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. China is a highly polluted region, particularly the North China Plain (NCP).However, emission reductions have been occurring in China for about the last10 years; these reduction measures have been in effect since 2006 for SO2 emissions andsince 2010 for NOx emissions. Recent studies have shown adecrease in the NO2 tropospheric column since 2013 that has been attributed tothe reduction in NOx emissions. Quantifying how these emissionreductions translate regarding ozone concentrations remains unclear due toapparent inconsistencies between surface and satellite observations. In thisstudy, we use the lower tropospheric (LT) columns (surface – 6 km a.s.l. – above sea level)derived from the IASI-A satellite instrument to describe the variability andtrend in LT ozone over the NCP for the 2008–2016 period. First, we investigate the IASIretrieval stability and robustness based on the influence of atmosphericconditions (thermal conditions and aerosol loading) and retrieval sensitivitychanges. We compare IASI-A observations with the independent IASI-Binstrument aboard the Metop-B satellite as well as comparing them with surface and ozonesondemeasurements. The conclusion from this evaluation is that the LT ozone columnsretrieved from IASI-A are reliable for deriving a trend representative of thelower/free troposphere (3–5 km). Deseasonalized monthly time series of LTozone show two distinct periods: the first period (2008–2012) with nosignificant trend (&lt;−0.1 % yr−1) and a second period (2013–2016) with ahighly significant negative trend of −1.2 % yr−1, which leads to anoverall significant trend of −0.77 % yr−1 for the 2008–2016 period. Weexplore the dynamical and chemical factors that could explain these negativetrends using a multivariate linear regression model and chemistry transportmodel simulations to evaluate the sensitivity of ozone to the reduction inNOx emissions. The results show that the negativetrend observed from IASI for the 2013–2016 period is almost equally attributed tolarge-scale dynamical processes and emissions reduction, with the large El Niñoevent in 2015–2016 and the reduction of NOx emissions beingthe main contributors. For the entire 2008–2016 period, large-scaledynamical processes explain more than half of the observed trend, with apossible reduction of the stratosphere–troposphere exchanges being themain contributor. Large-scale transport and advection, evaluated using CO asa proxy, only contributes to a small part of the trends (∼10 %). However,a residual significant negative trend remains; this shows the limitation oflinear regression models regarding their ability to account for nonlinear processes such as ozonechemistry and stresses the need for a detailed evaluation of changes in chemicalregimes with the altitude.

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