Ozone and Brewer-Dobson Notes 12 Feb 2020

ESSE 4230/ ESS 5230 : Remote Sensing of the Atmosphere : Ozone and Brewer-Dobson Notes 12 Feb 2020

Passive Sensing – Extinction and Scattering. (Stephens, Chapter 6). Also see Houghton et al, Chapter 10.

Sun is the radiation source. Measurements from ground (Dobson, Brewer) and limb measurements from satellites.

Aerosols, Ozone, other trace gases.

Beer’s Law https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law

DOAS - Differential Optical Absorption Spectroscopy https://en.wikipedia.org/wiki/Differential_optical_absorption_spectroscopy

Ozone - Dobson and Brewer Spectrophotometers http://www.ndaccdemo.org/ Doesn’t seem to be useful

http://www.kippzonen.com/Product/50/Brewer-MkIII-Spectrophotometer#.Wp7Nt2rFKpo

http://kippzonen-brewer.com/

-2 -1 Basis is Beer-Lambert law. Iλ is radiance intensity (Wm sr )

dIλ = - σext,λ(s)Iλ ds ; d(ln Iλ)/ds = - σext,λ(s)

-1 The extinction coefficient (m ) is due to scattering and absorption, σext,λ = σsca,λ + kλ , but excludes scattering into path, and emission, wavelengths in uv and visible.

Simpler than RTE – dLλ/ds = -kλρa(Lλ - Jλ); wavelengths IR and micro-wave. Different usage of kλ, volume, mass, number, extinction or just absorption.

Solution can be written Iλ(s) = Iλ(s0) exp(-τλ) where optical thickness τλ = ∫ σext,λ(s’)ds’ (dimensionless) and the integral is from s0 to s.

If s0 is above the atmosphere and we define an optical depth τλ at zenith then, when sun is at angle θ0 to the vertical, we have

Iλ(τλ) = Iλ(τλ = 0) exp(-τλ/cos θ0)

Or ln(Iλ(τλ)) = ln(Iλ(τλ = 0)) - τλ/cos θ0

So plotting ln(Iλ) vs sec θ0 as the sun moves across the sky will, if the air mass is clear and stable!, allow one to determine τλ and Iλ(τλ = 0). (Fig 6.1 from Stephens). Data from Manua Loa, Lambertian plot. Assumes output voltage proportional to radiance.

What causes the extinction, τλ = ∫ σext,λ(s’)ds’? Scattering (Rayleigh) by molecules and aerosol scattering plus absorption by some gases – which varies with wavelength.

Atmospheric turbidity. Clean air, air pollution, volcanic and other dust. WMO solar monitoring network.

Total (Column) Ozone measurements – Dobson and Brewer spectrometers. https://en.wikipedia.org/wiki/Dobson_unit

From Wikipedia, the free encyclopedia

The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer. One Dobson unit is

equal to the number of ozone molecules needed to create a pure layer of ozone 0.01 millimeters thick at STP - standard conditions for temperature and pressure.[1]

The Dobson Unit is defined as the thickness (in units of 10 µm) of that layer of pure gas which would be formed by the total column amount at STP.[2] This is sometimes referred to as a 'milli-atmo-centimeter.' A typical column amount of 300 DU of atmospheric ozone therefore would form a 3 mm layer of pure gas at the surface of the Earth if its temperature and pressure conformed to STP. The Dobson unit is named after Gordon Dobson, a researcher at the University of Oxford who in the 1920s built the first instrument to measure total ozone from the ground, making use of a double prism monochromator to measure the differential absorption of different bands of solar ultraviolet radiation by the ozone layer. This instrument, called the Dobson ozone spectrophotometer, has formed the backbone of the global network for monitoring atmospheric ozone[3] and was the source of the discovery in 1984 of the Antarctic ozone hole.[4]

• One mol m−2 is approximately equivalent to 2,241 DU. • One DU is equivalent to 0.4462 mmol m−2. • One DU is also equivalent to 2.687×1020 molecules per square metre.

Mole =

The mole is the unit of measurement for amount of substance in the International System of Units (SI). The unit is defined as the amount or sample of a chemical substance that contains as many constitutive particles, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon- 12 (12C), the isotope of carbon with standard atomic weight 12 by definition. This number is expressed by the Avogadro constant, which has a value of approximately 6.022140857×1023 mol−1. The mole is an SI base unit, with the unit symbol mol.

Ozone

NASA uses a baseline value of 220 DU for ozone. This was chosen as the starting point for observations of the Antarctic ozone hole, since values of less than 220 Dobson units were not found before 1979. Also, from direct measurements over Antarctica, a column ozone level of less than 220 Dobson units is a result of the ozone loss from chlorine and bromine compounds.

Dobson and Brewer Spectrophotometers

http://kippzonen-brewer.com/

The Brewer MkIII Spectrophotometer is a sophisticated optical instrument. It provides near- simultaneous observations of the Total Ozone Column (TOC) and Sulphur Dioxide (SO2) between the instrument and the sun. It can also make high resolution spectral measurements of the ultraviolet (UV) radiation in the direct sun beam or from the whole sky.

Kipp & Zonen Brewer Spectrophotometer The Brewer makes measurements by a combination of firmware running autonomously within the spectrophotometer and software running on a computer that must be connected to the Brewer. When the Brewer has been installed and levelled; the latitude, longitude, altitude, date and time are entered via the software. An algorithm calculates the position of the sun and the Brewer moves to point in that direction. After alignment is optimized the Brewer will automatically point at the sun throughout the day when it is above the horizon.

The spectrophotometer is mounted onto the azimuth tracker, which has a stepping motor and drive system controlled by the firmware. The tracker has a very rugged and stable tripod stand with adjustable feet for levelling.

The direct sun is viewed through a flat quartz window. A stepping motor controlled by the firmware moves the zenith prism so that the direct radiation from the sun passes through the fore-optics to the spectrometers.

The spectrophotometer contains two modified Ebert f/6 spectrometers in series. This reduces the unwanted ‘stray’ light by a factor of 1000 compared to a single spectrometer. Each utilises a 3600 line/mm holographic diffraction grating operated in the first order.

Slides from Tom McElroy.

https://community.wmo.int/meetings/13th-biennial-wmo-gaw-brewer-users-group-meeting

Some material at https://community.wmo.int/stratospheric-ozone-and-ultraviolet-radiation-research

APPENDIX I - Dobson/Brewer Spectrophotometers

The measurement of total column ozone with spectrometers is an analytical technique with a long historical background. The first measurements with a Dobson Ozone Spectrophotometer were conducted in the mid-twenties. A global network of Dobson instruments was established during the years following the first International Geophysical Year in 1957. A modern instrument, the Brewer Ozone Spectrophotometer, was developed in the nineteen seventies and introduced into the global network in 1982 with the delivery of instruments to Greece, Sweden, Germany and Canada

The basic measurement principle of both types of spectrometers is the same. The thickness of the ozone layer is determined by comparing the intensity of solar radiation that has passed through the atmosphere at wavelengths in the ultraviolet that are strongly and weakly absorbed by ozone. The Dobson utilizes a kind of internal virtual 'ozone layer' (a variable attenuator called an 'optical wedge') to measure the intensity ratio of two wavelengths, the Brewer directly measures the intensity of light at a number of different wavelengths in the ultraviolet. The physical principles and instrumental characteristics of these methods are well known and acknowledged and therefore do not need any further justification as a primary technique for the NDACC. It is, however, important, to characterize the individual instruments of both spectrometer types and to determine their specific calibration constants.

https://www.esrl.noaa.gov/gmd/grad/neubrew/MkIV.jsp *** A useful explanation of the instrument.

The Brewer Spectrophotometer is a scientific instrument which measures Ultra Violet, or Visible, radiation in the solar spectrum. By examining the differential absorption of select wavelengths in the UVB portion of the spectrum, determinations of Total Column Ozone and Total Column Sulphur Dioxide are inferred. In addition, especially with the MKIII double monochromator instrument, accurate spectral intensity profiles of UV radiation in the 286.5 nm to 363 nm range are measured. Then the MKIV instrument measures intensity of radiation in the visible part of the spectrum (430-450nm) and uses differential absorption in this region to infer Total Column Nitrogen Dioxide.

Calculation of Ozone column is similar to that with the DIAL lidar, except that absorption is just over one pass through the air mass and we don’t need to worry about scattering from the target.

Two Hartley Higgins uv wavelengths, radiances I and I’ can lead to difference in radiance (Stephens p272-274).

Starting from ln(Iλ(τλ)) = ln(Iλ(τλ = 0)) - τλ/cos θ0 ; and set mr = sec θ0, relative air mass except for low sun elevation

In the following, Why log10? If L1 = log10 A. then ln A = 2.3026 L1

Why τd,… “decadic optical depth” based on log10 rather than ln.

Why???? ln(Iλ(τλ)) = ln(Iλ(τλ = 0)) - τλ/cos θ0

or, I suppose, log10(Iλ(τλ)) = log10(Iλ(τdλ = 0)) - τdλ/cos θ0

“ln = 2.303 log10”, so presumably τλ = τdλ x 2.303

Note that this assumes τdO3 = X0kd for Ozone amount. kd called the “decadic optical depth” presumably because of log10?

------http://exp-studies.tor.ec.gc.ca/clf2/e/main.html - a better source!

https://woudc.org/home.php , https://woudc.org/data/explore.php - seems hard to work with,

#TIMESTAMP UTCOffset Date -05:24:11 1/7/2010

* NdFilter is not in UV; F324 is in UV only

#OBSERVATIONS Time WLcode ObsCode Airmass ColumnO3 StdDevO3 ColumnSO StdDevSO2ZA NdFilter TempC F324 9:34:55 9 UV 3.487 393 2 74.009 8 58.3 9:51:04 9 UV 3.204 405 2 72.424 11 64.2 9:59:59 9 ZS 3.076 426.8 3.4 -11.3 1.3 71.614 0 11 10:53:40 9 UV 2.598 393 2 67.838 10 56.8 11:06:34 9 UV 2.535 392 2 67.235 9 61.9 11:15:31 9 ZS 2.501 418.4 2.8 -8 0.9 66.89 0 8 11:20:49 9 UV 2.484 397 2 66.715 8 66.7

Brewer MKV 29

#LOCATION (Alert)

Latitude Longitude Height

82.45 -62.51 220

* Time reported is Solar Time. Subtract UTCOffset for UTC.

#TIMESTAMP UTCOffset Date -04:18:35 1/13/2014

* NdFilter is not in UV; F324 is in UV only

#OBSERVATIONS

Time WLcode ObsCode Airmass ColumnO3 StdDevO3 ColumnSO StdDevSO2ZA NdFilter TempC F324 0:04:12 9 FM 2.357 310.7 1.6 6.5 2 65.325 0 18 1:37:57 9 FM 2.573 320.9 10.6 3.8 2.4 67.599 0 19 2:31:34 9 FM 2.75 320.5 6.8 5.2 3.5 69.185 0 19 3:31:32 9 FM 2.997 325.6 10.9 -0.8 4.1 71.076 0 17 17:13:59 9 FM 2.802 336.4 8.8 0.8 5.8 69.613 0 18 18:43:50 9 FM 2.515 320.9 9.9 10.1 6.5 67.031 0 19 19:55:45 9 FM 2.364 338.1 8.9 3.1 5 65.405 0 15

More column ozone, from satellites https://ozonewatch.gsfc.nasa.gov/ Each hemisphere, up to date. https://ozonewatch.gsfc.nasa.gov/facts/NH.html https://aura.gsfc.nasa.gov/about.html

So how do we measure Ozone from satellites? OMI About OMI - from https://aura.gsfc.nasa.gov/omi.html

The OMI instrument can distinguish between aerosol types, such as smoke, dust, and sulfates, and measures cloud pressure and coverage, which provides data to derive tropospheric ozone.

OMI continues the TOMS record for total ozone and other atmospheric parameters related to ozone chemistry and climate. OMI measurements are highly synergistic with the other instruments on the Aura platform.

The OMI instrument employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation in the visible and ultraviolet. The hyperspectral capabilities improve the accuracy and precision of the total ozone amounts and also allow for accurate radiometric and wavelength self calibration over the long term.

The instrument is a contribution of the Netherlands's Agency for Aerospace Programs (NIVR) in collaboration with the Finnish Meteorological Institute (FMI) to the Aura mission. OMI Instrument Science

OMI derives its heritage from NASA's Total Ozone Mapping Spectrometer(TOMS) instrument and the European Space Agency(ESA) Global Ozone Monitering Experiment (GOME) instrument (on the ERS-2 satellite). It can measure many more atmospheric constituents than TOMS and provides much better ground resolution than GOME (13 km x 25 km for OMI vs. 40 km x 320 km for GOME).

OMI is a key instrument on Aura for monitoring the recovery of the ozone layer in response to the phase out of chemicals, such as CFCs, agreed to by the nations of the world in the Montreal protocol and later modifications to it at Copenhagen and London.

OMI measures criteria pollutants such as O3, NO2, SO2, and aerosols. The US Environmental Protection Agency (EPA) has designated these atmospheric constituents as posing serious threats to human health and agricultural productivity. These measurements are made at near urban scale resolution and track industrial pollution and biomass burning.

OMI detects volcanic ash and sulfur dioxide produced in volcanic eruptions with up to at least 100 times more sensitivity than TOMS. These measurements are important for aircraft safety.

OMI measures ozone profiles (in the UV) complimentary to those measured by TES and HIRDLS (in the IR) and MLS (in the microwave).

OMI measures BrO, formaldehyde, and OClO which all play a role in chemistry of the stratosphere and troposphere.

OMI measures the total column amount of atmospheric ozone NO2 as well as lower atmospheric dust, smoke, and other aerosols.

OMI link:

OMI Parameters

The instrument observes Earth's backscattered radiation with a wide-field telescope feeding two imaging grating spectrometers. Each spectrometer employs a CCD detector.

Onboard calibration includes a white light source, LEDs, and a multi-surface solar-calibration diffuser. A depolarizer removes the polarization from the backscattered radiation.

• OMI is a nadir-viewing wide-field-imaging spectrometer, giving daily global coverage. • OMI measures the key air quality components such as nitrogen dioxide(NO2), sulfur dioxide (SO2), bromine oxide(BrO), OClO, and aerosol characteristics. • OMI provides mapping of pollution products from an urban to super-regional scale. https://www.cfa.harvard.edu/~xliu/papers/liu_acp-OMPROF_10-2521-2010.pdf

Ozone profile retrievals from the Ozone Monitoring Instrument X. Liu1,2,3, P. K. Bhartia3, K. Chance2, R. J. D. Spurr4, and T. P. Kurosu21Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, Baltimore, Maryland, USA 2Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA 3NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 4RT Solutions Inc., Cambridge, Massachusetts, USA

Received: 18 Aug 2009 – Discussion started: 27 Oct 2009 Revised: 06 Feb 2010 – Accepted: 24 Feb 2010 – Published: 12 Mar 2010 Abstract. Ozone profiles from the surface to about 60 km are retrieved from Ozone Monitoring Instrument (OMI) ultraviolet radiances using the optimal estimation technique. OMI provides daily ozone profiles for the entire sunlit portion of the earth at a horizontal resolution of 13 km×48 km for the nadir position. The retrieved profiles have sufficient accuracy in the troposphere to see ozone perturbations caused by convection, biomass burning and anthropogenic pollution, and to track their spatiotemporal transport. However, to achieve such accuracy it has been necessary to calibrate OMI radiances carefully (using two days of Aura/Microwave Limb Sounder data taken in the tropics). The retrieved profiles contain ~6–7 degrees of freedom for signal, with 5–7 in the stratosphere and 0–1.5 in the troposphere. Vertical resolution varies from 7–11 km in the stratosphere to 10–14 km in the troposphere. Retrieval precisions range from 1% in the middle stratosphere to 10% in the lower stratosphere and troposphere. Solution errors (i.e., root sum square of precisions and smoothing errors) vary from 1–6% in the middle stratosphere to 6–35% in the troposphere, and are dominated by smoothing errors. Total, stratospheric, and tropospheric ozone columns can be retrieved with solution errors typically in the few Dobson unit range at solar zenith angles less than 80°.

OMI is a Dutch-Finnish built nadir-viewing pushbroom UV/visible instrument that measures backscattered radiances in three channels covering the 270–500 nm wavelength range (UV-1: 270– 310 nm, UV-2: 310–365 nm, visible: 350–500 nm) at spectral resolution of 0.42–0.63 nm (Levelt et al., 2006). OMI has a very wide field-of-view (114◦ ) with a cross-track swath width of 2600 km. Measurements across the track are binned into 60 positions for the UV-2 and visible channels and into 30 positions for the UV-1 channel (larger bins due to weaker signals). This results in daily global coverage with a spatial resolution of 13 km×24 km (along × across track) at nadir position for UV-2 and visible channels and 13 km×48 km for the UV-1 channel.

Pushbroom? http://www.geo.oregonstate.edu/classes/geo444_544/LECTURES/lecture6.pdf https://en.wikipedia.org/wiki/Push_broom_scanner

Information + maps + polar plots https://ozonewatch.gsfc.nasa.gov/facts/NH.html , https://ozonewatch.gsfc.nasa.gov/monthly/NH.html https://ozonewatch.gsfc.nasa.gov/meteorology/NH.html ; https://ozonewatch.gsfc.nasa.gov/facts/vortex_NH.html ; PV on 460K isentropic surface Potential Vorticity, PV ~ (ζ + f)∂θ/∂z/ρ = -g(ζ + f)∂θ/∂p often defined on isentropic surfaces. An approximately conservative quantity. 460K is about 18 km (standard atmosphere) https://en.wikipedia.org/wiki/International_Standard_Atmosphere#/media/File:Comparison_Interna tional_Standard_Atmosphere_space_diving.svg Isentropic Surfaces: Unsaturated air parcels move along isentropic surfaces thus allowing a conceptually easy way to identify airstreams and to visualize regions of rising and sinking air. However, the convenience of isentropic analysis is diminished when latent heat is released by the condensation of moisture. Polar Vortex: https://weather.gc.ca/data/analysis/sai_100.gif https://weather.gc.ca/mainmenu/modelling_menu_e.html

https://www.cfa.harvard.edu/~xliu/papers/liu_acp-OMPROF_10-2521-2010.pdf

Retrieval notes.

Assorted links: https://www.atmos-meas-tech.net/7/1395/2014/amt-7-1395-2014.pdf Paper on O3 measurements.

Ozone data, global and Canada: https://exp-studies.tor.ec.gc.ca/e/ozone/ozoneworld.htm

https://exp-studies.tor.ec.gc.ca/e/ozone/Curr_map.htm

https://eospso.nasa.gov/ https://www.cpc.ncep.noaa.gov/products/stratosphere/tovsto/

https://www.cpc.ncep.noaa.gov/products/stratosphere/tovsto/tovsto_info.shtml

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Separation of wavelengths.

https://en.wikipedia.org/wiki/Dispersive_prism wavelength dependent refractive index

https://en.wikipedia.org/wiki/Diffraction

https://en.wikipedia.org/wiki/Diffraction_grating

https://en.wikipedia.org/wiki/Optical_spectrometer

A diffraction grating is an optical component with a regular pattern. The form of the light diffracted by a grating depends on the structure of the elements and the number of elements present, but all gratings have intensity maxima at angles θm which are given by the grating equation

d(sin θm + sin θi) = m λ

where θi is the angle at which the light is incident, d is the separation of grating elements, and m is an integer which can be positive or negative. The incident light can have multiple wavelengths and different components will be enhanced in the outgoing light at different angles.

The light diffracted by a grating is found by summing the light diffracted from each of the elements, and is essentially a convolution of diffraction and interference patterns. When light is normally incident on the grating, the diffracted light has maxima at angles θm given by: d sin θm = mλ. https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=9026

Some notes from Jim Whiteway

Next step is to take logs of the ratio PλON/PλOFF ln( PλON/PλOFF) = ln(βON/βOFF) - 2∫α'ON- α'OFF + N(z)(σON - σOFF)dz ' where integral is from 0 to z.

With Δσ = σON - σOFF constant and other assumptions, differentiation wrt z will give a result for the Ozone number density, N(z).

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