Investigating the diurnal variability of trace gases in the TROpical West PACific measured by solar absorption spectrometry in the infrared (TROPAC)

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The ocean in the West Pacific with temperatures of 30°C year-round is the warmest ocean worldwide. In the tropical West Pacific the boundary layer air temperature is highest globally and overhead ozone lowest. Due to the general low and mid-tropospheric advection from the east by the Walker circulation across the Pacific, air above the tropical West Pacific has been in a clean, warm and humid environment for a long time.Thus, the loss of odd oxygen and ozone precursors like NOx proceeded longer than elsewhere in the tropics, leading to very low ozone concentrations. This increases the lifetimes of short-lived biogenic and anthropogenic chemical species. Furthermore, high sea surface temperatures favor strong convective activity in the tropical West Pacific, which can lead to low ozone mixing ratios in the convective outflow regions in the upper troposphere. The West Pacific warm pool is also a major source region for stratospheric air. Hence, the region where the lifetimes of short lives species is increased and the source region of stratospheric air are coincident. Thus, the composition of the tropospheric atmosphere in this region determines the global stratospheric composition to a large extent.Ozone is an important trace gas for the climate assessment because of feedback processes among changing temperature, dynamics and ozone. Since the West Pacific warm pool is the major source region for stratospheric air, the knowledge of ozone and other shorter lived trace gases is also important to understand the transport of trace gases into the stratosphere. The aim of our project is to measure the diurnal variability of ozone and other trace gases using the high resolution solar absorption FTIR-spectrometry. The measurements yield the total columns densities of up to 20 trace gases. For a few trace gases, like O3, the analysis of the spectral line shape allows to retrieve the concentration profiles in up to about 4 atmospheric height layers. The observations will be complemented by ozone balloon sondes, continuous measurements of the UV-radiation, and modelling activities using a chemistry transport model. The observations are planned for August to October 2022, the analysis and interpretation from November 2022 until January 2023.