Lower tropospheric ozone over the North China Plain: variability and trends revealed by IASI satellite observations for 2008–2016

China is a highly polluted region, particularly the North China Plain (NCP). However, emission reductions have been occurring in China for about the last 10 years; these reduction measures have been in effect since 2006 for SO2 emissions and since 2010 for NOx emissions. Recent studies have shown a decrease in the NO2 tropospheric column since 2013 that has been attributed to the reduction in NOx emissions. Quantifying how these emission reductions translate regarding ozone concentrations remains unclear due to apparent inconsistencies between surface and satellite observations. In this study, we use the lower tropospheric (LT) columns (surface – 6&thinsp;km&thinsp;a.s.l. – above sea level) derived from the IASI-A satellite instrument to describe the variability and trend in LT ozone over the NCP for the 2008–2016 period. First, we investigate the IASI retrieval stability and robustness based on the influence of atmospheric conditions (thermal conditions and aerosol loading) and retrieval sensitivity changes. We compare IASI-A observations with the independent IASI-B instrument aboard the Metop-B satellite as well as comparing them with surface and ozonesonde measurements. The conclusion from this evaluation is that the LT ozone columns retrieved from IASI-A are reliable for deriving a trend representative of the lower/free troposphere (3–5&thinsp;km). Deseasonalized monthly time series of LT ozone show two distinct periods: the first period (2008–2012) with no significant trend (&lt; − 0.1&thinsp;%&thinsp;yr−1) and a second period (2013–2016) with a highly significant negative trend of −1.2&thinsp;%&thinsp;yr−1, which leads to an overall significant trend of −0.77&thinsp;%&thinsp;yr−1 for the 2008–2016 period. We explore the dynamical and chemical factors that could explain these negative trends using a multivariate linear regression model and chemistry transport model simulations to evaluate the sensitivity of ozone to the reduction in NOx emissions. The results show that the negative trend observed from IASI for the 2013–2016 period is almost equally attributed to large-scale dynamical processes and emissions reduction, with the large El Niño event in 2015–2016 and the reduction of NOx emissions being the main contributors. For the entire 2008–2016 period, large-scale dynamical processes explain more than half of the observed trend, with a possible reduction of the stratosphere–troposphere exchanges being the main contributor. Large-scale transport and advection, evaluated using CO as a proxy, only contributes to a small part of the trends ( ∼ 10&thinsp;%). However, a residual significant negative trend remains; this shows the limitation of linear regression models regarding their ability to account for nonlinear processes such as ozone chemistry and stresses the need for a detailed evaluation of changes in chemical regimes with the altitude.</p

Development of cave foot deformity in failed repair of the Achilles tendon

Two cases of failed primary repair of the Achilles tendon are reported. Cave foot deformity as an additional clinical sign of this condition is described. A possible biomechanical hypothesis is formulated, and a surgical procedure for correction of the symptoms is described

Decadal trends in global CO emissions as seen by MOPITT

International audienceNegative trends of carbon monoxide (CO) concentrations are observed in the recent decade by both surface measurements and satellite retrievals over many regions of the globe, but they are not well explained by current emission inventories. Here, we analyse the observed CO concentration decline with an atmospheric inversion that simultaneously optimizes the two main CO sources (surface emissions and atmospheric hydrocarbon oxidations) and the main CO sink (atmospheric hydroxyl radical OH oxidation). Satellite CO column retrievals from Measurements of Pollution in the Troposphere (MOPITT), version 6, and surface observations of methane and methyl chloroform mole fractions are assimilated jointly for the period covering 2002-2011. Compared to the model simulation prescribed with prior emission inventories , trends in the optimized CO concentrations show better agreement with that of independent surface in situ measurements. At the global scale, the atmospheric inversion primarily interprets the CO concentration decline as a decrease in the CO emissions (−2.3 % yr −1), more than twice the negative trend estimated by the prior emission inventories (−1.0 % yr −1). The spatial distribution of the inferred decrease in CO emissions indicates contributions from western Europe (−4.0 % yr −1), the United States (−4.6 % yr −1) and East Asia (−1.2 % yr −1), where anthropogenic fuel combustion generally dominates the overall CO emissions, and also from Australia (−5.3 % yr −1), the Indo-China Peninsula (−5.6 % yr −1), Indonesia (−6.7 % yr −1), and South America (−3 % yr −1), where CO emissions are mostly due to biomass burning. In contradiction with the bottom-up inventories that report an increase of 2 % yr −1 over China during the study period, a significant emission decrease of 1.1 % yr −1 is inferred by the inversion. A large decrease in CO emission factors due to technology improvements would outweigh the increase in carbon fuel combustions and may explain this decrease. Independent satellite formaldehyde (CH 2 O) column retrievals confirm the absence of large-scale trends in the atmospheric source of CO. However, it should be noted that the CH 2 O retrievals are not assimilated and OH concentrations are optimized at a very large scale in this study

HCFC-22 emissions at global and regional scales between 1995 and 2010: Trends and variability

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A three-dimensional synthesis inversion of the molecular hydrogen cycle: Sources and sinks budget and implications for the soil uptake

International audienceOur understanding of the global budget of atmospheric hydrogen (H2) contains large uncertainties. An atmospheric Bayesian inversion of H2 sources and sinks is presented for the period 1991–2004, based on a two networks of flask measurement stations. The types of fluxes and the spatial scales potentially resolvable by the inversion are first estimated from an analysis of the correlations of errors between the different processes and regions emitting or absorbing H2. Then, the estimated budget of H2 and its uncertainties is presented and discussed, for five groups of fluxes and three groups of large regions, in terms of mean fluxes, seasonal and interannual variations, and long‐term trends. One main focus of the study is the improvement of the estimate of H2 soil uptake, which is the largest sink of H2. Various sensitivity tests are performed defining an ensemble of more than 20 inversions. We show that inferring a robust estimate of the H2 soil uptake requires to prescribe the prior magnitude of some other sources and sinks with a small uncertainty. Doing so an estimate of the H2 soil uptake of −62 ± 3 Tg y−1 is inferred for the period 1991–2004 (the uncertainty is the residual error after inversion). The inferred soil H2 sink presents a negative long‐term trend that is qualitatively consistent with a bottom‐up process‐based model