Monitoring Ozone in the Lower Troposphere with IASI: Analysis of the Dynamical and Chemical Processes Controlling the Ozone Distribution over Eastern Asia

Tropospheric ozone plays a major role in air quality by affecting human health and causing damages to ecosystems. In this framework, our main concern is to regionally characterize major pollution events and the inter-annual evolution of tropospheric ozone. For our studies, we primarily use IASI space-borne observations for monitoring daily distributions of ozone in the lower troposphere from the regional to the continental scale. IASI has been operating aboard the MetOp platform since October 2006. It is a nadir-viewing Fourier transform spectrometer measuring thermal infrared radiances. We have developed an altitude-dependent Tikhonov-Phillips retrieval algorithm optimized to maximize the information one can extract from the lower part of the troposphere. Capabilities to monitor lower tropospheric ozone for air quality concerns have been demonstrated [1,2] with good performances in terms of vertical sensitivity. As well, we have shown that assimilation of IASI ozone observations in air quality models over Europe allows one to improve significantly simulated ozone distribution, even at the surface [3]. The step forward to use these observations is a systematic cross-analysis of the observed ozone distribution with model simulations in order to evaluate the processes that control the spatial and temporal distribution of lower tropospheric ozone, especially when large ozone amounts are observed. The part of the dynamical processes (vertical transport, impact of large source plumes, etc) versus the chemical processes (photochemical production) has to be quantified. For example, first analyses over the Mediterranean Basin in Europe that exhibits high tropospheric ozone especially during summer months, shows that the vertical transport from the upper to the lower troposphere combined with North-West to South-East transport play a significant part in the observed variability of ozone. In this paper, we focus mainly on Eastern Asia and Chinese megacities, one of the largest pollution source regions in the world. We give an overview of the situation over Eastern Asia and also at the scale of the megacities for the year 2008. The observed ozone variations are analyzed accounting for the variations of the vertical sensitivity of the observations. Actually, observations provide a view of different parts of the troposphere depending on the thermodynamical conditions. This needs to be considered carefully in the interpretation of the observations and the comparison with simulations. We evaluate in particular the influence of transport on the variability of ozone depending on the time scale considered. A first analysis of seasonal to daily variations of lower tropospheric ozone observed with IASI shows that the monthly variations of ozone are controlled by the Asian summer monsoon whereas the daily variations of ozone, especially in the North of China, are driven by stratospheric intrusion in winter and more likely by local photochemical production in spring and summer. [1] Eremenko et al., Geophys. Res. Lett., 35, L1885, 2008. [2] Dufour et al.. Atmos. Chem. Phys., 10, 3787-3801, 2010. [3] Coman et al., Atmos. Chem. Phys., 11, 2513-2532, 2012.