Thessaloniki '91 field measurement campaign—II. Ozone formation in the greater Thessaloniki area

Abstract To assess the potential of photosmog formation and the transport of ozone in the Greater Thessaloniki Area, ozone was measured together with meteorological parameters (wind speed, wind direction, temperature) at nine temporary stations, spread over an area of about 30 × 30 km 2 , in September/October 1991. On three selected days during the campaign the vertical ozone concentrations were measured using a tethered balloon and a small ozone sensor. Ozone levels in the morning in the Greater Thessaloniki Area in September/October have been found to be affected by two interacting sunlight-induced meteorological events, the break-up of the nocturnal inversion layer and the onset of the sea breeze. Ozone volume fractions over 90 ppb were measured at the various stations in the afternoon. The average afternoon concentrations ranged between 50 and 60 ppbv. The daily formation of ozone in the early morning hours and the fairly rapid decrease in the late afternoon result in the typical sigmoidal curves of the variation of ozone concentration with time at those stations which were located around the Bay of Thessaloniki. The local topographical and meteorological conditions gave rise to higher ozone concentrations during the night at those stations which were located at the western slope of the Hortiatis mountain range above the level of nocturnal inversion at about 200 m. Below the nocturnal inversion layer, the ozone concentration decreased rapidly in the evening and at night due to dry deposition and/or to chemical reactions. With the onset of the sea breeze around 8:00 LST in the morning, air masses already loaded with photo-oxidants of the previous day were transported back from the Thermaikos Bay to the Greater Thessaloniki Area. During stormy weather with wind speeds of 15–25 m s −1 low background concentrations of ozone between 30 and 40 ppbv were recorded at all measuring sites during day and night.