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«Abstract Ground-based Fourier-transform infrared (FTIR) solar absorption spec- troscopy has led to a number of significant advances in our ...»

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The OASIS Observatory Using Ground-

Based Solar Absorption Fourier-Transform

Infrared Spectroscopy in the Suburbs


of Paris (Creteil-France)

P. Chelin, C. Viatte, M. Ray, M. Eremenko, J. Cuesta, F. Hase, J. Orphal,

and J.-M. Flaud


Ground-based Fourier-transform infrared (FTIR) solar absorption spec-

troscopy has led to a number of significant advances in our understanding of the

atmosphere by providing information on the vertical distribution of various trace

gases. Previously used to analyse solar absorption spectra measured at high- resolution in unpolluted sites, the retrieval code PROFFIT has been adapted to deal with spectra recorded at medium spectral resolution with a Bruker Optics Vertex 80 FTIR spectrometer. As one of the major instruments of the experimental observatory named OASIS (Observations of the Atmosphere by Solar Infrared Spectroscopy), this instrument is dedicated to the study of air composition in the P. Chelin (*), M. Ray, M. Eremenko, J. Cuesta, and J.-M. Flaud ` ´ Laboratoire Inter-Universitaire des Systemes Atmospheriques (LISA) CNRS UMR 7583,

´ ´ ´

Universite Paris-Est Creteil, Universite Paris Diderot, Institut Pierre-Simon Laplace, 61

´´ ´

Avenue du General de Gaulle, 94010 Creteil Cedex, France e-mail: pascale.chelin@lisa.u-pec.fr C. Viatte ` ´ Laboratoire Inter-Universitaire des Systemes Atmospheriques (LISA) CNRS UMR 7583,

´ ´ ´

Universite Paris-Est Creteil, Universite Paris Diderot, Institut Pierre-Simon Laplace, 61

´´ ´

Avenue du General de Gaulle, 94010 Creteil Cedex, France Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA F. Hase Institute for Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany J. Orphal ` ´ Laboratoire Inter-Universitaire des Systemes Atmospheriques (LISA) CNRS UMR 7583,

´ ´ ´

Universite Paris-Est Creteil, Universite Paris Diderot, Institut Pierre-Simon Laplace, 61

´´ ´

Avenue du General de Gaulle, 94010 Creteil Cedex, France Institute for Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany ´ E. Jimenez et al. (eds.), Environment, Energy and Climate Change I: 21 Environmental Chemistry of Pollutants and Wastes, Hdb Env Chem (2015) 32: 21–52, DOI 10.1007/698_2014_270, © Springer-Verlag Berlin Heidelberg 2014, Published online: 29 July 2014 22

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suburbs of Paris. Accurate measurements of the most important atmospheric pollutants are indeed essential to improve the understanding and modelling of urban air pollution processes. Located in an urban region, OASIS enables to monitor key pollutants such as NOx, O3, CO and VOCs. In this chapter, 5 years intercomparison study with on-ground and satellite measurements for O3 and CO is reported, demonstrating the performances of a medium-resolution ground-based instrument and especially confirming its capability for tropospheric ozone monitoring.

Keywords Air quality in megacity, Carbon monoxide, IR spectroscopy, Ozone, Remote sensing, Solar occultation

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1 Introduction Ozone (O3) plays an important role in the Earth’s atmosphere. In the stratosphere, its presence is vital for life on Earth because it absorbs harmful ultraviolet radiation [1]; in the troposphere, it is involved in photochemical processes, a key parameter for both air quality and climate issues [2]. In the boundary-layer, O3 is harmful to humans [3], animals and vegetation [4]. In the upper troposphere, O3 impacts radiative forcing [5–7]. O3 also controls the oxidizing capacity of the atmosphere [8]. In addition to O3, carbon monoxide (CO) is also involved in tropospheric photochemical processes: in fact O3 production takes place when CO and hydrocarbons are photo-oxidized in the presence of nitrogen oxides (NOx). Also CO is an excellent tropospheric air-mass tracer due to its rather long lifetime of 2 months on average.

In the international Network for the Detection of Atmospheric Composition Change (NDACC), around 20 high-quality, remote-sensing IR research stations employ ground-based Fourier-transform infrared (FTIR) solar absorption spectroscopy to observe and to analyse the physical and chemical states of the stratosphere and upper troposphere and to assess the impact of stratospheric changes on the underlying troposphere and on global climate. These high spectral resolution FTIR stations are preferentially operated at remote sites far away from sources of air ˜ pollution and often are located at high altitudes (e.g. Izana, Jungfraujoch, Zugspitze, Table Mountain, etc.) so that measurements of tropospheric ozone are obviously limited at these stations. Hence, in order to perform air quality research in large megacities, we have assessed the capability of a medium-resolution FTIR solar absorption spectrometer for monitoring pollutants, especially O3 and CO. We have demonstrated that such an observatory, named OASIS (Observations of the ´ Atmosphere by Solar Infrared Spectroscopy) installed in Creteil near Paris (France) ´ since 2008, is able to continuously monitor tropospheric ozone over Creteil with good accuracy as documented by a first analysis of information content in OASIS ozone retrievals [9].

24 P. Chelin et al.

The purpose of the present study is to confirm and expand this preliminary result by incorporating time series of total and tropospheric ozone during 5 years of measurements from February 2009 to July 2013 and by intercomparing satellite observations and in situ measurements. Another new aspect is to complement the O3 observations with CO total columns (and its seasonal cycle) that are measured simultaneously with O3 by taking advantage of the wide spectral range covered by the OASIS instrument.

This chapter firstly describes in detail the OASIS instrumentation (Sect. 2) and outlines the radiative transfer equation and the radiative transfer model and retrieval code applied (Sect. 3). In this latter section, a discussion on the separation of tropospheric and stratospheric columns of ozone based on the information content analysis is included. The overall results derived from the OASIS measurements are presented in Sect. 4 for the total columns of O3 and CO and for the tropospheric ozone column. Also columns derived from OASIS measurements are compared with correlative satellite and ground-based observations. Finally, Sect. 5 is devoted to the conclusions.

2 OASIS Instrumentation

The OASIS observatory (48.79 N, 2.44 E, 56 m above sea level) was established in 2008 with the installation of a medium-resolution, Vertex model Fouriertransform infrared spectrometer manufactured by Bruker Optics (Ettlingen, Germany). This instrument collects infrared atmospheric absorption spectra using the sun as light source (Fig. 1) and monitors continuously the concentrations of important atmospheric constituents, such as H2O, CO2, CH4, N2O, O2, NH3, OCS, O3 and CO. The observatory comprises an automatized cupola (Sirius 3.5 “School Model” observatory, 3.25 m high and 3.5 m in diameter) in which the upper part, the dome, equipped with a mobile aperture, rotates in order to be aligned with the solar tracker and the sun. The drive engine of the dome is fed by a battery recharged by two solar panels.

2.1 Sun Tracker

The alt-azimutal solar tracker in OASIS is the A547N model manufactured by Bruker Optics. To reach a tracking precision of Æ2 min of arc, the solar tracker uses a quadrant diode to register deviations from the precalculated pointing direction of the tracking system. The diode signal is then fed into the control loop of the tracker under cloudless blue sky conditions. In case of overcast sky, the diode signal drops down, and if it is under a minimal value, the sun tracker sustains tracking according to astronomic calculation.

The OASIS Observatory Using Ground-Based Solar Absorption Fourier-Transform... 25 Fig. 1 Interior view of the OASIS observatory showing the FTIR spectrometer and the sun tracker The solar light that is analysed by the quadrant diode is decoupled from the parallel beam a few cm in front of the entrance aperture by a small planar mirror, so that only a small subsection of the full beam diameter is used. Both tracker mirrors are coated with protected aluminium to cover the entire spectral range from 700 to 40,000 cmÀ1. However, we experienced corrosion problems with the Al mirrors.

Visible damage appeared on the mirror’s surface like white coatings after several months of use. Likely these problems are due to high humidity levels and large temperature excursions inside the cupola and maybe also to the presence of corrosive pollutants in the urban area. In the near future, the replacement of the current Al mirrors by bare gold-coated mirrors which cover the spectral range from 10 to 15,800 cmÀ1 will be tested. Moreover, two heaters have been installed to generate a less hostile environment during the cold season: one heater diminishes the overall humidity inside the cupola, the other one circulates dry air around the two mirrors. Indeed, relative global humidity dropped from 80 to 30%.

26 P. Chelin et al.

Fig. 2 Ground-based atmospheric solar spectrum recorded with an entrance aperture diameter of

1.5 mm and a maximum nominal spectral resolution of 0.075 cmÀ1 A software provided by Bruker controls the sun tracker from the OASIS computer. The latter is also controlled by internet connection which permits the remote control of all the instruments from any computer in the university LAN.

2.2 Fourier-Transform Infrared Spectrometer As one of the major instruments of the OASIS observatory, the Fourier-transform spectrometer is dedicated to the measurement of atmospheric spectra in solar occultation geometry using the infrared spectral region in order to investigate the air composition in the suburbs of Paris. The optical key element of the OASIS spectrometer is a linear Michelson interferometer that records single-sided interferograms with a maximum optical path difference of 12 cm. Infrared solar absorption spectra are nominally recorded on a DTGS (deuterated triglycine sulphate) detector using a potassium bromide (KBr) beam splitter to cover the spectral region from 700 to 11,000 cmÀ1 (0.9–14.3 μm). It is a dry air purged spectrometer to always maintain the humidity in the optical benchmark at values lower than 10%.

During low temperature conditions, the spectrometer is placed inside a wood container with an additional heater.

To achieve a sufficiently high signal-to-noise ratio, each spectrum is produced by co-adding 30, 12-cm optical path difference scans, resulting in one interferogram recorded over a period of approximately 10 min (Fig. 2). Each interferogram is Fourier-transformed into a spectrum without further numerical apodization (i.e. unapodized/boxcar apodization).

The OASIS Observatory Using Ground-Based Solar Absorption Fourier-Transform... 27 Fig. 3 Gas cell spectra recorded with OCS at low pressure for the determination of the ILS parameters

2.3 Spectrometer Accuracy Any quantitative trace gas retrieval depends on a proper knowledge of the spectral characteristics of the instrument. For FTIR spectrometers, the Instrumental Line Shape (ILS) is advantageously separated into one part which refers to the inherent self-apodization due to the circular field stop. This contribution can be calculated easily using the spectrometer’s field of view and the optical path difference of the interferometer. The second component of the ILS can be described by a complex modulation efficiency (represented by a modulation amplitude and a phase error, both functions of the optical path difference) which result from misalignments and optical aberrations [10]. In our case, the determination of the instrumental line shape (ILS) is made with a 25 cm long gas cell (using CaF2 windows) filled with OCS (carbonyl sulphide) at very low pressure, which is put in the interferometer’s sample compartment (Fig. 3). To analyse the results and to obtain the modulation efficiency and the phase error, the version 12 of the LINEFIT programme [11] is used.

The zero transmittance baseline has also been checked in an atmospheric spectrum on the ν2 fundamental band of H2O centred around 1,595 cmÀ1 that is totally saturated as showed in Fig. 4. The accuracy on the zero transmittance is better than 0.15% checked between 1,600 and 1,700 cmÀ1. This is important since it demonstrates a very low nonlinearity of the detector.

28 P. Chelin et al.

Fig. 4 Atmospheric spectrum with zero transmittance in the ν2 band of H2O 3 Radiative Transfer Equation and Retrieval Code In order to retrieve the concentrations of the target species in the atmosphere, one needs first to calculate a synthetic atmospheric spectrum using a radiative transfer algorithm. This is done using the PROFile ForWarD (PROFFWD) model of the PROFFIT 9.6 code [12] widely used by the NDACC community to retrieve trace gases from high-resolution FTIR measurements.

3.1 Forward Calculation of a Synthetic Spectrum When the sun is used as the external source, the radiative transfer model is based on

the integrated form of the Beer–Lambert law accounting only for absorption [13]:

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3.2 Retrieval Code PROFFIT (PROFile FIT) [12] is the retrieval tool used to analyse the solar absorption spectra measured with high-resolution ground-based FTIR spectrometers. It is here adapted to the medium spectral resolution of the instrument. Note that to achieve sufficient information content in the retrievals, the micro-windows have to be widened as compared to high-resolution retrievals (as described in Sect. 3.3).

The retrieval of trace gas profiles from such spectra is an ill-posed problem and needs a constrained nonlinear least squares fitting technique [15]. For this purpose, an analytical altitude-dependent regularization method with the regularization matrix containing first- and second-order Tikhonov constraints [16] is used, together with altitude-dependent coefficients that are optimized to maximize the information content of the retrievals.

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