Abstract
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| DU decade−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| DU yr−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.
Highlights
O3 plays a key role throughout the whole troposphere where it is produced by the photochemical oxidation of carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and methane (CH4) in the presence of nitrogen oxides (NOx) (e.g., Logan et al, 1981)
Despite the weak contribution of trends to the total variation in the middle–low troposphere (MLT) O3 columns at a global scale, the results demonstrate the possibility of differentiating significant trends from the explanatory variables, especially in summer at mid- and high latitudes of the Northern Hemisphere (NH) (∼ 40–75◦ N) where the contribution and the sensitivity of trends are the largest
The results clearly demonstrate the possibility of determining such a trend amplitude from the available Infrared Atmospheric Sounding Interferometer (IASI) dataset and the used multi-linear regression (MLR) model at northern mid- and high latitudes in summer, while much larger measurement periods are necessary elsewhere
Summary
O3 plays a key role throughout the whole troposphere where it is produced by the photochemical oxidation of carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and methane (CH4) in the presence of nitrogen oxides (NOx) (e.g., Logan et al, 1981). All these datasets suffer from a lack of homogeneity in terms of measurement methods (instrument and algorithm) and spatio-temporal samplings (e.g., Doughty et al, 2011; Heue et al, 2016; Leventidou et al, 2017) Those limitations, in addition to the large natural interannual variability (IAV) and decadal variations in tropospheric O3 levels (due to large-scale dynamical modes of O3 variations and to changes in stratospheric O3, in stratosphere–troposphere exchanges, in precursor emissions and in their geographical patterns), introduce strong biases in trends determined from independent studies and datasets (e.g., Zbinden et al, 2006; Thouret et al, 2006; Logan et al, 2012; Parrish et al, 2012 and references therein). The O3–CO correlations, enhancement ratios and covariance are examined for characterizing the origin of the air masses in regions of positive and negative trends
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