difference in
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1631:{\displaystyle \sigma _{i0}={\frac {\Delta \sigma _{i}}{\mathrm {amf} _{i}(\theta _{2})-\mathrm {amf} _{i}(\theta _{1})}}}
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before passing through the troposphere but takes different paths through the stratosphere as shown in the figure.
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456:{\displaystyle I=I_{0}\exp \left(\sum _{i}\beta _{i}\sigma _{i}\right)=I_{0}\prod _{i}e^{\beta _{i}\sigma _{i}}}
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The change in intensity of a beam of radiation as it travels through a medium that is not emitting is given by
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1282:{\displaystyle \delta _{d}(\lambda )=\sum _{i}\beta _{i}^{*}(\lambda )\,\Delta \sigma _{i}}
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660:. Since these don't supply much information, the spectrum can be divided into two parts:
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is that which remains and we shall call the differential cross-section. Therefore:
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by assuming that it does not change significantly with the path—i.e. that it is a
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180:{\displaystyle I=I_{0}\exp \left(\sum _{i}\int \rho _{i}\beta _{i}\,ds\right)}
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1407:{\displaystyle \sigma _{i0}=\mathrm {amf} _{i}(\theta )\sigma _{i\theta }}
71:
Long-path DOAS System at the Cape Verde
Atmospheric Observatory (CVAO) at
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1507:. Airmass factors can be determined by radiative transfer calculations.
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and not the top, there is no way to determine the initial intensity,
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and spectral features, all the species could be solved for by simple
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If that was all there was to it, given any spectrum with sufficient
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Some algebra shows the vertical column density to be given by:
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must be determined to figure out where the column begins.
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DOAS and atmospheric chemistry group at IUP, Heidelberg
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1668:is the angle at the first measurement geometry and
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differential optical absorption spectroscopy (DOAS)
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277:. Since the DOAS method is used to measure total
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1738:: CS1 maint: multiple names: authors list (
1708:Differential Optical Absorption Spectroscopy
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1423:is the airmass factor of species
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327:The new, considerably simplified
1728:. Vol. 26. pp. 93–108.
1719:. Vol. 32. pp. 83–99.
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1706:Platt, U.; Stutz, J. (2008).
1450:{\displaystyle \sigma _{i0}}
1688:{\displaystyle \theta _{2}}
1661:{\displaystyle \theta _{1}}
1457:is the vertical column and
1192:{\displaystyle \delta _{d}}
265:is the path. The subscript
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813:{\displaystyle \beta ^{*}}
1754:DOAS group at IUP, Bremen
1500:{\displaystyle \theta }
786:{\displaystyle \alpha }
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243:{\displaystyle \beta }
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215:{\displaystyle \rho }
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18:atmospheric chemistry
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1779:Remote sensing
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65:
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58:Bromine oxide
55:
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1726:J. Atm. Chem
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1717:J. Atm. Chem
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54:Iodine oxide
46:Formaldehyde
42:Nitrous acid
38:water vapour
21:
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1710:. Springer.
73:SĂŁo Vicente
50:Tetraoxygen
26:trace gases
1768:Categories
1700:References
658:wavelength
476:atmosphere
468:resolution
256:scattering
252:absorption
77:Cape Verde
1734:cite news
1677:θ
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296:σ
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228:substance
210:ρ
200:radiation
196:intensity
157:β
147:ρ
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134:∑
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93:Beers law
329:equation
275:constant
271:integral
250:is the
224:density
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198:of the
194:is the
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773:where
190:where
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30:ozone
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261:and
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687:exp
358:exp
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