The Long-Term coupling between column ozone and tropopause properties
The horizontal and vertical distributions of ozone are very important in determining atmospheric chemical composition, radiation and circulation. This is because ozone absorbs harmful ultraviolet radiation and determines the stratospheric temperature profile. Ozone also plays a critical role in the production of hydroxyl radicals (OH), which are important oxidants, producing photochemical smog in the troposphere.
The observational data of the vertical temperature distribution and column ozone, obtained from two stations in Iran, are analyzed in order to explore the tropopause variations in conjunction with the dynamical variability in column ozone. The data used in this study are daily vertical temperature profiles in Tehran (Mehrabad station) and Esfahan station; and total ozone content (TOC) that extracted from TOMS(Total Ozone Mapping Spectrometer)site
,obtained for the period 1989-1999. The profiles of temperature have been measured by high-resolution radiosonde ascents at two stations. Radiosond was launched twice a day (0000 and 1200 UTC). For all soundings, the thermal tropopause was defined according to the standard World Meteorological Organization (WMO) criterion (UNEP/WMO 1957) as the lowest level in the free troposphere where the temperature lapse rate becomes smaller than 2 K km-1 for a layer of at least 2-km thickness. In order to avoid interference from photochemical changes (which follow the annual cycle induced from solar radiation), averaging was performed separately for periods around summer [May–June–July (MJJ)] and around winter [November–December–January (NDJ)] during the period 1989–1999.
From the analysis presented, it is evident that the summer distribution of the frequency of occurrence of the tropopause height (TH) over Tehran has maximum around 12-13 km in summer. The radiosonde ascents performed at Tehran are subsequently examined by focusing on the appearance of the second thermal tropopause. The frequency occurrence of the second thermal tropopause is less frequent during the recent time period. An investigation of the seasonal variability in the frequency of occurrence of the second thermal tropopause showed that it becomes less frequent as time goes on, in all seasons.
Total ozone concentration has seasonal variation. Significant differences are observed between the values of TOC in the MJJ and NDJ periods over Tehran. In the MJJ period is around 270 DU and in the NDJ period, TOC is 320 DU. The linear regression analyses of the monthly mean column ozone and tropopause height (TH) indicate the inverse relationship between column ozone and tropopause height that the tropopause variations might be responsible for about a quarter of the observed TOC trend over Tehran.