عنوان مقاله [English]
نویسنده [English]چکیده [English]
In this study, the daily Total Ozone Column (TOC) measured by the instruments of TOMS (2001) and OMI (2005-2011) satellites and Brewer ground station (2002-2004) is used to investigate the extreme ozone mini-holes over Esfahan. Based on previous reports on validation of the TOC data products, it is found that there is no problem with homogenization of data records, which was provided by the above measuring instruments. Firstly, it is shown that the TOC monthly mean and standard deviation over central Iran depend on the seasonal cycle with maximum values of 298 and 27 DU in winter and minimum values of 270 and 8 DU in summer, respectively. The difference between the maximum and minimum climatological monthly means is 53 DU. Regarding the absolute values of TOC, the maximum (minimum) amplitude is related to the winter season with 169 DU in Feb (summer with 39 DU in Aug). Due to the minus twice standard deviation of the monthly average which is known as the threshold chosen to identify the possible ozone mini-holes, 25 events are detected during the study period with maximum concentrations, of which 16 and 7 cases occurred in autumn and winter seasons, respectively. The most occurrences of ozone mini-hole are seen in 2005 and 2011 with 7 and 6 events, respectively. It is worthwhile to mention that the lowest levels of ozone in Arctic were also seen during the two mentioned years from 2001 to 2011. Nevertheless, no mini-holes were detected for three years 2003, 2004 and 2009. The range of ozone negative anomalies is confined from around 24% in winter (Jan) to 6% in summer (Aug). However, it was reported that ozone mini-holes in some regions have reduced the TOC up to 40% of climatology mean of mid and high latitudes over the northern hemisphere. It is found that during ozone mini-hole events, the Tropopause Height (TH) tends to move upwards (with a maximum of 5.5 km higher than monthly average on 10 March 2008) which in turn leads to decrease in the temperature and pressure of TH. Similar to its seasonal cycle, the low observed values of the tropopause temperature and pressure in summer is stronger than winter season. In general, the ranges of temperature (pressure) in the thermal tropopause during low ozone events becomes from -2.3°C (-27 hPa) in February 2006 to -15.5°C (-115 hPa) in March 2008. However, the mentioned above pattern almost explains the maximum events, the observed ozone mini-holes in January 2002 don not show similar anomalies in TH. It is more probably that low ozone events during the January of 2002 are more related to the meridional transport of air masses with climatology low ozone from the subtropical latitude which is poleward near the tropopause. Backward trajectory analysis also showed that the origins of poor ozone air masses in the spring/summer (autumn/winter) seasons are related to the eastern areas (western areas) of Iran. On 7 Jan 2002 at 16 km altitude (on 16 Oct 2011 at 22km altitude), the lower part of trajectory analysis, is more characterized by horizontal movement of poor ozone air mass from lower latitude (higher latitude). During the two extreme low ozone events over Esfahan which approximately correspond to the deepest events and eventful periods, two broad ridges are seen over coastal line of North-West Europe along with two deep troughs in the eastern-central Mediterranean Sea. The blocking ozone mini-holes over North-West Europe are related to the upward movement of geopotential height in the upper troposphere and lower stratosphere (UTLS) region which is in agreement with both the advection of poor ozone air from the sub-tropical (7 Jan 2002) and the higher latitudes (16 Oct 2011) toward the mid latitudes over central Iran.