| 04 Dec 2025
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 NOx, this suppression limits upper-tropospheric ozone and OH production. GEOS-Chem sensitivity simulations confirm that removing lightning NOx 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.
In “Tropospheric Low Ozone and Its Diurnal Cycle over the Western Pacific Warm Pool from Solar Absorption FTIR observation”, Sun et al. present FTIR-based tropospheric ozone column measurements over the Palau Atmospheric Observatory in the Pacific Warm Pool between September and October 2022. Low ozone is observed and attributed to the low precursor availability, including low lightning activity, and the transport of clean marine air. The authors find a pronounced diurnal cycle of ozone, which cannot be reproduced by GEOS-Chem model simulations.
The paper is well-written and presents interesting observations in a part of the globe that is currently still understudied, but highly important to atmospheric chemistry processes with global implications. I have a few questions and comments. Once these are addressed, the paper will be a valuable addition to the current literature.
Specific comments:
L. 31 f.: An important removal process for O3, that’s missing here, is photolysis and subsequent reaction with H2O, particularly in the marine boundary layer.
L. 33 f.: The reaction of NO with O3 is not really a sink of O3 because NO2 can be photolyzed back to O3 and the interconversion occurs on a short timescale.
L. 250 / Figure 2: Which area does the model represent? Is it the grid extracted at the observatory? How do the error bars look like?
L. 267 / Figure 3: Are these hourly averages including all observation days? I recommend adding error bars to the plot or showing them in the Supplement.
L. 270: Is there really any significant variation in the model throughout the day? Please add error bars.
L. 276: What about the diurnal cycle of other trace gases? And it would be interesting to look at the diurnal cycle of ozone production and loss, since you have the GEOS Chem outputs.
L. 291 f.: Is the pattern significant given the large variability in the data points? Please add error bars.
L. 319: HCHO is not only a precursor, but also an important by-product of O3 formation from VOCs.
L. 325: NOx itself is not removed by precipitation; it can only be removed indirectly after formation of HNO3.
L. 325: Is Rex et al., 2014 really the correct citation here regarding the washout of soluble species?
L. 328 f.: HCHO might not be transported over long distances, but it can be formed locally from longer-lived VOCs, including CH4.
L. 368 / Figure 6: Could the authors also show a panel for NOx to highlight the changes introduced by the sensitivity run?
L. 371: Is the difference really only 7 pptv? If so, that would show that lightning has basically no impact on O3 (< 0.1%). Or is 7 ppbv meant here? However, that would be a surprisingly large impact.
L. 373: I recommend adding a sensitivity study with enhanced lightning, e.g. doubled or tripled lightning NOx emissions. The Warm Pool experiences very low lightning activity and therefore it would be interesting to show the impact of lightning enhancements on O3 and OH as well.
Technical comments:
L. 226 f.: Please double-check the sentence: double use of “available”.
L. 406: Do you mean day-to-day variation here? “Daily” implies “Diurnal” to me.
L. 440: Same here; day-to-day would be better than daily.