Tropospheric Low Ozone and Its Diurnal Cycle over the Western Pacific Warm Pool from Solar Absorption FTIR observations

Abstract. We present observations of the daytime diurnal cycle of tropospheric column ozone over Palau in the tropical Pacific Warm Pool, based on high-resolution solar absorption Fourier Transform Infrared (FTIR) spectrometry during September–October 2022. The tropospheric column-averaged ozone (surface–10.2 km) showed a distinct diurnal cycle, with concentrations increasing from morning to a midday maximum and declining in the afternoon, primarily reflecting near-surface variability. Relative comparisons with ozonesonde profiles confirm this diurnal pattern. GEOS-Chem model simulations reproduce the daily mean variability but are not able to capture the observed diurnal cycle, underscoring the need for improved representation of local photochemistry and boundary-layer processes in models.

Palau exhibited persistently low column-averaged ozone between 20–30 ppb during the campaign period, reflecting limited precursor availability, efficient convective washout, and advection of clean marine air from the eastern Pacific. Satellite and reanalysis data indicate low aerosol loadings and large cloud droplets, which suppress convective electrification and reduce lightning activity. With lightning providing a key natural source of NO_x, this suppression limits upper-tropospheric ozone and OH production. GEOS-Chem sensitivity simulations confirm that removing lightning NO_x emissions further decreases both species, underscoring how aerosol–cloud interactions indirectly shape a chemically low-oxidizing environment. Given that the Tropical Western Pacific (TWP) is a major pathway for troposphere-to-stratosphere transport, the persistence of low ozone and OH suggests that air can ascend into the stratosphere before reactive species are removed by oxidation, thereby influencing the chemical composition of the lower stratosphere.