For further information please contact: Ivonne Trebs Tel.: Fax: ONLINE MEASUREMENTS OF AMMONIA, ACIDIC TRACE GASES AND AEROSOL INORGANIC IONIC SPECIES IN THE AMAZON BASIN UNDER BIOMASS BURNING AND BACKGROUND CONDITIONS I. Introduction III. Field setupIV. Overview of preliminary results References: J. J. Slanina et al. (2000): The continuous analysis of nitrate and ammonium by the steam jet aerosol collector (SJAC): extension and validation of the methodology, pp , Atmospheric Environment 35 J.H. Seinfeld and S.N. Pandis (1998): Thermodynamics of aerosols, in Atmospheric Cemistry and Physics- from Air Pollution to Climate Change, pp , John Wiley & Sons, Inc. VI. Processed data: trace gasesV. Processed data: aerosol species II. Method Water-soluble inorganic aerosol species and soluble gases, such as NH 3 and HNO 3, are expected to play a major role in the nucleation and growth of cloud droplets under clean and polluted conditions. We measured diel and seasonal variations in the mixing ratios of ammonia (NH 3 ), nitric acid (HNO 3 ), nitrous acid (HONO), hydrochloric acid (HCl) and the aerosol species ammonium (NH 4 + ), nitrate (NO 3 – ), nitrite (NO 2 - ), chloride (Cl - ) and sulfate (SO 4 2- ) in the Amazon Basin (Rondônia, Brazil) from September to November 2002 (LBA-SMOCC ( * ) ). Sampling was performed using a wet- annular denuder in combination with a Steam- Jet Aerosol Collector (SJAC) followed by online analysis. Measurements were supported by monitoring of meteorological quantities (e.g. relative humidity and air temperature). Our online measurements provide important information of the gas-aerosol interactions due to temperature and humidity changes within the planetary boundary layer. ( * ) LBA = Large Scale Biosphere-Atmosphere experiment in Amazonia SMOCC = Smoke Aerosols, Clouds, Rainfall and Climate-Aerosols from Biomass Burning Perturb Global and Regional Climate The sampled air (~17l min -1 ) is stripped from atmospheric trace gases using a wet annular denuder system employing a M carbonate absorption solution. After the air passed the denuder it enters a mixing reservoir where steam is injected. Aerosol particles grow rapidly within 0.1s into droplets of at least 2  m diameter which are collected in a cyclone. Gas and aerosol sample are analyzed separately using ion chromatography with chemical suppression for anions and flow injection analysis for ammonium. The optimal time resolution of the measurements is 20 min. The detection limit varies according to the ion analyzed from 30 ppt to 50 ppt in ambient air. The sampling system was described earlier in detail (Slanina et al., 2000). Figure 1: Scheme of the denuder- SJAC system Figure 2: Inlet system Sampling was performed on a pasture site (10.46°S, 62.22°W) which had been deforested 25 years ago. The instrument was located in an air conditioned hut. To minimize losses of atmospheric trace gases (especially HNO 3 ) and to match an optimal sampling fetch a polyethylene inlet pipe (h= 530 cm, d= 7 cm) was used (Figure 2). The properties of the air passing the pipe were monitored with an air velocity sensor (Velocicalc, TSI Instruments). The air flow was generated by a ventilator in the pipe bottom and was adjusted to meet (as close as possible) laminar conditions to minimize the contact of the air with the walls of the pipe. The average air velocity measured during the experiment was 1 ms -1. Assuming the walls of the pipe as a total sink of atmospheric trace gases the losses calculated ranged from 5 % to 39 %. Since a polyethylene surface is not considered to be a total sink of the measured gases the actual losses are expected to be much lower. The calculation of maximum aerosol losses due to non-isokinetic sampling between inlet pipe and the inlet of the sampling system itself resulted in 5%. This inlet system provided a useful tool to verify and optimize the sampling performance. Figure 3: Preliminary median NH 3, HNO 3, HONO and HCl mixing ratios (median, 25 % and 75 % quartiles) Figure 4: Preliminary median aerosol NH 4 +, NO 3 -, NO 2 - and Cl - mixing ratios (median, 25 % and 75 % quartiles) VIII. Conclusions Figure 6: NH 3 and HNO 3 mixing ratios for the Sept (biomass burning season) Figure 7: HONO and HCl mixing ratios for the Sept (biomass burning season) Figure 5: Aerosol NH 4 +, NO 3 - and SO 4 2- mixing ratios for the Sept (biomass burning season) In contrary to aerosol species gaseous HNO 3 and HCl mixing ratios (Figure 6 and 7) showed highest values during the day. This may be due to (1) mixing processes in the turbulent boundary layer during daytime and a stable thermal stratification of the nocturnal surface layer at nighttime (limiting HNO 3 and HCl supply from residual layer), (2) higher temperature and lower relative humidity during daytime and (3) daytime photochemistry. Both, NH 3 (Figure 6) and aerosol NH 4 + (Figure 5) revealed daily peaks between 6:00 and 12:00. Currently, the sharp decrease after 12:00 cannot be explained and needs further investigations. HONO (Figure 7) shows depletion at day time due to photolysis and probable formation at night by reaction of NO X with surface water. Mixing ratios of aerosol NH 4 + and NO 3 - (Figure 5) revealed diel variations. For NO 3 - we found low mixing ratio levels during the day and high values during nighttime which may be result of the strong relative humidity increase at night. Aerosol NH 4 + did not exactly follow the behavior of NO 3 - and increased during daytime with maxima between 6:00 and 12:00. This unexpected finding might have been caused by NH 4 + levels which were ~10 times higher than NO 3 - and therefore less dependent from meteorology (e.g. relative humidity) at day and nighttime. For the measured particulate SO 4 2- a pronounced diel variation could not be found and levels were lower than NH 4 + and NO 3 - mixing ratios. VII. Example for gas-aerosol interactions Figure 7: Aerosol [NO 3 - ] : [sumNO 3 - ] (aerosol NO HNO 3 ) versus relative humidity measured in the inlet pipe for the data presented in Figures 5 and 6 (100 data points) Gaseous and particulate NO 3 - versus relative humidity (measured in the inlet pipe) is shown in Figure 7. Relative humidity above 80 % corresponds to nighttime and below 80 % to daytime. The ratio aerosol [NO 3 - ] : sum [NO 3 - ] rises with increasing relative humidity. This indicates that at night most of the NO 3 - is present in the aerosol phase. The thermodynamic equilibrium between gaseous NH 3, HNO 3 and aerosol NH 4 NO 3 is shifted towards the aerosol phase at higher relative humidity (Seinfeld and Pandis, 1998). Since relative humidity at night exceeded the deliquescence point of the aerosols, NH 4 NO 3 should be found in the aqueous state. Also, other factors such as air temperature should influence these gas-aerosol interactions.