21 Jul 2023
 | 21 Jul 2023

Air Mass Transport to the Tropical West Pacific Troposphere inferred from Ozone and Relative Humidity Balloon Observations above Palau

Katrin Müller, Ingo Wohltmann, Peter von der Gathen, and Markus Rex

Abstract. Due to the unique local air chemistry, the transport history of tropospheric air masses above the remote tropical West Pacific (TWP) is reflected by local ozone (O3) and relative humidity (RH) characteristics. In boreal winter, the TWP is the main global entry point for air masses into the stratosphere and therefore a key region of atmospheric chemistry and dynamics. However, a long-term in situ monitoring of tropospheric O3 to assess the variability of TWP air masses and the respective controlling processes has yet been missing. The aim of our study was to identify air masses with different origins and pathways to the TWP and their seasonality using the new Palau time series (2016–2019) of mostly fortnightly Electrochemical Concentration Cell ozone and radio soundings. Based on monthly statistics of O3 volume mixing ratios and RH we defined a free tropospheric locally-controlled background and analyzed anomalies for both tracers in the 5–10 km altitude range. We found that anomalously high O3 indicates a remote origin, while RH is controlled by a range of dynamical processes resulting in a bimodality in RH anomalies. The Palau time series confirms a year-round presence of low O3 background air masses and a seasonal mid-tropospheric cycle in O3 with a prominent anti-correlation between O3 volume mixing ratios and RH. We assumed five different types of air masses with differing tracer characteristics and origin which we validated by analyzing backward trajectories calculated with the transport module of the Lagrangian chemistry and transport model ATLAS. The main result is a clear separation of origin and pathways for the two most contrasting types of air masses, i.e. ozone-poor and humid versus ozone-rich and dry air. Both, potential vorticity and air mass origin analyses, reveal no indication for stratospheric influence for the ozone-rich dry air masses. Rather, we found indications for O3 production due to biomass burning or anthropogenic pollution at the origins of these air masses and drying due to clear sky subsidence during long-range transport. The seasonal occurrence is tied to the position of the Intertropical Convergence Zone which opens a pathway from potential source regions which are confirmed by the trajectory analysis.

We conclude, that dominant ozone-poor and humid air masses are of local or Pacific convective origin and occur year-round, but dominate from August until October. Anomalously dry and ozone-rich air is generated in Tropical Asia and subsequently transported to the TWP via an anti-cyclonic route, mostly from February to April. The areas of origin suggest different sources of ground pollution as a cause for O3 production. We propose large-scale descent within the tropical troposphere and subsequent radiative cooling in connection with the Hadley circulation as responsible for the vertical displacement and dehydration.

Katrin Müller et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1518', Anonymous Referee #1, 01 Sep 2023
  • RC2: 'Comment on egusphere-2023-1518', Anonymous Referee #2, 05 Nov 2023

Katrin Müller et al.

Katrin Müller et al.


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Short summary
The transport history of tropospheric air masses above the remote tropical West Pacific is studied by local ozone and relative humidity profile measurements from Palau. A prominent anti-correlation between both tracers separates air masses of different origin and genesis. Back trajectories confirm a local convective origin of the year-round humid ozone-poor background. Anomalously dry ozone-rich air is generated in Tropical Asia by pollution and dehydrated during transport via radiative cooling.