Special Case  HIGH CONCENTRATIONS OF OZONE IN THE NIGHT FOR EFFECTS OF CONVECTIVE SYSTEM

1. Special Case HIGH CONCENTRATIONS OF OZONE IN THE NIGHT FOR EFFECTS OF CONVECTIVE SYSTEM A.M. Cordova1,2, L.V. Gatti1, M.A. Silva2, P. Artaxo3 1 Instituto de Pesquisa Energéticas e Nucleares (IPEN), São Paulo, Brazil 2 Instituto de Astronomia e Geofisica, Universidade de São Paulo, São Paulo, Brazil 3 Instituto de Física, Universidade de São Paulo, São Paulo, Brazil e-mail: amcleal @net.ipen.br Laboratório de Química Atmosférica INTRODUCTION Ozone measurements were made in Abracos pasture site as part of the LBA AMC experiment in Rondônia, Brazil (10°46.42’S, 62°20.22’W), from February to May 1999. In this period, the average ozone concentration at night was around 5 ppb. The maximum concentration during daytime used to occur between 13:00 – 15:00 LT with an average value of about 20 ppb. A distinct episode was observed on the February 17th and 18th, showing high concentrations at night and part of the daytime. This episode started at 23:30 LT of February 17th, followed by increasing ozone concentrations to values up to 22 – 30 ppb. The concentration remained in this range until 18:00 LT of February 18th. Figure 3. Profiler at the Ji-Paraná Airport showing downdrafts. Figure 2. Ozone distributions in Rondônia during Feb 12-28, 1999. EXPERIMENT Ozone concentrations have been measured at the surface, 4.0 m above ground, using an ultraviolet absorption instrument (ThermoEnvironment Instrument, Model 49C). The limit detection were 0.05 ppb. The meteorological date were Abracos Pasture tower (LBA-EUSTACH). Figure 1. Ozone concentration in Rondônia during wet season (Feb - May 99). Figure 4. Ozone concentrations, rain, air and surface temperature in the Abracos site in Rondônia on Feb 17-18, 1999. The meteorological data were collected at every 30 min. Figure 5. Relative humidity and net radiation in Abracos site in Rondônia Feb 17-18, 1999. The data were collected at every 30 min in the tower. A B RESULTS AND DISCUSSION Ozone measurements were made at every 5 min during 4 months (Figure 1). There were diurnal variations with minimum concentrations during the night and maximum concentrations in the daytime. The day maximum is reached around 14:00-15:00 LT, when the mixing ratio reached values about 20 ppb (6.5 to 30 ppb). The average ozone concentration at night was around 5 ppb (0.5 to 12 ppb). A distinct episode was observed on the February 17th and 18th, showing high concentrations at night and part of the daytime. Figure 2 shows the ozone distributions for February and increased of ozone concentration for this periods. This episode started at 23:30 LT of February 17th, followed by increasing ozone concentrations values up to 22 – 30 ppb. The concentration remained in this range until 18:00 LT of February 18th. On Feb 17th, the temperature during the day was 35°C, and the relative humidity was 70%. The rain episode between 15:00-18:00 LT accumulated 12.14 mm of rain. During the early morning of Feb 18th two rain episodes were observed, the first between 00:30 –02:30 LT and the second between 3:00 – 5:30 LT (Figure 4). The last one was very important with 19.32 mm of accumulated rain. The maximum radiation in Feb 18th, was 293 W/m2, having no correlation with the ozone concentrations observed during the day (Figure 5). This increase in ozone concentration in Feb 17th, can be related to convective system formed in the northeast region of the Rondonia State, organized into a oriented N-S line. Figure 6 presents GOES-8 satellite image in the infrared, the squall line is formed in Pará State and cloud cluster forming in the east of Bolivia (Figure 6A), and it advances in southwest direction to Rondônia State (Figure 6B). The dry air and the downdraft were intensified and the convergence ahead of the gust front ended up occurring in a more humid area over Rondônia (Figure 6C). The system’s southern portion reached ABRACOS site producing lightning and heavy rain during the night of 17/18 February. At the ABRACOS site, wind direction changes from north to east at about 1:00 LT with enhanced wind speeds thereafter (Figure 6D). Rain starts a few minutes after the gust front arrival(Figure 6E). The profiler of the vertical velocity obtained at the Ji-Paraná Airport shows that downdrafts persisted from just after 1 to 6 LT (Figure 3). The subsidence generated over Rondônia by squall line and other convective systems and the further downdrafts from convective activities suggest a mechanism where high ozone concentration from medium troposphere would be transported to the surface levels. C D E F Figure 6. GOES-8 satellite infrared image sequence showing: (A,B) squall line at northeastern Amazonas and a cloud cluster forming in the east of Bolivia suggesting induction of subsidence over Rondônia; (C,D) propagation of squall line and intense convective activities over Abracos site; (E) clouds associated to rain observation (19mm/4hr); (F) decaying convective activities. Acknowledgements :FAPESP