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Atmospheric chemistry nadir observation from space now spans nearly forty years. The early UVN (UV nadir missions) mainly focused on measuring ozone. The selected wavelengths between 250 and 360 nm measured by the series of SBUV (Solar Backscatter Ultraviolet) and TOMS (Total Ozone Mapping Spectrometer) permitted in addition to ozone, detection of SO2 as well as aerosol extinction. The introduction of DOAS (Differential Optical Absorption Spectroscopy) type UVN instruments in 1995 (GOME: Global Ozone Monitoring instrument) with larger and continuous spectral coverage extended largely the list of observable atmospheric trace gases that also include BrO, NO2, OClO, H2CO, glyoxal, water vapor, IO. Nadir observations in generally provide only column information (total column, vertical column density) from trace gases, but sophisticated retrieval schemes permit the separation of tropospheric (up to about 8-15 km) and stratospheric contents (above 15 km). Vertically resolved ozone profiles can be derived from the UV absorption edge (about 8 km vertical resolution). Additionally the UVN sensors provide information on aerosols and clouds.

The combination of UVN observations from different missions now allows the detection of decadal trends in atmospheric constituents and aerosols that are relevant to climate change and air quality. Natural processes like volcanic emissions (SO2, aerosols) play an important role and their impact our climate can be investigated.

The main scientific achievements from UVN are in the area of:

  • Stratospheric ozone dynamics and chemistry (recovery of the ozone layer and climate interaction)
  • Air quality (atmospheric lifetime of reactive species, regional distribution and transport, chemistry)
  • Natural emission and hazards
  • Cloud and aerosol physics and chemistry

Chapter Editor

Mark Weber (U. Bremen)

Contributors

Udo Friess (U. Heidelberg), Diego Loyola (DLR), Thomas Wagner (MPIC)

 

 

Overview and timelines of past and future UVN (ultraviolet nadir) sounders. The earliest measurements were provided by the BUV instrument starting in 1970. The routine daily atmospheric observations started with SBUV and TOMS aboard Nimbus-7 by the end of 1978. DOAS type observations from space were first provided with GOME in 1995. "DOAS type" means that continuous atmospheric spectra across the UV and/or the visible spectral range are recorded that allow the detection of many more trace gases in addition to ozone and SO2 (sulphur dioxide). Current and past missions have been operating in a near polar low-earth orbit (LEO). For most points of the earth at most one measurement per day at nearly identical local times are available. Some of the future missions like Sentinel 4 (UVN), MP-GEOSat (GEMS), and TEMPO will be observing from a geostationary orbit that will allow local measurements at different times of the day, which will be particularly useful for monitoring regional air quality. Links to the various UVN satellite missions can be found in the "Link" section.

 

Typical UV/visible spectra as measured with the GOME spectrometer. The reflectance is obtained by dividing the nadir observed spectrum (I) by the solar spectrum (F). Under cloudy sky condition the reflectance is high across the UV and visible spectral range (white clouds), while the clear-sky reflectance peaks in the near UV (blue). The peaking in the blue part of the spectrum is due to Rayleigh-scattering ("why is the sky blue?") and explains why our planet is called the "blue planet". The reflectance spectra show several major atmospheric absorbers like ozone (O3), water (H2O), and oxygen (O2) that along with the minor absorbers (not directly visible here) can be retrieved from DOAS typeUVN data. The sharp drop below about 340 nm is due to ozone absorption and explains why ozone is vital in protecting the earth surface from the harmful UV radiation. Adapted from Burrows et al. (1999).

 

Most UV nadir sounders routinely observe directly the sun (spectral solar irradiance) about once a day. From the emission core of the solar Fraunhofer Mg II doublet observed near 280 nm the so-called Mg II index can be derived. This index is frequently used as a proxy for UV solar spectral irradiance variation. From the series of UVNs (SBUV, SBUV/2, GOME, SCIAMACHY, OMI, and GOME-2) and dedicated solar missions (SOLSTICE, SUSIM) a composite Mg II index (Viereck et al., 2004, Snow et al., 2014) can be derived which shows the variations with the 11-year sunspot cycle (22 year magnetic cycle) and solar rotation (~27 days) . Values indicated are the numbers of days for each satellite (series) contributing to the composite index. Missing values were filled using scaled F30 cm radio flux data obtained from ground observations. The black curve shows the timeseries twice smoothed with a 55-day boxcar (removing the 27 day solar rotation signal). The solar maximum in solar cycle 24 (reached in 2014) is lower than in the three previous cycles 21-23. Daily updates are available at http://www.iup.uni-bremen.de/UVSAT/Datasets/mgii. Courtesy: Mark Weber.

 

Schematics of the GOME spectrometer. A very similar design is used for SCIAMACHY and GOME-2. The only moving part is the scan mirror (red arrow) that can be rotated to pick up the signal from the nadir Earth view, the sun, and the internal Pt/Ne/Cr hallow discharge lamp. Dispersion into four spectral channels occurs with the predisperser- and band-separator prisms. In each of the optical channel gratings leads to further dispersions thus achieving moderately high spectral resolution (0.2-0.4 nm) in the spectrum recorded by a linear Si reticon array detector. The Pt/Ne/Cr lamp is used to calibrate the instrument. Some of the signal is diverted to Polarisation Measurement Devices (PMD) that are used for polarization corrections and cloud detection. Adapted from Burrows et al. (1999)

 

Fingerprint of molecules observable in the near UV and visible spectrum. Absorption cross-section of molecules that can be observed by DOAS type UVN and from other platforms (ground, air- and ballon-borne) are shown. From space the molecules O3, NO2, SO2, BrO, OClO, H2O, O4, O2, IO, HCHO, and (CHO)2 have been successfully observed. The favorite method to retrieve molecular abundances from the UV/Vis spectrum is the Differential Optical Absorption Spectroscopy (DOAS). Courtesy Udo Friess (U Heidelberg).

 

The prime UVN retrieval method is the DOAS (Differential Optical Absorption Spectroscopy) approach here illustrated for UV total ozone retrieval. From both nadir spectrum (upper left panel) and ozone absorption cross-section a polynomial (broad-scale features) is subtracted to obtain the differential absorption (bottom panel). The differential ozone absorption cross-section has been scaled to match the differential nadir spectrum in a linear least squares regression. The scaling factor provides the slant column density that has to be converted into vertical column density (independent of viewing geometry) by calculation of an air mass factor (AMF) using an atmospheric radiation transfer model (RTM). Ozone concentration as a function of altitude (ozone profile) can be estimated using a different approach, in most cases an optimal estimation approach (e.g. Barthia et al., 2013, Miles et al., 2015). From Dameris and Loyola (2009).

 

The DOAS principle can be applied to retrieve various atmospheric trace gases. The spectral signature and fits in selected spectral region are shown for many atmospheric constituents in the small panels. The large middle panel is the satellite nadir backscatter spectrum from which the trace gas amounts are estimated. From Wagner et al. (2008).

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