The Tropical West Pacific (TWP) is the key entry point of air into the stratosphere during Northern Hemispheric winter. Our previous work showed how transport to the TWP troposphere is modulated by the movement of the Intertropical Convergence Zone, allowing transport of polluted air masses from tropical Asia during spring (Müller et al., 2023a, Müller et al. 2023b). Here, we present insights on air mass transport to the upper troposphere/lower stratosphere (UTLS) in this region using the ozone and relative humidity time series (2016-2023) from the Palau Atmospheric Observatory (PAO), located in the center of the tropical warm pool (7°N, 134°E). We further give an overview of all observations performed at the PAO during the ACCLIP summer campaign 2022, including ozone, CFH and COBALD sonde and lidar measurements.In analogy to Müller et al. 2023b, Lagrangian backward trajectories calculated are used to identify transport from remote locations. The time series already allows some interpretation regarding interannual variability mainly to the ENSO cycle. Here, we show anomalies during the 2016 strong El Nino and potentially the El Nino in the upcoming winter 2023. The PAO proves to be an excellent site to study UTLS composition and dynamics in the TWP region.

The eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 was one of the most explosive eruptions of the last decades. The unprecedented amount of water vapor injected into the stratosphere increased the stratospheric water vapor burden by about 10%. Using model runs from the ATLAS chemistry and transport model and Microwave Limb Sounder (MLS) satellite observations, we show that while 20-40% more water vapor than usual was entrained into the Antarctic polar vortex as it formed (e.g., typical values of 4.6 ppm at 21.5 km increased to 6.7 ppm), the direct effect of the increased water vapor on Antarctic ozone depletion was minor. This is caused by the very low temperatures in the vortex, which limit water vapor to the saturation pressure and tend to reset any anomalies in water vapor by dehydration before they can have an effect on ozone loss.

We present a fast model for stratospheric ozone chemistry based on a neural network approach. The model is intended to replace the detailed chemistry schemes of chemistry and transport models (CTMs), general circulation models (GCMs) or Earth system models (ESMs), which are computationally very expensive. The neural network (NN) model estimates the rate of change of ozone in 24 hours at a grid point and is trained on data of the detailed full chemistry model of the ATLAS chemistry and transport model (CTM). The benefit of this surrogate models is a much lower computation time (minutes instead of days) while the same level of accuracy is achieved. This represents a necessary step from understanding the chemistry and building sophisticated CTMs towards the usage of this knowledge in climate models, which is only feasible if much lower computation times can be achieved. Modelling of the Earth system is a complex task and models usually contain a large number of sub-modules and parameterizations. This applies for example to the atmosphere, hydrosphere, solid earth and the ice sheets. Atmospheric chemistry is complex and usually involves dozens of species and hundreds of reactions with a wide range of concentrations and lifetimes. This project concentrates on the estimation of the rate of change of ozone in the extrapolar stratosphere. The dynamics from the polar regions and from other layers of the atmosphere regarding the ozone change are not treated within this work. The ATLAS model is a Lagrangian CTM for stratospheric chemistry. It solves a coupled differential equation system using a stiff solver and a variable time-step. The stratospheric chemistry scheme of ATLAS has 46 active species, 171 reactions and heterogeneous chemistry on polar stratospheric clouds. It is not using the concept of chemical families. The application of the ATLAS CTM has high requirements on computational power. This is the reason why the coupling of full chemical models to climate models is generally not feasible with respect to the computation time of a global climate model. However, the incorporation of detailed chemistry is often desirable, in order to account for various feed-backs between chemistry, atmosphere and ocean. These complex chemical models motivate the formulation of faster but still accurate surrogate models, that are tailored to the coupling into earth climate models. This project builds on the SWIFT project, which has a polar and an extrapolar surrogate model for the stratospheric ozone chemistry. We investigate an alternative approach to the polynomial approach used by extrapolar SWIFT by exploiting the improved approximation capability of NN with respect to nonlinear contexts.

Motivated by previous measurements of very low tropospheric ozone concentrations in the Tropical West Pacific (TWP) and the implied low oxidizing capacity of this key region for transport into the stratosphere (e.g. Rex et al. 2014), we set up an atmospheric research station in Palau (7° N 134° E). Our analysis of regular balloon-borne tropospheric ozone observations at Palau from 01/2016-10/2019 confirms the year-round dominance of a low ozone background in the mid-troposphere. Layers of enhanced ozone are often anti-correlated with water vapor and occur frequently. Moreover, the occurrence of respective layers shows a strong seasonality. Dry and ozone-rich air masses between 5 and 10 km altitude were observed in 71 % of the profiles from February until April compared to 25 % from August until October. By defining monthly atmospheric background profiles for ozone and relative humidity based on observed statistics, we found that the deviations from this background reveal a bimodal distribution of RH anomalies. A previously proposed universal bimodal structure of free tropospheric ozone in the TWP could not be verified (Pan et al. 2015). Back trajectory calculations confirm that throughout the year the mid-tropospheric background is controlled by local convective processes and the origin of air masses is thus close to or East of Palau in the Pacific Ocean. Dry and ozone-rich air originates in tropical Asia and reaches Palau in anticyclonic conditions over an area stretching from India to the Philippines. This supports the hypothesis of several studies which attribute ozone enhancement against the low ozone background to remote pollution events on the ground such as biomass burning (e.g. Andersen et al. 2016). A potential vorticity analysis revealed no stratospheric influence and we thus propose large-scale descent within the tropical troposphere as responsible for dehydration of air masses on their way to Palau.

In the Antarctic ozone hole, ozone mixing ratios have been decreasing to extremely low values of 0.01-0.1 ppm in nearly all spring seasons since the late 1980s, corresponding to 95-99 % local chemical loss. In contrast, Arctic ozone loss has been much more limited and mixing ratios have never before fallen below 0.5 ppm. In Arctic spring 2020, however, ozone sonde measurements in the most depleted parts of the polar vortex show a highly depleted layer, with ozone loss averaged over sondes peaking at 93 % at 18 km. Typical minimum mixing ratios of 0.2 ppm were observed, with individual profiles showing values as low as 0.13 ppm (96 % loss). The reason for the unprecedented chemical loss was an unusually strong, long-lasting and cold polar vortex, showing that for individual winters the effect of the slow decline of ozone-depleting substances on ozone depletion may be counteracted by low temperatures.