Tuesday, December 6, 2022

Kinetic Studies of the Tropospheric HOx Chemistry


 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) + H2O2 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].

 

 

 

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