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Volume 18, issue 9
Atmos. Chem. Phys., 18, 6867-6885, 2018
https://doi.org/10.5194/acp-18-6867-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Quadrennial Ozone Symposium 2016 – Status and trends...

Atmos. Chem. Phys., 18, 6867-6885, 2018
https://doi.org/10.5194/acp-18-6867-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 May 2018

Research article | 16 May 2018

Decrease in tropospheric O3 levels in the Northern Hemisphere observed by IASI

Catherine Wespes1, Daniel Hurtmans1, Cathy Clerbaux1,2, Anne Boynard2, and Pierre-François Coheur1 Catherine Wespes et al.
  • 1Spectroscopie de l'Atmosphère, Service de Chimie Quantique et Photophysique, Faculté des Sciences, Université Libre de Bruxelles (ULB), Brussels, Belgium
  • 2LATMOS/IPSL, UPMC Univ. Paris 06 Sorbonne Universités, UVSQ, CNRS, Paris, France

Abstract. In this study, we describe the recent changes in the tropospheric ozone (O3) columns measured by the Infrared Atmospheric Sounding Interferometer (IASI), onboard the Metop satellite, during the first 9 years of operation (January 2008 to May 2017). Using appropriate multivariate regression methods, we differentiate significant linear trends from other sources of O3 variations captured by IASI. The geographical patterns of the adjusted O3 trends are provided and discussed on the global scale. Given the large contribution of the natural variability in comparison with that of the trend (25–85% vs. 15–50%, respectively) to the total O3 variations, we estimate that additional years of IASI measurements are generally required to detect the estimated O3 trends with high precision. Globally, additional 6 months to 6 years of measurements, depending on the regions and the seasons, are needed to detect a trend of |5|DUdecade−1. An exception is interestingly found during summer at mid- and high latitudes of the Northern Hemisphere (NH; ∼ 40 to ∼75°N), where the large absolute fitted trend values (∼ |0.5|DUyr−1 on average) combined with the small model residuals (∼ 10%) allow for detection of a band-like pattern of significant negative trends. Despite no consensus in terms of tropospheric O3 trends having been reached from the available independent datasets (UV or IR satellites, O3 sondes, aircrafts, ground-based measurements, etc.) for the reasons that are discussed in the text, this finding is consistent with the reported decrease in O3 precursor emissions in recent years, especially in Europe and USA. The influence of continental pollution on that latitudinal band is further investigated and supported by the analysis of the O3–CO relationship (in terms of correlation coefficient, regression slope and covariance) that we found to be the strongest at northern midlatitudes in summer.

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