For further information please contact: Ivonne TrebsE-mail: ivonne@mpch-mainz.mpg.deTel.: +49-6131-305-306Fax: +49-6131-305-579

I. Trebs(1), F.X. Meixner(1), R.P. Otjes(2), J.J. Slanina(2), P.A.C. Jongejan(2), M. A. L. Moura(3), R. S. da Silva Jr.(3), O.L. Mayol-Bracero(4), P. Artaxo(5), M.O. Andreae(1)

(1) Max Planck Institute for Chemistry, Biogeochemistry Department, Mainz, Germany (2) Energy research Centre of the Netherlands, Petten, Netherlands (3) Universidad Federal de Alagoas, AL, Brasil (4) Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, Puerto Rico, USA (5) Instituto de Física, Universidad de São Paulo, São Paulo, Brasil

ONLINE MEASUREMENTS OF AMMONIA, ACIDIC TRACE GASES ANDONLINE MEASUREMENTS OF AMMONIA, ACIDIC TRACE GASES AND

AEROSOL INORGANIC IONIC SPECIES IN THE AMAZON BASIN UNDERAEROSOL INORGANIC IONIC SPECIES IN THE AMAZON BASIN UNDER

BIOMASS BURNING AND BACKGROUND CONDITIONSBIOMASS BURNING AND BACKGROUND CONDITIONS

I. Introduction

III. Field setup IV. 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. 2319-2330, 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.491- 541, 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 NH3 and HNO3, 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 (NH3), nitric acid (HNO3), nitrous acid (HONO), hydrochloric acid (HCl) and the aerosol species ammonium (NH4

+), nitrate (NO3–), nitrite (NO2

-), chloride (Cl-) and sulfate (SO4

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 10-4 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 HNO3) 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 NH3, HNO3, HONO and HCl mixing ratios (median, 25 % and 75 % quartiles)

Figure 4: Preliminary median aerosol NH4+, NO3

-, NO2- and

Cl- mixing ratios (median, 25 % and 75 % quartiles)

Figure 3 and Figure 4 show mixing ratios of trace gases and inorganic aerosol compounds during the dry/biomass burning season (12th to 23rd Sep. 2002), the transition period (07th to14th Oct. 2002) and the wet season (1st to 14th Nov. 2002). The dominating compounds were ammonia (NH 3) and aerosol ammonium (NH4

+) whose mixing ratios were an order of magnitude larger compared to the other species. The calculated median mixing ratio levels decreased steadily from the biomass burning season through the transition period to the wet season, by approximately 75% for trace gases and by 50% and 75% for aerosol NH4

+ and other inorganic aerosol species, respectively.

VIII. Conclusions

Figure 6: NH3 and HNO3 mixing ratios for the 18- 20 Sept. 2002 (biomass burning season)

Figure 7: HONO and HCl mixing ratios for the 18- 20 Sept. 2002 (biomass burning season)

Figure 5: Aerosol NH4+, NO3

- and SO42- mixing ratios for the 18- 20

Sept. 2002 (biomass burning season) In contrary to aerosol species gaseous HNO3 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, NH3 (Figure 6) and aerosol NH4

+ (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 NOX with surface water.

Mixing ratios of aerosol NH4+

and NO3-

(Figure 5) revealed diel variations. For NO3-

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 NH4

+ did not exactly

follow the behavior of NO3- and increased

during daytime with maxima between 6:00 and 12:00. This unexpected finding might have been caused by NH4

+ levels which were

~10 times higher than NO3- and therefore less

dependent from meteorology (e.g. relative humidity) at day and nighttime. For the measured particulate SO4

2- a pronounced diel variation could not be found and levels were lower than NH4

+ and NO3- mixing ratios.

• Aerosol and trace gas levels measured during biomass burning season declined by at least 50 % through transition period to wet season

• NH3 and aerosol NH4+ were the dominating compounds measured during the experiment

• Mixing ratios of gaseous HNO3 and particulate NO3- were dependent on relative humidity and on

day/ night time boundary layer meteorology• Evaporation of NH3 and HNO3 from aerosols contributed to increased mixing ratios in the

turbulent boundary layer at day time • Increase of relative humidity to ~100% at night time promoted the formation of aerosol NH4NO3

due to gas-aerosol interactions

VII. Example for gas-aerosol interactions

Figure 7: Aerosol [NO3-] : [sumNO3

-] (aerosol NO3- + HNO3) versus

relative humidity measured in the inlet pipe for the data presented in Figures 5 and 6 (100 data points)

R2 = 0.875

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Gaseous and particulate NO3- 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 [NO3

-] : sum [NO3-]

rises with increasing relative humidity. This indicates that at night most of the NO3

- is present in the aerosol phase. The thermodynamic equilibrium between gaseous NH3, HNO3 and aerosol NH4NO3 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, NH4NO3 should be found in the aqueous state. Also, other factors such as air temperature should influence these gas-aerosol interactions.

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