A detailed description of the WAD/SJAC and the analytical procedures is given elsewhere (Slanina et al., 2001; Wyers et al., 1993). A simplified sketch of the sampling system is shown in Fig. 2. The air flow through the instrument (∼17 l min−1, STP) was generated by a scroll pump outside the wooden house and could be adjusted with a needle valve. The flow was measured continuously (1 min time resolution) with a mass flow meter (Bronkhorst, F-112ACHA-55-V). After the sample air passed the steel elbow and/or pre-impactor and the PFA Teflon tubing, it entered a horizontally aligned WAD that scavenges soluble gaseous species. Trace gases (such as NH3, HNO3, HNO2, HCl and SO2) were collected in a 10−4 M NaHCO3 absorption solution. The liquid input was controlled automatically by an infrared sensor and a switching valve and the liquid was continuously pumped out of the denuder at a flow rate of 0.5– 0.6 ml min−1 by a peristaltic pump. The liquid effluent was collected in a sample reservoir (“gas sample”, see Fig. 2). Artifacts due to evaporation of aerosol phase species in the WAD can be excluded because the characteristic time for formation/evaporation of NH4NO3 is >10 s (Dlugi, 1993), while the mean residence time of the sample air in the WAD is ∼0.002 s (annulus volume: 0.0018 l, flow rate: ∼17 l min−1). After the WAD, the air entered a reservoir where it was mixed with steam of highly purified water. The supersaturation causes aerosol particles to grow rapidly (within 0.1 s) into droplets of at least 2μm diameter. These droplets, containing the dissolved aerosol species were then collected in a cyclone (Khlystov et al., 1995). The cyclone effluent (“aerosol sample”) was transferred into the sample reservoir by a peristaltic pump at a flow rate of 0.5–0.6 ml min−1.

We measured the mixing ratios of ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), hydrochloric acid (HCl), sulfur dioxide (SO2 and the corresponding water-soluble inorganic aerosol species, ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), chloride (Cl- and sulfate (SO42-), and their diel and seasonal variations at a pasture site in the Amazon Basin (Rondônia, Brazil). This study was conducted within the framework of LBA-SMOCC (Large Scale Biosphere Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate). Sampling was performed from 12 September to 14 November 2002, extending from the dry season (extensive biomass burning activity), through the transition period to the wet season (background conditions). Measurements were made continuously using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC) followed by suitable on-line analysis. A detailed description and verification of the inlet system for simultaneous sampling of soluble gases and aerosol compounds is presented. Overall measurement uncertainties of the ambient mixing ratios usually remained below 15%. The limit of detection (LOD) was determined for each single data point measured during the field experiment. Median LOD values (3σ-definition) were ≤0.015ppb for acidic trace gases and aerosol anions and ≤0.118ppb for NH3 and aerosol NH4+. Mixing ratios of acidic trace gases remained below 1ppb throughout the measurement period, while NH3 levels were an order of magnitude higher. Accordingly, mixing ratios of NH4+ exceeded those of other inorganic aerosol contributors by a factor of 4 to 10. During the wet season, mixing ratios decreased by nearly a factor of 3 for all compounds compared to those observed when intensive biomass burning took place. Additionally, N-containing gas and aerosol species featured pronounced diel variations. This is attributed to strong relative humidity and temperature variations between day and night as well as to changing photochemistry and stability conditions of the planetary boundary layer. HONO exhibited a characteristic diel cycle with high mixing ratios at nighttime and was not completely depleted by photolysis during daylight hours.

Five new instruments for semicontinuous measurements of fine particle (PM2.5) nitrate and sulfate were deployed in the Atlanta Supersite Experiment during an intensive study in August 1999. The instruments measured bulk aerosol chemical composition at rates ranging from every 5 min to once per hour. The techniques included a filter sampling system with automated water extraction and online ion chromatographic (IC) analysis, two systems that directly collected particles into water for IC analysis, and two techniques that converted aerosol nitrate or sulfate either catalytically or by flash vaporization to gaseous products that were measured with gas analyzers. During the one‐month study, 15‐min integrated nitrate concentrations were low, ranging from about 0.1 to 3.5 μg m −3 with a mean value of 0.5 μg m −3 . Ten‐minute integrated sulfate concentrations varied between 0.3 and 40 μg m −3 with a mean of 14 μg m −3 . By the end of the one‐month study most instruments were in close agreement, with r‐squared values between instrument pairs typically ranging from 0.7 to 0.94. Based on comparison between individual semicontinuous devices and 24‐hour integrated filter measurements, most instruments were within 20–30% for nitrate (∼0.1–0.2 μg m −3 ) and 10–15% for sulfate (1–2 μg m −3 ). Within 95% confidence intervals, linear regression fits suggest that no biases existed between the semicontinuous techniques and the 24‐hour integrated filter measurements of nitrate and sulfate;, however, for nitrate, the semicontinuous intercomparisons showed significantly less variability than intercomparisons amongst the 24‐hour integrated filters.

Data obtained during the 1999 Atlanta Supersite Experiment are used to test the validity of the assumption of thermodynamic equilibrium between fine particulate (PM2.5) nitrate (NO3-) and ammonium (NH4+) and gas-phase nitric acid (HNO3(g)) and ammonia (NH3(g)). Equilibrium is tested by first calculating the equilibrium concentrations of HNO3(g) and NH3(g) implied by the PM2.5 inorganic composition (i.e, Na+,NH4(+),Cl-, NO3-, and SO42-), temperature, and relative humidity observed at the site. These calculated equilibrium concentrations are then compared to the corresponding observed gas-phase concentrations. The observed PM2.5 composition is based on the 5-min averaged measurements of the Georgia Tech PILS [Weber et al., 2001], while the observed HNO3(g) and NH3(g) concentrations are based on the measurements of Edgerton et al. [2000a] and Slanina et al. [2001], respectively. The equilibrium gas-phase concentrations are calculated using the ISORROPIA model of Nenes et al. [1998]. Out of the entire Atlanta Supersite database, we were able to identify 272 five-minute intervals with overlapping measurements of PM2.5 composition, HNO3(g) and NH3(g). Initial calculations using these 272 data points suggest an absence of thermodynamic equilibrium with the calculated equilibrium NH3(g) generally less than its observed concentration and predicted HNO3(g) generally greater than the observed concentration. However, relatively small downward adjustments in the measured PM2.5 SO42- (or apparent acidity) bring the calculated and measured NH3(g) and HNO3(g) into agreement. Moreover, with the exception of 31 of the 272 data points with either anomalously low observed concentrations of SO42- or NH3(g), there is a close correspondence between the SO42-(or acidity) correction needed for HNO3 and that needed for NH3(slope of 1.04, intercept of similar to0, and r(2)=0.96). The average relative corrections required for equilibrium with HNO3 and NH3 are -14.1% and -13.7%, respectively; significantly larger than the estimated uncertainty arising from random errors in the measurement. One interpretation of our results is that thermodynamic equilibrium does in fact apply to the inorganic PM2.5 composition during the Atlanta Supersite Experiment and either (1) the PM2.5 SO42- concentration measured by the PILS was systematically overestimated by similar to15% or (2) the PM2.5 PILS systematically underestimated the concentration of the alkaline components by similar to15%; and/or 3. The ISORROPIA model systematically underestimated the pH of the PM2.5 encountered during the experiment.

Classical methodology based on the application of filters for sampling, followed by extraction and analysis, introduces severe artifacts for semi-volatile compounds like ammonium nitrate. These filter methods do not meet the requirements for the assessment of the impact of aerosols on acidification, air quality and especially on the radiative balance, in terms of required speed, detection limits and selectivity. These artifacts are avoided by using a steam jet aerosol collector sampler, based on scavenging of aerosols by droplet formation, in combination with on-line analytical techniques such as ion-chromatography for nitrate and membrane separation followed by conductivity detection for ammonium. The SJAC sampler combines very low blanks with high efficiency of collection of particles. The ammonium detector and the IC system, based on 1-point internal standard calibration in combination with correction for curved calibration graphs, enables detection of ammonium and nitrate at background conditions, the detection limit is about 0.02μgm−3 of ammonium and nitrate. Accuracy is, depending on ambient concentration, in the order of 5–10% relative, at a range of 0.05–50μgm−3. The time resolution is 15–120min, depending on required detection limit, and is short enough for continuously monitoring the chemical composition of aerosols. Quality assurance and quality control experiments and intercomparison experiments with classical filter methods, thermo-denuder systems, denuder difference methods and other continuous monitoring techniques have shown that the results are reliable. The instrument has successfully been employed in field campaigns in Europe and the US.

Due to the lack of in-situ measurements of multiphase processes in mixed clouds, the fate of the scavenged pollutant material in mixed phase clouds is poorly understood. In the few numerical models which treat the ice phase in clouds, and the associated chemical processes, it is assumed either that chemical species are completely incorporated into the ice phase upon freezing of droplets or that the incorporation process follows a Henry-like equilibrium. However, it is not known whether this is true or whether part of the material is expelled, redistributed or changed in any way during the transfer process to the ice phase. The scavenging of chemical species in mixed clouds was studied during the EU project CIME (Cloud Ice Mountain Experiment) at the Puy de Dôme, central France (1465 m a.s.l.).

During recent years, it has become clear that ammonia is an important gas in relation to different environmental issues, such as acidification, eutrophication, human health and climate change (through particle formation). Therefore, there is a growing need to develop and apply instrumentation suitable for research into emission, dispersion, conversion and deposition of ammonia and ammonium. Recently, several instruments were developed suitable for measuring concentrations in ambient conditions even at very low levels, such as ammonia sensors suitable for monitoring and research, deposition measuring systems and aerosol samplers for on-line measurement of aerosol composition. These instruments have been tested and applied in a number of field studies. These studies include dry deposition measurements, ammonium nitrate studies in relation to the (in)direct aerosol effect, emission studies and policy evaluation with concentration and deposition monitoring data. The policy evaluation study showed that the measures to reduce ammonia emissions were not as successful as projected beforehand by statistical studies.

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dome in the centre of France. The content of residual aerosol particles, of H2O2 and NH3 in cloud droplets was evaluated by evaporating the drops larger than 5 mum in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 mum diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud.

An automated system for the collection, in-line preconcentration and analysis of acidifying atmospheric gases is described. Collection of HCl, HNO2, HNO3 and SO2 from ambient air is carried out with an annular wet denuder. After addition of an internal standard to the sample now and conductivity suppression, preconcentration of the sample flow is performed by pumping a measured volume of the sample flow through an anion exchange concentrator column. Following the concentration step, the anions of the corresponding acids are analyzed by ion chromatography with conductivity detection.The manifold described with two sample loops and two concentrator columns operating in parallel allows automated collection and analysis of two air samples per hour.A computer programme has been developed for automated performance. It allows the system to run unattendedly during extended periods as all the functions, including data processing, are computer-controlled.Factors affecting the performance are reported. The detection limits achieved are 5.4, 12 and 17 ng/m(3) for HCl, HNO2, HNO3 and SO2 respectively, with relative standard deviations better than 4% at 1-2 mu g/m(3). Ambient measurements for Petten. The Netherlands, are presented which show the capability of the system for continuous monitoring of atmospheric acidifying gases.

A combination of ion chromatography (IC) and a so-called wet denuder system for the measurement of the deposition velocities of components such as HCl, HNO3 and SO2 was tested. For this type of measurement, a precision of better than 5% is required when analysing the absorption solutions of the wet denuder systems. The accuracy and precision of an IC system constructed from commercially available components was tested in the concentration range 10–5000 μg l−1 for chloride, nitrate and sulphate. The output of conductivity, UV and ion-selective electrode detectors was linearized. Setting the calibration accuracy at 5%, a precision of 5% was obtained for sulphate and nitrate at a concentration of 30 μg l−1, and for chloride at 50 μg l−1. A precision of 1% was attained at concentrations of 60 and 400 μg l−1 for sulphate and nitrate, respectively. Accuracies of 5% and 2% were achieved at concentrations of 100 and 200 μg l−1 for sulphate, nitrate and chloride, respectively. Setting the calibration accuracy at 2%, a precision of 5% was achieved at a concentration of 20 μg l−1 for sulphate, nitrate and chloride and a precision of 1 % at concentrations of 60,400 and 500 μg l−1 for sulphate, nitrate and chloride, respectively. An accuracy of 5% was obtained at concentrations of 30,100 and 100 μg l−1 for sulphate, nitrate and chloride, respectively. At a concentration of 200 μg l−1 of these components the accuracy was 2% or better.

An ion chromatographic system is described which is composed of a high-performance separator column, a micro-membrane suppressor system, dedicated software and various detection systems. The system is built for the analysis of fluoride, chloride, nitrate and sulphate in precipitation samples. In this system conductivity detection is used in series with UV detection (for nitrate) and an ion-selective electrode (for fluoride).