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Aircraft-borne spectroscopic limb measurements of trace gases absorbing in the UV-A spectral range [Elektronische Ressource] : investigations of bromine monoxide in the Arctic troposphere / put forward by Cristina Prados-Román

236 pages
Dissertationsubmitted to theCombined Faculties for the Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural SciencesPut forward byM.Sc. - Physics: Cristina Prados-Rom´anBorn in: Madrid, SpainOral examination: 9.12.2010Aircraft-borne spectroscopic limb measurements oftrace gases absorbing in the UV-A spectral range.Investigations ofbromine monoxide in the Arctic troposphere.Referees: Prof. Dr. Klaus PfeilstickerProf. Dr. Ulrich PlattAircraft-borne spectroscopic limb measurements of trace gases absorbing inthe UV-A spectral range. Investigations of bromine monoxide in the Arctictroposphere.Reactive halogen species (i.e., RHS=X, XO, X , XY, OXO, HOX, XONO , XNO , with2 2 2X,Y being I, Br and Cl) are known to be key compounds for the oxidation capacity of thetroposphere, affecting the lifetime of relevant species such as O , HO , NO , hydrocarbons,3 x xdimethylsulfide and gaseous elementary mercury. Furthermore, recent observations link iodinespecies to the formation of new aerosol particles. This work aims at the characterization of theabundance of BrO in the Arctic troposphere during the spring season, when the auto-catalyticrelease of bromine species from sea ice related halides is known to cause tropospheric OzoneDepletion Events (ODEs).A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals(e.g.
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Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
Put forward by
M.Sc. - Physics: Cristina Prados-Rom´an
Born in: Madrid, Spain
Oral examination: 9.12.2010Aircraft-borne spectroscopic limb measurements of
trace gases absorbing in the UV-A spectral range.
Investigations of
bromine monoxide in the Arctic troposphere.
Referees: Prof. Dr. Klaus Pfeilsticker
Prof. Dr. Ulrich PlattAircraft-borne spectroscopic limb measurements of trace gases absorbing in
the UV-A spectral range. Investigations of bromine monoxide in the Arctic
troposphere.
Reactive halogen species (i.e., RHS=X, XO, X , XY, OXO, HOX, XONO , XNO , with2 2 2
X,Y being I, Br and Cl) are known to be key compounds for the oxidation capacity of the
troposphere, affecting the lifetime of relevant species such as O , HO , NO , hydrocarbons,3 x x
dimethylsulfide and gaseous elementary mercury. Furthermore, recent observations link iodine
species to the formation of new aerosol particles. This work aims at the characterization of the
abundance of BrO in the Arctic troposphere during the spring season, when the auto-catalytic
release of bromine species from sea ice related halides is known to cause tropospheric Ozone
Depletion Events (ODEs).
A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals
(e.g., O , BrO, IO) was deployed on the DLR-Falcon (Deutsches Zentrum fu¨r Luft- und3
oRaumfahrt) aircraft and tested during a field campaign that took place at Svalbard (78 N) in
spring 2007. Herein, a new algorithm for inferring concentration vertical profiles of tropospheric
trace gases from aircraft-borne DOAS limb observations is presented, characterized and
validated through the profile retrieval of the UV-A absorber O . The method is then applied4
for retrieving tropospheric vertical profiles of BrO during the polar campaign.
For deployments during ODEs, the retrieved BrO vertical profiles consistently indicate high
BrO mixing ratios (∼ 15 pptv) within the boundary layer, low BrO mixing ratios (≤ 1.5 pptv)
in the free troposphere, occasionally enhanced BrO mixing ratios in the upper troposphere
(∼ 1.5 pptv), and increasing BrO mixing ratios with altitude in the lowermost stratosphere.
These findings are well in agreement with satellite and balloon-borne soundings of total and
partial BrO atmospheric column densities. Moreover, the capabilities of the aircraft-borne
measurements are further exploited by analyzing the sources, photochemistry and transport
processes of BrO in the boundary layer above the sea ice.
Flugzeuggestu¨tzte Streulicht-Spektroskopie von UV-A absorbierenden
Spurengasen. Erforschung von Brommonoxid in der arktischen Troposph¨are.
Reaktive Halogenverbindungen (RHS=X, XO, X , XY, OXO, HOX, XONO , XNO , mit2 2 2
X,Y entweder I, Br oder Cl) haben eine Schlu¨sselfunktion fu¨r die Oxidationskapazit¨at der
Troposph¨are, insbesondere indem sie die Lebensdauer wichtiger Schadstoffe wie O , HO , NO ,3 x x
Kohlenwasserstoffe, Dimethylsulfid and gasf¨ormiges, elementares Quecksilber beeinflussen. Des
weiteren,weisenneuereBeobachtungenaufeinewichtigeRollevonIodverbindungenbeiderBil-
dungvonAerosolpartikelnhin.ZieldieserArbeitistes,dieVerteilungvonBrOinderarktischen
Troposph¨are zu charakterisieren, wenn im Fru¨hling Bromverbindungen auto-katalytisch aus
Haliden u¨ber dem Meereis freigesetzt werden und es dadurch zu troposph¨arischem Ozonabbau
(Ozone Depletion Events - ODEs) kommt.
Ein neuartiger mini-DOAS Spektrometer wurde auf dem Falcon-Flugzeug des DLR (Deutsches
Zentrum fu¨r Luft- und Raumfahrt) eingesetzt und w¨ahrend einer Messkampagne bei Svalbard
o(78 N) im Fru¨hling 2007 getestet. Der Spektrometer ist auf den Nachweis von Radikalen
ausgelegt, die im UV und sichtbaren Spektralbereich absorbieren, und kann Streulicht-
beobachtungen in verschiedenen Sichtgeometrien durchfu¨hren. In dieser Arbeit wird ein neuer4
Algorithmus zur Bestimmung von vertikalen Konzentrationsprofilen troposph¨arischer Spuren-
gase aus flugzeuggestu¨tzte DOAS Streulichtbeobachtungen vorgeschlagen, charakterisiert und
mittels der Profilbestimmung von O validiert. Anschließend wird die Methode zur Auswertung4
troposph¨arischer BrO-Profile w¨ahrend der Kampagne am Pol verwendet.
Bei Auftreten von ODEs, weisen die BrO Vertikalprofile durchgehend hohe BrO Konzen-
trationen (∼ 15 pptv) in der planetaren Grenzschicht, niedrige BrO Konzentrationen (≤
1.5 pptv) in der freien Troposph¨are, gelegentlich erh¨ohte BrO Konzentrationen in der
oberen Troposph¨are (∼ 1.5 pptv) sowie mit der H¨ohe ansteigende BrO Konzentrationen
in der unteren Stratosph¨are auf. Diese Befunde werden von Satelliten- und Ballonmessun-
gen der BrO Gesamt- and Teils¨aulendichte best¨atigt. Weiterhin werden die M¨oglichkeiten
der flugzeuggestu¨tzten Messungen zur Erforschung von Brom-Quellen sowie der relevanten
PhotochemieundTransportprozesseu¨berdemMeereisinderplanetarenGrenzschichterkundet.Contents
1 Introduction 1
2 Physics of radiation and molecular absorption 5
2.1 Interaction of electromagnetic radiation with matter . . . . . . . . . . . . . . . . 5
2.2 Solar radiation and the Earth-atmosphere system . . . . . . . . . . . . . . . . . . 8
2.3 Radiative transfer in the Earth’s atmosphere . . . . . . . . . . . . . . . . . . . . 10
2.3.1 Scattering of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.2 Absorption and emission of light . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.3 Radiative transfer equation . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Atmospheric structure and dynamics 21
3.1 Vertical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Atmospheric circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.1 Tropospheric circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.2 Stratospheric circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2.3 Stability and vertical transport . . . . . . . . . . . . . . . . . . . . . . . . 28
4 Atmospheric chemistry 35
4.1 Ozone photochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Hydrogen oxide radicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3 Nitrogen oxide radicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.4 Halogen oxide radicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.4.1 Reactive halogen species in the troposphere . . . . . . . . . . . . . . . . . 49
4.4.2 Reactive halogen species in the stratosphere . . . . . . . . . . . . . . . . . 60
4.5 Aerosol particles and heterogeneous chemistry . . . . . . . . . . . . . . . . . . . . 65
5 The Arctic environment 71
5.1 Defining the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.1 Geographical extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.2 The Arctic Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.1.3 Sea ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.1.4 Particles and pollution in the Arctic atmosphere . . . . . . . . . . . . . . 80
5.2 Halogen species in the Arctic atmosphere . . . . . . . . . . . . . . . . . . . . . . 81
5.2.1 Polar Stratospheric Clouds and the Ozone Hole . . . . . . . . . . . . . . . 81
5.2.2 Bromine explosion and Ozone Depletion Events . . . . . . . . . . . . . . . 85
5.3 Arctic and Climate Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
iii CONTENTS
6 Remote sensing of the atmosphere 97
6.1 Basics of the optical absorption spectroscopy . . . . . . . . . . . . . . . . . . . . 97
6.2 Differential Optical Absorption Spectroscopy in the UV/vis . . . . . . . . . . . . 99
6.2.1 The DOAS principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.2.2 Experimental DOAS measurements setups . . . . . . . . . . . . . . . . . . 108
6.3 Radiative transfer modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7 Aircraft deployment of the mini-DOAS instrument 115
7.1 Description of the mini-DOAS instrument . . . . . . . . . . . . . . . . . . . . . . 115
7.2 Deployment on the DLR-Falcon aircraft . . . . . . . . . . . . . . . . . . . . . . . 118
7.3 Aircraft-borne limb DOAS measurements in the Arctic . . . . . . . . . . . . . . . 120
7.3.1 The ASTAR 2007 campaign . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.3.2 The GRACE 2008 campaign . . . . . . . . . . . . . . . . . . . . . . . . . 121
8 Retrieval of tropospheric trace gas abundances 129
8.1 Spectral retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
8.1.1 BrO spectral retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
8.1.2 OClO spectral retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.1.3 O spectral retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384
8.2 Tropospheric vertical profile retrieval . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.2.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.2.2 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
8.2.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9 Bromine monoxide in the Arctic troposphere 155
9.1 Tropospheric vertical profile of BrO volume mixing ratios . . . . . . . . . . . . . 156
9.1.1 BrO vmr vertical profile in context with other trace gases . . . . . . . . . 162
9.1.2 Satellite validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
9.2 Sources, photochemistry and transport processes . . . . . . . . . . . . . . . . . . 167
9.2.1 BrO and the dynamical tropopause . . . . . . . . . . . . . . . . . . . . . . 169
9.2.2 BrO in the polar marine boundary layer . . . . . . . . . . . . . . . . . . . 173
10 Conclusion 193
Publications 199
List of Figures 204
List of Tables 205
Bibliography 225
Acknowledgements 228Chapter 1
Introduction
A novel limb scanning mini-DOAS spectrometer measuring scattered skylight was deployed
on the DLR-Falcon (Deutsches Zentrum fu¨r Luft- und Raumfahrt) aircraft for the detection
of UV/vis absorbing radicals (e.g., O , BrO, IO, HNO ). The instrument was tested during3 2
two campaigns based in the Arctic region within the framework of the International Polar
Year 2007/08, and as part of the POLARCAT project (“Polar Study using Aircraft, Remote
Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport”).
Main objectives during these campaigns were to examine the capabilities of the mini-DOAS
aircraft-borne instrument, and to perform spectral and vertical profile retrievals of tropospheric
trace gases.
The Differential Optical Absorption Spectroscopy (DOAS) is a well known and established
atmospheric measurement technique (Platt and Stutz, 2008). In many applications using scat-
tered skylight, the main challenge of the remote sensing DOAS method consists in retrieving
trace gas concentrations from the measured differential Slant Column Densities (dSCDs). Trace
gas concentrations are inferred by consecutively probing the air masses at different viewing
geometries, and a subsequent mathematical inversion of the whole set of observations (e.g.,
Rodgers, 2000). In the best case scenario, the sampling is arranged so that the amount of pieces
of independent information on the multi-dimensional (spatial and temporal) distribution of the
targeted species is maximized. In practice however, the degrees of freedom are often limited
since the changing viewing geometries are predetermined by movements of the light source
(e.g., by celestial light sources), by displacements of the instrument platform (ships, aircrafts,
balloons, satellites, etc), by the change of the viewing direction of the light receiving telescope,
or by a combination of all of the above. Gathering the information often requires sampling over
a large spatial or temporal domain of the atmosphere, in which the radiative transfer (RT) may
change considerably as well. The need of dealing with these observational limitations correctly,
and of accounting for the atmospheric RT of each individual measurement properly, defines
12 CHAPTER 1. INTRODUCTION
a rather complicated (and in general ill-posed) mathematical inversion problem. As solutions
largely depend on the individual kind of observations, different strategies have been developed
to solve these ill-posed inversion problems (e.g., Rodgers, 2000). This work reports on aircraft-
borne observations of important and rare trace gases absorbing in the UV-A spectral range
(e.g., tropospheric BrO), monitored in a heterogeneously scattering atmosphere (the Arctic
troposphere). Herein, a dedicated method for the profile retrieval of trace gases constrained by
means of measured relative radiances is introduced and validated. In a similar way as in the
recently published work of Vlemmix et al. (2010), the observed (relative) radiances are used
to describe the scattering processes in the atmosphere during the time of the measurements.
Unlike Vlemmix et al. (2010), here not just the total aerosol optical thickness is inferred, but
vertical profiles of the extinction coefficient (E ) of aerosol and cloud particles (from now onM
referred to as ‘aerosols’). The targeted trace gas profile inversion, constrained by the retrieved
aerosolE , is then addressed with a regularization approach using no a priori knowledge of itsM
vertical distribution (e.g., Phillips, 1962; Rodgers, 2000).
The validity of the novel algorithm and the capabilities of the instrument are demonstrated
for deployments of the mini-DOAS instrument on the DLR-Falcon aircraft during the ASTAR
2007 campaign (“Arctic Study of Tropospheric Aerosol, Clouds and Radiation”). The campaign
o owas based on Spitsbergen (78 N, 18 E) and took place during March and April 2007. During
this field campaign, target trace gases to be detected from the boundary layer (BL) up to the
upper troposphere/lowermost stratosphere (UT/LS) with the mini-DOAS instrument were O ,3
NO , BrO, OClO, IO, OIO, HONO, C H O , CH O, H O and O .2 2 2 2 2 2 4
Since recent studies point out the relevance of halogens for the tropospheric photochemistry
(e.g., Von Glasow and Crutzen, 2007), this work focuses on the detection and retrieval of
bromine monoxide (BrO). Indeed, reactive halogen species (i.e., RHS=X, XO, X , XY, OXO,2
HOX, XONO , XNO , with X,Y as I, Br and Cl) are known to be key species, e.g., for the2 2
oxidation capacity of the troposphere and for the lifetime limitation of other species such us
O , HO , NO , hydrocarbons and dimethylsulfide. RHS are also known to be involved in new3 x x
particle formation (by iodine compounds, e.g., O’Dowd et al., ). Moreover, RHS are related to
atmospheric mercury depletion events that eventually yield scavenge of Hg by snow and parti-
cles, and deposition of toxic mercury to the polar ecosystems (e.g., Steffen et al., 2008). Also
characteristic (but not unique) of polar regions are the ozone depletion events (ODEs) occurring
in the low troposphere during springtime. These ODEs are linked to halogen activation in
−auto-catalyticcyclesinvolvingsalthalidesaerosols(e.g.,Br ),andtakeplaceoverareascovered
by first-year sea ice, i.e., sea ice lasting less than one melting season (e.g., Simpson et al., 2007,
and references therein). While the horizontal extent of the BrO associated with this type of
sea ice is fairly well captured by total column satellite measurements (e.g., SCIAMACHY,