1. Study of NO x nocturnal events observed at the Abracos Pasture Site, Rondonia Gatti, Luciana V. (1), Cordova, Ana Maria (1), Yamazaki, Amelia (1), Artaxo, Paulo (2), Silva Dias, Maria A. F. (3), Meixner, Franz X. (4), Kirkman Grant A. (4), Fitzjarrald, David.(5) Fisch, Gilberto (6) 1- Instituto de Pesquisas Energeticas e Nucleares (IPEN), Sao Paulo, Brazil, 2- Instituto de Fisica, Universidade de Sao Paulo, Brazil 3- Instituto Astronomico e Geofisico (IAG), Universidade de Sao Paulo, Brazil, 4- Max Plank Institute for Chemistry, Mainz, Germany 5– University of Albany, United States, 6- CTA/CPTEC e-mail: lvgatti@net.ipen.br INTRODUCTION NO and NO 2 concentrations were measured during Wet (7 Feb to 18 May, 1999) and Dry (12 Sept to 28 Oct, 1999) seasons in the Abracos Pasture site, Rondonia. During both seasons very high nighttime NO and NO 2 concentrations were observed. This process occurs mainly from 6:00 PM to 12:00 PM and from 6:00 AM to 7:00 AM. The measured concentrations in these short episodes can be up to 10 times higher than the daily average. Acknowledgements : Acknowledgements : FAPESP Laboratório de Química Atmosférica RESULTS IT is possible to see in figure 1 and 2 that this process occurs frequently, but the intensity changes every day. In the begining of the night (18:00 to 24:00LT) the NO and NO 2 concentrations avergaes 0.34 and 1.21 ppb in wet season, while in the dry season the values are 0.015 and 3.52 ppb. Latter in the night, (0:00 to 6:00LT) the NO and NO 2 concentrations were measured at 0.25 and 0.75 ppb in wet season, and 0.041 and 2.78 ppb in dry season (figure 3). This enhancement process also occurs with aerosol particles. This fact makes clear that trace gas chemistry is not causing this enhancement, but instead, dynamic caused are responsible for this nocturnal peaks (figure 4 and 5). The occurrence of this high nocturnal short episodes is associated with the dynamics of the nocturnal boundary layer, with the accompanying wind velocity variations associated to mechanical turbulence. The other possibility is that during the night, when net radiation is reduced, there is eventually much reduced cooling at soil surface to maintain the stable potential temperture gradiente. This could cause an enhanced heat, trace gases and aerosol fluxes. The formation of the nocturnal boundary layer represents a decoupling from the atmosphere above it, and induces an increase in the concentration of NO, NO 2 and aerosols. The decoupling provides an increase in wind shear and induces mechanical turbulence, which provides mixing in a deeper layer thus reducing the concentration measured at ground level. Figure 1- NO 2 concentrations during all wet season (7 Feb to 18 May). The data were collected at every 5 min. Figure 2- NO and NO 2 concentrations during all dry season (12 Set to 28 Oct). The data were collected at every 5 min. Figure 3- Diurnal variation of NO 2 concentrations during all wet and dry season. Figure 4 – NO, NO 2 concentrations and CPC (particles/cm³) during dry season (12 Set to 28 Oct). The data were collected at every 5 min to NOx and 20 sec for CPC. Figure 5 – NO, NO 2 concentrations, CPC (particles/cm³) and black carbon during wet season (24 - 25 March). The data were collected at every 5 min to Nox, 20 sec for CPC and 20min for Black Carbon. Figure 6 – NO, NO 2 concentrations, Wind speed 5 m and 50 m during wet season (17 Feb). The data were collected at every 5 min to Nox, 1 min wind speed 5 m and 15 min for 50 m10.