WP 4.1: Data acquisition, optimisation of long-term series, 
and delivery to central database
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Observations of key parameters at 29 stations
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Objectives
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Observation of target stratospheric variables
GEOmon sites and data Delivery
- Stratospheric observations at 29 stations covering all latitudes are supplied.
- Preliminary data are delivered weekly to the GEOmon data Centre in near-real time
(within weeks of the measurement) whenever possible. - The SAOZ web site also provides near-real time data (not GEOmon funded)
- Preliminary data are then revised on an annual bases and send to the NDACC data centre.
Measured Species
- Stratospheric O3 plays a major role in the climate system. Its absorption of UV and visible sunlight warms the stratosphere, and this temperature inversion helps define the top of the convectively-stirred troposphere. Via its infrared bands it also acts as a conventional greenhouse gas, which acts in the opposite sense to absorption of sunlight that would otherwise heat the surface, so that changes in O3 in the lowest stratosphere have the largest radiative forcing. At the poles in spring, the large cooling associated with ozone loss due to CFC derivatives (in Antarctica the ozone hole) causes changes in density which propagate to the surface during stratospheric warmings, thereby affecting the North Atlantic Anomaly in surface climate and the wind strength in the southern ocean.
- BrO and inorganic chlorine compounds (HCl and ClNO3, often known as Cly) are the main agents of ozone loss from CFCs (containing chlorine) and Halons (containing bromine), so any attempt to model and monitor ozone loss must include their measured values.
- Polar Stratospheric Clouds (PSCs) form the surfaces which convert the less reactive inorganic chlorine compounds (HCl and ClNO3) into Cl2, which becomes the more reactive Cl in sunlight, so measurements of PSCs are essential to understanding polar ozone loss.
- NO2 is bound up with natural ozone loss, so any trends in NO2 must be included in any attempts to model anthropogenic ozone loss. Provided stratospheric aerosol is also monitored, trends in NO2 are also a useful diagnostic of trends in the overall circulation of the stratosphere, the circulation that brings CFCs and greenhouse gases into the stratosphere from below, and returns ozone to reinforce pollution.
- HF is the end product of the fluorine in CFCs, so is an excellent diagnostic of the reduction in the sum total of CFCs that we expect from the success of the Montreal Protocol. Because it is more inert than HCl, it is also useful in diagnosing the descent in polar winter that is part of the overall stratospheric circulation.
- Stratospheric H2O is the source of OH and HO2, which also cause natural ozone loss; it is a greenhouse gas with a contribution to radiative forcing at the surface; and may well be an indicator of the overall stratospheric circulation. The significant trend in stratospheric H2O observed over the last 50 years is not understood, and has recently seemed to reverse.
- Stratospheric sulphate aerosol, much of which is of volcanic origin, reflects sunlight thereby cooling the surface. It also has some infrared absorption bands, so that the stratosphere is warmed following a large volcanic eruption.
- Temperature in the stratosphere is obviously the main local climate variable, but the stratosphere also radiates to the troposphere so affects general climate. Temperatures in the lower and middle stratosphere (to about 30 hPa) are well-measured by radiosondes on meteorological balloons, data from which is already archived and analysed by other organisations.
Instruments
- Ground-based UV-visible spectrometers observe sunlight scattered from the zenith sky. By measuring at twilight, path lengths through the stratosphere are long so absorption by stratospheric gases is large. The increased density of the troposphere means that little light penetrates to the scattering point via a slant path through the troposphere, so absorption by the same gas in the troposphere (e.g. NO2 and O3 during pollution events) is small. Because there is no unique path for the scattered sunlight, the slant amounts measured must be converted to vertical amounts via a radiative transfer model. Gases with absorption bands in the UV and visible include O3, NO2 and BrO. Observation of scattered sunlight also means that measurements can be made in any cloud condition, and at the polar circle in midwinter.
- Ground-based Fourier Transform Infrared (FTIR) spectrometers observe the direct sun or moon to measure the absorption lines of trace gases. If the spectral resolution is sufficiently high that pressure-broadened widths are determined, some profile information is found. Otherwise, not saturated lines must be observed, to measure the vertical amounts without knowledge of the vertical profile. A large variety of trace gases have infrared absorption bands, for GEOmon we concentrate on HCl, ClNO3 and HF, called hereinafter ‘Cly, Fy’. The observations are necessarily restricted to clear skies.
- Lidars transmit pulses of laser light upwards, and measure the light scattered back to the ground by air molecules, clouds and aerosols. The backscattered light is measured with fine time resolution, so that a vertical profile of scattering and absorption can be determined from the time of flight. If the laser is at a wavelength with little absorption by ozone, then in the absence of clouds and aerosol in the upper stratosphere the intensity of backscattered light is a measure of the density of air molecules, which can be inverted to a temperature profile. If the temperature profile in the lower and middle stratosphere is known from radiosonde ascents or climatology, the intensity is a measure of clouds and aerosol. If a second wavelength with significant ozone absorption is transmitted, ozone profiles can be determined. Lidar observations are restricted to periods with small amounts of tropospheric cloud. By day the signal to noise ratio is much smaller.
- Ground-based microwave spectro-radiometers observe radiation emitted at rotational absorption lines of trace gases. By measuring the variation in emission across the pressure-broadened width of the line, the pressure and so the altitude of the emission is deduced. The vertical resolution of the measurement is typically 5 km. Several trace gases have microwave lines that can be observed from the ground; in GEOmon we concentrate on stratospheric H2O.
- Balloon-borne Flash sondes measure the absorption of UV light by H2O over a short path in situ. Hence they have excellent vertical resolution and a good absolute calibration.
Software for the analysis of remote sensing observations
- QDOAS is a UV-visible trace gas retrieval software based on the Differential Optical Absorption Spectroscopy (DOAS) method. The software comes with a new graphical user interface written in QT and is compatible with most commonly used operating systems (Linux, Windows, MacOS, etc). In its current Beta version, it incorporates most of the functionality of the previous Windoas software. Data file formats from a number of different instruments are already supported including SAOZ, RASAS, GOME, SCIAMACHY, GOME-2, etc. The user manual from the original Windoas is available at the Belgian Institute for Space Aeronomy and a new QDOAS version of this manual is under preparation.
- New software for chemically-modified Langley plots takes this scheme to improve the accuracy of measurements of NO2 by ground-based UV-visible spectrometers, developed earlier at BAS, and recodes it for efficient calculation at any site and any season. The new well-documented software has been distributed to project partners, though has only been fully used for the BAS site so far.
