Characterization of ozone and the oxidizing capacity of the tropical West Pacific troposphere

Abstract

Motivated by previous measurements of very low tropospheric O3 concentrations in the Tropical West Pacific (TWP) and the implied low oxidizing capacity of this key region for transport into the stratosphere, we set up an atmospheric research station in the center of the warm pool in Palau (7°N 134°E) and established a continuous balloon-borne O3 measurement program with Electrochemical Concentration Cell (ECC) ozonesondes. Preparation, execution and quality management of the soundings have been an integral part of this doctoral project, with this final thesis as the first presentation of the unprecedented 4-year time series. Furthermore, we show the development and first measurements of an instrumental device for in-flight monitoring of the so-called background current in ECC sondes. Our analysis of the O3 observations at Palau from 01/2016-10/2019 confirms the year-round dominance of a humid, low O3 background in the mid-troposphere in this part of the TWP. Distinct layers of enhanced O3 are often anti-correlated with water vapour (H2O), occur frequently and show a strong seasonality. In the 5-10 km altitude range, these so called O3+RH- layers were observed in 71 % of profiles from February until April (FMA) compared to 25 % from August until October (ASO). We quantified the seasonal differences by defining a monthly atmospheric background profile for O3 and relative humidity (RH) based on observed statistics. Back trajectory calculations confirm that the mid-tropospheric background is controlled by local convective processes and the origin of air masses is the Pacific Ocean, mainly East of Palau. Dry and ozone-rich air (O3+RH-) originates in tropical Asia and reaches Palau in anticyclonic conditions. This supports several studies which attribute O3 enhancement against the low O3 background to pollution events on the ground. We found no stratospheric influence and thus propose large-scale descent within the tropical troposphere as responsible for dehydration of air masses. The relevance of O3+RH- layers led us to investigate their influence on the otherwise low oxidizing capacity of the TWP. Chemical box model calculations for clear sky conditions at 400 hPa in Palau in ASO revealed that the predominant anti-correlation of O3 with H2O buffers the effect of both constituents on daily mean OH concentrations.