OHP, France, 43.94°N, 5.71°E

Introduction

The "Observatoire de Haute-Provence", OHP, Southern France (43.94°N, 5.71°E), is part of the "Observatoire Astronomique de Marseille Provence", OAMP. OHP is one of the primary alpine stations of the international Network for the Detection of Atmospheric Composition Change (NDACC) and is also part of the network of WMO's Global Atmosphere Watch (GAW). The station also reports the stratospheric column amount of ozone and NO2 (stratospheric reference provided by a SAOZ instrument operated by CNRS at OHP since 1992), as well as ancillary data as vertical distribution of pressure/temperature (radio-sondes and temperature LIDAR), ozone (electrochemical ozone sondes, LIDAR, SAOZ and Dobson), and aerosols information (CIMEL instrument operated by Lille University in the AERONET network context), very useful for the MAXDOAS analysis and for the interpretation of the results. Because OHP alternates between background and polluted conditions (essentially due to transport from polluted neighbourhoods), it is a very interesting location for tropospheric observations, providing thus support to validate satellites.

View of the "Observatoire de Haute-Provence" site

Instrument and operation

BIRA-IASB has participated to the NDSC/SCUVS-3 intercomparison campaign that held at the Observatoire de Haute Provence for three weeks in June 1996. The purpose of this exercise was to compare NO2 and O3 slant columns from UV-Visible measurements but in parallel to the NDSC activities, BIRA-IASB contributed also with other European groups to the intercomparison of BrO measurements. Even though the site was not appropriate for the detection of this species, it was a good opportunity to discuss about the measurement techniques and to improve the DOAS retrieval algorithms for BrO.

In 1998, BIRA-IASB came back to OHP with a innovative instrument optimized precisely for the detection of the BrO. In order to reduce the impact of the polarization and of the Ring effect on twilight observations, the spectrometer is mounted on a rotating plate, fully automated by software to follow the solar azimuth. The instrument is placed on the top of the geophysics station. At the end of 2002, the instrument has been improved by the addition of a mirror in order to complete zenith-sky measurements with off-axis ones. These latter measurements are important in order to separate the tropospheric and the stratospheric components from the slant columns. The instrument left OHP in July 2002 to participate to field campaigns organized in the Po valley (Milan area, Italy) in Summer 2002 and Autumn 2003 in the frame of the European project FORMAT (FORMAldehyde as a tracer of oxidation in the Troposphere) and also to the NDSC intercomparison campaign at Andoya (Norway, 69°N) in Winter 2003. The instrument in its current version reintegrated OHP in 2005.

Data

BIRA-IASB provides the GEOMON data base (http://www.geomon.eu) with BrO 90°-80° differential slant columns densities measured at OHP.

Scientific highlights

Bromine monoxide (BrO) is a key atmospheric trace gas known to cause significant ozone destruction in both the stratosphere and troposphere. At OHP, low resolution vertical profiles of BrO have been retrieved by applying a profiling technique to the ground-based MAXDOAS observations performed since 2005. Such a profiling technique enables to separate the tropospheric and stratospheric contributions to the BrO total columns. The retrieved profiles have been used to validate SCIAMACHY limb and nadir measurements. In the following figure, the 15-27 km BrO partial columns calculated from SCIAMACHY limb (version 3.2 of the IUP-Bremen scientific product) and ground-based UV-visible profiles are compared for the 2005-2006 period. Both data sets are in good agreement, with SCIAMACHY higher than the ground-based observations by 1 18%. The seasonality in the BrO column is also consistently captured in both data sets (more details in Hendrick et al., 2008).

Comparison of the 15-27 km BrO partial columns calculated from SCIAMACHY limb and ground-based UV-visible profiles at OHP for the 2005-2006 period

The mean level of total BrO VCDs from SCIAMACHY and ground-based evaluations is consistent. The seasonal variation (and even short-term variations) of the total BrO VCDs is captured in a similar way by SCIAMACHY and ground-based instruments. However, systematic discrepancies tend to appear in summer conditions when both satellite and ground-based retrievals have their largest uncertainties.

Moreover, data from the MAXDOAS instrument of OHP are currently used in the O3MSAF framework to test and set up a method for the validation of the operational GOME-2 NO2 product (Lambert et al., 2008; Pinardi et al., 2011). Although it is largely rural, OHP can occasionally be influenced by polluted air masses transported from neighbouring cities, providing interesting test cases for GOME-2 sensitivity to tropospheric NO2. Figure 1 shows a timeseries of the ground-based MAXDOAS and the GOME-2 tropospheric NO2 columns over OHP from June 2007 to March 2011. One can see that pollution episodes are well captured by GOME-2, although the scatter of the individual comparison points is relatively large. These results are qualitatively similar to those obtained in previous validation exercises (e.g. Brinksma et al., 2008; Celarier et al., 2008; Irie et al., 2008). However, in our study a much longer comparison dataset is available (~4 yr) and averaging can be performed, in order to limit the influence of temporal and spatial sensitivity differences on the comparison. A more detailed discussion can be found in Valks et al., 2011 and Pinardi et al., 2011 .

List of publications