Abstract
Abstract. We present a new multispectral approach for observing lowermost tropospheric ozone from space by synergism of atmospheric radiances in the thermal infrared (TIR) observed by IASI (Infrared Atmospheric Sounding Interferometer) and earth reflectances in the ultraviolet (UV) measured by GOME-2 (Global Ozone Monitoring Experiment-2). Both instruments are onboard the series of MetOp satellites (in orbit since 2006 and expected until 2022) and their scanning capabilities offer global coverage every day, with a relatively fine ground pixel resolution (12 km-diameter pixels spaced by 25 km for IASI at nadir). Our technique uses altitude-dependent Tikhonov–Phillips-type constraints, which optimize sensitivity to lower tropospheric ozone. It integrates the VLIDORT (Vector Linearized Discrete Ordinate Radiative Transfer) and KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) radiative transfer codes for simulating UV reflectance and TIR radiance, respectively. We have used our method to analyse real observations over Europe during an ozone pollution episode in the summer of 2009. The results show that the multispectral synergism of IASI (TIR) and GOME-2 (UV) enables the observation of the spatial distribution of ozone plumes in the lowermost troposphere (LMT, from the surface up to 3 km a.s.l., above sea level), in good agreement with the CHIMERE regional chemistry-transport model. In this case study, when high ozone concentrations extend vertically above 3 km a.s.l., they are similarly observed over land by both the multispectral and IASI retrievals. On the other hand, ozone plumes located below 3 km a.s.l. are only clearly depicted by the multispectral retrieval (both over land and over ocean). This is achieved by a clear enhancement of sensitivity to ozone in the lowest atmospheric layers. The multispectral sensitivity in the LMT peaks at 2 to 2.5 km a.s.l. over land, while sensitivity for IASI or GOME-2 only peaks at 3 to 4 km a.s.l. at lowest (above the LMT). The degrees of freedom for the multispectral retrieval increase by 0.1 (40% in relative terms) with respect to IASI only retrievals for the LMT. Validations with ozonesondes (over Europe during summer 2009) show that our synergetic approach for combining IASI (TIR) and GOME-2 (UV) measurements retrieves lowermost tropospheric ozone with a mean bias of 1% and a precision of 16%, when smoothing by the retrieval vertical sensitivity (1% mean bias and 21% precision for direct comparisons).
Highlights
Tropospheric ozone is currently one of the air pollutants posing greatest threats to human health (e.g. Gryparis et al, 2004; and Ito et al, 2005) and ecosystems (e.g. Fuhrer and Achermann, 1994; USEPA, 1996; and EEA, 2011)
We present a new multispectral approach to observe lowermost tropospheric ozone using the synergy of thermal infrared (TIR) atmospheric radiances observed by Infrared Atmospheric Sounding Interferometer (IASI) and UV earth reflectances measured by GOME-2
One may identify ozone plumes located below 3 km a.s.l. as they are clearly depicted by IASI + GOME-2, but not by IASI
Summary
Tropospheric ozone is currently one of the air pollutants posing greatest threats to human health (e.g. Gryparis et al, 2004; and Ito et al, 2005) and ecosystems (e.g. Fuhrer and Achermann, 1994; USEPA, 1996; and EEA, 2011). We present a new multispectral approach to observe lowermost tropospheric ozone using the synergy of TIR atmospheric radiances observed by IASI and UV earth reflectances measured by GOME-2 Both instruments are onboard the MetOp satellite series (in orbit since 2006 and expected until 2022 with MetOp-C) and they are both well suited for monitoring air quality at regional and global scales resulting from their excellent scanning capabilities. The IASI + GOME-2 multispectral scheme is constructed by adapting and merging together 2 state-of-the-art and thoroughly validated methods to retrieve ozone profiles using either only IASI observations in the spectral TIR region (described by Eremenko et al, 2008, hereafter referred as “IASI” method) or only GOME-2 measurements in the UV (called in the following “GOME-2” retrieval) This last one is based on Cai et al (2012) for forward calculations, but here with a 1/8 finer pixel resolution for spatial consistency with IASI pixels and a Tikhonov–Phillips-type regularization. We describe how these 2 methods are joined together into the IASI + GOME-2 algorithm (Sect. 2.1) and the specific developments necessary for building this multispectral inversion approach (i.e. regularization constraints and adequate fitting variables, Sects. 2.2 and 2.3)
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