Seasonal and intra-seasonal variations of ozone over the Indochina Peninsula

1. Introduction

Atmospheric ozone plays an important role in determining dynamical and thermal structures of the atmosphere through radiative and chemical processes. Therefore, it is important to know the 3-dimensional distribution of ozone and its temporal variability on the global basis. However, continuous ozone observations in the upper air have not been carried out so far over the Indochina Peninsula. We have performed continuous ozonesonde observations in Hanoi, Vietnam since September 2004 as a part of SOWER (Soundings of Ozone and Water in the Equatorial Region) project. Here, we introduce preliminary results of the observations.

2. Observation

We started monthly ozonesonde observations in Hanoi in September 2004. The observation frequency was increased to twice-monthly in February 2006, which has been partly supported by SHADOZ (Southern Hemisphere ADditional OZonesondes) and KAGI21 (Kyoto university Active Geosphere Investigations for the 21st Century COE Program). We also conducted intensive observation campaigns three times in winter, which were a part of the SOWER's multi-station observations over the western Pacific, the Maritime Continent and the Indochina Peninsula. A period of each campaign was about 2 weeks and in each campaign we launched ozone and water vapor sondes at an interval of a few days.

3. Seasonal variation of ozone

First we described seasonal variations of ozone from the troposphere to the stratosphere. In the stratosphere, we found a clear seasonal cycle in ozone mixing ratio above 25 km height with maxima in summer and with minima in winter, while we did not find significant seasonal variation below 25 km. This clear seasonal cycle is consistent with well-known seasonal variation of stratospheric ozone. In the upper troposphere (10-15 km), we found the similar seasonal variation with maxima in summer and with minima in winter. In the lower troposphere (0-5 km), the large values in ozone mixing ratio were found in winter to spring (late dry season). This ozone increase is possibly related to the biomass burnings which frequently occur in this season. In the middle troposphere (5-10 km), the seasonal variability was not significant.

4. Intra-seasonal variation of ozone

A prominent intra-seasonal variability was found in the upper tropospheric ozone. The backward trajectory analysis showed that this variability caused by the difference of transportation paths, that is, the air parcels with large ozone mixing ratio were transported from the mid-latitude region west of the Indochina Peninsula, while the small values of ozone mixing ratio tended to be transported from the Maritime Continent. The result suggests that the alternation of the mid-latitude stratospheric air and the equatorial tropospheric air takes place over Hanoi with the intra-seasonal time scale. Therefore, the Indochina Peninsula is one of the key regions for investigating the quasi-horizontal stratosphere-troposphere exchange.

5. Summary

We have conducted almost 3 years continuous ozonesonde observations in Hanoi, Vietnam. Seasonal variations in ozone in the troposphere and the stratosphere were shown as preliminary results of the observations. Annual cycle with maxima in summer and with minima in winter is found in the middle stratosphere and the upper troposphere, while the maxima in winter to spring (late dry season) were found in the lower troposphere. The upper tropospheric ozone showed a strong intra-seasonal variability. This variability was caused by the difference of transportation paths.