Multi axis differential optical absorption spectroscopy (MAX-DOAS)

International audienceMulti Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) in the atmosphere is a novel measurement technique that represents a significant advance on the well-established zenith scattered sunlight DOAS instruments which are mainly sensitive to stratospheric absorbers. MAX-DOAS utilizes scattered sunlight received from multiple viewing directions. The spatial distribution of various trace gases close to the instrument can be derived by combining several viewing directions. Ground based MAX-DOAS is highly sensitive to absorbers in the lowest few kilometres of the atmosphere and vertical profile information can be retrieved by combining the measurements with Radiative Transfer Model (RTM) calculations. The potential of the technique for a wide variety of studies of tropospheric trace species and its (few) limitations are discussed. A Monte Carlo RTM is applied to calculate Airmass Factors (AMF) for the various viewing geometries of MAX-DOAS. Airmass Factors can be used to quantify the light path length within the absorber layers. The airmass factor dependencies on the viewing direction and the influence of several parameters (trace gas profile, ground albedo, aerosol profile and type, solar zenith and azimuth angles) are investigated. In addition we give a brief description of the instrumental MAX-DOAS systems realised and deployed so far. The results of the RTM studies are compared to several examples of recent MAX-DOAS field experiments and an outlook for future possible applications is given

Multi axis differential optical absorption spectroscopy (MAX-DOAS)

Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) in the atmosphere is a novel measurement technique that represents a significant advance on the well-established zenith scattered sunlight DOAS instruments which are mainly sensitive to stratospheric absorbers. MAX-DOAS utilizes scattered sunlight received from multiple viewing directions. The spatial distribution of various trace gases close to the instrument can be derived by combining several viewing directions. Ground based MAX-DOAS is highly sensitive to absorbers in the lowest few kilometres of the atmosphere and vertical profile information can be retrieved by combining the measurements with Radiative Transfer Model (RTM) calculations. The potential of the technique for a wide variety of studies of tropospheric trace species and its (few) limitations are discussed. A Monte Carlo RTM is applied to calculate Airmass Factors (AMF) for the various viewing geometries of MAX-DOAS. Airmass Factors can be used to quantify the light path length within the absorber layers. The airmass factor dependencies on the viewing direction and the influence of several parameters (trace gas profile, ground albedo, aerosol profile and type, solar zenith and azimuth angles) are investigated. In addition we give a brief description of the instrumental MAX-DOAS systems realised and deployed so far. The results of the RTM studies are compared to several examples of recent MAX-DOAS field experiments and an outlook for future possible applications is given

Stratospheric chlorine activation in the Arctic winters 1995/96–2001/02 derived from GOME OClO measurements

In this article, we present satellite observations of OClO from the years 1995–2002. The focus is on observations of OClO in the Arctic wintertime stratosphere, which are compared to results of the SLIMCAT model and to observations in the Antarctic. In particular, we investigated the beginning and ending of the chlorine activation in the Arctic stratosphere. The Slant Column Densities (SCDs) of OClO increase significantly, when the temperature falls below the threshold for formation of Polar Stratospheric Clouds. The time for decrease of the OClO-SCDs in the deactivation phase (early spring) varies strongly and is related to the degree of denitrification. In the Arctic, chlorine activation can be further increased when there is strong activity of stratospheric mountain waves

Balloon-borne limb profiling of UV/vis skylight radiances, O₃, NO₂, and BrO: technical set-up and validation of the method

A novel light-weight, elevation scanning and absolutely calibrated UV/vis spectrometer and its application to balloon-borne limb radiance and trace gas profile measurements is described. Its performance and the novel method of balloon-borne UV/vis limb trace gas measurements has been tested against simultaneous observations of the same atmospheric parameters available from either (a) in-situ instrumentation (cf., by an electrochemical cell (ECC) ozone sonde also deployed aboard the gondola) or (b) trace gas profiles inferred from UV/vis/near IR solar occultation measurements performed on the same payload. The novel technique is also cross validated with radiative transfer modeling. Reasonable agreement is found (a) between measured and simulated limb radiances and (b) inferred limb O₃, NO₂, and BrO and correlative profile measurements when properly accounting for all relevant atmospheric parameters (temperature, pressure, aerosol extinction, and major absorbers)

Intercomparison exercise between different radiative transfer models used for the interpretation of ground-based zenith-sky and multi-axis DOAS observations

We present the results of an intercomparison exercisebetween six different radiative transfer (RT) models carriedout in the framework of QUILT, an EU funded projectbased on the exploitation of the Network for the Detection ofStratospheric Change (NDSC). RT modelling is an importantstep in the interpretation of Differential Optical AbsorptionSpectroscopy (DOAS) observations. It allows the conversionof slant column densities (SCDs) into vertical column densities(VCDs) using calculated air mass factors (AMFs). Theoriginality of our study resides in comparing SCD simulationsin multi-axis (MAX) geometry (trace gases: NO2 andHCHO) and in taking into account photochemical enhancementfor calculating SCDs of rapidly photolysing species(BrO, NO2, and OClO) in zenith-sky geometry. Concerningthe zenith-sky simulations, the different models agree generallywell, especially below 90 SZA. At higher SZA, largerdiscrepancies are obtained with relative differences rangingbetween 2% and 14% in some cases. In MAX geometry,good agreement is found between the models with the calculatedNO2 and HCHO SCDs differing by no more than 5%in the elevation and solar zenith angle (SZA) ranges investigated(5 20 and 35 85 , respectively). The impacts ofaerosol scattering, ground albedo, and relative azimuth onMAX simulations have also been tested. Significant discrepanciesappear for the aerosol effect, suggesting differencesbetween models in the treatment of aerosol scattering. A better agreement is found in case of the ground albedo and relativeazimuth effects. The complete set of initialization dataand results have been made publicly available through theQUILT project web site (http://nadir.nilu.no/quilt/), enablingthe testing of other RT codes designed for the calculation ofSCDs/AMFs