Origin of Tropospheric Air Masses in the Tropical West Pacific
identified by Balloon-borne Ozone and Water Vapor Measurements from
Palau
Abstract
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.