introduction to measurement techniques in environmental physics summer term 2006

Introduction to Measurement Techniques in Environmental Physics, C. v. Savigny, Summer Term 2006

Introduction to Measurement Techniques in Environmental Physics

Summer term 2006

Postgraduate Programme in Environmental Physics

University of Bremen

Atmospheric Remote Sensing II

Christian von Savigny

Date 9 – 11 11 – 13 14 – 16

April 19 Atmospheric Remote Sensing I (Savigny)

Oceanography (Mertens)

Atmospheric Remote Sensing II (Savigny)

April 26 DOAS (Richter) Radioactivity (Fischer)

Measurement techniques in Meteorology (Richter)

May 3 Chemical measurement

techniques (Richter)

Soil gas ex- change (Savigny)

Measurement Techniques in Soil physics (Fischer)


Overview – Lecture 2

• Principle of wavelength pairing to remotely sense atmospheric O3 and also other constituents

• Overview of retrieval Techniques for atmospheric remote sensing of atmospheric constituents

- Emission measurements- Nadir-Backscatter- Occultation- Limb-scattering

• Retrieval example: onion-peel inversion of solar occultation measurements


Principle of wavelength pairs (online - off-line)

C

ross

sec

ton

1 2

dxxndxxn

eI

I

I

I 21

20

10

2

1

dxxn

eII2

202

dxxn

eII1

101

dxxn

eI

I

I

I 21

20

10

2

1

dxxn

I

I

I

I21

20

10

2

1 lnln

Note: Absorption cross section typically also dependent on x

Complication: Light path dependent on

Known from measurements of solar spectrum

Absorber column amount along effective light path


Dobson’s spectrophotometer I

Quartz platesAjustable wedgeFixed slitsPrismsDetector: photomultiplier

Org. photographic plate

Detailed description also to be found in script


Dobson’s spectrophotometer II

Name WL 1 [nm] WL 2 [nm]

A 305.5 325.4

B 308.8 329.1

C 311.45 332.4

D 317.6 339.8

C’ 332.4 453.6

Longest existing ozone time series

Used wavelength pairs


Overview of satellite observations geometries

Measured signal:Reflected and scattered sunlight

Measured signal:Directly transmitted solar radiation

Measured signal:Scattered solar radiation


Change in O2(1):

AO2[O2(1)]: volume emission rate, measured quantity

Steady state: [O3] = AO2[O2(1)] / JO3

retrieve [O3]

Ozone measurements using emissions

• Usually measure longwave radiation thermally emitted by the atmosphere along the line of sight of the instrument• Infrared (9.6 μm ozone band), microwave, and NIR wavelengths (760nm, 1270 nm)• Limb sounding or nadir sounding viewing geometries• Used to retrieve ozone profiles and total columns

Example: Mesospheric O3 profiles retrieved from O2(1) measurements

Main source of O2(1) in dayglow: photolysis of ozone

O3 + h O2(1) JO3

O2(1) O2 + h AO2

ΔOAOJdt

ΔOd 12O3O

12

23

Simplified chemistry !!


Backscatter UV Ozone measurements I

• Solar UV radiation reflected from the surface and backscattered by atmosphere or clouds is absorbed by ozone in the Hartley-Huggins bands (< 350 nm)

• Note:most ozone lies in stratospheremost of the backscattered UV radiation comes from the tropospherelittle absorption by ozone occurs in the tropospherelittle scattering occurs in the stratosphereradiation reaching the satellite passes through the ozone layer twice

• Backscatter UV measurements allow retrieval of total O3 columns and also vertical profiles, but with poor vertical resolution (7 – 10 km)

• Measure O3 slant column and use Radiative transfer model to convert to vertical column


Examples:

BUV (Backscatter Ultraviolet) instrument on Nimbus 4, 1970-1977

SBUV (Solar Backscatter Ultraviolet) instrument on Nimbus 7, operated from 1978 to 1990

SBUV/2 (Solar Backscatter Ultraviolet 2) instrument on the NOAA polar orbiter satellites: NOAA-11 (1989 -1994), NOAA-14 (in orbit) can measure ozone profiles as well as columns

TOMS (Total Ozone Mapping Spectrometer) first on Nimbus 7, operated from 1978 to 1993. Then three subsequent versions: Meteor 3 (1991-1994), ADEOS (1997), Earth Probe (1996-). Measures total ozone columns.

GOME (Global Ozone Monitoring Experiment) launched on ESA's ERS-2 satellite in 1995 employs a nadir-viewing BUV technique that measures radiances from 240 to 793 nm. Measures O3 columns and profiles, as well as columns of NO2, H2O, SO2, BrO, OClO.

Backscatter UV Ozone measurements II


Solar occultation measurements I

I0() spectrum at the highest tangent altitude with negligible atmospheric extinction

I(,zi) spectrum at tangent altitude zi within the atmosphere

λI

THλ,Iexp

0

i)THτ(λ,- i

)LoS(TH

λext,

i

dxxkexp LoS: line of sight

With kext, being the total extinction coefficient at position s along the line of sight LoS.

Extinction is usually due to Rayleigh-scattering, aerosol scattering and absorption by minor constituents:

xkxkxkxk gasesλ

aerosolλ

Rayleighλλext,


Disadvantage of solar occultation measurements:

The occultation condition has to be met: Measurements only possible during orbital sunrises/sunsets

For typical Low Earth Orbits there are 14–15 orbital sunrises and sunsets per day

poor geographical coverage

Solar occultation measurements II

If we assume that the cross-section does not depend on x, i.e., not on temperature and/or pressure, then

iO

THLoS

Oiozone THcσdxxnσTHλ,τ

3

i

3 With the column density c(zi)

The measurement provides a set of column densities integrated along the line of sight for different tangent altitudes zi.

Inversion to get vertical O3 profile

Due to the different spectral characteristics of the different absorbers and scatterers the optical depth due to, e.g., O3 can be extracted.

LoS

Oiozone dxxnxσTHλ,τ

3

Absorption cross-section

O3 Number density


Solar occultation measurements III

Examples:

SAGE (Stratospheric Aerosol and Gas Experiment) Series provided constinuous observations since 1984 to date

Latest instrument is SAGE III on a Russian Meteor-3M spacecraft

HALOE (Halogen Occultation Experiment) on UARS (Upper Atmosphere Research Satellite) operated from 1991 until end of 2005, employing IR wavelengths

POAM (Polar Ozone and Aerosol Measurement) series use UV-visible solar occultation to measure profiles of ozone, H2O, NO2, aerosols

GOMOS (Global Ozone Monitoring by Occultation of Stars) on Envisat will performs UV-visible occultation using stars

SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) on Envisat performs solar and lunar occultation

measurements providing e.g., O3, NO2, and (nighttime) NO3 profiles.


Limb scatter measurements I

• Radiation is detected which is scattered into the field of view of the instrument along the line of sight and also transmitted from the scattering point to the instrument

• Solar radiation pickts up absorption signatures along the way

• Also: Light path can be modified by atmospheric absorption

• Vertical profiles of several trace constituents can be retrieved fom limb-scattered spectra emplying a radiative transfer model (RTM) to simulate the measurements

Retrieval without forward model not possible

0

10

20

30

40

50

60

70

80

90

10-6

10-5

10-4

10-3

10-2

10-1

305 nm

280 nm

Limb radiance [a.u.]

Tang

ent h

eigh

t [km

] “Knee”

Optically thin regime

Optically thick regime

At 280 nmModelled limb-radiance profiles


Examples:

SME (Solar Mesosphere Explorer) launched in 1981, carried the first ever limb scatter satellite instruments. Mesospheric O3 profiles were retrieved using the Ultraviolet Spectrometer and stratospheric NO2 profiles were retrieved using the Visible Spectrometer

MSX satellite – launched in 1996 , carried a suite of UV/visible sensors (UVISI)

SOLSE (Shuttle Ozone Limb Sounding Experiment) flown on the Space Shuttle flight in 1997. Provided good ozone profiles with high vertical resolution down to the tropopause

OSIRIS (Optical Spectrograph and Infrared Imager System) launched in 2001 on Odin satellite. Retrieval of vertical profiles of O3, NO2, OClO, BrO

SCIAMACHY (Scanning Imaging Absorption SpectroMeter for Atmospheric CHartographY), launched on Envisat in 2002. Retrieval of vertical profiles of O3, NO2, OClO, BrO and aerosols

Limb scatter measurements II


TECHNIQUE ADVANTAGES DISADVANTAGES

Emission • doesn’t require sunlight • slightly less accurate than• long time series backscatter UV• simple retrieval technique • long horizontal path for limb obs.• provides global maps twicea day (good spatial coverage)

Backscatter UV • accurate • requires sunlight, so can’t be• long time series used at night or over winter poles• good horizontal resolution • poor vertical resolution below thedue to nadir viewing ozone peak (~30 km) due to the

effects of multiple scattering andreduced sensitivity to the profile

shape

Occultation • simple equipment • can only be made at satellite• simple retrieval technique sunrise and sunset, which limits• good vertical resolution number and location of meas.• self-calibrating • long horizontal path

Limb Scattering • excellent spatial coverage • complex viewing geometry• good vertical resolution• data can be taken nearly • poor horizontal resolutioncontinuously

Advantages and disadvantages of measurement techniques:


Example: Onion peel inversion of occultation observations

Earth x1x2x3x4x5

(TH1)(TH2)(TH3)(TH4)(TH5)

•

•

•

My

y

y

y

y

3

1

1

Nx

x

x

x

x

3

1

1

xy

A

xi : O3 concentration at altitude zi

yj : O3 column density at tangent height THj

MNMM

N

N

aaa

aaa

aaa

21

22221

11211

A

a11 a21/2a22

a32/2

The matrix elements aij correspond to geometrical path lengths through the layers

N

jjiji xay

1

Sun


yx -1A

Inverse of A exists if the determinant of A is not zero

Standard approach: least squares solution (assume N=M)

Mixay j

N

jijii ,,1,

1

Minimize:

by:

M

i

M

i

N

jijiji yxa

1 1

2

1

22

01

2

1

M

i

N

jijij

k

yxax

This leads to:

01 1

M

iik

N

jijij ayxa

yx TT AAA

yx TT AAA

-1

Unconstrained least squares solution


Constrained least squares solution

M

i

N

jjj

M

i

N

jijij

N

jji xxyxaxx

1

2

11

1

2

1

2

1

2

Add additional condition to constrain the solution, e.g.:

is a smoothing or constraint coefficient coefficient

02

11

1

2

1

N

jjj

M

i

N

jijij

k

xxyxax

02 111 1

kkk

M

iik

N

jijij xxxayxa

0 xyx

HAAA TT yx TT AHAA

-1


End of lecture

introduction to measurement techniques in environmental physics summer term 2006

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