1.0 Introduction
The
general goal of this work has been to clarify the kinetic and mechanism of some
of the most relevant chemical processes taking place in the Earth’s atmosphere.
In order to achieve this goal, I have focused on those chemicals that rule the
chemistry of the atmosphere like the Hydroxil (OH) radical, the Ozone molecule
and the Nitrogen dioxide. This dissertation is composed of this introduction
and six chapters that cover respectively:
- Chapter One: O(1D) Quantum Yield from Ozone
Photolysis in the near UV region between 305 and 375 nm.
- Chapter Two: A Pulsed Laser Photolysis-Pulsed Laser
Induced Fluorescence Study of the Kinetic of the Gas-Phase Reaction of OH
with NO2
- Chapter Three: Constraining the reaction mechanism of
the three-body recombination reaction between the Hydroxyl radical and
Nitrogen Dioxide. Vibrational deactivation and Isotopic substitution
Experiments.
- Chapter Four: Kinetic and Mechanism of the reaction NO+
OH
- Chapter Five:
Vibrational Deactivation Studies of OH (v=1-5) with Nitrogen and
Oxygen.
- Chapter Six: Atmospheric
Implication and Final Remarks.
1.1 OH Production in the Troposphere
The
Hydroxyl (OH) radical is an extremely reactive chemical that play a significant
role in the day-time atmospheric chemistry. The primary source of tropospheric
OH is the reaction of H2O with O(1D)-atoms [Levy II,
1972] which are formed by solar photolysis of ozone:
O3
+ h O2 + O(1D) (1.1.1)
O(1D)
+ H2O2 OH
(1.1.2)
A
significant part of this study addresses some aspects of OH production from
this sequence of reactions measuring the O(1D) quantum yield in the
near UV. While photolysis in the 200-300
nm bands is important in the stratospheric chemistry of ozone, absorption in
the weak tail (300-350 nm) dominates the photochemical activity in the
troposphere [Molina and Molina, 1986]. This is because of the dramatic increase in the intensity of the
actinic flux at wavelengths longer than 300 nm as shown if Fig. 1.1.1.
An
accurate determination of the quantum yield for O(1D) from ozone
photolysis in the region 300-350 nm is critical to quantify the actual
production of OH radicals in the troposphere. The major channel for the
production of O(1D) is a spin-allowed process (1.1.3):
O3 + h O(1D)
+ O2 (1)
(1.1.3)
O3 + h O(3P)
+ O2 (3-) (1.1.4)
O3 + h O(1D)
+ O2 (3-) (1.1.5)
The
spin-allowed dissociation channel of tropospheric importance in ozone
photolysis is reaction (1.1.4) but this reaction cannot affect the HOx
tropospheric cycle, while the spin-forbidden reaction (1.1.5) can represent a
source for tropospheric OH radical generation in the troposphere. Reaction
(1.1.3) has a thermodynamic threshold around 310 nm at 0 K [Heicklen, 1976].
This means that the sum of the potential energy of the two fragments [O(1D)
and O2 (1)] becomes larger than the energy of a 310 nm
photon. At room temperature, this threshold is shifted to the red by about 10
nm due to the vibrational excitation of the ozone molecule. Thus, the quantum
yield for the production of O(1D) from ozone photolysis should be
zero for wavelengths > 325 nm. However the production of O(1D)
has been observed in laboratory experiments at wavelengths longer than 325 nm
by several authors [Silvente et al. 1997; Amerrding et al., 1990; This work].
According to 1997 NASA recommendations [De
More et al., 1997], reaction (1.1.3) takes place only at 325 nm. The
production of O(1D) in the region between 310 and 325 nm is
attributed to the photolysis of vibrationally excited ozone. This possesses
enough vibrational energy to produce O(1D) after photolysis even at
wavelengths between 310 and 325. O(1D) formation beyond 325 nm
cannot be attributed to photolysis of vibrationally excited ozone at room
temperature: observations of O(1D) beyond 325 nm can only be
explained by a spin-forbidden process that results in the formation of O(1D).
The
wavelength dependence of the O(1D) quantum yield of reaction (1.1.5)
is temperature independent. However since the quantum yield for reaction
(1.1.3) depends on photolysis of vibrationally excited ozone it is temperature
dependent [Moortgat et al, 1977]. Consequently the importance of reaction
(1.1.3) may increase at low temperatures.
We
determined accurately the quantum yield for O(1D) in the near UV. In
this study we verify that reaction (1.1.3) can be a potential source for O(1D)
in the troposphere.
1.2 HOX tropospheric cycle
At sufficiently high NO
concentrations ([NO]>10 ppt) the OH radical is regenerated from the reaction
between the HO2 radical and NO:
NO + HO2 OH +
NO2 (1.2.1)
NO2 + h NO +
O(3P) (1.2.2)
O(3P) + O2
O3 (1.2.3)
This sequence of reactions has a major role in the production of
tropospheric ozone and seems to be the most important pathway for OH radical
regeneration in urban environments [Seinfeld, 1995]. In urban contexts,
combustion of fossil fuels provides the major atmospheric NOx
emission [Logan, 1983]. A pictorial representation of the tropospheric OH cycle
is reproduced in Fig. 1.2.1. In this laboratory the reaction between HO2
and NO is extensively used to investigate possible OH re-generation channel. An
accurate determination of the reaction between OH and NO becomes critical for
such a task since NO is present in the system. The primary source for
tropospheric HO2 radical is photo-oxidation of formaldehyde:
HCHO + h H + HCO
(1.2.4)
H + O2 HO2 (1.2.5)
While the OH initiated oxidation of CO is an important
process in the interconversion of the OH radical to the HO2 radical.
CO + OH CO2
+ H (1.2.6)
H + O2 + M M
+ HO2 (1.2.7)
In an
overall view, the steady-state concentrations of OH can be strictly related to
the HO2 production.
1.3 OHX and NOx Atmospheric Sinks
The three-body recombination of OH with NO2
is the major pathway for permanent
NOx and HOx removal as illustrated in Fig. 1.6.1. Nitric
acid is thought to be the most stable product for this reaction under
conditions representative of the atmosphere. Once is formed HNO3 is
quickly removed from the troposphere via either dry or wet deposition. Since
its high solubility in water HONO3 is incorporated in water droplets
almost quantitavely. It has been estimated that HNO3 contributes to
almost 30% of the total acidity in the acid rain [Seinfeld, 1985].
OH+NO2+(M)
Products +(M)
(1.3.1)
In
the polluted troposphere reaction (1.3.1) can act as a major sink for OH. The
pressure dependence of the absolute rate constant for this association reaction
lies in the falloff region between second and third order kinetics over the
range of pressures and temperatures encountered in the atmosphere. In spite of
this fact there is substantial amount of experimental data for this reaction
available in the current literature, still the mechanism of the OH
recombination with NO2 it is not fully understood.
Most
of the controversy regarding the OH+NO2 reaction is not related with its role in the
atmosphere but with the reaction mechanism. It has been argued that an
alternative reaction pathway may exist leading to the formation of the elusive
isomer of nitric acid HOONO.
OH+NO2 HNO3 (1.3.2)
OH+NO2
HOONO (1.3.3)
This possible reaction’s channel had been first
hypothesized by Ian Smith in 1984, but only in recent years this hypothesis has
been extensively investigated [Golden et al., 2001]. Due to the significance of
the issue experimentalists have been able to observe the formation of HOONO as
product OH+NO2 [Nizkorodov et
al, 2002, Bean et al., 2003]. Although very little information is available on
the possible formation of this species in the atmosphere.
The
energetic of this reaction, as proposed by J. Barker [Barker et al, 2002], is
summarized in Fig. 1.3.2. The nitric acid is by far the most stable product but
the energy of the HOONO adduct is still lower than the sum of the energies of
the reactants.
The
formation of the unstable species HOONO could explain the discrepancies between
the high-pressure studies performed by the group lead by Horst Hippler [Fulle
et al., 1998; Forster et al., 1995] and the relatively low pressure
observations performed by a number of investigators. The kinetic measurements
observed by the German group lead by Professor Hippler Horst under high pressure
are currently being reviewed [Hippler et al., 2002]. It seems that indirect
experimental evidences of the adduct formation may be observed under extreme
conditions of pressure and temperature. Nevertheless the importance of this
pathway under atmospheric conditions is still to prove being the yield in HOONO
less than a small fraction.
Several
theoretical and experimental paper, quoted later in the discussion, point out
that the HOONO channel is either null or marginal under atmospheric conditions,
while may become important in extreme temperature or pressure.
We performed a comprehensive series
of kinetic runs under atmosphere-like conditions determining the absolute rate
coefficient for the OH recombination with NO2 under up to 600 Torr
of Helium, Nitrogen, and Oxygen at room temperature. In addition to that we
reported some measurements performed at 273 K, considered the average
temperature of the troposphere in most atmospheric models.
Since
our work is motivated by the need of reliable kinetic data-base for
tropospheric and stratospheric modeling, we also examined the potential effect
of water vapor on the rate of this reaction as hypothesized by Evanseck and
co-workers [Davey et al., 2000].
Besides
being a pathway for the permanent removal of NOx and HOx
reaction (1.3.1) influences significantly the incremental reactivity of several
organics [Yang et al., 1995; Bergin et al., 1998]. In some recent EPA funded
studies investigator have pointed out that one of the major uncertainties in
determining the effect of organic compounds on the tropospheric ozone cycle is
related to the rate coefficient for this reaction.
In
the latest portion of the study we tried to constrain the mechanism for the
recombination reaction between OH and NO2 using two novel
approaches:
- The Vibrational deactivation technique as a tool for the
determination of the high pressure limit;
- The Isotopic substitution technique.
While these two approaches were useful to clarify
significant portion of the mechanism, still a certain degree of uncertainty
remains causing the need for further attention.
1.4
Global relevance of OH chemistry
The
HOx radicals, hydroxyl (OH) and hydro-peroxy (HO2), play
a central role in the chemistry of both the troposphere and stratosphere
[Wayne, 2000]. In the troposphere, OH is the primary oxidizing agent,
reactively removing most trace gases such as CO, methane,
hydrofluorochlorocarbons (HCFC) and non-methane hydrocarbons.
The
reaction with the hydroxyl radical is the main sink for a number of gaseous
species in the Earth's atmosphere, determining their residence time or
atmospheric lifetime. The residence time is used in estimating global budges
and often to evaluate the impact of human activities on the Earth' s
atmosphere. The general definition of atmospheric lifetime is derived by the
solution of the differential equation representative of the mass balance for
the generic atmospheric species M:
(d[M]/dt)=P+I-R-O (1.4.1)
where
P= production rate
I= inflow rate
R= removal rate
O= outflow rate
in an unmixed system equation (1.4.1) becomes
(d[M]/dt)=P-R (1.4.2)
and the time constant is defined as:
=[M]/P=[M]/R (1.4.3)
The
term R present in this expression includes removal both via deposition and
chemical removal. For most of the volatile species the deposition term is
negligible if compared with the chemical removal term. The chemical removal
term may be subsequently split in two components:
Rchemical=R photolysis+Rreaction (1.4.4)
R photolysis represents the loss of
the species [M] due to photolysis and the Rreaction component accounts for the loss of [M] due to
chemical reaction. In most cases the reaction with OH is the dominant factor in
the Rreaction term. Hence the lifetime for the generic
species M in the atmosphere may be rewritten only as function of the average OH
concentration times the kinetic constant of the reaction between the OH radical
and the species.
(OH)=kM+OH(M)-1
* [OH]-1 (1.4.5)
From
expression (1.4.5) becomes clear the importance of knowing accurately the two
quantities as the average OH concentration and the absolute rate coefficient of
the OH initiated oxidation of the species M.
The estimation of average tropospheric OH
concentration is a different issue to be resolved with appropriate modeling and
extensive field campaigns. A catalytic cycle, involving both OH and HO2,
plays an important role in determining tropospheric ozone levels. The
effectiveness of this cycle is moderated by the reaction between the
hydro-peroxyl radical and nitrogen monoxide. The interest in OH and HO2
chemistry [Chris et al., 1995] and in their actual tropospheric levels
[Crosley, 1995] has greatly risen in the last ten years. Concentration profiles
of these radicals have recently been estimated [Poppe et al., 1995], although
this subject is still under the scrutiny of the scientific community. A number
of models have been proposed for the average profiles of tropospheric OH and HO2 [Thompson,
1995]. These profiles suggest concentrations of 1x106 (molecules/cm3)
for OH and 3x108 (molecules/cm3) for HO2 at 0
km and 45o N, with calculated peak concentrations in the tropics
considerably higher.
One
of the goals of this project was to accurately determine the rate of reaction
of OH with Nitrogen Oxides. This project is in fact part of an on-going effort,
performed in this laboratory, leading to a critical revision of the database
for the reaction between the OH radical and most of the environmentally
important chemicals.
1.5 Ozone
Ozone
is a bend molecule made up of 3 oxygen atoms forming a ~117 degrees angle with
each other as predicted by the VESR model. Its chemical formula is O3
and it is a gas naturally present in the atmosphere. The distance between two
oxygen atoms in the ozone molecule has been estimated around 0.126 nm. Since
its natural distribution in the Earth's atmosphere (Fig. 1.5.1), and since the
importance of ozone on human activities, ozone related issues are usually
split, in two broad categories:
- The Tropospheric Ozone
- The Stratospheric Ozone
In the troposphere (0-10 Km) exposure to ozone induces effects
on health and the environment, causing respiratory difficulties in sensitive
people and possible damage to vegetation and ecosystems. Threshold values set
for the protection of human health, vegetation and ecosystems are exceeded
frequently in most European countries and in some regions of the USA with
adverse effect on human population.
Ozone
in the troposphere is also of relevance to the climate change issue since ozone
is a greenhouse gas. It is currently estimated that tropospheric ozone adds 0.4
W.m-2 to the current enhanced climate forcing of 2.45 W.m-2.
While the total forcing is mainly a result of the increase in long-lived
compounds (CO2, CH4,
N2O, halocarbons) [Pitts and Pitts, 1986].
This profile (Fig 1.5.1) shows how
the amount of ozone varies with height in the atmosphere. Note that most of the
ozone is in the lower stratosphere, at an altitude of about 20-25 kilometers
(12-15 miles) above sea level. This is the so-called "ozone layer."
It acts as a shield by absorbing biologically active ultraviolet light ( UV-B)
from the sun. If the ozone layer is depleted, more of this UV-B radiation reaches
the surface of the earth. Increased exposure to UV-B has harmful effects on
plants and animals, including humans. The chlorine and bromine in
human-produced chemicals such as the ones known as chlorofluorocarbons (CFCs)
and halons are depleting ozone in the stratosphere through a catalytic odd
oxygen cycle that can be written in is general form [Seinfeld and Pandis,
1998]:
X+O3-> XO+O2
(1.5.1)
XO+O3-> X+
2O2 (1.5.2)
net:
2O3->3O2
(1.5.3)
Where X may be substituted in the
chemical equation by Cl, Br, H, NO or
OH. Since the net result of the cycle is the net destruction of ozone and the
regeneration of the generic catalyst X this cycle is called the catalytic ozone
cycle. A Iodine (IOx) cycle has also been suggested, but its
importance result to be marginal since the Iodine bearing species have a short
tropospheric lifetime. From an energetic point of view the role of the catalyst
is to lower the activation energy of the overall odd oxygen destruction.
1.6.1
Ozone Cross Section
The absorption cross section of
a compound represents the amount of photon absorbed by a molecule of that
compound at a specific wavelength. The Beer-Lambert’s laws relates the
wavelength and temperature dependent absorption cross section with the
concentration of a chemical species.
N**l=log(Io/I) (1.6.1)
Where I and Io are the intensities of the
transmitted and incident light, respectively. N is the number of molecules, l(T)
is the cross section, and l is the path-length. The product N**l is
also referred as absorbance (A).
The cross section is an
extremely important characteristic for a molecule of atmospheric relevance
since they interact with sunlight. Absorption cross section may show
temperature dependence as in the case of the Ozone molecule. Orphal [Orphal,
2003] recently reviewed both Ozone and nitrogen dioxide cross sections. The cross section of ozone in the UV-visible
range has been classified into four systems ranging from shorter to longer
wavelength:
(a) The Hartley bands,
(b) The Huggins bands,
(c) The Chappius bands
and
(d) The Wulfs bands.
Among these bands, the
strongest is the Hartley bands, which extend from about 200-300 nm, peaking at
around 255 nm. There is a residual vibrational structure despite its smooth
shape. This is explained by the quasiperiodical orbits of the electronic
wave-packets before dissociation in the upper state, a structure that is
slightly temperature-dependent [Orphal, 2003]. At present, there is no
calculation that can predict the absorption cross-sections of ozone within
experimental accuracy despite the fact that the structure of the Hartley band
is theoretically well understood.
The Huggins
bands consist of a series of individual peaks from 300-390 nm. Because of the
changing slope of the Hartley band and to the sharpening of the individual
bands at lower temperature, there is a substantial temperature dependence in
the Huggins band. Moreover, as a result of the drastic change of cross-section
with wavelength, the Huggins band system is extremely difficult to be measured
at once. Currently, Huggins bands are used for spectroscopic remote-sensing O3
by various experimental techniques. A significant section of this thesis
deals with the O1D quantum yield from Ozone photolysis in the
Huggins band. This topic has been quite controversial for a while now, since
part of the uncertainty in the experiments depends on a not well established
information on the structure of the cross section.
The Chappuis
band is a thousand times weaker than the Hartley band; it is a broad structure
in the visible region at about 380-800 nm. Here, the residual structures arise
from quantum mechanical interferences between two interacting excited
electronic states, showing a little variation with temperature. Atmospheric
remote-sensing of O3 uses the region between 400 and 500 nm.
In the O(1D)
quantum yield study, the focus is in the region between 305 and 375 nm mainly
occupied by the Hartley bands. An
accurate value for the cross section of ozone is critical for a proper
interpretation of the lif measurements of the O(1D) quantum yield
[Bauer et al, 2000]. There is a number of published ozone cross sections at
sufficiently high resolutions but only three of them cover the range of
wavelengths covered in our investigation in detail:
(a) Vogit et al., 1999;
(b) Malicet et al., 1995;
(c
) Brion et al., 1998.
Two more authors have subsequently
measured the UV-visible ozone cross section but their results are not yet being
published:
(d) Richter, 1995;
(e) Bogumil et al., 2003.
1.7.1
Temperature Dependence of
Ozone Cross Section
Orphal
[2003] compared the relative change of the O3 absorption cross-sections
with temperature, distinctly pointing out the identification of regions where
the effect of temperature is dominant. In the Hartley band (240-310 nm), the
cross-sections slightly increase as temperature decreases at wavelengths below
260 nm. It starts to decreases significantly at lower temperature at
wavelengths above 260 nm. At present, there are no accurate theoretical
predictions for this kind of behavior. But this is the same effect as observed
upon a small wavelength shift of the absorption cross-sections towards longer
wavelengths when the temperature decreases. The Harttley band system results to
be the most interesting in terms of temperature dependence since it is the
region in which the temperature has the higher effect.
In the Chappuis band
(400-790), there is a very small change of the peak cross-sections with
temperature. Further, there is a good agreement on the increase of differential
structure with decreasing temperature. The magnitude of these features is in
general agreement. However, there is a statistically significant disagreement
between the available laboratory cross-sections on the relative temperature
dependence in the 400-500 regions, where the temperature dependence is
irregular.
Integrated cross-sections
differences as a function of the temperature are significant in the Huggins
band region [Orphal, 2003]. Otherwise,
there are relative changes of the spectrum, which are observed in the peak
region of the Hartley band, and in the Chappuis band. Due to an overall
decrease of the cross-sections together with strong changes in the differential
structure of the bands, the region of the Huggins band is the most difficult
region to be characterized.
1.8.1 Nitrogen Oxides
Nitrogen dioxide belongs
to a family of highly reactive gases called nitrogen oxides (NOx).
These gases form when fuel is burned at high temperatures, and come principally
from motor vehicle exhaust and stationary sources such as electric utilities
and industrial boilers. Nitrogen dioxide is a brownish gas, and it is a strong
oxidizing agent that reacts with the OH radical to form nitric acid, as well as
organic nitrates [Atkinson et al., 1982]. It also plays a major role in the
atmospheric chain of reactions that produce ground-level ozone [Atkinson,
1984].
In regard of human health nitrogen dioxide can irritate the
lungs and lower resistance to respiratory infections such as influenza. The
effects of short-term exposure are still unclear, but continued or frequent
exposure to concentrations that are typically much higher than those normally
found in the ambient air may cause increased incidence of acute respiratory
illness in children. EPA's health-based national air quality standard for NO2
is 0.053 ppm (measured as an annual arithmetic mean concentration). Nitrogen
oxides take part in the catalytic ozone cycle. Once present in the atmosphere,
nitrogen oxides can significantly contribute to a number of phenomena adverse
to the environment: such as acid rain and eutrophication in coastal waters.
Nationally, annual NO2 concentrations remained
relatively constant throughout the 1980's, followed by decreasing
concentrations in the 1990's. Average NO2 concentrations in 1995
were 14 percent lower than the average concentrations recorded in 1986. The two
primary sources of the NOx emissions in 1995 were fuel combustion
(46 percent) and transportation (49 percent). Between 1986 and 1995, emissions
from fuel combustion decreased 6 percent, and emissions from highway vehicles
decreased 2 percent. Overall, national total NOx emissions decreased
3 percent in the last 5 years.
Tropospheric
chemistry is non-linear, involving a large number of compounds emitted at the
surface, and is complicated by interactions between different phases including
gas, liquid, aerosol, and various surfaces [Madronich, 1993]. Tropospheric
nitrogen oxides originate primarily from the heating of air to temperatures
where the Zeldovich mechanism becomes operative; these temperatures are reached
during most combustion processes and lightning. Additional NOx
sources may be associated with bacterial processes in soils [Madronich, 1993].
Once in the atmosphere, NO and NO2 partake in many
chemical reactions: some of these are simple NO - NO2 interconversions, while
others are actual NOx sinks [Madronich, 1993]. Specifically, the
reaction removes NOx quickly, with about 1 day lifetime for typical
mid-latitude conditions. The short NOx lifetime has one important
implication: If the sources of NOx are not geographically uniform,
the global NOx distributions will be highly variable, being very
sensitive to both chemical and meteorological processes. NOx levels
are seen to span about 3 orders of magnitude, and can be on either side of the
ozone net -production threshold [Madronich, 1993].
1.9.1 NO2
Cross Section
The
NO2 absorption
cross-section in the 240-790 nm region is separated into two principal systems:
the D-X band which is below 250 nm and the broad B-X and A-X band systems found
in the 300-790 nm region, with a maximum at around 400 nm. However, it is
impossible to predict the spectrum of NO2 from molecular quantum
theory within experimental accuracy because of the complexity of its excited
electronic states.
There are no measurements of absolute absorption
cross-sections of NO2 at very high spectral resolution in the region
between 250-790 nm. The cross-sections measurements have been limited to lower
spectral resolution because it was believed that it was sufficient enough to
use low-resolution spectra. However, lately, many sets of very high-resolution
measurements of NO2 cross-sections
are being published by different authors. A more detailed comparison between
reference NO2 spectra and our measured spectrum is discussed in the
NO+OH chapter.
1.10.1. Temperature Dependence of NO2
The
NO2 absorption cross-section depends on temperature, both in absolute
value and in shape. If in the data analysis a cross-section is used that is
appropriate for low stratospheric temperatures, a significant NO2
absorption in the warm boundary layer will not only increase the measured NO2
column, but also lead to a distinct residual structure originating from the
mismatch in absorption cross-section [Richter et al., 2001]. This difference
signal can be simulated by orthogonalizing two NO2 cross-sections
taken at different temperatures. From the magnitude of the cross-section
difference (2 ´ 10-20 cm2/molec peak to peak) one can
roughly estimate an expected absorption of 4 ´ 10-4 for a large
tropospheric column of 2 ´ 1016 molec/cm2 [Richter et
al., 2001].
In a work by
Richter et al. (2001), three alternative approaches have been studied that can
be used to discern the tropospheric NO2: the wavelength method,
extensively described in Richter and Burrows (2000), a method based on the
temperature dependence of the NO2 absorption cross-section and a
method using results from the 3-D CTM SLIMCAT.
State of the
art chemical transport models such as SLIMCAT [Chipperfield, 1999] have been
shown to provide good estimates of the stratospheric columns of many species
including NO2. These models are driven by meteorological wind fields,
and therefore are based on a realistic representation of stratospheric
dynamics. In principle, the SLIMCAT values can be converted to the expected
stratospheric columns and then subtracted from the measurements to yield the
tropospheric columns. But in practice, it turns out that relatively small
uncertainties in the absolute amount of NO in model or measurement can have a
large impact on the retrieved tropospheric columns [Richter et al., 2002]..
The
absorption cross-section of NO has a very structured temperature dependence
[Richter et al., 2002]. However, as the most pronounced of these features
correlate with instrumental features of GOME, retrieval can be performed on a
subset of the lines only. As a result, only very qualitative retrievals have been
possible, showing that a temperature signal is in the measurements, but clearly
too noisy to be used for a quantitative retrieval. This should improve
significantly for other instruments.
References (Additional)
Chipperfield,
M. P. (1999) Multiannual Simulations with a Three-Dimensional Chemical
Transport Model, J. Geophys. Res., 104, 1781-1805
Madronich, S. (1993). Tropospheric
photochemistry and its response to UV changes. In The role of the
stratosphere in global change. Vol. 18. NATO-ASI Series, ed. M-L. Chanin,
437-61. Amsterdam: Springer-Verlag.
Richter, A.
and Burrows, J. P. (2000) A multi-wavelength approach to the retrieval of
tropospheric NO2 from GOME measurements, Proceedings of the
ERS-ENVISAT symposium, Gothenburg October 2000
Richter, A., Nüß, H.,
Sinnhuber, B., Wagner, T. & Burrows, J. P. (2001) Annual Report: Quantification of Tropospheric Measurements
from Nadir Viewing UV/visible InstrumentsInstitute of Environmental Physics, University of
Bremen, Kufsteinerstr. Bremen, Germany and Institute
of Environmental Physics, University of Heidelberg, Germany
_________
(2002) Determining Tropospheric Constituent Columns from UV/visible Nadir
Satellite Measurements. Available at
[http://nadir.nilu.no/poet/EUROTRAC_0203_richter.pdf.]. Accessed [03/11/03].
